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NBS TECHNICAL NOTE 1179

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

A Calibration Service for

Wattmeters and

Watthour Meters

VOD
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act ol Congress on March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology

and facilitate their effective application for public benefit. To this end, the Bureau conducts

research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau's technical work is per- formed by the National Measurement Laboratory, the National Engineering Laboratory, and

the Institute for Computer Sciences and Technology.

THE NATIONAL MEASUREMENT LABORATORY provides the national system of

physical and chemical and materials measurement; coordinates the system with measurement

systems of other nations and furnishes essential services leading to accurate and uniform

physical and chemical measurement throughout the Nation's scientific community, industry,

and commerce; conducts materials research leading to improved methods of measurement,

standards, and data on the properties of materials needed by industry, commerce, educational

institutions, and Government; provides advisory and research services to other Government

agencies; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Absolute Physical Quantities^ — Radiation Research — Chemical Physics —

Analytical Chemistry — Materials Science

THE NATIONAL ENGINEERING LABORATORY provides technology and technical ser- vices to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement capabilities;

provides engineering measurement traceability services; develops test methods and proposes

engineering standards and code changes; develops and proposes new engineering practices;

and develops and improves mechanisms to transfer results of its research to the ultimate user. The Laboratory consists of the following centers:

Applied Mathematics — Electronics and Electrical Engineering^ — Manufacturing

Engineering — Building Technology — Fire Research — Chemical Engineering^

THE INSTITUTE FOR COMPUTER SCIENCES AND TECHNOLOGY conducts

research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and

economy in Government operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing the Federal Information Processing Standards Program, developing Federal ADP standards

guidelines, and managing Federal participation in ADP voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and

provides the technical foundation for computer-related policies of the Federal Government.

The Institute consists of the following centers:

Programming Science and Technology — Computer Systems Engineering.

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Washington, DC 20234. ^Some divisions within the center are located at Boulder, CO 80303.
A Calibration Service for

Wattmeters and

Wattiiour IVIeters

J. D. Ramboz and R. C. McAuliff

HATIQ^iAL BUKt, Of STA'i'DARD!

AUG 2 5,1^"

Electrosystems Division

Center for Electronics and Electrical

Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

^^^EAu of

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued July 1983
NATIONAL BUREAU OF STANDARDS TECHNICAL NOTE 1179

Natl. Bur. Stand. (U.S.), Tech. Note 1179, 111 pages (July 1983)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1983

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402

Price $4.75

(Add 25 percent for other than U.S. mailing)
TABLE OF CONTENTS

Page

LIST OF FIGURES vi

LIST OF TABLES ix

Abstract 1

1. INTRODUCTION 1

2. NATIONAL STANDARDS FOR ENERGY 2

3. WATTHOUR METER CALIBRATION SERVICES AT NBS 5

4. CALIBRATION OF THE NBS WATTHOUR METER REFERENCE BANK 6

4.1 Reference Watthour Meters -- General Description 8

4.2 Current Comparator Power Bridge 9

4.3 Thermal Power/Energy Meter 12

4.4 Calibration of the NBS Reference Bank 14

5. ENERGY COMPARISON SYSTEM 15

5.1 Watthour Meter Comparison Philosophy 15

5.1.1 Primary Circuit Configuration 17

5.1.2 Comparison System for Watthour Meters

With Registers 17

5.1.3 Comparison System for Watthour Meters

With Pulse Outputs 21

5.2 Comparison System Hardware 23

5.2.1 Power Supply and Control Hardware and Circuits 23

5.2.2 Precision Voltage and Current Transformers 24

5.2.3 Signal Conditioning and Pulse-Counting Electronics .... 27

5.2.4 Auxiliary Electronic Circuits 30

5.2.4.1 Line Frequency Circuits 30

5.2.4.2 Gated Relay 30

5.3 Test Bench Wiring and Features 32

6. PROCEDURE FOR CALIBRATING WATTHOUR METERS 32

6.1 Initial Inspection and Preparation 32

6.2 Installation of the Watthour Meter 33

6.3 Testing Procedures 33

6.4 Data Analysis 35

7. REPORT OF CALIBRATION 36

m
TABLE OF CONTENTS (cont.)

Page

8. ERROR ANALYSIS 36

8.1 Current Comparator Power Bridge 40

8.2 Thermal Power/Energy Standard 43

8.3 NBS Reference Bank 43

8.4 Energy MAP Transport Standards 43

8.5 Calibration of Customers' Meters 46

8.5.1 High Accuracy Calibrations 46

8.5.2 Routine Calibrations 47

9. ELECTRICAL ENERGY MEASUREMENT ASSURANCE PROGRAM 47

9.1 Transport Standards 50

9.2 Measurement Process 50

9.3 MAP Data Analysis 51 9.4 MAP Results 52

10. INTERNATIONAL COMPARISONS OF ELECTRICAL ENERGY STANDARDS 52

11. SUMMARY 58

12. ACKNOWLEDGMENTS 59

13. REFERENCES 60

APPENDICES

APPENDIX A — NBS CALIBRATION SERVICES OF WATTHOUR METERS 61

A.l Technical Information 61

A. 2 Calibration Requests, Watthour Meter Shipping,

Insurance and Risk of Loss 61

APPENDIX B — WATTHOUR METER COMPARISON SYSTEM HARDWARE

AND CIRCUITS 65

B.l Power Supply and Control Hardware Circuits 65

B.2 Signal Conditioning and Pulse-Counting Electronics 72

B.2.1 Input Connections 77

B.2. 2 Signal Conditioning Circuits 77

B.2. 3 Pulse Adder and Gate Circuits 81

B.2. 4 Display Select Gates 84

B.2. 5 Auxiliary Electronic Circuits 87

IV
TABLE OF CONTENTS (cont.)

Page

B.2.5.1 Line Frequency Circuits 87

B.2,5,2 Standard Second Signal Circuit 87

B.2.5.3 Auxiliary Schmitt Trigger Circuit 91

B.2,5.4 Variable Threshold Schmitt Trigger Circuit ... 91

B.2.5.5 Pulse Delay Circuit 91

B.2.5.6 Pulse Divide-by-10 Circuit 91 B.2.5.7 Miscellaneous Gate Circuits 94

B.2.5.8 Reset and Start Circuit 94

APPENDIX C — INTEGRATED CIRCUIT COMPONENT LISTING 97

APPENDIX D — DETAILED PROCEDURE FOR THE CALIBRATION

OF WATTHOUR METERS 99

D.l Installation of and Connections to the Watthour Meter 99

D.2 Testing Procedures 100
LIST OF FIGURES

Page

Figure 2-1. Major standards used by NBS in the calibration support

of electrical energy and power 3

Figure 4-1. Diagram of the major measurement steps used to calibrate

the NBS reference bank watthour meters. Also shown are subsequent calibrations of customers' meters 7

Figure 4-2. Current comparator power bridge configuration with the

dc-to-ac thermoelement comparator and dc voltage

measurement instrumentation (for unity power factor) .... 10

Figure 4-3. Simplified schematic of the current comparator power

bridge used at half power factor. Polarity of the

transformer providing voltage Vg as shown is for a lagging power factor; for a leading power factor,

the transformer is changed to make it non-inverting 11

Figure 4-4. Principal components of the thermal wattmeter 13

Figure 5-1. View of the four rotating watthour meters inside the

thermally controlled enclosure. These meters comprise

the NBS reference bank 16

Figure 5-2. Basic wiring connections for the voltage and

current circuits for the four reference watthour

meters and the watthour meter being tested 18

Figure 5-3. Basic configuration for the comparison calibration

of a watthour meter having registers 19

Figure 5-4. Basic configuration for the comparison calibration of a watthour meter having electrical pulse outputs 22

Figure 5-5. Details of the current circuit showing the two-stage

amplifier-aided precision current transformer 26

Figure 5-6. View of the equipment panels for the signal conditioning

and pulse-counting circuitry 28

Figure 5-7. Simplified block diagram for the signal conditioning

and pulse-counting electronics 29

Figure 5-8. Typical wave shape of the output of a slotted-disc type

watthour meter (top) and signal conditioned output

after the Schmitt trigger circuit (bottom) 31

Figure 6-1. Typical data worksheet for a watthour meter having

electrical pulse outputs 34

VI
LIST OF FIGURES (cont.)

Page

Figure 7-1 (a). Report of Calibration for a typical watthour meter .... 37

Figure 7-1 (b). Report of Calibration for a typical watthour meter .... 38

Figure 8-1. Major steps and uncertainties for the entire calibrations

process for watthour meters at NBS. This figure

traces the measurement chain from the NBS power bridge

to the routine calibration of customers' watthour

meters. The uncertainties (in ppm) of each measurement

step for unity and half power factors are shown in

parentheses, where the first value is the estimated

systematic uncertainty and the second value is the

estimated 3a//R random component of uncertainty.

The values used for the transport standard are typical

random uncertainties at NBS; for the actual values

for each MAP transport standard, see table 8-3 39

Figure 8-2. Measurement results for the NBS reference bank at

unity power factor. Residual standard deviation is

0.0020% 44

•Figure 8-3. Measurement results for the NBS reference bank at

half power factor. Residual standard deviation is

0.0068% 45

Figure 8-4. Measurement results for the NBS monitor watthour

meter (check standard) at unity power factor.

Residual standard deviation is 0.0039% 48

Figure 8-5. Measurement results for the NBS monitor watthour

meter (check standard) at half power factor lagging.

Residual standard deviation is 0.0066% 49

Figure 9-1. Measurement results from the MAP for electric energy

for three large utility companies. Tests made at

unity power factor. (See eq (9-2).) 53

Figure 9-2. Measurement results from the MAP for electric energy

for the same three utility companies as shown in

figure 9-1. Tests made at half power factor lagging.

(See eq (9-2).) 54

Figure 10-1. Results from the international comparison of electrical

power and energy at unity and half power factors 56

vn
LIST OF FIGURES (cont.)

Page

Figure B-1 . Watthour meter calibration system power sources,

switch selectable between the 208-V three-phase power

lines or the three-phase electronic power supply 66

Figure B-2. Voltage, current, and power factor controls 67

Figure B-3. Phasor diagram for the output voltage from the

current adjustment controls and isolating inversion

transformer (transformer TIO of fig. B-2) 69

Figure B-4. Phasor diagram for the output voltage Vp for the

voltage circuits for the watthour calibration system .... 70

Figure B-5. Phasor diagram showing the current, voltage, and

power factor relationships for power factors of

unity, half lag, and half lead 71

Figure B-6. Switch selectable system output voltages for

120, 240, or 480 V 73

Figure B-7. System current supply transformers and link terminals

for output currents from 3 to about 1 00 A 74

Figure B-8. Current supply transformer linking arrangements

for different current ranges. Light lines are permanently wired; heavy lines are externally

connected jumpers used to complete the circuits 75

Figure B-9. Detailed interconnection diagram for the

signal conditioning and pulse-counting electronics 76

Figure B-10. Pulse input connections for the four stations of

the test watthour meters 78

Figure B-1 1 . Pulse input connections for the four reference

watthour meters of the NBS reference bank 79

Figure B-12. Signal conditioning circuits for the wave shaping

of the output pulses from the four test and four

reference channels (circuit boards E and F) 80

Figure B-1 3. Pulse-adding circuits for the conditioned outputs

of the four reference watthour meters 82

Figure B-14. Simple clock circuit used to drive the 4-bit cascaded

priority registers in the pulse-adding and gating

circuits 83

vm
LIST OF FIGURES (cont.)

Page

Figure B-15. Pulse-gating and clocking circuit for the four test

channels 85

Figure B-16. Counter display selector circuits 86

Figure B-17. Schmitt triggers on the auxiliary board 88

Figure B-18. Line frequency phase-locked-loop circuitry.

The 60-Hz input line frequency is regenerated into

a phase-locked 1000-Hz signal 89

Figure B-19. A retriggerable monostable multivibrator used to

generate a standard one-second pulse from WWV

broadcast signal 90

Figure B-20. Pulse delay circuit; delays input pulse by 10 ms

to 10 s, adjustable by the 10-kJ^ variable resistor 92

Figure B-21 . Pulse divide-by-10 circuit where the output pulse

rate is one-tenth that of its input 93

Figure B-22. Miscellaneous gate circuitry and gated relay power 95

Figure B-23. Reset and start circuitry 96

LIST OF TABLES

Page

Table 5-1. Available test voltages in volts 25

Table 5-2. Available test currents in amperes 27

Table 8-1. Systematic uncertainties for current comparator

power bridge in ppm at unity power factor 41

Table 8-2. Systematic uncertainties for current comparator

power bridge in ppm at 0.5 power factor 42

Table 8-3. Random uncertainties of five typical MAP transport

standards 46

Table 10-1. Calibrations of the PTB Wattmeter 57

Table 10-2. NBS/NRC(IEE) Comparison using commercial wattmeters 58

1x
LIST OF TABLES (cont.)

Page

Table C-1 . Integrated circuit (IC) component listing for signal

conditioning and pulse-counting electronics . . e 97

Table C-2. IC nomenclature for components used in signal conditioning

and pulse-counting electronics 98

Table C-3. Circuit board nomenclature, functions, and figure

reference 98
A CALIBRATION SERVICE FOR WATTMETERS AND WATTHOUR METERS

J. D. Ramboz and R. C. McAuliff

An NBS calibration service for wattmeters and watthour

meters is described. The service offers measurements of percentage

registration for watthour meters and percentage correction for

wattmeters over a range of voltages and currents at a frequency of

60 Hz, Measurements are limited to power factors of 1.0 and 0.5,

leading and lagging. The Measurement Assurance Program (MAP) for

electric energy is discussed. National standards for electric

energy, NBS services, special equipment and instruments, and

measurement methods and procedures are described, as are error estimates and quality control. A representative Report of

Calibration is included.

Key words: calibration; electric power and energy; electric

standards; NBS services; watthour meters; wattmeters.

1. INTRODUCTION

The unit of ac electrical energy is established, maintained, and

disseminated by the National Bureau of Standards (NBS) and is used by watthour

meter manufacturers, electric utilities, and state utility commissions to ensure

the accuracy of the 93 million watthour meters which register the electrical

energy sold in the United States. At the present time, the annual revenue

from the sale of this energy exceeds $80 billion.

The energy unit has previously been established and disseminated by NBS

with an uncertainty of 0.05%. This uncertainty was appropriate when considered

with respect to the performance expected from both standard- and service-type

watthour meters. Recently, however, there have been improvements in both

meter types, and a more accurate unit of energy is justified. Furthermore,

statistical meter testing techniques, adopted by many electric utilities as a substitute for periodic testing of all meters, have demonstrated the need for

an improved energy standard. With recent improvements in the power and energy

measuring apparatus at NBS as well as refinements in the test procedures and

statistical treatment of the data, the units of power and energy can now be

disseminated by NBS with an uncertainty of 0.01% or better.

Traditionally, a constant power-time interval method has been used at NBS

to establish the unit of ac energy. This method made use of an electrodynamic

wattmeter to transfer a power measurement from dc to ac [1].^ The wattmeter

was then used to measure a constant ac power during an accurately measured

time interval to calibrate a group of standard watthour meters with an amount

of energy which was known in terms of the units of voltage, resistance, and

time interval as maintained at NBS.

^Numbers in brackets refer to the literature references listed in section 13,
Two recent developments have made advances to the state-of-the-art in

power and energy measurements. The first, a current comparator power bridge

was developed in 1974 which made possible calibrations of high quality watt

and watthour meters to uncertainties in the order of 0.003%. The second was the development of a precise power and energy meter by Physikalisch-Technische

Bundesanstalt, Institut Berlin (PTB(IB)), which uses a multiple junction

thermocouple and automatic balancing electronics to linearize certain

thermal nonlinearities. These features accompanied with a precise analog-todigital conversion and a microprocessor-based arithmetic unit give digital

readings of voltage, current, power, and energy. The device can be calibrated

with dc techniques and can achieve overall uncertainties also in the order of

0.003%.

Both of these devices now reside at NBS and are used periodically to

calibrate the NBS reference bank of four watthour meters, the latter of which

serve as comparison standards for the routine calibration of watthour meters

submitted to NBS. This report discusses these instruments, the procedure and

rationale of their use, and discusses the calibrations of watthour meters in

general. An error analysis is included which traces the accumulation of

systematic and random uncertainties from the basic standards to the final

calibrations. The system hardware and test procedures are described in detail,

along with discussions of the Measurement Assurance Program (MAP) for electric

energy and the recently completed international comparison results between the

three national laboratories of the United States, Germany, and Canada.

2. NATIONAL STANDARDS FOR ENERGY2

The electric power and energy metering in the U.S. at 60 Hz is the final

result of a long series or chain of measurements made at NBS. The measurements

and the standards and instruments calibrated by NBS achieve much of their

economic importance from the necessity for accurate energy metering. Standard

watthour meters are normally calibrated to an uncertainty of 0.05%. For special

high accuracy tests, 0.01% can be achieved. At these accuracy levels, it is

especially important to understand the basis for accurate power and energy

measurements, and to make best use of the NBS services and standards [3].

Figure 2-1 shows the major electrical standards which are used by NBS in

its program to establish and disseminate electric power and energy standards.

The connecting lines and arrows indicate the important relationships between

them. For clarity, some of the minor relationships and the kinds of calibrations

performed are not shown.

The internationally accepted prototype standards of mass, length, and

time are shown on the top row of the figure. The meter is defined as a certain

number of wavelengths of the orange-red line of krypton 86. The atomic second

is defined in terms of a specific transition period of cesium 133, and stable

oscillators serve as working standards of time and frequency. The kilogram is

still defined as the mass of the prototype Pt-Ir standard.

^Portions of this section were excerpted from an article, "Precision Power

and Energy Measurements," by F. L. Hermach. See reference 2.
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Two experiments are performed at NBS to determine the basic electrical

units in terms of these three standards and two measured constants, the speed

of light in vacuum (c) and the acceleration of gravity (g). They are reasonably

simple in principle, but extremely difficult and involved in practice because

of the accuracy required. One experiment consists of constructing a precision

calculable capacitor and computing its capacitance in electromagnetic units.

It has a value of about 0.5 pF from the calculation using length and the speed

of light. With remarkable bridges, step up is made to two 1000-pF capacitors

across two 100,000-$^ resistors down to lOOO-fj resistors of known ac-dc

difference, and then down to l-n resistors [4]. The final step is made because

the ^-^ Thomas-type resistors are the most stable resistance standards known.

The average resistance of a group of such resistors serves to maintain the ohm

at NBS between each absolute determination.

The second experiment consists of "weighing the ampere" with a current

balance, and is thus based directly on the definition of the ampere in terms

of the force between current-carrying conductors. One conductor (a coil in

reality) is suspended from one arm of a balance, and the change of force when

its current is reversed is measured in terms of the acceleration of gravity on

a known mass [5].

The parameter of dc voltage is maintained with a high precision and stability

in terms of a measured frequency by means of the ac Josephson effect. A bank

of saturated standard cells is monitored frequently and corrections are made

for any changes in their emf's. An intercomparison is made between the absolute

ampere determination of the current balance and a 1-fi resistor by passing

the current through the resistor. The resulting voltage drop is compared to

the standard cell bank.

At NBS the ohm is accurately known (in terms of the defined "absolute"

value) to better than 0.1 parts per million (ppm). The ampere and volt are accurate to better than 5 ppm, but the volt is maintained (thanks to the Josephson

apparatus) with a precision and stability of better than 0.05 ppm.

Direct reading ratio-sets were developed at NBS to calibrate resistors by

the substitution method, and to step up and down on the resistance scale. No line

leads to them in figure 2-1 because their accuracy depends on ratios of resistors

or turns ratio on a current comparator, not on the unit of resistance. Standard

cells are calibrated by connecting the known and unknown cells in opposition

and measuring the small voltage difference with a potentiometer. Potentiometers

and volt boxes are calibrated with universal and direct reading ratio-sets.

With these the user can then extend the dc voltage scale very accurately.

AC-DC transfer instruments are comparators for determining the equality of

ac and dc currents, voltages, and powers. At NBS they serve chiefly to determine

how well other ac-dc comparators do this. Since such comparators are very stable,

the user can then make accurate ac measurements based on these known dc standards.

They are also an important step in establishing ac power and energy standards.

For many years an electrodynamic wattmeter was the basic ac-dc transfer

standard for ac power and energy measurements at NBS, and it was used to calibrate

the NBS reference group of standard watthour meters periodically [6]. Now,

however, a thermocouple transfer voltmeter coupled with a current comparator

serves the same functions, and a thermocouple wattmeter is used for the ac
calibration of the NBS reference bank of four watthour meters. Special high

accuracy calibrations are also performed using the thermal wattmeter to

uncertainties of 100 ppm (0.01%) or better. The calibration and use of the thermal

wattmeter, the NBS reference bank of watthour meters, and associated apparatus is

discussed in sections 4 and 5 of this report.

Other meter laboratories maintain groups of standard watthour meters, because

these determine the commercially important unit of energy directly and simply.

These laboratories send one or more meters to NBS or an independent laboratory

for periodic calibration. These are the standards shown in the lower right

block of figure 2-1. NBS now also provides a Measurement Assurance Program

(MAP) for electric energy in which an NBS watthour meter is calibrated as an "unknown" by the meter laboratory with the user's equipment and procedures.

The test results are evaluated with NBS help. The calibration and MAP services

are discussed in detail in sections 3 and 9 of this report.

3. WATTHOUR METER CALIBRATION SERVICES AT NBS^

The National Bureau of Standards has the responsibility to establish and

maintain the legal electrical units and, in addition, to make them available

for use by industry, science, and government at all levels. Dissemination

takes place in three ways: by routine calibration of high accuracy electrical

and electronic standards and measurement apparatus, by in-situ calibrations

performed on equipment which for technical reasons cannot be moved to or calibrated at NBS, and through Measurement Assurance Programs.

Routine calibrations of electrical energy standards (i.e., watthour meters)

submitted to NBS are performed on a cost reimbursable basis using permanent

facilities at the Bureau. These services are intended to support primary

standards laboratories rather than to assign values to apparatus used by secondary

laboratories. Accordingly, NBS will calibrate only standards and apparatus of

the highest quality except under unusual circumstances such as to fulfill legal

requirements or to resolve certain technical disputes. Those requiring support

for secondary activities are encouraged to seek help from the numerous commercial

calibration sources available. NBS may be of some assistance in locating a convenient source.

Normally, values of registration are reported with an uncertainty of ±0.05%.

This includes a portion due to the systematic uncertainties of the calibration

process and a portion due to the random components of the process. In instances

where the randomness is too great, the total uncertainty may be increased to

encompass the measured data.

Special high accuracy tests can be performed on quality wattmeters and

watthour meters where systematic uncertainties of the test are less than ±0.01%.

The overall uncertainty is a function of the random components. The random

^Portions of this section were excerpted from NBS Special Publication 250,

"Calibration and Related Measurement Services of the National Bureau of

Standards, 1980 Edition." See reference 7.
components are determined at the time of the test. Because high accuracy is

required of these watthour meters, only those meters having electrical pulse

outputs or high resolution registers (digital display) generally qualify for

the special service. NBS should be consulted prior to submitting the meter to ensure that the particular type has the precision and stability to justify this

high accuracy test. The voltage and current ranges are limited and power factors

are normally confined to unity, 0.5 current lagging, and 0.5 current leading.

The NBS Measurement Assurance Program (MAP) for electric energy (NBS

Technical Note 930) is designed to evaluate energy measuring equipment. An

NBS-owned transport standard watthour meter is shipped to a customer, and a tie to the U.S. national energy unit is made without the down-time encountered

when meters are calibrated at NBS. In addition, and more important, for those

who calibrate reference standard watthour meters, a MAP standard can be used

to evaluate an entire measuring system. The MAP program is discussed in detail

in section 8 of this report.

Additional information regarding test point and shipping instructions are given in appendix A of this report.

4. CALIBRATION OF THE NBS WATTHOUR METER REFERENCE BANK

The major portion of the calibration workload of watthour meters submitted

to NBS is done by comparing four NBS reference watthour meters and the customer's

meter. A general description of the reference watthour meters is given. This

section of the report also describes the equipment and process by which the

four reference meters are periodically calibrated.

The block diagram of figure 4-1 shows the three principal steps, the current

comparator power bridge, the thermal wattmeter, and the NBS reference bank.

Electrical power is established using the NBS power bridge [8]. The thermal

wattmeter is used as an intermediate step between the power bridge and the

reference bank as a matter of timeliness and convenience. Although its use adds small uncertainties (discussed later) to the process, its use speeds the

calibration process considerably.

The reference bank consists of four rotating-type watthour meters. These are each calibrated a minimum of four times a year (quarterly) using the thermal

wattmeter. Each meter calibration consists of at least four independent measurements

at the three power factor conditions of unity, half lagging and half leading.

The thermal wattmeter provides a convenient and relatively fast means to perform

these measurements. To simplify the number of required measurements, the NBS

reference bank is always calibrated under the standard conditions of 120 V, 5 A,

60 Hz and 25°C. Ranging for other voltage and current test requirements is

accomplished using precision voltage and current transformers which are discussed

later.

Although the reference bank could be calibrated directly with the current

comparator power bridge without the use of the thermal wattmeter (and indeed

this was done before the thermal wattmeter became available), the time required

to do so exceeded many times that required by using the thermal wattmeter as

an intermediate step. As discussed in section 8, this intermediate step adds

an uncertainty of about 10 ppm. The decreased calibration time is judged as

an advantageous compromise.
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4.1 Reference Watthour Meters -- General Description

The four reference standards are commercial induction type watthour meters.

They were selected because of their simplicity and stability. They are two-stator,

rotating disk-type meters having no mechanical registers, but rather an electrical

pulse output.

The watthour meters contain two horizontal disks mechanically connected

by a common vertical central spindle. The driving disk generates a torque

proportional to the instantaneous product of the voltage and current and thus

causes the spindle to rotate. Two magnet-dumping assemblies are mounted near the driving disk opposite each other and are located 90 mechanical degrees

from the stators. Such an arrangement balances the driving and damping forces

about the spindle so there is no net lateral thrust. The spindle and disk

assembly are magnetically floated to minimize frictional forces which would

result from mechanical bearings.

A signal -generating disk is mounted on the spindle about 7 cm above the

driving disk. The signal -generating disk contains 500 slots near its outer

edge equally spaced around its perimeter. Located above the disk is a miniature

lamp which illuminates a small portion of the disk slot. Beneath the disk and

directly in line with the lamp is a photoelectric sensor. When rotating, the

signal -generating disk acts as an optical shutter between the lamp and photo-sensor,

producing a pulse output whose frequency is proportional to the speed of rotation.

The reference watthour meters have nominal ratings of 5 A and either 120 or

240 V. At unity power factor and at rated current and voltage, the disk rotation

rate is (5/9)it rad/s (16-2/3 revolutions per minute). The watthour constants

(Kh) are nominally 0.6 Wh/rev for an input of 120 V and 1.2 Wh/rev for 240-V

input.

For operation in the reference bank at NBS, the watthour meters are always

operated at a nominal 5 A and 120 V. Their temperatures are held constant by

placing them in a thermally insulated and heated enclosure which is maintained

at 25°C ±0.2°C. Furthermore, power is applied to the units continuously so that internal heating is constant. The internal lamp is also continuously lit

so that its thermal contributions are insignificant. Each of the watthour

meters is encased in a heavy cast aluminum housing which provides additional

thermal lag time to any small temperature fluctuations which might occur within

the insulated enclosure. (Figure 5-1 shows the four reference watthour meters

inside the enclosure.)

Each watthour meter has a small bubble level built onto it. It is essential

that the units be calibrated and used in the same vertical alignment so that

the driving disk position in respect to the stator coils, damping magnets, and

pivot bearings be constant. It is not critically important that the spindle,

for example, be perfectly vertical, but rather that it is in the same location

in use as it was when calibrated.
4.2 Current Comparator Power Bridge

The unit of electrical power (and when integrated over time, energy) is

established using the NBS current comparator power bridge shown in figure 4-2.

It requires only knowledge of dc voltage, ac impedance, dc resistance, current

ratio, and ac-dc voltage difference.

The dc voltage supply is set to precisely 120 V with the use of a precision

voltage divider, potentiometer, and standard cell. This known dc voltage is

also applied to the input of the differential thermoelement voltage comparator

(DTVC) whose output is adjusted for a null. The DTVC is then switched to the

ac voltage and the ac supply is adjusted until the null is again achieved. At

this point, the ac voltage equals 120 V rms to within experimental error of

the apparatus. This ac voltage is applied to the voltage input of the thermal

wattmeter being calibrated.

The same 120-V ac voltage is applied across a precision ac resistor, as shown in figure 4-2, for unity power factor. The current through the precision

resistor is nominally 50 mA. The current comparator has a turns ratio of

100:1. The comparator is balanced by adjusting the 5-A current which also

flows through the thermal wattmeter current circuit. By this means, a precise

current of 5 A is established. The power applied to the thermal wattmeter is

then calculated (at unity power factor) by

Pa = -^(1 -^C^a) > (4-1)

where Pg is the apparent power applied to the thermal wattmeter,

N is the current comparator turns ratio, 100:1 in this instance,

Vdc is the value of dc voltage, 120 V nominally,

R is the dc value of the resistor (for unity power factor),

2.4 kfi nominally, and

Cy^a is a correction term accounting for the parameters of voltage,

resistance, and current comparator turns ratio. (C^^a typically amounts

to about 13 ppm for the NBS comparator at unity power factor.)

For half power factors, the same circuit is used with the addition of a voltage transformer and phase shifting capacitor as shown in figure 4-3. The

value of resistance is changed to 4.8 kn and the current is 25 mA at 120 V.

The capacitor is chosen so that total current through the comparator's coil is

50 mA and the phase angle between the impressed voltage and current is it/3 rad

(or 60°). The procedure is the same as for the unity power factor test. The

input power applied to the thermal wattmeter is calculated exactly as in eq (4-1)

except that additional corrections must be added for the transformer and capacitor

phase shifting network.
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4.3 Thermal Power/Energy Meter

The thermal power/energy meter"^ shown in figure 4-1 is used in the overall

calibration process as a transfer standard between the current comparator power

bridge and the NBS reference bank. It is also the reference standard for

special high-accuracy tests with systematic uncertainties of less than 100 ppm.

The principle of operation makes use of the fact that the ac current can be

compared to a dc current which is flowing through the heater of a thermoelement

by comparing temperatures. Using this principle and appropriate summing and

differencing circuits, a dc feedback current is added to the heater of the

thermal element to continuously balance the temperature. In this way, the

product of the input currents and an input voltage is converted to a dc current

and the value of a resistor. A full discussion of this process is given in [9].

A simplified block diagram is shown in figure 4-4 for the thermal wattmeter.

Voltage and current ranging is accomplished using precision voltage and current

transformers. The signals then are applied to the thermal ac-dc transfer unit

where an electronic control circuit continuously balances the ac by an equivalent

dc voltage at the thermoelement's heater. The dc is measured using a precision

analog-to-digital converter. The thermal wattmeter's ac-dc transfer unit feeds

its output to the analog-to-digital converter. From this digital information,

proper calculations can be performed by a microprocessor-based arithmetic unit

to determine power precisely, accounting for scaling factors of the input

transformers, amplifier gains, and other factors. By incorporating a precision

time base circuit, time integration is possible so that electrical energy can

be determined. Averaging times of 1, 10, and 100 s can be selected. When the

1 00-s averaging time has been selected, the instrument has a resolution of

about 1 ppm of apparent power.

Because the circuits beyond the input transformers can be used for either

an ac or dc input voltage or current, it is possible to calibrate the thermal

watthour by dc measurement techniques. When performed with care, the unit can

be calibrated with uncertainties of no greater than 10 to 15 ppm, about the

same uncertainty as that for the current comparator power bridge. The process

of calibration is, however, judged more fundamental when the current comparator

power bridge is employed as the basic standard as shown by figure 4-1. Slightly more uncertainty is finally accumulated using the thermal wattmeter, but the

confidence in the overall chain of calibration steps is adequate to meet present

demands and an overall time savings is realized.

The thermal wattmeter is calibrated using the current comparator power

bridge at least annually. The power bridge could be used to perform calibrations

of high accuracy for special tests, but it is more convenient to use the thermal

wattmeter for these tests.

^The thermal power/energy meter can function as a wattmeter or a watthour meter

When used as a wattmeter, it will be referred to as a thermal wattmeter; when

used as a watthour meter, it will be referred to as a thermal watthour meter,

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4.4 Calibration of the NBS Reference Bank

The NBS reference bank is calibrated at least four times a year using the

thermal watthour meter. Standard conditions of 120 V, 5 A, 60 Hz input power

of 600 and 300 W (for power factors of unity and half, leading and lagging)

and a temperature of 25°C are used. After a suitable warm-up period, each of

the four reference standards is individually calibrated. A time interval

equivalent to at least 120 revolutions is used to ensure suitable resolution

from the rotating standards of the reference bank.

The individually adjustable voltage and current power supplies in the NBS

test setup are used to apply power to the thermal watthour meter and the NBS

reference bank. (The supplies and controls, along with the auxiliary electronics,

are described and discussed in section 5.) The potential circuits of the

thermal wattmeter and the NBS reference standards are connected in parallel

and excited with 120 V ac. The current circuits of the thermal wattmeter and

the primary of a precision current transformer are connected in series. The

secondary of the current transformer is connected to the NBS reference standard

current circuits. The current is set to a value of 5 A. Power factors are

set individually for unity, 0.5 lagging and 0.5 leading.

A preset counter in the NBS test console is set to a value of 60,000

which represents exactly 120 revolutions of one NBS reference standard. The

output of one of the four NBS reference standards is connected to the input of

the preset counter. The output from the preset counter generates a gate whose

period is determined by the time required to count the 60,000. The outputs of

each of the four NBS reference standards are recorded on individual counters.

The preset counter output gate serves as a start and stop gate for these four

counters as well as for the thermal watthour meter.

The thermal watt/watthour meter is operated as an energy reference (watthour

meter) by integrating the power during the test interval. Its digital display

reads in units of energy. Nominally, the amount of energy required for 120

revolutions of a NBS reference standard is 259.2 kJ (72 Wh).

The percentage registration for each of the four NBS reference standards is

calculated from the ratio of the total number of pulses counted for each meter

to the energy as read on the thermal wattmeter times an appropriate constant.

This can be expressed as

Rsx = (Cx/J) H , (4-2)

where Rjx is the percentage registration, (x = 1, 2, 3, or 4 signifies each meter),

Cx is the counter reading from one of the four NBS reference standards

(x = 1, 2, 3, or 4),

J is the energy display of the thermal wattmeter in kilojoules, and

H is a factor which accounts for the rotating meter constants, conversion

from units of kilojoules to watthours and a factor of 100 to convert

to percent.

14
The factor H is calculated from

Kc /36OO X 100\ Kc (4-3A) ,^^^,

V-nf [

1 000 1 = -^3(360)

where Kg is the disk constant for the rotating reference standard (0.6 Wh/revolution

for the NBS reference meters),

ng is the number of electrical pulses per revolution of the disk in the NBS

reference standard (ng = 500 pulses/revolution for the NBS reference meters),

3600 is a factor to convert the energy units of joules to watthours,

1000 is a factor to convert kilojoules to joules, and

100 is a factor to express the results in percent.

When the term H is evaluated with the constants of Kg = 0.6 Wh/rev and

ng = 500 pulses/rev, then

H = 0.432 , (4-3B)

and further, from eq (4-2), the percentage registration can be expressed as

Rsx = 0.432 (Cx/J) . (4-4)

For a nominal count of Cx = 60,000 pulses and J = 259.2 kJ, Rgx = 100%. The values

of Rgx are used when the NBS reference bank is used to calibrate the customer's

watthour meters to determine the value for the preset comparator setting.

5. ENERGY COMPARISON SYSTEM

Watthour meters submitted to NBS for calibration are generally tested by a direct comparison with a bank of four reference watthour meters. A description of

the reference bank is given in the preceding section and figure 5-1 shows the bank

in its temperature-controlled enclosure. A description of the comparison philosophy,

power supplies and controls, signal conditioning and comparison electronics are given.

5.1 Watthour Meter Comparison Philosophy

There are three basic types of watthour meters which are submitted for

calibration:

1. Rotating induction types with mechanical registers,

2. Rotating induction types without mechanical registers, but with electrical

pulse outputs, and

3. Electronic types with pulse outputs, or digital readouts.

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The following discussion of the comparison measurement philosophy is divided

into two parts: one discusses meters with mechanical registers and the other

discusses meters without mechanical registers. Much of the hardware, supplies,

and instrumentation used are the same for both types of meters; however, the

different outputs do lead to some differences in the calibration circuit.

5.1.1 Primary Circuit Configuration

Figure 5-2 shows the primary connections that are used for all watthour

meter tests and calibrations. Basically, the voltage circuits of the reference

watthour meters and the unit being tested are connected in parallel so that

each meter has the same voltage applied. By the same reasoning, the current

circuits of the reference and the units being tested are connected in series

so that the same current flows through each of the instruments. In this manner,

equal "apparent" input power is "dissipated" in the reference and test watthour

meters. Note, however, that the input circuits of the reference watthour

meters have both a precision voltage and a precision current transformer.

This arrangement permits a range of voltage and current values to be selected

for the watthour meter being tested, while at the same time maintaining a constant input voltage of 1 20 V and current of 5 A for the reference meters.

This is desirable because the reference watthour meters are calibrated only at

a minimal number of test conditions, namely, inputs of 1 20 V and 5 A at the

required power factors. The transformer characteristics are known and the errors have been minimized so that their effect is negligible.

The circuit also contains three meters which measure the input values of

voltage, current, and power to the reference watthour meters. The voltage and

current supplies are separately adjusted (magnitude and phase) to achieve the

desired test conditions for the reference watthour meters. Reference conditions

are normally 120 V, 5 A, and 600 W for unity power factor, or 300 W for half-power

factor.

Power factor is not displayed. The product of the voltage and current

indications is the apparent power. The wattmeter indicates the real power. By

definition, the ratio of the real power to the apparent power is the power factor.

In all instances, "phantom loading" of the watthour meters is utilized, that

is, the voltage and current sources are separate. By this means, the apparent

power for 120 V and 5 A is 600 W; however, the only power dissipated is in the

leads and the watthour meter coils and is small compared to 600 W. This minimizes

the size and power ratings of the supplies and controls and eliminates the

need for large stable loads.

5.1.2 Comparison System for Watthour Meters With Registers

The configuration for the comparison calibration of watthour meters having

registers which are visually read is shown in figure 5-3. The primary wiring for

the watthour meter being tested is fundamentally as that shown in figure 5-2,

with the addition of a relay-controlled switch in the voltage circuit. The

purpose of this relay is to apply voltage to the meter at the beginning of the

test and interrupt the voltage at the end of the test. The time interval

during which the voltage is applied (and hence the time that the meter under

test is running) is controlled by the output of a digital preset comparator.

For sufficient resolution and accuracy, it is desirable to obtain a minimum of

30 revolutions for each of the watthour meters being tested.

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Figure 5-3. Basic configuration for the comparison calibration of a watthour meter having registers.

19
As seen in figure 5-3, the outputs from the NBS reference bank of watthour

meters go into a totalizer where they are summed. With meters having 500 output

pulses per revolution, the output per meter for 30 revolutions is 15,000 counts.

For four nominally equal meters, the totalized count for 30 revolutions is

60,000 counts. When a value, for example, of 60,000 is dialed into the preset

digital comparator, and the test is begun by resetting the comparator, the

relay in the potential circuit for the meter under test is closed, thus applying

voltage to that instrument. Meanwhile, the NBS reference bank is continuously

running and its output is counted and fed to the comparator. When a count of

60,000 is achieved in the comparator, the relay is opened, the meter under

test stops, and its registers are read and recorded. The exact value which is

to be set into the preset comparator is calculated from the following expression:

C = ^ X ^ X ^ X ps ns(Rsl ^ ^s2

-^ ^3 " h^^^M » ^5-1^

Va Ia Ks l -J

where

C = the value to be set into the comparator,

Vr = voltage range of meter being tested,

V/\ = nominal voltage applied to meter being tested,

Ir = current range of meter being tested,

I/\ = nominal current applied through meter being tested,

K-t = disk constant of meter being tested in watthours per revolution.

Kg = disk constant of reference meters in watthours per revolution,

Ps = number of revolutions of the reference watthour meters,

ng = number of output pulses per revolution for the reference meter, and

'^sl » '^s2> Rs3» '^s4 " ^^^ percentage of registration for each of the four

reference watthour meters.

Equation (5-1) can be somewhat simplified by substituting specific values

for the NBS reference bank watthour meters operation, namely. Kg = 0.6 watthour

per revolution, and n^ = 500 pulses per revolution. When this is done, the

preset comparator setting for the reference watthour meter in terms of the

number of revolutions, rg, can be expressed as

C . !B X Ifx K, hr^i%, - R32 - Rs3 ' ^34)] . (5-2) MM -^

The values that are used for Rsi , Rs2, Rs3» and Rs4 are derived from the

calibration of the four NBS reference watthour meters at specific power factors.

This is discussed in section 4.

20
Once the test is complete using the preset comparator's values as calculated

using eq (5-1), the average number of revolutions that the reference watthour

meters have rotated is exactly 30 (to within experimental error). Therefore,

the percentage registration for the watthour meter being calibrated can be

calculated from the following expression:

D =_B.x-K-x—^x—''xlOO. (5-3)

t ^^ h h r^s

where R^ = percentage registration for the watthour meter being tested, and

rt = number of revolutions of the watthour meter being tested.

The remaining terms have been described earlier. For example, if Ts = 30, Kg = 0.6,

Vr = V/\, Ir = I/\, and K^ = 0.6 watthour per revolution, and if the meter under

test achieved 30.027 revolutions during the time that the reference meters

revolved 30 revolutions, then the percentage registration would be R^ = 100.09.

The time required for a test can be derived from the equation

t

= ^Q X ^s x ^s ^i, (5-4)

's Vs X Is X (PF)

where t^ = the time that the watthour meters being tested are running in minutes,

Vg = the voltage applied to the reference watthour meters,

Ig = the current flowing through the reference watthour meters, and

(PF) = the power factor.

The factor of 60 in eq (5-4) changes the time units from hours to minutes. The

remaining terms are as previously described.

5.1.3 Comparison System for Watthour Meters With Pulse Outputs

The configuration for the calibration of watthour meters having electrical

pulse outputs is shown in figure 5-4. The primary wiring for the potential

and current circuits for the four reference watthour meters and the meter

being tested is as shown in figure 5-2. The sources and controls are identical

to those described previously as is the use of the NBS reference bank, totalizer,

and preset comparator. The difference is seen in the connection of the watthour

meter being tested. With this configuration, the excitation voltage and current

to the meter being tested are continuously applied. This approach minimizes

two sources of uncertainty. First, measurements are taken only after self-heating

effects have stabilized. Second, inertial effects which occur when rotating

meters are started and stopped do not influence the measurement.

The counter monitoring the output of the meter being tested is gated by the

preset comparator. The gate time is given by eq (5-4) and is usually no less

than the time required for the meter being tested to achieve approximately 30

revolutions. Hence the output pulses from the watthour meter being calibrated

are accumulated for one gate period of tg minutes.

21
3-PHASE60HZ POWER

11

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Figure 5-4. Basic configuration for the comparison calibration of a watthour meter having electrical pulse outputs.

22
The preset comparator input value is calculated as before in eq (5-1).

The percentage registration for the meter being calibrated is

R. =

-rr^ X 7^ X -^

X

-^ X 1 00 , (^-^)

t Va Ia Ks nt^s

where R^ = percentage registration for the watt hour meter being calibrated,

Vr = voltage range of the meter being tested,

Va = nominal voltage applied to meter being tested,

Ir = current range of the meter being tested,

Ia = nominal current flowing through meter being tested,

K-t = disk constant of meter being tested.

Kg = disk constant of reference watthour meter,

Nt = number of output pulses from meter being tested,

n^ = number of pulses per revolution for meter being tested, and

Tg = number of revolutions of the reference watthour meters.

For example, if an electronic watthour meter that had a full-scale voltage

range of 120 V, a current range of 5 A, an equivalent disk constant of 0.6 and

gave 1000 output pulses per equivalent revolution were operated at a test

current of 3 A, then from eq (5-2), the preset comparator value for 50 revolutions

of reference watthour meter bank would be nominally 100,000. From eq (5-5),

the percentage registration for an output pulse count of N^ = 29,970 would be

Rt = 99.90%.

5.2 Comparison System Hardware

The watthour meter calibration hardware consists of: 1) two adjustable power

sources, one for the voltage inputs to the watthour meters and the second for

the current inputs; 2) a test table which provides for convenient connections to

meters being tested; 3) precision voltage and current transformers for testing

at levels different from those used with the NBS reference watthour meters;

and 4) the comparison electronics for reading the output pulses of both the

reference meters and meters being tested. This group of equipment is discussed

in this section. Circuit details are shown and discussed in appendix B.

5.2.1 Power Supply and Control Hardware and Circuits

The four NBS reference watthour meters and the meters being tested derive

their electrical stimulus from two separate sources, one for the voltage circuit

and the other for the current circuit. By this means, "phantom" loading is

achieved eliminating the need to either supply or dissipate large amounts of

electrical power. This also eliminates the need for a variety of high power

23
loads. Both the voltage and current sources are driven from the line power.

During the warm-up period, power is derived directly from the commercial power

lines through suitable controls and transformers. During calibration, system

power is obtained from a three-phase electronic power supply whose input is

single phase 120-V 60-Hz power. This electronic supply generates a nearly

sinusoidal, three-phase voltage in a wye-connected output whose frequency is

tightly synchronized with the input power line. The major advantage of using

this electronic power source rather than the three-phase power lines is its

low distortion (less than 0.75% total harmonic distortion) and its amplitude

stability (within 0.5% for 10 hours, 1% for 10 days). Either the three-phase

power line or the electronic power source can be selected using a four-pole

double-throw switch which then routes the power to the voltage, current, and

power factor controls.

The output from the voltage control circuit feeds directly into a step-up

transformer having turns ratios of 1:1, 1:2, and 1:4, thus providing output

voltages of at least 120, 240, and 480 V, respectively. The primary is fused

and isolated from the secondary. The three output voltages are switch selectable

and are applied to the watthour meter under test and to the precision potential

transformer which feeds the NBS reference bank watthour meters.

The output of the current control circuit is connected to two step-down

transformers through a link arrangement. The transformers are rated at 240 B input on each primary and 2-V output on each secondary. These transformers are used for output currents of 3 A and greater. For currents less than 3 A, these

transformers are bypassed and the output from the current control circuit is fed

directly to the watthour meters. When used, the transformers provide for the

following output current ranges: 3 to 25 A, 20 to 40 A, and 30 A to the maximum

attainable. The maximum short circuit current is near 100 A; however, the

usable maximum current depends on the total impedance of the current circuit.

The step-down transformers used to supply the test current are never used

at more than about half their rated input voltage. The phase integrity for both

the step-up voltage transformer and the step-down current transformers is such

that the vector relationship (shown in appendix B) is nominally maintained,

that is, no phase inversions are made by these transformers.

5.2.2 Precision Voltage and Current Transformers

Because the four reference watthour meters in the NBS bank are operated

under standard conditions of 1 20 V at 5 A, precision voltage and current transformers

are employed between the circuits supplying the watthour meters being tested and

the four reference watthour meters to allow tests at other voltages and currents.

Two precision voltage transformers are used, one at a time, depending on voltage range, and each has a multi -tapped primary and fixed secondary. The

secondary voltage is always set to 1 20 V by the system voltage and phase control

and read by a system voltmeter also shown in figure 5-2. The voltage transformers

have the primary taps listed in table 5-1. The input voltage is nominally as stated when there is 120 V on the secondary. The installed transformer is

generally used. The second transformer is portable and connected into the

system when needed, particularly for voltages of 208 and 277 V.

24
Table 5-1. Available test voltages in volts

Installed transformer Portable transformer

primary voltage taps

110 250

115 260

120 440

125 460

130 480

220 500

230 520

240

primary voltage taps

120 277

208 480

240 600

For tests which require 1 20 V to be applied to the watthour meters being

calibrated, neither potential transformer is used, but rather the meters and

the reference bank are energized directly from the voltage supply.

The secondary of the installed voltage transformer is burden compensated

to reduce its errors. No subsequent corrections need be applied. No compensation

is used for the portable transformer, and corrections must be applied when it

is used. The secondary burden for the voltage transformer consists of the

reference bank of four watthour meters, a voltmeter, and a wattmeter, as shown

in figure 5-2.

To enable the NBS reference bank to operate at a level of 5 A, a precision

current transformer is employed which has a multi -tapped primary. It is a two-stage, amplifier-aided transformer as shown in figure 5-5. The advantage

of a transformer of this design lies in the tertiary winding which senses the errors in the secondary current, and supplies a correcting current via the

feedback amplifier [10,11]. The result is a transformer having errors on the

order of the square of errors in a conventional transformer of comparable

size. With this design, errors of less than a part per million can be achieved

over a wide range of burdens.

The precision current transformer has primary taps selected to provide a wide range of currents from 0.5 to 100 A. Table 5-2 gives the values of primary

current for a secondary current of 5 A.

25
FROM ADJUSTABLE CURRENT SUPPLY

V V

TAPPED PRIMARY

SECONDARY

C0RE1

TERTIARY

CORE 2

'mm]

±6 i

AMMETER

i3nF

WATTMETER

iHT

^6 6

WhM

WATTHOUR METER BEING TESTED

WhM

iH

WhM

iH^

WhM

V- Y

NBS REFERENCE BANK

a

WhM

Figure 5-5. Details of the current circuit showing the two-stage

amplifier-aided precision current transformer.

26
Table 5-2. Available test currents in amperes

Primary current taps

0.5 10

0.75 12.5

1 15

1.25 20

2 25

2.5 30

3 37.5

3.75 40

4 50

5 75

7.5 100

The operational amplifier used with the current transformer has a balance

trim adjustment which is set such that the amplifier's dc output is nulled. A

pair of binding posts on the panel provide for connection to the amplifier

output. A switch is also provided which disconnects the tertiary winding;

this function is used principally for troubleshooting purposes. In normal

operation, the switch is at the closed position.

Because of the large number of primary turns (1200), a relatively large

voltage (for a current transformer) exists across the primary, and capacitive

errors can result. This transformer has been compensated to reduce such errors;

however, for the compensation to be effective, the marked terminals of the

primary and secondary windings must both be grounded.

The secondary burden for the current transformer consists of the reference

bank of four watthour meters, an ammeter, and a wattmeter. These are shown in

the block diagram of figures 5-2 and 5-5.

5.2.3 Signal Conditioning and Pulse-Counting Electronics

The signal conditioning and pulse-counting electronics process the pulse

outputs from the NBS reference bank to gate the 120-V potential circuit for

watthour meters with registers or to condition and count the pulses from watthour

meters having pulsed outputs. Figure 5-6 shows a photograph of the equipment

rack housing these electronics and shows the counters and the panels where

connections are made to the circuits. Figure 5-7 shows a simplified block

diagram of the signal conditioning and pulse-counting electronics.

Connections are made to watthour meters being calibrated by suitable

connectors on the front panels. The fundamental flow of signals is from left

to right as drawn in figure 5-7. Watthour meters' output pulses are fed to

signal conditioners, adders, gates, display selectors, and finally to electronic

counters. The output of the adder in the reference channels feeds the sum of

the four reference watthour meters to a preset comparator, whose output generates

a suitable gate for stopping the counters when the preset value is reached.

27
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The wave shape of the "pulse outputs" from watthour meters having slotted

discs are near sinusoidal and as such do not serve well as triggering sources

for the subsequent pulse adders and electronic counters. To improve the pulse wave shapes and to make them compatible with TTL circuitry, signal conditioning

circuits are used.

The signal conditioners consist of a comparator made of an operational amplifier

and variable resistor, and a Schmitt trigger. The amplifier and resistor create

a variable threshold feature which is required because the input sine wave is

offset and never passes through zero volts. Figure 5-8 shows typical wave shapes

of the output of a slotted-disc type watthour meter at the input to the signal

conditioner (top trace) and the output of the Schmitt trigger (bottom trace).

This square wave is adjusted for symmetry by the variable threshold resistor

and is fully TTL compatible for the subsequent circuits. Symmetry is not

essential but is a convenient basis for adjustment.

The pulse adder circuit serves two purposes. First, it provides an output

which is the "sum" of the four reference channels (derived from the four reference

watthour meters), and second, it individually gates the four test channels.

The reference channel and the test channel outputs are conditioned so that

relatively narrow pulses, about 20 ys wide, appear at the outputs independent

of the input pulse widths, which typically are no shorter than about 10 to 30 ms.

The input pulse widths are determined by the rotational speed of watthour meters

with disks and by internal circuits for electronic-type meters.

As shown in figure 5-7, the outputs of the four reference channels and

the four test channels are directed to the eight inputs on the display-select

circuit. The purpose of this circuit is to select electronically the reference

channels or the test channels for display on the electronic counters.

5.2.4 Auxiliary Electronic Circuits

Auxiliary electronic circuits are used to shape, generate, and gate signals

such as the standard frequency and the line frequency. These circuits and

their functions are discussed below.

5.2.4.1 Line Frequency Circuits

It is desirable to measure the test frequency at which the reference and

test watthour meters are being used. The NBS laboratory has available a standard

frequency of 1 kHz. Comparison against this standard frequency is accomplished

as follows.

A phase-locked-loop circuit is used to generate an output signal near 1 kHz

which is phase-locked to the line frequency. This signal and the standard 1 kHz

are connected to separate electronic counters which are gated during the test.

By comparing the two counters' readings, an accurate measurement of the mean

line frequency can be obtained.

5.2.4.2 Gated Relay

The TTL driven relay is used to control a +50-V dc line which controls

mercury-wetted relays on the test bench. These relays are used to open and

close the voltage circuits to watthour meters having readable registers. This

then starts the watthour meters being calibrated and stops them at the end of

the test, and is described in section 5.1.2.

30
Figure 5-8. Typical wave shape of the output of a slotted-disc type

watthour meter (top) and signal conditioned output

after the Schmitt trigger circuit (bottom).

31
5.3 Test Bench Wiring and Features

The test bench provides access to the voltage and current circuits. It

has four stations allowing four meters to be calibrated simultaneously.

The current circuit is a continuous series loop including the four stations

and the precision current transformer primary. The voltage circuits are wired

in parallel so that the same voltage appears across each station and across

the primary of the precision potential transformer. Each station's voltage

lead goes through a relay and switch. The relays are controlled by the preset

comparator and closed at the beginning of the test and opened at the conclusion.

Thus for watthour meters having registers, these relays start and stop the

meters by applying and removing the voltage to the meter. A switch in parallel

with the relay contacts allows the relays to be bypassed. This is desirable

while the meters are "warming up" prior to calibration and also for electronictype watthour meters that do not require starting and stopping. The latter

are tested with voltage applied continuously.

If all the stations are not in use, the current connections at the unused

stations must be shorted with a heavy link to provide circuit continuity. The

voltage terminals are left open at the unused stations.

Leveling pads are available at each station with built-in bubble levels.

These pads are used for rotating standards which require leveling before use.

6. PROCEDURE FOR CALIBRATING WATTHOUR METERS

The procedure for the calibration of watthour meters involves several steps

including preparation, laboratory measurements, data recording, and issuance of

the Report of Calibration.

6.1 Initial Inspection and Preparation

When a watthour meter is received, it is physically examined for damage or other factors that may be apparent which could affect its performance and calibration

The customer's order is examined to ascertain what is desired in the way of

requested calibration ranges, etc. Presuming that no apparent damage is evident

and that the request is understood and falls within the NBS calibration capability,

the necessary documents are prepared. If damage is evident or suspected, or

if there are questions regarding test conditions, etc., the customer is contacted

and the problems are discussed and resolved prior to beginning the calibration.

A file is maintained for each meter submitted to NBS for calibration. It

records the NBS test folder number and the date of tests, as well as the meter's

nomenclature and identification (i.e., serial number or other unique identification).

A data worksheet is prepared which indicates all pertinent data regarding

the instrument, including meter manufacturer, model, type, serial number, and

any known modifications. The test number, date of test, and laboratory personnel

performing the calibration are each noted. The customer submitting the meter

for calibration is also listed. The data columns are headed with the parameters

of the test, namely,

32
Preset comparator value

Voltage range

Applied voltage

Current range

Applied current

Power factor

Counter reading or register reading

Line frequency, and

Standard frequency.

For each day of calibration, the ambient room temperature, relative humidity,

and the air temperature of the reference bank enclosure are each recorded.

Figure 6-1 shows a typical data worksheet.

The preset comparator value is calculated by using eq (5-1). The preset

value is recorded on the data sheet for each set of test conditions. After

the above preparations have been made, the watthour meter can be installed.

6.2 Installation of the Watthour Meter

The installation involves selecting the test bench station at which the

watthour meter is to be calibrated, leveling the meter (if required), and

connecting the proper leads from the test bench to the meter. Station four is

usually reserved for an NBS monitor standard which is used and calibrated with

each daily run. On unused bench positions, the current terminals are shorted

and the voltage terminals are left open. Voltage and current are then increased

to the appropriate operating levels (normally 120 V, 5 A at 0.5 power factor)

and the test meters are allowed to warm up from one to four hours depending

upon the type of meter and test.

6.3 Testing Procedures

The power supply controls are set to zero and the necessary taps changed

according to the test point conditions listed on the prepared data sheet. The

comparator is set to the preset value of the test point as recorded on the

data sheet. For meters having registers, the voltage switch is turned to

"off" and the registers are zeroed. The test is initiated with the "start"

button which clears and enables the comparator and the counters.

When the test ends, the meter value (as indicated on the counter or the

meter register) and line and standard frequencies are each recorded. The test

is then repeated. The meter and frequency readings are again recorded when

the second run ends. After the second run, the voltage and current are reduced

to zero and the appropriate connections are changed for the next set of test

conditions. This is repeated until a pair of readings have been obtained for

each test condition. A second set of data is obtained for each test point at

least one day later so that a minimum of four readings have been made, two

pairs, on two different days. At the end of the test or day, the power supplies

are reduced to provide "idle" power for the meters overnight and for weekends

and holidays. This is important, especially for rotating standards.

33
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(3-75) Company XYZ UT3:UTTf C6^\-?/»MY Serial Number OOOO |

Preset Value

Voltage

Range

Applied

Voltage

Current

Range

Applied

Current Power

Factor 0000

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WATTIHOUR METE R CALIBRAT ION

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Figut"e 6-1. Typi(:al dat.a work Sheet 1For a watthouj" meter havin(3 electrical pulse outputs.

34
6.4 Data Analysis

After the second set of data has been recorded, the average of the four

readings is obtained and the percentage registration is computed. The percentage

registration calculation is made using the following relationship:

R = ( Rdg 1 + Rdg 2 + Rdg 3 + Rdg 4)/4 x 100 , (6-1)

Nominal Count

where Rdg 1, Rdg 2, Rdg 3, and Rdg 4 are the four readings taken during calibration,

and the denominator is the expected nominal count. As an example, for the

data given in figure 6-1, the first four data readings are 30007, 30007, 30009,

and 30003, which are all counter readings for the four tests at 120 V, 5 A,

and unity power factor. The nominal count (denominator) is 30000. Therefore,

the percentage registration is 100.0217.

The mean test (line) frequency can be calculated from the ratios of the

line and standard frequency counters. If the 60-Hz line frequency were exact,

then the two counter readings would be the same (within extremely small errors

due to the NBS standard frequency and the ±1 count of each counter). The

line frequency can be calculated from the following relationship:

f - ( Cl + C2 + C3 + C4 ) ^ .^ (6-2)

line -(S^ + S2 + S3 + S4) ^ 60 ,

where C] , C2, C3, and C4 are the counter readings for the line frequency, and

Si , S2, S3, and S4 are the counter readings for the standard frequency during

the calibration period. For the example given above from data of figure 6-1,

the four counter readings for the "line frequency" and the four standard

frequencies lead to

f ^ (215811 + 215815 + 215917 + 215937) (6-3)

1^""^ ~ (215815 + 215814 + 215909 + 215911)

= 60.002 Hz ,

which is only about 36 ppm greater than the nominal value of 60 Hz. The test

frequency is generally very close to nominal 60 Hz and is generally of no great concern in the routine calibration of watthour meters inasmuch as the

frequency corrections are usually small.

After the completion of the test, the test parameters and the four counter

readings (or four register readings) are entered into a computer. The computer

calculates the percentage registration and prepares the Report of Calibration.

As part of the computer computations, the data for the monitor standard are also entered and its registration calculated. The standard deviations are computed for both the data of the meter being tested and the monitor.

35
The computer results are checked to ensure that errors were not made when

entering the data into the computer. The results are also compared to previous

calibrations of the same instrument to detect abnormal drifts or changes. The

results from the monitor standard are examined to ensure that the calibration

process was in control. If all factors seem acceptable, the Report of Calibration

is prepared giving the test conditions and results. The Report is then reviewed

by no fewer than two persons for accuracy in all aspects.

7. REPORT OF CALIBRATION

For each watthour meter that is suitable for test and submitted to NBS, a Report of Calibration is prepared. This Report is a legal document attesting

to the results of the measurements made at NBS on that particular instrument

under the test condition specified at the time of test. A typical example of

a Report of Calibration is shown in figure 7-1 (a). The report consists of a cover page which specifies the instrument being calibrated by manufacturer,

model or type, and serial number. The voltage and current ranges as well as the frequency are stated. The company or utility which submitted the instrument

is clearly given by name and address.

Specific details of the test conditions are given such as laboratory

temperature, warm-up time, the number of disk revolutions for rotating type

meters, and any special electrical connections or conditions. The estimated

uncertainty in assigning the values of registration (derived from the systematic

and random components of error estimates) is given. The cover sheet also

gives the NBS test number, the customer's order number, and the date on which

the Report was prepared.

The second page of the Report (fig. 7-l(b)) gives values of percentage

registration at specific voltages, currents, and power factors. The voltage

and current columns are divided into the full-scale range and the nominal

voltage and current applied during the test.

The reported values of percentage registration are computed as discussed

in sections 5 and 6 of this report. The values are usually reported to the

nearest hundredth of one percent (two significant decimal places).

8. ERROR ANALYSIS

An error analysis is presented herein which begins at the fundamental

electrical standards at NBS and progresses through the calibration of customers'

meters. Figure 8-1 shows the basic chain of steps which includes the NBS

reference bank of four meters, the Measurement Assurance Program (MAP) transport

meters, routine calibrations of customers' meters, and special high accuracy

tests.

In figure 8-1, a set of uncertainty values is given between each step.

The value outside the parenthesis is the power factor and is either 1 or 0.5.

The first value inside the parenthesis is the estimated systematic component

of uncertainty assigned at that step of the measurement process. The second

value is the estimated random uncertainty of the process and is expressed as three standard deviations, 3Sf|,, of the mean. Both the systematic and random

36
(REV. tm)

U.S. DEPARTMENT OF COMMERCE

NATIONAI. BUREAU OF STANDARDS

WASHINGTON, O.C. 20234

REPORT OF CALIBRATION

PORTABLE STANDARD AC WATTHOUR METER

120, 240, 480 Volts, 1.0, 5.0, 15.0, 50.0 Amperes, 60 Hz Rheed-Wright Meter Company Type UR-100

Serial No. 0000

Submitted by

XYZ Utility Company

1234 Main Street Somewhere, New York 12345

This watthour meter was tested at rated frequency with alternating current of practically sinusoidal waveform. Testing was performed in July 1983 at a room temperature of approximately

23°C. The instrument was placed on a level table.

The meter was energized for four hours before the test. Tests

were run so that the meter disk completed close to 30 revolutions

during the test period. The leads in series with the voltage

coil of the meter had negligible resistance. At 0.5 power factor

the current lagged the voltage, except as noted.

The results shown in the attached table are the averages of four

or more runs at each test point. The values of registration

have an uncertainty of 0.05%. This figure includes allowances

(available on request) for both the random and systematic errors

of the calibration process.

For the Director

National Engineering Laboratory

Cal E. Braytor, Group Leader

Applied Electrical Measurements

Electrosystems Division

Test No.: 722/225888

Order No.: ABC123

Date: July 31, 1983

Figure 7-1 (a). Report of Calibration for a typical watthour meter.

37
XYZ Utility Company

Intrument Type UR-100

NBS Test No, 722/225888

Test Dates 15 July to 22 July 1983

Percentage Registration

Voltage Current Power Instrument Serial Number

Range Applied Range Applied Factor 0000

Volts Amperes

120 120 1.0 0.5 1.0 100.12

120 120 5.0 2.5 0.5 100.13

120 120 5.0 2.5 1.0 100.10

120 120 5.0 5.0 0.5 100.08

120 120 5.0 5.0 1.0 100.10

120 120 5.0 5.00 0.5LEAD 100.11

240 240 15.0 3.00 1.0 100.08

240 240 15.0 30.00 0.5 100.09

240 240 15.0 30.00 1.0 100.13

240 240 50.0 50.00 0.5 100.12

240 240 50.0 50.00 1.0 100.11

240 277 5.0 2.50 1.0 100.13

480 480 5.0 2.50 1.0 100.10

Figure 7-1 (b). Report of Calibration for a typical watthour meter,

38
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39
components are given in parts per million (ppm). All uncertainties are measured

in terms of real power or energy. For example, the first process step is the

calibration of the thermal watt/watthour standard by the current comparator

power bridge, which at unity power factor has an estimated systematic uncertainty

of 13 ppm and an estimated random uncertainty, 3sm, of 10 ppm.

The systematic and random components of uncertainties are combined using

the relationship shown below:

Uj = SU + 3S[n , (8-1)

where

Uj = the total uncertainty of a particular measurement step,

SU = the systematic uncertainty associated with that step, and

Sfyi = the standard deviation of the mean.

Sm = S//TT , (8-2)

where

s = the standard deviation of an observation, and

N = the total number of independent observations used to calculate x".

s = [s(x^-x)'/(N-l)]''\ (8-3)

where

x^ = the i^*^ observation, and

X = the mean of the values of x-j .

As can be seen, the random uncertainty varies inversely with the square-root

of the number of measurements performed in a set. Also, a limit of three standard

deviations is used to encompass the randomness of the process. For the process

shown in figure 8-1, the total uncertainty for any one step is used as the

systematic uncertainty for the following step.

8.1 Current Comparator Power Bridge

The watthour meter calibration chain begins with an NBS current comparator

power bridge [8] which establishes an ac voltage (at power frequency which is

compared to dc standards through a thermal transfer process. An ac resistor

provides a proportional in-phase ac current that is scaled to 5 A by the current

comparator. This process was discussed in detail in section 4.1 of this report.

The uncertainties that are attributed to the current comparator power bridge

involve systematic effects that include inaccuracies in the reference standards

used in addition to bias from individual instruments. These combine with random

uncertainties when using the power bridge to calibrate the thermal power/energy

standard. .^
The estimated uncertainties from systematic effects in the current comparator

system at unity power factor are listed in table 8-1. Contributions from voltage,

current ratio, and the ac resistor are evaluated from information derived from

calibrated system components or from direct measurement. Drifts or aging

effects between calibrations of the system components are not included.

As table 8-1 shows, uncertainties in the dc voltage determinations are attributed to a standard cell, potentiometer, voltage divider, a reversing switch,

and power supply regulation. A reversing switch is used for dc voltage reversal

into the thermal transfer device. The error caused by the voltage drop across

the switch due to an estimated switch contact resistance of 3 mfi is about 0.5 ppm.

Another 0.5 ppm is added for the dc power supply regulation characteristics

during thermal element switching. The amount of uncertainty in the dc voltage

determination is estimated as 5.1 ppm. The ac-dc difference of the differential

thermocouple comparison process has been determined to be about 2 ppm when

establishing the value of ac voltage. Each of the individual contributions

are combined in a root-sum-square manner. The total uncertainty in the voltage

determination is estimated to be 5.5 ppm. Because the applied energy is proportional

to the square of the voltage, the uncertainty in energy is taken as twice that

of the voltage uncertainty resulting in an uncertainty estimate of 11 ppm.

Table 8-1. Systematic uncertainties for current comparator

power bridge in ppm at unity power factor

Item Magnitude Subtotal Total

DC voltage

standard cell 4

potentiometer 3

voltage divider 1 reversing switch 0.5

dc power supply regulation 0.5

root-sum-of-squares 5.1

AC voltage

dc/ac difference of DTVC 2

root-sum-of-squares (dc + ac) 5.5

Total voltage 2 x (dc + ac) 11

Current ratio 1 .1

Resistor 6.4

Total systematic uncertainty bound

root-of-sum-of-squares 13

Uncertainties in determining current ratio are attributed to the fact that

an exact balance cannot be attained to better than about 20 yV. This results

in an unbalance uncertainty of 0.1 ppm. Current comparator transformer

corrections amount to about 1 ppm and other effects amount to less than 0.5 ppm

41
in the worst case. The total uncertainty assigned for the current ratio is

1.1 ppm (table 8-1). The ac resistor, when calibrated, had an ac-dc difference

uncertainty of 5 ppm and a dc resistance uncertainty of 4 ppm. These combine

to an uncertainty of 6.4 ppm. The total systematic uncertainty bound, calculated

by a root-sum-square method from the above contribution, is 13 ppm for unity

power factor.

Table 8-2 lists the estimated uncertainties for the current comparator

system at half power factor. Additional terms are those for the "voltage

transformer quadrature correction" and the "capacitor conductance" calibrations.

The uncertainty in the transformer's quadrature correction is 5.3 ppm.

The uncertainty in the capacitor conductance is 19.6 ppm. Also note that

the uncertainty values associated with the current ratio are somewhat larger

under the conditions of half power factor than at unity power factor due to

unbalance conditions. The overall estimated systematic uncertainty assigned

to the current comparator power bridge is 25 ppm at half power factor. When

using the current comparator at 0.5 power factor leading, the voltage transformer

is simply reversed in polarity, and hence the same uncertainty applies to a leading or lagging power factor.

Table 8-2. Systematic uncertainties for current comparator

power bridge in ppm at 0.5 power factor

Item Magnitude Subtotal Total

DC voltage

same as table 8-1 5.1

AC voltage

dc/ac difference of OTVC 2 root-sum-of-squares (dc + ac) 5.5

Total voltage 2 x (dc + ac) 11

Current ratio 4.1

Resistor

same as table 8-1 6.4

Voltage transformer quadrature 5.3 5.3

Capacitor conductance 19.6 19.6

Total systematic uncertainty bound

root-of-sum-of-squares 25

42
8.2 Thermal Power/Energy Standard

The thermal power/energy standard is the reference standard for the rest

of the measurement chain and is discussed in section 4.2 of this report and

also in [9]. Its calibration and associated uncertainty are verified annually

by the power bridge.

The systematic uncertainties, as stated above, are 13 ppm and 25 ppm at

unity and half power factors, respectively. The random components are based on the results of the international comparison [12]. Based on nine independent

measurements, the Ss^ figures for both unity and half power factors are estimated to be no greater than 10 ppm. Therefore, the total uncertainties of

the calibration are 23 ppm at unity and 35 ppm at half power factor. These

values are assigned as the systematic uncertainties for the thermal power/energy

standard when calibrating the four NBS rotating standard watthour meters.

8.3 NBS Reference Bank

The unit of energy is maintained by the mean registration of a bank of

four reference standard watthour meters. They are each periodically calibrated

(every three months) by the thermal watt/watthour standard. A least-squares

linear fit as a function of time is made to the mean registration for the bank

order to compensate for drift in the individual meters. The value assigned to

the bank is the predicted value from the fit at the time of calibration. The

estimated random error associated with the assigned value depends on the number

of data points and the scatter of points about the fitted line as estimated by

the residual standard deviation of the fit. Figures 8-2 and 8-3 show the results

of these calibrations.

Based on twelve intercomparisons over three years, the standard deviation

of the assigned value, s^i, is 11 ppm for unity power factor and 39 ppm for half

power factor. The limits to random error for the calibration of the bank,

expressed as 3S[y,, become 33 and 117 ppm at unity and half power factor, respectively,

The total uncertainty of the values assigned to the reference bank can be computed

from eq (8-1). These values, 56 ppm at unity power factor and 152 ppm at half

power factor, then become the systematic uncertainties for measurements made by

the NBS reference bank.

8.4 Energy MAP Transport Standards

The NBS reference bank is used in the Energy Measurement Assurance Program

(MAP) to assign values of registration to the transport standards. The MAP

process is discussed thoroughly in [14] and an overview is presented in

section 9 of this report.

As discussed in section 8.3, the assigned systematic uncertainties associated

with the reference bank are 56 ppm and 152 ppm, respectively, for unity and half

power factor. The random components, however, require special consideration

since the MAP transport standards are used only to transfer the unit from NBS

to a participating laboratory. Long-term drifts in these standards are much

more difficult to assess because they undergo unknown environmental changes

during shipping. Short-term performance is more important because the transport

standards are calibrated before and after shipment. Therefore, the random

43
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error associated with measurements made at NBS is estimated by pooling the

short-term standard deviations calculated for each MAP comparison. These

short-term values are computed from four independent observations taken over

a two-week interval, and the pooled or assigned standard deviation for each

transport standard is the weighted root -mean-square result of all of the

short-term standard deviations which have been calculated for that standard.

The random component of uncertainty of a MAP transfer depends not only on the random error associated with the NBS process but also on the random error associated with measurements made at the participating laboratory. Transport

standard performance is shown in table 8-3, where the first figure in each

column represents the random component of uncertainty associated with the NBS

calibration of each transport standard. The values in parentheses are typical

components of random uncertainties encountered at the participating laboratories,

Values are calculated from pooled short-term standard deviations where N = 4.

Table 8-3. Random uncertainties of five typical

MAP transport standards

Transport standard Pooled 3Sjt^ in ppm

No. Type Unity power factor Half power factor

155 (283)

151 (114)

91 (152)

129 (139)

117 (162)

125

8.5 Calibration of Customers' Meters

An alternate method of transferring the unit of energy involves the calibration

of a customer's watthour meter at NBS. There are two services available -- high

accuracy calibrations and routine calibrations.

8.5.1 High Accuracy Calibrations

The high accuracy calibration service compares customers' wattmeters or watthour meters directly to the thermal watt/watthour standard. The systematic

uncertainties of this process are 23 ppm and 35 ppm for unity and half power

factor, respectively. The random uncertainties will depend upon the individual

meters submitted for test and are shown in figure 8-1 as the variables X and Y. The total uncertainty at each power factor will be computed by eq (8-1), where

s is the standard deviation estimated during the two- to three-week testing

period. This service is limited to two test points (1.0 and 0.5 power factor)

allowing up to nine independent measurements at each point. Estimated total

uncertainties similar to those assigned to the reference bank are expected.

46

089 Electronic 106 (237)

092 Electronic 65 (120)

420 Electronic 73 (137)

628 Electromechanical 106 (210)

681 Electromechanical 109 (112)

Weighted rms 95
8,5.2 Routine Calibrations

The routine calibration service allows up to three customers' watthour meters

to be tested simultaneously at a wide variety of test points. Because only two

independent measurements are made at each point, it becomes very difficult to

predict the random component of error with a high degree of confidence. Therefore,

certain assumptions are made regarding "typical" random errors. As mentioned

earlier, short-term estimates of random error of several types of NBS MAP

transport standards are summarized in table 8-3. It is felt that these transport

standards are typical of the meters submitted for test at NBS. Moreover, most

watthour meters sent to NBS for calibration are used very much like the MAP

transport standards in that they are calibrated by the customer's reference

standards, shipped to NBS for test and then recalibrated upon return to the

customer's laboratory (the short-term performance is of primary interest). The

data analysis for these meters can thus make use of the extensive statistical

studies available for the MAP transport standards. The weighted "rms" values

at the bottom of table 8-3 are used for meters tested at NBS. The major difference

between the error analyses of the MAP and the routine calibration service is that

the routine calibrations are based on two independent observations instead of the

four used in MAP. The NBS values shown in table 8-3 must therefore be multiplied

by the square root of two. This yields typical random limits for the routine

calibration service of 134 ppm at unity and 170 ppm at half power factor which

are shown in figure 8-1. These, combined with the reference bank systematic

uncertainties, give total uncertainties of 190 ppm at unity and 322 ppm at

half power factor.

The 0.05% accuracy statement presently reported for this calibration service

is based on a statistical study of watthour meters in use 15 years ago. It is

expected that this statement will be modified in the near future to reflect

the improvements in both systematic and random uncertainties that have occurred

over the past several years.

The quality of this calibration service is monitored by a check standard

which is a commercial watthour meter tested simultaneously with each customer's

meter. The performance of this meter over an eight -month period is shown in

figures 8-4 and 8-5. The 3$^^ limits for the average of two independent observations

based on 114 averages are 84 ppm at unity and 141 ppm at half power factor.

This is somewhat better than the random limits assigned to the calibration

process and indicates that these limits are reasonable assessments of the

process precision. A rigorous test of process control is made by comparing a short-term standard deviation (determined from the most recent tests) to the

long-term value by an "F" test similar to the one described in [13].

9. ELECTRICAL ENERGY MEASUREMENT ASSURANCE PROGRAM

The fundamental concepts of a Measurement Assurance Program began at NBS

in conjunction with the National Conference of Standards Laboratories (NCSL) in

the early 1970's [14]. A laboratory participating in this program periodically

determines its measurement process offset from NBS by testing one or more NBS

transport standards as part of that laboratory's normal workload. Using the

NBS standard greatly relieves the laboratory of the cost and responsibility of

maintaining their own transfer standard and of determining the standard's

characteristics, especially those which might be affected by the shipping

47
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49
environments. By carrying out the transfer of the measurement unit (in this

instance, electrical energy) onsite, with the procedures normally used for its

regular workload, statements can be made regarding the uncertainty of its

regular measurement process. When this is accompanied by a program of continuing

surveillance of the workload by remeasuring a set of their working standards,

the laboratory can gain assurance of the validity of their calibration uncertainty

statements, and a degree of quality control is being exercised. A feasibility

study of alternate methods of dissemination of the unit of energy was made by an EEI-NBS Research Associate in the early 1970's [15]. The MAP for electrical

energy began in 1975 [13].

Before shipment to the participating laboratory is made, a set of measurements

is made at NBS. The instrument is then shipped and the laboratory performs its

set of measurements and returns the instrument to NBS. NBS then makes a second

set of measurements and the results are analyzed. Upon completion of the analysis,

a report is issued giving the difference between the measurement parameter

maintained by the participating laboratory and the legal unit maintained by NBS.

The report also includes a complete error analysis of the measurement process.

9.1 Transport Standards

The transport standards used for the electrical energy Measurement Assurance

Program are high quality commercial watthour meters, similar to those used by

the utilities. The transport standards have been well characterized by a careful measurement program at NBS.

9.2 Measurement Process

Three sets of measurements are made on each transport standard used for

the electrical energy MAP. The first set is made at NBS by the process discussed

in section 6 of this report. It is compared directly to the NBS reference

bank of four watthour meters. Four readings are made, two each on two different

days. The unit is then shipped to the participating laboratory. That laboratory measures the transport standard with the techniques and apparatus normally

used for their workload. Eight measurements are made in four sets of two

measurements each. Each of the four sets are done on different days. Upon

return to NBS, an additional two sets of two measurements are made. In all,

each laboratory has made eight measurements each to complete the data set.

The registration of the transport standard may be affected by variations

in the test conditions and the test environment at the participating laboratory.

The laboratory is therefore asked to record the average value of each parameter

that might affect the watthour meter's registration during the test. The

accepted standard test conditions for the transport standards are listed below:

Input voltage: 120 V Input current: 5 A

Input power: 600 W, and 300 W

Power factor: 1 , 0.5

Frequency: 60 Hz

Waveform: Sinusoidal

Temperature: 25°C.

Other information such as lead resistances, circuit used for testing, long-term

standard deviations of the participating laboratory, etc., are sought and

space is provided on the reporting form.

50
9.3 MAP Data Analysis

During the data analysis, a normalizing procedure is used which compensates

for the effects of the above parameters if different than their nominal values.

This procedure uses coefficients for each of the parameters which were determined

earlier for each standard. By employing such a normalizing procedure, the overall

accuracy is improved. The laboratory computes and reports their values for the

registration noted as ru^Q, When the results are processed at NBS, the corrections

are performed to compute the normalized registration, Ri_ab> by the relationship

RlAB = f'LAB + PlAB ' (9-1)

where P|_/\b is the normalizing term. (See [13] for a more complete explanation.)

The difference in percentage registrations assigned at the participating

laboratory and at NBS is given by

aR = Rlab - RnBS ' (9-2)

where Rf^BS ""^ ^^^ normalized mean of the registrations, rj^BS computed before

shipment, and r'f^iBs computed after return to NBS.

The total uncertainty of aR using a three standard deviation limit is

calculated from

hR= SUnbs ^fl s2 + s2 V'' ' (9-3)

\/n LAB NBS/

where SUf^BS ""s the systematic uncertainty of the NBS calibration process,

^LAB ""s the standard deviation of the participants measurement process,

^NBS "i^ the standard deviation of the NBS measurement process, and

N is the number of independent observations made at each laboratory.

(An independent observation for these measurements is defined as a set of two or more readings separated by at least several hours

from any other set of readings.)

Systematic errors in a laboratory process can be determined by calibrating

the NBS transport standard watthour meter as a working standard. If the laboratory

corrections are adjusted so that aR = 0, the uncertainty due to systematic

errors of their process can be reduced to the uncertainty, U^, of the NBS

transport standard. Random errors can be determined through local statistical

analysis by the participant using a group working standard calibrated many times

over a long period. Using this type of information, obtained from the MAP report

and local statistical analysis, the laboratory can determine the uncertainty,

Ulab> """ their calibration process, where

51
2L_AB

UlAB = U^R + 3

-^— . (9-4)

The reader is encouraged to study NBS Technical Note 930 "A Measurement

Assurance Program for Electric Energy" (see [13]).

9.4 MAP Results

Results for electric energy MAP show an overall improvement in transferring

the standard energy unit from NBS. The program can thus decrease measurement

uncertainties and tie the utility industry closer to the legal unit of energy. For

purposes of illustrating this improvement, typical data are shown in figures 9-1

and 9-2 for measurements made at unity power factor and half power factor lagging,

respectively. For each figure, three different symbols designate different

laboratories. The symbol location indicates the mean value of the measured

difference, aR, between NBS and the utility. The length of the vertical bar of

each point represents the total uncertainty assigned to that set of measurements,

previously defined as U^r, eq (9-3).

For measurements made at unity power factor (fig. 9-1), all three laboratories

show a convergence towards zero difference. Initial measurements indicated a difference in excess of 1000 ppm (0.1%). After four MAP interchanges, the mean differences were improved to be between about 200 to 400 ppm (0.02 to 0.04%).

The more recent data show the mean to lie within the total uncertainty (i.e.,

the length of the uncertainty "bars" pass through zero difference).

For half power factor lagging (fig. 9-2), the same converging trend is evident.

In the case of the data represented by the symbols o and a, the trend is a settling

of the means toward zero difference. For the data shown by the symbol o , the

uncertainty is larger, but of more concern; there is an apparent "drift" in the

process causing the mean difference to increase rather than decrease. Even though

the total uncertainty still encompasses zero difference, the results suggest that

this measurement process may not be in control and further investigation should

be made.

Because the electric energy MAP has been in operation for only about five

years, an abundance of data is not available. However, as time progresses

charts similar to figures 9-1 and 9-2 will show the degree of overall improvement

that the MAP process has made in electric energy measurement.

10. INTERNATIONAL COMPARISONS OF ELECTRICAL ENERGY STANDARDS

Two international comparisons of electrical power measurements were made in

1976. The most notable involved the national laboratories of Germany, Canada,

and the United States. Namely the Physikalisch-Technische Bundesanstalt-Institut

Berlin (PTB(IB)), the National Research Council of Canada (NRC(EE)), and the

National Bureau of Standards (NBS). The second comparison was carried out

between NBS and NRC(EE). Both are documented in [12],

The ensuing discussion of this section is a direct synopsis of that paper

with emphasis being placed on the NBS results in comparison to the other two

laboratories. This comparison improves the confidence of electrical power and

energy measurements made at NBS.

52
UNITY POWER FACTOR

+ .200

+ .180

+ .160

+ .140

+ .120

+ .100

a. + .080

<

CO

QQ

+ .060

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+ .040 S

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11 II ~

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1975 1976 1977 1978 1979 1980 1981

DATE OF TEST

Figure 9-1. Measurement results from the MAP for electric energy for

three large utility companies. Tests made at unity power

factor. (See eq (9-2).)

53
HALF POWER FACTOR

+ .160

+ .140

+ .120

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1 1975 1976 1977 1978 1979

DATE OF TEST

1980 1981

Figure 9-2. Measurement results from the MAP for electric energy for

the same three utility companies as shown in figure 9-1.

Tests made at half power factor lagging. (See eq (9-2).)

54
Prior to 1976, a precision comparison of electrical power between laboratories

was not practical inasmuch as a considerable number of sensitive instruments

would have had to be shipped. This is because ac power is a derived quantity

involving basic standards of dc voltage, resistance, and ac-dc transfer. This

situation was altered significantly by the development of a sufficiently accurate,

stable, transportable thermal wattmeter at PTB(IB). The wattmeter is based on

a thermal ac-dc transfer principle [9] which is discussed in section 4 of this

report. The wattmeter was calibrated at PTB(IB), then shipped to NBS and NRC(EE),

where repeated sets of measurements were made. The instrument was then returned

to PTB(IB), where it was recalibrated.

The measurement method that NBS used is described in detail in section 4

of this report, and the appropriate errors are given and discussed in section 8,

As discussed therein, the NBS current comparator power bridge is employed as the NBS basis of establishing the legal unit of electrical power. It was this same power bridge and related apparatus that was used to perform the measurements

for the international comparisons.

The PTB(IB) thermal wattmeter provides means for the precise measurement

of voltage and current as well as for power. Though these parameters were indeed

measured and analyzed during the comparison process, only the power measurement

results will be given here inasmuch as it is power and energy measurements that are of prime concern in this report.

The conditions of test were essentially the same at all laboratories, that

is, 120 V, 5 A, 60 Hz, and either 600 W or 300 W, depending on the power factor.

During the calibrations of the thermal wattmeter, readings were recorded over a period of several days. The average errors of the instrument as well as three

standard deviation limits of the random uncertainty were calculated from these

data.

The listed quantities are defined as follows.

Errors: (P^ - Pa)/S ,

where Pj^ is the active power indicated by the wattmeter, P/\ is the active power

applied to the wattmeter, and S is the apparent power.

The results are shown in table 10-1 and are illustrated in figure 10-1. The

calibrations PTB(IB)-1 are those made by PTB at the beginning of the comparison

and PTB{IB)-2 at the end. In deriving the averages quoted, each of the PTB

calibrations is given half weight. It is apparent from the two PTB values

that the wattmeter calibration was relatively unaffected by either the transport

conditions experienced between laboratories or the time period of four months

taken to accomplish the intercomparison.

55
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56
Table 10-1. Calibrations of the PTB Wattmeter

(In ppm of apparent power at 120 V, 5 A, and 60 Hz)

Random un limitscertainty

^ m'

Number of

measurements

-0.5 1 .0 0.5 -0.5 1.0 0.5

3

4

9

2

3 3

7 1 9 6

2 2

7

12

5

9

7 7 12 9

10 5

9 9

Laboratory Errors^

Power Factor -0.5(1ag) 1.0 +0.5(1ead)

PTB(IB)-1 + 7(8) + 5(8) + 7(8)

NBS +20(16) + 2(17) +21(14)

NRC(EE) +20(25) +14(39) +36(21)

PTB(IB)-2 + 7(7) - 1(7) +14(7)

Average +16 +6 +22

^Total uncertainty is shown in parenthesis after each error value.

Examining the results, no laboratory differed by more than ±15 ppm from the

average, with reference to apparent power at unity and half power factors (both

leading and lagging). This is consistent with estimates of error limits of each

of the three participants. At unity power factor, NBS agreed exactly with the

average of the PTB(IB) results.

At half power factor (both leading and lagging) the errors as measured by all

three laboratories indicated a positive shift with respect to the values at unity

power factor. About 10 ppm of this shift was later determined to be the result of

a nonlinearity of the thermal wattmeter. However, a residual discrepancy appears

as an offset of the PTB(IB) results as compared to those results reported from

both NBS and NRC(EE). The cause of this offset is unknown at the time of the

writing of the paper [12].

The results of a separate comparison between NBS and NRC(EE) using two commercial

wattmeters of the time division multiplier (TDM) type are shown in table 10-2. The

reported difference is computed from the average value obtained by the base laboratory,

at the beginning and end of the comparison period, and is compared with the value

obtained at the other laboratory. The average value was used because instabilities

of the order of 50 ppm were observed for both instruments during the exchange

period. While the commercial meters were not as stable as the PTB(IB) instrument,

the results are still very impressive. It is apparent that commercial power

instruments warrant measurements of higher accuracy than is now generally quoted.

57
Table 10-2. NBS/NRC(IEE) Comparison using commercial wattmeters

(In ppm of apparent power at 120 V, 5 A, and 60 Hz)

Wattmeter (NBS-NRC) at power factor

-0.5(lag) 1_^^ +0.5(leacl)

1 +30-5+16

2 -30 -40 -58

Average -22 -21

11. SUMMARY

The basic concepts and philosophy of extending the measurement chain from

the fundamental units of mass, length, and time to the calibration of NBS

reference watthour meters and then to the ultimate calibration of^watthour meters

sent to NBS is presented. The hardware used in such a chain of measurements and

the test procedures employed are discussed in detail. The associated uncertainty

of each step in the chain is examined and these results are documented herein.

A sample Report of Calibration is included with an accompanying explanation.

The NBS Measurement Assurance Program (MAP) for electric energy is discussed

with results shown over the period since MAP's establishment in 1975 to the

present time. The results given show that the utilities participating in the

program have adjusted their values so that their measurement uncertainties have

decreased from typically ±0.10% to values typically ±0.02%. This five-fold

improvement demonstrates the value of the MAP to the electric utility industry.

Results of international comparisons for energy measurements are given. The

national laboratories of the United States (NBS), Canada (NRC), and West Germany

(PTB) each participated. All laboratories' results agreed to within ±15 ppm

(+0.0015%) on the average with reference to apparent power at power factors

of 1 , 0.5 lagging, and 0.5 leading. These results were consistent with the

uncertainty limits estimated by each of the participants.

With recent improvements of hardware, standards, measurement procedure,

and data analysis, electric energy calibrations with systematic uncertainties

of less than ±100 ppm (±0.01%) can be performed. Where the random component

of uncertainty is small, the overall uncertainty may be less than ±100 ppm.

Based on this performance, NBS is now offering special high accuracy calibration

of quality watthour meters to uncertainties of ±100 ppm.

58
12. ACKNOWLEDGMENTS

The authors would like to express gratitude to the following staff members

at the National Bureau of Standards for their help in preparing this report:

Mr. N. M. Oldham, who helped design the present system and, in particular, much

of the electronic hardware, provided much useful information as well as reviewing

the manuscript thoroughly; Mr. T. M. Souders who designed the current transformer

and provided information regarding transformers and their errors; Dr. K. J, Lentner

who designed the current comparator power bridge and assessed its errors;

Mr. J. D. Neal who constructed most of the hardware; Mr. J. Jones who operated

the system and gave practical information regarding its use; and Mrs. C. Croarkin

who performed statistical analyses of much of the data and summarized the results

to a useful form. Dr. R. E. Hebner, Group Leader of the Applied Electrical

Measurements Group, provided overall guidance and made many helpful suggestions.

Appreciation is given to Mrs. S. Kelley, Mrs. L. Bequette, Mrs. B. Meiselman,

Mrs. 6. Frey, Mrs. J. Palla, and Mrs. B. Oravec, who prepared the manuscript with

the finest of attitudes and quality.

59
13. REFERENCES

[1] Park, J. H. and Lewis, A. B., Standard electrodynamic wattmeter and ac-dc

transfer instrument, J. Res. Nat. Bur. Stand. (U.S.), Vol. 25, pp. 545-579

(Nov. 1940).

[2] Hermach, F. L., Precision power and energy measurements, NBS Dimensions,

Nat. Bur. Stand. (U.S.) (June 1979).

[3] Hermach, F. L., Electrical calibration accuracies at NBS -- normal and best

capabilities, Instr. Technology, pp. 63-66, Nov. 1967.

[4] Cutkosky, R. D., New NBS measurements of the absolute farad and ohm,

IEEE Trans. Instrum. Meas .

, Vol. IM-23, No. 4, pp. 305-309, Dec. 1974.

[5] Driscoll, R. L. and Olsen, P. T., The NBS Ampere in Absolute Measurement,

Report to Comite' Consultatif d' Electricite', Comite' International

des Poids et Measures, 12th Session, Sevres, France, 1968.

[6] Spinks, A. W. and Zapf, T. L., Precise comparison method of testing ac watthour meters, J. Res. Nat. Bur. Stand. (U.S.), 53, 95 (1954).

[7] Calibration and related measurement services of the National Bureau of

Standards, Nat. Bur. Stand. (U.S.), Spec. Publ . 250 (1980).

[8] Lentner, K. J., A Current Comparator System to Establish the Unit of Electrical

Energy of 60 Hz, IEEE Trans. Instrum. Meas., Vol. IM-23, No. 4, pp. 334-336,

Dec. 1974.

[9] Schuster, G., Thermal Instrument for Measurement of Voltage, Current, Power,

and Energy at Power Frequencies, IEEE Trans. Instrum. Meas., Vol. IM-29,

No. 3, pp. 153-157, Sept. 1980.

[10] Souders, T. M., Wide-Band Two-Stage Current Transformers of High Accuracy,

IEEE Trans. Instrum. Meas., Vol. IM-21 , No. 4, pp. 340-345, Nov. 1972.

[11] Petersons, 0., A Self-Balancing Current Comparator, IEEE Trans. Instrum.

Meas., Vol. IM-15, pp. 62-71, May/June 1966.

[12] McAuliff, R. C, Lentner, K. J., Moore, W.J.M., and Schuster, G. An

International Comparison of Power Measurements at 120 V, 5 A, and 60 Hz,

IEEE Trans. Instrum. Meas. Vol. IM-27, No. 4, pp. 445-449, Dec. 1978.

[13] Oldham, N. M., A measurement assurance program for electric energy,

Nat. Bur. Stand. (U.S.), Tech. Note 930 (Sept. 1976).

[14] Cameron, J. M., Measurement Assurance, Nat. Bur. Stand. (U.S.), NBSIR 77-1240

(April 1977).

[15] Houghton, S. R., Transfer of the Kilowatthour, IEEE Trans. Power Appar. Syst.

Vol. PAS-94, No. 4, July/August 1975, pp. 1232-1239.

60
APPENDIX A*

NBS CALIBRATION SERVICES OF WATTHOUR METERS

A.l Technical Information

Only portable standard watthour meters (rotating standards and electronic

types) will be accepted for test. These tests consist of determinations of

the percentage registration of the meter "as received." If meters are to be

cleaned and adjusted this must be done before the Requests from Federal agencies,

or from State agencies, for calibrations are submitted for test. The Bureau

does not undertake the cleaning and adjustment of meters and does not knowingly

begin tests on faulty meters.

Before tests can be started, the test conditions must be completely specified

by the user as to current and voltage ranges to be tested, frequency, applied

voltage and current, and power factor. Test voltages should be chosen from

the following values: 1, 2, or 4 times 110, 115, 120, 125 and 130 V (but not

to exceed 480 V). Test currents should be chosen from the following values:

1, 10, or 100 times 0.5, 0.75, 1, 1.25, 1.5, 2, 2.5, 3, 3.75, 4, 5, 7.5 A (but

not to exceed 50 A). Tests at other voltages or at power factors other than

1.0, 0.5 current lagging and 0.5 current leading will be considered as special

tests. Unless otherwise specified, test runs on portable standard watthour

meters are of approximately 100 seconds duration. The meters are energized

for at least four hours at rated voltage and current on one range before starting

the test. Normally values are reported with an uncertainty of ±0.05%.

A. 2 Calibration Requests, Watthour Meter Shipping,

Insurance and Risk of Loss

A formal purchase order for the calibration or test should be sent before

or at the time the standard is shipped. This should provide clear identification

of the apparatus being submitted and give separate instructions for return

shipment, mailing of report, and billing. To minimize the time during which

equipment is out of service, usually one can arrange to delay shipment until

the test is scheduled to start.

Requests from Federal agencies, or from State agencies, for calibrations

or tests on material to be used on private or Federal contract work should be

accompanied either by purchase order or by letter or document authorizing the

cost of the work to be billed to the agency. The Bureau's acceptance of purchase

orders does not imply acceptance of any provisions set forth in the order

contrary to the policy, practice, or regulations of the National Bureau of

Standards or the U.S. Government. The purchase order should clearly state

*Portions of this section were excerpted from NBS Special Publication 250,i "Calibration and Related Measurement Services of the National Bureau of Standards,

1980 Edition." See reference 7.

61
special or necessary conditions of test where appropriate (i.e., operating

frequency, power factor, etc.). Requests for measurement services from

foreign sources should be sent to:

Office of Measurement Services

National Bureau of Standards

Washington, DC 20234 .

NBS staff will provide assistance for individual measurement problems. The

headquarters of the National Bureau of Standards is located in Gaithersburg,

Maryland, approximately 25 miles northwest of Washington, D.C. The calibration

of watthour meters is performed at the Gaithersburg site by the Electrosystems

Division (722) of the Center for Electronics and Electrical Engineering of NBS.

Inquiries may be made by directing correspondence to:

National Bureau of Standards

Electrosystems Division (722)

Metrology Building, Room B344

Washington, DC 20234 . v

The reader is encouraged to obtain a copy of the NBS Special Publication 250,

Calibration and Related Measurement Services of the National Bureau of Standards,

and is available at the following places:

Superintendent of Documents

Government Printing Office

Washington, DC 20402

Office of Measurement Services

National Bureau of Standards

Washington, DC 20234

Field Offices of the Office of Field Services

Department of Commerce

Program Information Office

National Bureau of Standards

Boulder, CO 80303

Federal Depository Libraries.

The appendix of SP 250 listing current services and fees is issued twice

yearly (June and December) and is available free from:

Office of Measurement Services

National Bureau of Standards

Washington, DC 20234

Program Information Office

National Bureau of Standards

Boulder, CO 80303 .

Scheduled work assignments for calibrations and other tests generally

will be made in the order in which confirmed requests are received. However,

62
Government work may be given priority. For the regular services, the workload

is usually such that the turnaround interval, between the date a customer's

apparatus is received and the date it is prepared for return shipment, will be

not more than 45 days. Some types of instruments may require a longer time,

particularly if their abnormal behavior requires reruns to check reliability.

The customer who can spare his instrument for only a short time usually can arrange by letter or phone for shipping it to NBS just as his assigned starting

date approaches. Generally, the acknowledgment of the purchase order gives

the expected completion date. When prearrangements are made, most meters can

be calibrated and returned within ten working days.

Limited staff precludes NBS from undertaking repair activities. Therefore,

all apparatus submitted for calibration should be free of defects and in proper

working order. Electrical contacts should be in proper condition both mechanically

and electrically. Due to the delicate nature of most watthour meters, it is

advisable to pack them extremely carefully. Special reusable shipping containers

customized for this purpose are a worthwhile consideration.

A report is issued upon the calibration of each meter. This report contains

the measured values of each of the appropriate attributes of the device and their

uncertainties relative to the legal units. Additional uncertainties associated

with certain other effects may be quantified from additional measurements. NBS

personnel can assist in setting up the appropriate experiments. Reports of

Calibration for watthour meters are discussed in section 7 of this report and

an example of a typical report is given.

NBS neither requires nor recommends intervals between NBS calibrations for

watthour meters. These calibration intervals depend upon the performance of the

individual standard and the accuracy requirements of its application. These must

both be determined by the user. Some state utility commissions have established

calibration intervals they have determined to be necessary for watthour meters.

Shipment of apparatus to NBS for calibration or other tests should be made

on.ly after the customer has accepted the estimate of cost and the tentative

scheduling. Repairs and adjustments on apparatus submitted should be attended

to by the owner since NBS will not undertake them except by special arrangement.

Apparatus not in good condition will not be calibrated. If defects are found

after calibration has begun, the effort may be terminated, a report will be

issued summarizing such information as has been found, and a fee may be charged

in accordance with the amount of work done.

The customer should pack apparatus sent to NBS so as to minimize the

likelihood of damage in shipment and handling. In every case, the sender should

consider the nature of the apparatus, pack it accordingly, and clearly label

shipments containing fragile instruments or materials. Care should be taken

in selecting the best mode of transportation.

To minimize damage during shipment resulting from inadequate packing, the

use of strong reusable containers is recommended. As an aid in preventing

loss of such containers, the customer's name should be legibly and permanently

marked on the outside. In order to prolong the container's use, the notation

REUSABLE CONTAINER, DO NOT DESTROY should be marked on the outside.

63
Shipping and insurance coverage instructions should be clearly and legibly

shown on the purchase order for the calibration or test. The customer must

pay shipping charges to and from NBS; shipments from NBS will be made collect.

The method of return transportation should be stated and it is recommended

that return shipments be insured, since NBS will not assume liability for

their loss or damage. For long-distance shipping it is found that air freight

provides an advantage in reduction of time in transit. If return shipment by

parcel post is requested or is a suitable mode of transportation, shipments

will be prepaid by NBS but without covering insurance. When no shipping or insurance instructions are furnished, return shipment will be made by common carrier collect and uninsured.

The risk of loss or damage in handling or testing of any item by NBS must

be assumed by the customer, except when it is determined by the Bureau that

such loss or damage was occasioned solely by the negligence of Bureau personnel,

In such cases, the owner may apply for reimbursement.

Shipments of watthour meters to NBS for calibration §,hould be directed to:

National Bureau of Standards

Electrosystems Division, MET B165

Rt. 1-270 and Quince Orchard Road

Gaithersburg, Maryland 20878 .

64
APPENDIX B

WATTHOUR METER COMPARISON SYSTEM HARDWARE

AND CIRCUITS

The watthour-meter calibration hardware consists of: 1) two adjustable power

sources, one for the voltage inputs to the watthour meters and the second for

the current inputs; 2) a test table which provides for convenient connections to

meters being tested; 3) precision voltage and current transformers for testing

at levels different from those used with NBS reference watthour meters; and 4) the

comparison electronics for reading the output pulses of both the reference meters

and meters being tested. This group of equipment will be discussed in detail in

this appendix.

8,1 Power Supply and Control Hardware Circuits

The four NBS reference watthour meters and the meters being tested derive

their electrical stimulus from two separate sources, one for the voltage circuits

and the other for the current circuits. Figure B-1 shows the basic circuit.

Power can be derived from either a commercial power line (208 V, three-phase),

or from a three-phase electronic power supply. The electronic power supply

provides a three-phase voltage of better stability and less distortion than

the voltage derived from the commercial power lines. Either the three-phase

power line or the electronic power source can be selected using a four-pole

double-throw switch as shown in figure B-1, which then routes the power to the

voltage, current, and power factor controls shown in figure B-2.

Voltage, current, and power factor can each be adjusted by the controls

shown in figure B-2. These controls are fed from a 208-V three-phase wye

connected line (with neutral). Each parameter has a coarse and a fine adjustment

configured from variable transformers. By the proper selection of the three-phase

lines and adjustments of the controls, the voltage to the watthour voltage

circuits can be set, the output voltage for the current transformer for the

current circuit can be set, and the power factor can be suitably adjusted.

Switch S3 selects the power factor condition of unity, lead, or lag and

feeds the properly phased voltage to transformers T4 and T5 for the current

adjust. Transformer T4 is tapped about 40% up and feeds T5 and a step-down

transformer T6. The secondary of transformer T6 is in series with the output

of T4 such that these voltages add. The combination of the 40% tap on T4,

transformer T5, and the step-down ratio of transformer T6 creates the fine

adjustment, having a voltage range of about 3% of a setting. Transformer TIO

is for isolation and phase inversion and has a nominal 1:1 ratio.

For the three conditions of power factor, the following pairs of phases are selected by switch S3 and these voltage phases are applied to the transformer T4.

Power factor Voltage phase

Unity

Lead

Lag

65

B to C

B to A

A to C
TO POWER SUPPLY CONTROLS

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ABCNG

3-PHASE

POWER

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AAA AA

ABCNG

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12OV6OHZ10

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selectable between the 208-V three-phase power lines or the three-phase electronic power supply.

66
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Figure B-3 is the phasor diagram which shows the voltage output phasors for

the three conditions of power factor from the current adjustment transformers.

The output voltage is connected to step-down current transformers discussed

later; the test current for the watthour meters will be in-phase with this

voltage thus generated by the current controlled adjustments. The current

phasor for unity power factor is in-phase with the line-to-line voltage Vq^.

(The subscripted notation designates the voltage from phase C to phase B,

namely, Vqr = (Vq - Vg). Similar notation is used later in this discussion

for the other phased voltages.) The current phasor for a leading power factor

is in-phase with V/\g and for a lagging power factor, with Vq/^. Note that

the phase difference between V/\3 and Vqq and the difference between Vq/^ and

Vqb is nominally 60°. This, as will be seen later, generates the required

phase shift for the power factors of 0.5 leading and lagging.

The voltage used for the watthour meter potential circuits is adjustable

in both its amplitude and phase. Transformers T7 and T8 are fed from phases A

and B of the input three-phase line. Transformers T8 and T9 are used for fine

adjustment. The output of these transformers serves as cTne side of the output

voltage for the watthour meter testing as well as a reference voltage for

transformers Tl , T2, and T3. The output from the secondary of transformer T3

is the other side for the watthour voltage circuits. Figure B-4 shows a phasor

diagram for the voltage adjustment output voltage Vp. The variable K] is a ratio of the output voltage from transformer T9 to the input voltage to transformer

T7, and, likewise, variable K2 is the ratio of the output voltage of transformer

T3 to the input of transformer Tl . When nominally set for an output voltage,

Vp, of 120 V, K^V/^R is about 60 V and K2VC is about 104 V with the phase angles

as shown in figure B-4. As can be seen from the vector diagram and figure B-2,

adjustments of either the "voltage" or "power factor" controls (transformers Tl

and T7, respectively) will cause the output voltage vector Vp to change in magnitude

and phase. Because transformer Tl has more effect over the voltage and less on the

power factor than does transformer T7, the former is labeled as the voltage control.

When the two phasor diagrams as shown in figures B-3 and B-4 are superimposed

as shown in figure B-5, the relationships between the current and voltage vectors

are apparent for the three conditions of power factor generally used in calibration

and testing (i.e., unity and ±0.5). Vectors with open arrows represent the circuit

currents and arrows which are solid represent circuit voltages. Voltage Vp is the

test voltage.

When the test voltage Vp is in-phase with iunity ^ power factor of 1 is

realized. When the power factor selector switch S3 is turned to the "lead"

position, the current vector leads the voltage vector Vp by nominally 60°.

When lag condition is selected, the current vector lags the voltage Vp by 60°.

Also illustrated by figure B-5 and shown below, the range of phase adjustment

for each of the power factor settings of the selector switch S3 is 90°.

Power factor Phase shift range ^ Nominal phase

Unity +30° to -60° 0°

Lead 0.5 0° to +90° +60°

Lag 0.5 -30° to -120° -60°

^A positive angle is considered as a counterclockwise rotation. Phase shifts

are with respect to the voltage vector Vp.

68
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Note that the angular range is 90° of phase adjustment for the voltage

vector, but for nominal conditions the angles are either 0° or ±60°. The

control latitude permits adjustments about the desired power factor and also

voltage over ranging, if necessary.

Note that the voltage vector Vp is nominally the same for all testing, i.e.,

its adjustment is always within a fixed range with respect to the three-phase

line voltages. Power factor selection is accomplished by switching the test

current vector into one of three possible angles as shown and discussed above.

The output from the voltage control circuit feeds a step-up transformer

having turns ratios of 1:1, 1:2, and 1:4, thus providing output voltages of at

least 120, 240, and 480 V, respectively. The three output voltages are switch

selectable as shown by figure B-6. The selected voltage is applied to the watthour

meter under test and to the precision voltage transformer which feeds the NBS

reference watthour meters.

The output of the current control circuit is connected to two step-down

transformers through a link arrangement. The transformers are rated at 240-V

input on each primary and 2-V output on each secondary. Figure B-7 shows the

basic arrangement and figure B-8 shows the link arrangements. These transformers

are used for output currents of 3 A and greater. For currents less than 3 A,

these transformers are bypassed and the output from the current control circuit

is fed directly to the watthour meters. When used for output currents of greater

than 3 A, the transformer primaries are connected in parallel and a voltage from

to 208 V can be impressed.

The four secondaries are then linked in three basic configurations such

that current from 3 A to near 100 A can be obtained. The maximum current into

a short circuit is near 100 A; however, typical maximum currents are about 30 to

50 A, depending on the total impedance of the current circuit.

B.2 Signal Conditioning and Pulse-Counting Electronics

The signal conditioning and pulse-counting electronics process the pulse

outputs from the NBS reference bank, meters, and to either gate the 120-V

potential circuit for watthour meters with registers or to condition and count

the pulse outputs for watthour meters having pulsed outputs. Figure 5-6 shows

a simplified block diagram of the signal conditioning and pulse-counting

electronics.

Connections are made to watthour meters being calibrated by suitable

connectors on the front panels. These provide for not only system input of the

pulses from the meters having such, but also provide for lamp and bias voltages

for meters requiring them. The fundamental flow of signals is from left to

right as drawn in figure 5-6. Watthour meters' output pulses are fed to signal

conditioners, adders, gates, display selectors, and finally to electronic counters.

The output of the adder in the reference channels feeds the sum of the four

reference watthour meters to a preset camparator, whose output generates a suitable gate for stopping the counters when the preset value is reached.

To facilitate this discussion, reference is made to figure B-9 which is a detailed block diagram expanded from that shown in figure 5-6. Here the blocks are arranged by circuit board and are identified as A through G. Coaxial connectors

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are used at certain strategic circuit locations for inputs, outputs, and at

monitoring points. Throughout the ensuing discussion, reference will be made

to figure B-9 in conjunction with individual drawings of particular circuits.

A major portion of the circuitry is located on circuit boards which are mounted in a chassis. Each board plugs into a 44-pin connector. The numbers

for the pins used are shown on each board. (A hybrid numbering system is

employed using numbers from 1 to 22, and 22 letters from A to Z, with the

letters G, I, 0, and Q being omitted from the sequence.) All pins are not used.

B.2.1 Input Connections

Figure B-10 shows the four multipin input connectors, eight coaxial

connectors, and the four variable resistors. Note that two kinds of inputs can

be accommodated, "single pulse" and "multiple pulse" inputs. For single pulse,

one pulse per watthour meter disc revolution is received. For multiple pulse, a number of pulses are received per revolution of the disc. Single-pulse operation

uses pin C of the connector while pin H is used for the multiple pulse inputs.

The multipin connector also provides for bias voltage and for lamp voltage as shown for the watthour meters being tested. The variable resistors are used to

optimize input pulse wave shape.

Figure B-11 shows the four multipin connectors for the input from the

reference bank of four watthour meter standards. These connectors are located

on a panel at the rear of the rack cabinet. Bias voltage and lamp voltage is

also passed through these connectors. Pulse shaping variable resistors are also seen with their adjustments located on the rear panel.

Except for location and one ground connection, the inputs for the four

input test stations and the inputs from the four reference watthour meters are the same. Pin E of the multipin connector is grounded for the test watthour

meters whereas it is not for the reference meters.

Signal outputs are picked up at the coaxial rear panel connectors as indicated on the detailed block diagram, figure B-9, and on figure B-12. Two

front panel, ganged, eight-position, selector switches are wired to the input

connector circuits so that any one of the test inputs or any one of the reference

inputs can be monitored, principally on an oscilloscope. These switch selectable

outputs connect to front panel connectors labeled SINGLE PULSE OUTPUTS and

MULTI PULSE OUTPUTS. Short jumper cables provide the interconnections of the

signal pulses to the inputs of the eight channels of signal conditioning.

B.2.2 Signal Conditioning Circuits

The wave shape of the "pulse outputs" from watthour meters having slotted

discs are near sinusoidal and as such do not serve well as triggering sources

for the subsequent pulse adders and electronic counters. To improve the pulse wave shape and to make them compatible with TTL circuitry, signal conditioning

circuits as shown in figure B-12 are used. There are two circuit boards, E and F, which are identical.

The signal conditioners consist of an operational amplifier and variable

resistor, and a Schmitt trigger. The amplifier and resistor create a variable

threshold feature. The input signal, though sinusoidal, can be offset from

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output pulses from the four test and four reference

channels (circuit boards E and F).

80
zero volts. Figure 5-8 shows typical wave shapes of the output of a slotted

disc-type watthour meter at the input to the signal conditioner and the output

of the Schmitt trigger. Note the square wave shape of the output. This square wave is adjusted for symmetry by the variable threshold resistor and is fully

TTL compatible for the subsequent circuits.

The numerals and/or letters in circles represent pin numbers on the

circuit board. The letter/numeral associated with each of the integrated

circuit (IC) components, for example, refers to the type of IC that is

employed. A list of these appears in appendix C. For example. El is one in

a package of four "dual 4-input positive NAND Schmitt trigger," type 7413.

The square wave outputs from the Schmitt triggers go to a switch which

can select any one of the eight signal conditioned channels. The switch output

feeds a connector labeled SIG COND MONITOR on the front panel. The four reference

channels go directly to the pulse adder circuit board as indicated by figure B-9.

The four test channels go to the pulse adder board through pairs of jumpered

connections which provide for channel flexibility and monitoring.

B.2.3 Pulse Adder and Gate Circuits

ce The pulse adder circuit serves two purposes. First, it provides an output

which is the "sum" of the four reference channels (derived from the four referen

watthour meters) and gates this output and, second, it individually gates the

four test channels. The reference channel and the test channel outputs are also

"clocked" so that relatively narrow pules, about 20 ys wide, appear at the outputs

independent of the input pulse width, which typically are no shorter than about

10 to 30 ms.

Figure B-13 shows the circuit used to sum the conditioned output pulse of

the four reference watthour meters. The four inputs are connected to pins C,

D, E, and F for standards 1, 2, 3, and 4, respectively. Each of these inputs

feeds into the "clock" input of a type D, positive edge triggered flip-flop,

with the D input held high by connecting it to the +5 V bus. The preset input

is left unconnected and floats to a high state. The clear inputs are at a high state initially. Initially, the Q outputs from each of the four type D flip-flops are in a low state and remain low until an input pulse appears at

the individual clock inputs. When the input leading edge goes positive, the

output at Q goes from low to high and will remain high until reset by the clear

input going to a low state, at which time Q also goes low again, waiting for the

next positive going leading edge to occur at the input. In this manner, each

input pulse is latched up until reset by the clear input.

The Q outputs from each flip-flop feed the inputs Dl , D2, D3, and 04 of a 4-bit cascaded priority register as shown in figure B-13. Input PO is held

low by grounding it. The "summed" output appears at PI which ties to pin 11 and pin M on the circuit board. A 50-kHz clock pulse, ck, is applied to the

strobe input, G. The simple 50-kHz clock is shown in figure B-14 and is contained

on the same circuit board. The priority register, thus configured, will scan the inputs from Dl to 04 with e'^ery strobe input pulse. If the input is high

(indicating a pulse had set the flip-flop high), then an output will appear

both at the PI output and at the respective "Y" output for a duration of about

one strobe period. The Y outputs in conjunction with the NAND gate and clock

pulse ck are used to reset the D-type flip-flop at the CLR input. This

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arrangement provides a 20-yS pulse out at PI for each input to the flip-flops,

regardless of when the input occurred. If inputs to the flip-flops are simultaneous, the priority register will "scan" them and provide individual

outputs, the lower priority output being one strobe period later.

The strobe rate is running more than 1000 times faster than the highest

watthour meter input pulse rate, thus there is no way in which input pulses can overrun themselves or be missed in the output. The "summed" output is buffered

and clocked by two cascaded NAND gates as shown by figure B-13 and appears at

pin 1 2 circuit board.

Four individual outputs are also available on pins 3, 4, 5, and 6, which

correspond to the reference watthour meters, 1, 2, 3, and 4, respectively. These

outputs are inverted and gated by NAND gates D3. The test gate is derived from

the preset counter of the comparator and is applied at pin B of the pulse adding

board. (See fig. B-9 for the preset counter location in the overall circuit.)

A second circuit very similar to the pulse adder described above is on the same circuit board and is used to process the four test channels. This is shown in

figure B-15. Positive edge triggered type-D flip-flops are used to hold the inputs

from each of the four test channels. The operation of these flip-flops and the

4-bit cascaded priority register is identical to that described above for the pulse

adding circuit. The D inputs to the flip-flops are each gated by the test gate

so that when it is in the high state, input pulses at the clock input will latch

the flip-flop with a high output on Q. Each Q output is tied directly to the

output pin on the board, namely, pins 16 through 19. The priority register and

NAND gates are clocked such that the D-type flip-flops are reset one clock period

after being set. Thus the circuit outputs appear as TTL pulses of approximately

the width of one clock period. Even though a summed output appears at the PI output of the priority register, it is not used. Output pulses can appear only

when the test gate, input pulses, and clock pulses are simultaneously present.

B.2.4 Display Select Gates

As shown by figure B-9, the outputs of the four reference channels and the

four test channels are directed to the eight inputs on the display-select

circuit board. The purpose of this circuit is to select electronically the

reference channels or the test channels for display on the electronic counters.

The circuit which does this is shown in figure B-16 and is comprised of NAND

gates for the electronic selection. Signal inputs from the four reference

channels are connected to pins 4 through 7. The four test channels connect to

pins 10 through 13. A double-pole, single-throw switch as shown in figure B-16

applies either a high (+5 V) or a low (0 V) to pins Y and 21. If Y is high,

then 21 is low, and vice versa. This logic controls all eight NAND gates to

either select the four test channels or the four reference channels.

The two hex inverters, G7, in the control select circuit (at pins Y and 21) serve as buffers between the switch and the input NAND gates; also, if this were

ever to be driven by TTL logic, it provides less fan-out loading to that logic.

The outputs appearing at pins 16 through 19 are inverted from the corresponding

inputs. Each of these four outputs tie to connectors and to a front panel rotary

switch marked COUNTER MONITOR which in turn feeds a front panel connector labeled

COUNTER INPUT MONITOR. The connectors Fl through F4 are normally connected to

the display counters 1 through 4.

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86
B.2.5 Auxiliary Electronic Circuits

A number of auxiliary electronic circuits are used to shape, generate, and

gate signals such as the standard frequency, and the line frequency. Figure B-9

shows these located on circuit boards B and C. These circuits and their functions

are discussed below.

B.2.5.1 Line Frequency Circuits

The NBS standard frequency of 1 kHz is a sinusoidal voltage which is fed

to a Schmitt trigger as shown in figure B-17 at pin 11. The shaped standard

frequency output (now at TTL levels) is connected to pin M and goes to the gate

board, circuit board C, as shown in figure B-9.

A 3.1 5-V line-frequency signal is obtained from a center-tapped filament

transformer. This provides an input of the test-line frequency. This is connected

to the input of another Schmitt trigger at pin 13 as shown in figure B-18. The

input Schmitt trigger shapes the sinusoidal waveform to a symmetrical square wave

compatible with TTL levels. A type 565A phase-locked-loop integrated circuit is

used, in conjunction with appropriate counters, to generate an output signal of

1 kHz which is phase-locked to the line frequency.

The 5.1 -kj^ resistor and the 300-pF capacitor at the 565A set up a free

running frequency of approximately 6 kHz. This signal is buffered by two

cascaded NAND gates and feeds two cascaded divide-by-10 decade counters. This

divided output of 60 Hz is fed back into the phase-locked-loop circuit to

complete the loop. The free running 6 kHz is now phase locked to the line

frequency. The 6-kHz signal also feeds into a divide-by-6 counter which then

has a nominal output frequency of 1 kHz at pin P. This output goes to the

gate board C as shown in figure B-9.

B.2.5. 2 Standard Second Signal Circuit

A type 74122 retriggerable monostable multivibrator is used to shape the

standard 1 -s tone "tics" from the WWV standard time broadcasts. The "tics"

are comprised of a five-cycle burst of a 1-kHz signal precisely gated to begin

every second. The circuit is shown in figure B-19. In order to generate a single pulse from this tone burst, the retriggerable monostable multivibrator

triggers on the first positive rise of the burst. If only one trigger were

received, the pulse width would be as determined by the following relationship:

tw = 0.32 RjCj (1 + 700/Rt) ,

where Rj is the value in ohms of the timing resistor and Cj is the value

capacitance in farads of the timing capacitor. For the circuit values shown,

Rj = 56 k^ and Cj = O.l yF. The normal pulse width is about 1.8 ms if the

multivibrator is not immediately retriggered. However, the second cycle of

the tone burst occurs 1 ms from the first, which is less than the 1.8 ms time

constant and therefore retriggers the multivibrator before it has time to

reset. This process continues so that the output is held high because of the

repeated retriggering of the multivibrator by the five cycles in the tone

burst. At approximately 1.8 ms after the fifth cycle, the output goes low.

The final pulse width under these conditions is 5 ms plus t^ or about 6.8 ms. The standard time signal has a longer burst every 60 seconds to mark the minutes.

87
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The multivibrator operates exactly the same for this longer burst except it is

held on longer and the output is the length of the burst plus 1.8 ms. The

"standard second" output can be used to gate counters and other electronic

apparatus used in the calibration of watthour meters.

B.2.5.3 Auxiliary Schmitt Trigger Circuit

An auxiliary Schmitt trigger is accessible through front panel connectors.

This circuit can be used to improve the shape of signals before further processing.

Occasionally, a watthour meter output will be such that wave shaping will be

necessary before further testing can be done. The circuit is located on board B and is shown by figure B-17.

B.2.5.4 Variable Threshold Schmitt Trigger Circuit

An auxiliary variable threshold Schmitt trigger is accessible by front panel

coaxial connectors. This circuit is identical to those used for the input signal

conditioners as described in section B.2.2. Sinusoidal input signals where

amplitudes are not symmetrical about zero volts can be handled by this circuit

to provide a symmetrical square wave output with TTL compatibility. Usual

practice is to adjust the variable resistor for waveform symmetry. Symmetry

is not essential, but does provide a convenient means for adjustment. The

circuit is shown in figure B-17.

B. 2.5.5 Pulse Delay Circuit

A pulse delay (pulse stretcher) circuit is included on circuit board B and

is used to lengthen certain pulses to a width determined by the adjustment of

a variable resistor. The circuit uses a type 555 timer as a one-shot multivibrator

as shown in figure B-20. The input at board pin 7 feeds to NAND gate which

helps to shape the input pulse and buffer the timer's input. The pulse delay

circuit can generate pulses of adjustable width from about 1 ms to 10 s.

An example for its use is in the calibrations of house-type watthour meters.

In some instances, there is only one slot or "optical hole" in the rotating

disc. If, under selected test conditions, the disc is rotating very slowly,

the transition of the optics from dark to light is poorly defined just as the

hole begins to pass the light and unstable multiple triggering can occur if

precautions are not taken. The delay circuit triggers once and then waits for

the selected time before resetting. Thus, multiple pulse outputs are avoided

and the disc revolutions can be counted electronically.

B.2.5.6 Pulse Divide-by-10 Circuit

A pulse divide-by-10 circuit is included on circuit board B. Front panel

coaxial connectors provide convenient access to the circuit input and output.

The purpose of this circuit is to provide a means of reducing the pulse rates,

if too great, by a factor of 10,

The circuit uses a decade counter as shown in figure B-21 . It is wired and

operates exactly the same as the divide-by-10 decade counters used in the line

frequency phase-locked-loop circuit described earlier. The circuit generates one output pulse for every ten input pulses. The circuit requires a TTL compatible

input.

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93
B.2.5.7 Miscellaneous Gate Circuits

The gates on circuit board C are of two types: 1) electronic non-inverting

NAND gates, and 2) a NAND gate-driven SPST relay. Figure B-22 shows the circuits

for each. The input controlling test gate is applied at connector pin C and

is derived from the preset counter (see fig. B-9),

The electronic gates provide the gating of three signals used in the

calibration of watthour meters. At connector pin C the summed output from the

pulse adder is connected. These pulses are allowed to pass through for a period of the test gate. The gated sum acts as the input to the preset counter

(see fig. B-9). The standard 1-kHz NBS frequency and the phase-locked 1-kHz

signal derived from the line frequency are also gated as shown by figure B-22.

The fourth electronic gate is currently not used.

The NAND gate-driven SPST relay is used to control a +50-V line which

feeds mercury-wetted relays on the test bench. The latter relays are used to

open and close the voltage circuits to the watthour meters having readable

registers. The SPST relay is closed for the length of time that the test gate

is high. This then starts the watthour meters being calibrated and stops them

at the end of the test.

The diode and the 0.1 -yF capacitor on the output of the relay provide

suppression of the transient voltage caused by opening the voltage to the bench

relays. Without these suppression components, contact arcing could occur at

the SPST relay contacts.

B.2.5.8 Reset and Start Circuit

The reset and start circuit shown in figure B-23 works in conjunction with

the preset counter. A single-pole, double-throw push button switch on the

front panel activates the reset and start operations. A pair of NAND gates

tied into the switch act as switch debounce circuits. The signal coming from

pin 20 of the comparator provides a "ready" indication on a panel -mounted LED.

The reset is connected to each of the four pulse counters as well as to

the two frequency counters and four relays in the test bench voltage circuits.

When the START button is pushed, it clears each of these six counters, starts

the preset comparator counter, and closes the test bench potential relays. When

the comparator reaches its preset values, the test gate goes to a low state

which, through the appropriate gates, stops the inputs to each of the counters

and opens the relays.

94
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APPENDIX C

INTEGRATED CIRCUIT COMPONENT LISTING

Table C-1 of this appendix lists the integrated circuit (IC) components

that are used in the signal conditioning and pulse-counting electronics circuitry,

The IC identifiers are shown on the figures in section 5 by a letter (B through

H) and a numeral (1 through 11). The letter designates the circuit board on which the IC is located. The numeral is a serial identifier. For example, IC

D4 is located on board D. From table C-1 it is seen that D4 is a type 7474.

Table C-2 gives nomenclature information regarding these ICs. Table C-3 lists

the circuit boards and relates them to the respective figures in section 5. It also provides a brief nomenclature and functional description for each board.

Table C-1. Integrated circuit (IC) component listing for

signal conditioning and pulse-counting electronics

Circuit board identifier

IC 1

C 2

M 3

P 4

N

E 5

N

T 6

1 7 D

E 8

N

T 9

IF 10

IE 11 R

E,F

74122 7403 7474 7413

555 (640-1) 7400 7413

741 7404 7400 7413

7400 7400 7474 7413

7413 7400 74278 741

7413 7400 7400 741

7490 7474 741

565 7400 741

7490 7474

7490 74278

7413 7404

7420 7403

7420 7400

7420

7404

7404

7400

7404

97
Table C-2. IC nomenclature for components used in signal

conditioning and pulse-counting electronics

IC type Nomenclature

555 Timer

565 Phase-locked loop

(640-1) Relay (Electromechanical component)

741 Operational amplifier

7400 Quad 2-input positi ve-NAND gates

7403 Quad 2-input positi ve-NAND gates

with open collector outputs

7404 Hex inverters

7413 Dual 4-input positive-NAND Schmitt triggers

7420 Dual 4-input positive-NAND gates

7474 Dual D-type positive-edge triggered

flip-flop with preset and clear

7490 Decade, di vide-by-1 2, and binary counter

74122 Retriggerable monostable multivibrators

with clear

74278 4-bit cascadable priority registers

Table C-3, Circuit board nomenclature, functions,

and figure reference

Board Figure Nomenclature and function

A B-9 Power supply connector board

B B-1

7

Signal -conditioning

Schmitt triggers

B-18 Signal -conditioning, linefrequency, phase-locked loop

B-19 Signal -conditioning,

standard second signal

B-20 Signal -conditioning,

pulse delay

B-21 Signal -conditioning,

pulse divide-by-10

C B-22 Gate board

D B-1

3

Pulse-adding, standard channels

B-14 Pulse-adding, clock

B-15 Pulse-adding, test channels

E,F B-12 Variable threshold

Schmitt triggers

G B-16 Display select gate board,

test gate buffer

H B-23 Comparator reset

98
APPENDIX D

DETAILED PROCEDURE FOR THE CALIBRATION

OF WATTHOUR METERS

D.l Installation of and Connections to the Watthour Meter

The installation involves the selection of the test bench station at which

the watthour meter is to be calibrated, leveling the meter (if required), and

connecting the proper leads from the test bench to the meter. Station four is

usually reserved for an NBS monitor standard which is used and calibrated with

each daily run. This provides an independent check on the system operation

and performance.

For rotating types of watthour meters, leveling is necessary. If the meter

has its own level indicator, the three adjustable supports of that station's pad

are adjusted to bring the instrument to a level position. If the meter does not

have a level indicator, then the bubble level on the pad is used.

Next, the potential and current leads are connected to the watthour meter

being calibrated. As a safety precaution all controls at the control console

(viz., voltage, power factor, and current) are set at their minimum settings

to preclude the chance of electrical shock. This also avoids the possibility

of subjecting the meters to surge currents which could cause magnetization

errors to occur, or of accidentally overranging the meters. The proper polarity

of the current and potential leads must be observed. A reversal of one set of

leads can cause serious errors, especially for rotating-type meters. Each

connection is to be tight and insulated.

For any of the four test bench positions not being used, the current

terminals are shorted. The voltage terminals of the same should be switched

to "off" to remove the voltage from the unused terminals on the bench. For

electronic-type watthour meters, it is necessary that auxiliary power is supplied

to the instrument, usually 120-V, 60-Hz single-phase power.

For those test stations being used, the voltage and current connections

on the test bench terminals and meter terminals are rechecked to be correct.

The meter terminals should be connected to the 120-V and 5-A taps. The voltage

switch on the test bench for the meters in the circuit should be in the "on"

position. The voltage should be connected and the current transformer should

be connected such that the 5-A tap is used. The power supply should be switched

to 120 V for the voltage supply and the connections on the current supply should

indicate 5 to 25 A position. (See figs. B-6 and B-8 in appendix B.)

The voltage and current are increased slowly. While still at low voltage

and current, rotary meters are inspected. If the meter disk runs in reverse,

return current and voltage to zero and change the polarity of the terminals

which appear to be causing the reversal. Then increase the voltage and current

to their proper values.

For watthour meters having pulse outputs, connection is made so that the

counter units are used. Initially the counter to be used is examined to verify

that it, as well as the counters for line and standard frequencies, is running.

99
The presence of bias and lamp voltages is verified by observing a pulse output

(usually on an oscilloscope) from the watthour meter after voltage and current

have been applied to the unit. The voltage and current are then increased to

120 V, 5 A, at 0.5 power factor lag. The meters are allowed to warm up from

one to four hours — usually for four hours.

D.2 Testing Procedures

Before beginning the test, the temperature of the NBS reference bank

watthour meters is checked. If it is within 25.0° ±0.2°C, calibration may

proceed.

The power supply controls are set to zero and the necessary taps changed

according to the test point conditions listed on the prepared data sheet.

Connection is made to the current and voltage taps of the meter and current

and voltage transformer taps, as necessary. Also, power supply connections

for the current and voltage supplies are changed when needed to provide the

required voltage and currents.

The power supply is set to 120 V, 5 A, and 0.5 power factor lag. Once

done, one may change power factor as needed. If the test is to be at 0.5 power

factor lag, the point is ready to be run. If the test is at unity power factor,

the current is reduced first to zero, the power factor switch changed to unity,

and current returned to 5.00 A. For a 0.5 power factor lead (or any other

leading power factor) the procedure done for unity power factor is repeated,

and adjustment of the watts indicator is made to achieve 0.5 power factor lead

(or any other leading power factor). If a lag value is other than 0.5 power

factor lag, adjust the watt indication for the required power factor.

The comparator is set to the preset value of the test point as recorded

on the data sheet. For rotary or disk-type meters, the voltage switch is

turned to "off" and the registers are zeroed. The test is now ready to run. The START button is pressed and the line and frequency counters are checked to ensure they are counting. For meters having registers, their operation is

checked. For meters having pulse outputs, the appropriate counter is checked

to ensure that it is counting.

When the test ends, the meter value (as indicated on the counter or the

meter register) and line and standard frequencies are each recorded. The

START button is again pressed and the test repeated. The meter and frequency

readings are recorded when the second run ends. The temperature of the room and reference bank are also noted.

After two or more runs, the voltage and current are reduced to zero and the

appropriate connections are changed for the next set of test conditions. This

is repeated until a pair of readings have been obtained for each test condition.

At the end of the test or day, the power supplies are set to about 75 V and 0.75 A on the 1 -A range at 0.5 power factor lag. This provides "idle"

power for the meter overnight and for weekends and holidays. This is important,

especially for rotating standards, A second set of data is obtained for each

test point at least one day later so that a minimum of four readings have been

made ~ two pairs -- on two different days.

100 ir us. GOVERNMENT PRINTING OmCE : 1983 O—380-997 (5136)
NBS-n4A REV. 2-8C U.S. DEPT. OF COMK/.

BIBLIOGRAPHIC DATA

SHEET fSee insirucvons)

1. PUBLICATION OR

REPORT NO.

1<BS TN 1179

2. Performing Organ. Report No. 3. Publication Date Julv 1933

4. TITLE AND SUBTITLE

A CALIBRATION SERVICE FOR WATTMETERS AND WATTHOUR METERS

5. AUTHOR(S)

J. D. Ramboz and R. C. McAuliff

6. PERFORMING ORGANIZATION (If joint or other tnan N6S, see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. ContracL/'Grant No.

8. Type of Report 6< Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State. ZIP;

Same as item 6,

10. SUPPLEMENTARY NOTES

Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-worc3 or iess factual summary of most significant informotion. If document includes a si gnificant

bibiiograony or literature survey, mention it here)

An NBS calibrati

meters is described,

registration for watt

wattmeters over a ran 60 Hz. Measurements

leading and lagging,

electric energy is di

energy, NBS services,

measurement methods a estimates and quality

Calibration is includ

on service for wattmeters and watthour

The service offers measurements of percentage

hour meters and percentage correction for

ge of voltages and currents at a frequency of are limited to power factors of 1.0 and 0.5,

The Measurement Assurance Program (MAP) for

scussed. National standards for electric

special equipment and instruments, and

nd procedures are described, as are error

control. A representative Report of

ed.

12. KEY WORDS (S.'x ic twe.'ve enffes, o'O" ccet'coi oroer; capitonze oniv proper ncnes; one seoorore ke>- yvoros Dv serr-,. co/on s . Calibration: electric powerwet and energy; electric standards; NbS services;

watthour meters; wattmeters.

rn. AVAILABILITY

i

' X ' Unhmted

I _ Fo' Of^'icia, Distrip-utior. Dc Not Release to NTIS

jJ^G'de- Frorr. Super inter.oent o^' DocuTients, U.S. Governr-.en: printing Of'^ce, Vvas^ingion. D.( 20402.

Oroe- Frorr. Nat.onai Tecnn.cal infcrp--3r,or Se-vice (\~:S,, Spr .-gf.eic. V ^.. 22!6i

14. NO. OF

PRINTED PAGES

111

15. Price

$4.75

,.:SCOMM-DC 6C43-P6C

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