A111D2 LE3TM3

EMX

NBS

PUBLICATIONS

NATL INST OF STANDARDS & TECH R.I.C.

A1 11 02623943

* Hillhouse, David L/A simplified system f a QC100 .U5753 NO. 1155 1982 V19 C.1 NBS-P NBS TECHNICAL NOTE 1155

"fffAU of

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

A Simplified System

for Calibration of CCVTs

in the Substation

QC

100

U5753

#1155

1982
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of 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 2 — 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 Engineering2 — Manufacturing

Engineering — Building Technology — Fire Research — Chemical Engineering2

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 A DP 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.

2 Some divisions within the center are located at Boulder, CO 80303.
A Simplified System

for Calibration of CCVTs in the Substation

David L. Hillhouse

Oskars Petersons

Wilbur C. Sze

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Bureau of Standards

Washington, DC 20234

Sponsored in part by:

Pennsylvania Power & Light Company

Two North Ninth Street Allentown, PA 18101

<"*AU O*

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued May 1982
National Bureau of Standards Technical Note 1155

Nat. Bur. Stand. (U.S.), Tech. Note 1155, 57 pages (May 1982)

CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1982
TABLE OF CONTENTS

Page

LIST OF FIGURES v

LIST OF TABLES vi

Abstract 1

1. INTRODUCTION 2

2. THE PROTOTYPE NBS SYSTEM 5

3. DESCRIPTION OF THE SIMPLIFIED SYSTEM 9

3.1 Calibration Procedure 10

3.2 System Components 12

3.2.1 Standard Divider 12

3.2.2 Voltage Comparator 14

3.2.3 Standard Transformer 14

3.2.4 Supply Transformer 17

3.3 System Configuration 17

3.3.1 Electrical Configuration 17

3.3.2 Physical Configuration 17

4. SYSTEM EVALUATION IS

4.1 Overview . 19

4.2 Voltage Dependence, Transfer Standard Divider 19

4.3 Standard Transformer T2 20

4.4 Voltage Comparator 20

4.4.1 Inductive Voltage Divider (IVD) T4 20

4.4.2 Quadrature Circuit 20

4.5 System Tests 21

4.5.1 Laboratory Tests 21

4.5.2 Field Tests 23

4.5.3 Summary 24

5. SUMMARY AND CONCLUSIONS 24

6. REFERENCES 29

APPENDIX A. SIMPLIFIED SYSTEM COMPONENT DETAILS 30

1. INTRODUCTION 30

2. DERIVATION OF SYSTEM EQUATIONS 30

2.1 Standard Divider Calibration 30

2.2 CCVT Calibration 32

m
TABLE OF CONTENTS (cont.)

Page

3. VOLTAGE COMPARATOR QUADRATURE CIRCUIT 33

4. VOLTAGE COMPARATOR DETECTOR CIRCUIT .... 35

5. CAPACITIVE TRANSFER STANDARD DIVIDER 37

6. COAXIAL CHOKES (GROUND VOLTAGE COMPENSATION) 39

APPENDIX B. A SAMPLE CCVT CALIBRATION PROCEDURE 45

1. CALIBRATE TRANSFER STANDARD DIVIDER (MEASUREMENT C3) ... 45

2. CALIBRATE CCVT (MEASUREMENT C4) 45

3. CALCULATE RESULTS 48

3.1 Standard Divider Ratio (p s

) 48

3.2 Ratio Correction Factor of Test CCVT 48

3.3 Phase Angle of Test CCVT 49

4. PRECAUTIONS 49

5. SUBSTATION SAFETY 49

6. MISCELLANEOUS 49

IV
LIST OF FIGURES

Page

*

Figure 1. Evolution of the metering accuracy CCVT: 3

(a) Carrier current coupling capacitor,

(b) Simple capacitive divider,

(c) Intermediate transformer added,

(d) Tuning reactor added to secondary,

(e) Modern metering accuracy CCVT.

Figure 2. Approximate layout for transporting CCVT calibration

system to the field in truck 6

Figure 3. Four-step calibration procedure for CCVTs, using the

prototype system 8

Figure 4. Schematic diagram, simplified calibration system: .... 11

(a) Measurement C3 - calibration of transfer

standard divider,

(b) Measurement C4 - calibration of CCVT.

Figure 5. Basic circuit of the modular capacitive standard divider. 13

Figure 6. Basic circuit of the voltage comparator 15

Figure 7. Voltage comparator assembly 16

Figure 8. Supply-standard transformer module (T1-T2) 18

Figure 9. Simple calibration circuit for the voltage

comparator quadrature circuit 22

Figure 10. Deviation of simplified system results from

prototype system results, field calibration

of CCVTs, May 1980 25

Figure 11. Deviation of simplified system results from

prototype system results, field calibration

of CCVTS, October 1980 26

Figure 12. Basic circuit of complete simplified system

for field calibration of CCVTs 31

Figure 13. Detailed diagram of voltage comparator

quadrature circuit, simplified system 34

Figure 14. Detailed circuit diagram of twin-T filter for

voltage comparator detector, simplified system 36
LIST OF FIGURES (cont.)

Page

Figure 15. Standard divider circuit, showing details of

auxiliary divider 38

Figure 16. Origin of ground voltages in the field

calibration of CCVTs 40

Figure 17. Function of coaxial chokes in eliminating

ground voltages 42

Figure 18. Coaxial choke test 44

Figure 19. Measurement C3 - calibration of transfer standard

divider 46

Figure 20. Measurement C4 - calibration of CCVT . . . 47

LIST OF TABLES

Table 1. Summary of error and fixed correction estimates,

simplified field calibration system for CCVTs 27

VI
A SIMPLIFIED SYSTEM FOR CALIBRATION OF CCVTs IN THE SUBSTATION

David L. Hi 11 house, Oskars Petersons, and Wilbur C. Sze

Coupling capacitor voltage transformers (CCVTs) are widely used for the revenue metering of energy exchanged

between utilities at EHV (345-500 kV) interties. These

devices are installed permanently in substations, and must

be calibrated there. Allowable error is ±0.3 percent and

±4.6 mrad (milliradians) .

NBS developed, and has had in operation for several

years, a prototype system for field calibration of these CCVTs.

This prototype system is more accurate, more complicated, more

bulky, and more costly than is essential for this application.

This report describes a simplified, lighter, and less

costly CCVT calibration system, newly developed and field

tested by NBS. The principal elements of this system are a portable reference standard transformer and moderate voltage

power supply (14.4 kV), a modular capacitive transfer standard

divider, and a voltage comparator. Results obtained with this

system agree with the prototype to within ±0.03 percent and

±0.1 mrad.

The prototype system is installed permanently in a dedicated calibration truck. The new system could operate

with a non-dedicated truck to transport the disassembled modular

divider, and a van to transport the rest of the components and

to serve as a field laboratory.

Key words: CCVTs; EHV revenue metering; energy metering;

field calibration; 500 kV; 500 kV substation measurements;

metering accuracy CCVTs.
1. INTRODUCTION

The coupling capacitor voltage transformer (CCVT) is a ratio

device for reducing transmission line alternating voltages

(230-765 kV rms, line-to-line) to 115 V rms to neutral for

relaying, and/or metering [l].l As its name implies, the CCVT

originated (in the 1920s) as a coupling capacitor used to couple

telephone carrier current from high-voltage transmission lines.

The basic circuits of its evolution into its present configuration

are shown in figure 1

.

In the early 1930' s, a low-side capacitor was added and the

resultant capacitive divider used for voltage monitoring and relaying

(fig. 1(b)). Since the voltage of the secondary tap had to be

several kilovolts in order to transfer significant energy, an intermediate iron-core transformer was added to reduce this voltage to

standard secondary levels (fig. 1(c)). To improve the accuracy and

greatly increase the energy transfer across the divider, a resonating

inductor was next placed in the secondary of this transformer

(fig. 1(d)) to tune out the high reactance of the capacitive divider.

In this form, the CCVT was accurate and stable enough for

relaying and for routine voltage measurement, and it was so used for

many years because of the cost advantage over conventional inductive

voltage transformers (VTs). It was not, however, accurate enough

for revenue metering (allowable errors of ±0.3 percent for ratio,

±4.6 mrad for phase angle) as specified in ANSI Std. C57. 13-1978.

Beginning about 1950, transmission voltages moved upward through

the EHV range (345-765 kV), and EHV circuits expanded to include

many interties between systems. This created a need for metering at

the EHV level. The increased cost and fragility of conventional VTs

at these voltage levels produced the incentive for improving CCVTs

to meet metering accuracy requirements. To accomplish this, the

tuning reactor was moved to the primary of the intermediate transformer,

stack capacitance and capacitive divider tap voltage were increased

significantly, and the low-side capacitor was made an integral part

of the stack capacitance instead of a separate unit. The basic

schematic circuit of the resultant present-day metering CCVT is

shown in figure 1(e). It consists of a modular capacitive divider

which reduces line voltage V] to voltage V2 (10-20 kV), a series

resonant inductor to tune out the high impedance of the divider, and

an intermediate voltage transformer to reduce V2 to voltage Vb (115 V).

As stated above, the principal impetus for applying the CCVT to

metering was the higher cost, bulk, and fragility of the conventional VT

lNumbers in brackets refer to the literature references listed at the

end of this report.
c

CD "O

13 CD

rWv-i

CD >

X

HH

C

(1) o

13 DQ

r-VW-i

>

r

l> i o

c

CD a

13 DQ

*-VWt

CDc

>

X

C

"w

o

X

CD

CO

CO DQ

UuUj i

H

rYYVi

I

H(-~W

CD

_ J

o

CD H(-ne C\J >

>

X o

C\J o

m

a

CD a

DQ ~~~^

> o O +JO

>> fD U Q_

s- o

o e

TO Ta> cl e 3

H 8

a> e E <D

i- <D i- -E 3

4- s_ o <u

*r— E S- o s_

•r—* fO

aa> ao

"a ai

•r- E

> 1-

•r- O

-o ^~

cu e > fO

•r- t. u ai

<T3 "r— O ip

L

, ... r- 5_ Q. a> E 4-> •r- E

00 •—

i

a • E r— O =•

C_> O) o

O <J

+-> fD ac

o

ra

i- E

O -r- -M S- O O)

fD +->

a.' a;

en ^

E S_

•r- ai e -a ^ c

t CO

CD DC

CDc d(-ne CO

>

X O

> mn u"D o
at voltages above 345 kV. Revenue metering was for many years

the exclusive province of the VT, for which its simplicity, stability,

and long life make it ideally suited. The VT depends primarily on its turns ratio to yield the actual voltage ratio. This ratio is

modified only slightly by leakage and magnetizing impedances and

stray capacitances, and has very little temperature, voltage, or aging dependence. In fact, a well-designed VT can be installed and

essentially "forgotten" for its 30-50-year lifetime, unless its burden

is changed.

The CCVT is a more complicated device whose basic accuracy and

stability depend strongly on the properties of its paper and oil

capacitor stack, and, to a lesser extent, on the inductance of its

series tuning reactor. The capacitor has considerable sensitivity to

temperature, voltage, aging, and proximate objects, although these

effects are mitigated considerably in the capacitance ratio by the

identical construction of both arms of the divider. In practice,

the mismatches in capacitor characteristics and ambient conditions

for the two arms are small, but not insignificant if ratio uncertainies

less than ±0.3 percent are required.

Aside from the properties discussed above, the CCVT has one other potentially serious weakness - it can fail without the failure

being detected immediately. Its capacitor stack consists of a large

number of individual capacitors ("rolls") in series, one or more of

which can short out, introducing a serious ratio error (typically

>0.3 percent per "roll" in a 500-kV CCVT) without detectably affecting

the stack's operation. On the other hand, if a turn shorts in a VT,

failure will be detected because it is immediate and catastrophic.

Theoretical properties aside, the CCVT is less thoroughly proven

than the VT, for which there is a record of over half a century of

development and application. Enough in-service and other data have

been accumulated on the CCVT to leave the prospective user with some reasonable doubts as to its reliability and stability [2, 3]. 2 Since the

device is installed permanently in the substation, it must be calibrated

there.

A prototype system for field calibration of CCVTs was developed

and has been in service at NBS for several years. This system is

described in [4] and [5]. Two other systems which have seen limited

application are described in [2] and [3].

Although the prototype system has operated quite successfully,

it is basically a research system. Experience has shown that it has

2See also [4], pp. 2-5.
a number of features probably not essential for routine calibration

service, and that a simplified system is feasible - the accuracy of the prototype system is better than

the field. This report describes such a simplified

which has since been developed at NBS.

2. THE PROTOTYPE NBS SYSTEM

particularly since

is required in

calibration system,

In order to better delineate the similarities and the differences

between the new system and the prototype, and to illustrate the

simplifications achieved, the prototype system will now be reviewed.

This system consists of five principal elements:

(1) A high-voltage compressed gas standard capacitor,

(2) A current comparator bridge,

(3) A 100-kV resonant power supply,

(4) Its control console, and

(5) A high-voltage modular capacitive divider,

which serves as a transfer standard.

Items (1), (2), (3), and (4) above are installed in a calibration

truck which serves as a completely self-contained field calibration

laboratory. Item (5), the modular divider, is broken down into its

component modules and transported to the field in the same truck.

This arrangement is shown in figure 2. When the system arrives at

the substation, the off-load hatch is opened, the divider modules

are removed, and the divider is assembled for use.

CCVT calibration with the prototype system consists of four

steps

Measurement 1

Measurement 2

Measurement 3

Measurement 4

Measurement of a series of one-module dividers, made

up of each standard divider module in turn, at above

rated voltage (125-150 percent), using the resonant power

supply.

Calibration of the compressed gas high-voltage capacitor

(C$t) against a reference standard capacitor (C$b)»

using the current comparator bridge.

Calibration of the ratio of the standard divider

against the ratio of compressed gas capacitor C5J

and reference capacitor C$M at 100 kV, using the

current comparator bridge and the high-voltage resonant

power supply.

Calibration of the CCVT by direct comparison with

the standard divider, using the current comparator

bridge.
Resonant Power

Supply 700kg

Supply-Standard

Transformer Module,

Simplified System

Divider Module Bridge

320kg 135kg

200kV Std. C (CST) 130kg

Divider Base Module

500kg

Divider

Modules

320kg Ea.

Resonant Power

Supply Control / Console 135kg

Plastic Bubble

Dome

Resonant

Power

Supply

200kV Std. C (CST)

Off-Load

Hatch

- Bridge, Control

Console Profiles

\\\\\\\\\\ YTTTTTY

Figure 2. Approximate layout for transporting CCVT calibration

system to the field in truck.
These steps are shown schematically in figure 3. The first

measurement is not actually part of the CCVT calibration. It is a preliminary step (proof test or "hipot") in which each capacitor module

in the transfer standard divider is subjected to up to 150 percent of

its rated voltage for five minutes, and the ratio of the resultant

one-module divider measured with Csj and C$m as references. This

gives assurance that the module is intact and safe for connection to

the utility bus. High voltage is furnished to C$j and module Cm by

the resonant power supply. Modules not being measured are bypassed

with shorting straps. This measurement will detect the shorting of one or more "rolls" in a given module. Note that the circuit

configuration is the same as for measurement 3.

The calibration of the CCVT is encompassed by measurements 2 through 4. Measurement 2 establishes the ratio P2 = (C$b/C$t) of

low-voltage reference capacitor C$b> mounted in the bridge, and high

voltage compressed gas capacitor C$y, using an auxiliary 250-V power

supply. (The ratio of interest is Csm/Cst» ^SM equals C$b to a few

parts per million. Equipment configuration makes it more convenient

to measure C$b/C$T') This measurement is required because C$t has

a temperature dependence of about 25 ppm/°C, and may drift during

a day's measurements. The low-voltage calibration values can be

used at high voltage because the voltage dependence of the compressed

gas capacitor is negligibly small [6]. Measurement 3 uses standard

current comparator bridge techniques [7,8], and the ratio p2 obtained

in measurement 2 to determine the ratio p$ of the transfer standard

divider at approximately 100 kV.

Measurement 4 is the final step, in which the on-line CCVT is

calibrated against the transfer standard divider, using the divider

ratio established in measurement 3. This is normally a 1:1 comparison

(p4 = 1.0), since the divider ratios are nominally equal to the

industry standard values, e.g., 1800, 2500, and 3750 to 1, for 230,

500 and 765 kV, respectively. The divider is connected to the test

piece bus for this measurement.

Since the ratio of the standard divider and the capacitance of the

compressed gas capacitor may drift slightly during a calibration,

measurements 2 and 3 are repeated afterward and the new values used

to interpolate any required corrections. Phase angle drift is

insignificant.

The system is arranged so that test results may be combined to

obtain the ratio correction factor (Ft) and phase angle (yt) of the

test piece, per conventional definitions for VTs (ANSI/IEEE C57. 13-1978)

Ft is obtained (after small corrections for divider voltage and temperature dependence, proximity effects, etc.) from the product of the bridge

ratio readings from measurements 2, 3, and 4. yt (mrad) is the sum of

the three corresponding bridge dissipation factor (DF) readings:
Hi

CD

(5O if

O

O CD -o

O

CO CD 2.2

EmoO

HiCD (D

10 ^ ££

°"S C o O 3

"^ "D

CO CD 2~

To "5 °"E

«'l

CO CO

CO O

CDO)

COO>

CDC>Ooo

.a

"cO o

+->

C7) o

S-o

ilCU

S-. aa>uo

s_a.

"E CD

CO O)

T3 CO c ~

co o co> _ "0

CO CD 2a:

2^CM

CD~

T~ g*CO - ^ CO o2CO D

Q--0

CO o o<>

CD

CO ~2. CO >

CD ,=:

CO CD 2o) E »- co 2 ° o w

-> CD |CO 5 C CO ^_^_ o2U -1 CD M—

CD DC < *-\(-

CO

CO o I

CO o ^_

„, <*> ,!_^ o

CD

q

O

COC "cO CD

CO "O

ii CD Q.CO gS-2 >o O co o ^ CD "O o E *

(— k- t- rn Q. CO

O

in -1- oo

T3c

CO

C\J

ell ! >

>-JCC/5 "I < nt6 i-CO o

Hi'

DCDCOw

CD CD O O o>

CD $ 2 -J 1!

x: o - o X co H- Q- O CO cOo -o 11

:= co O w. co CM CO »o

CO

4-> co • s- E

-Q (X) •i- +-> r— CO

<T3 >> O 00 Q_ CD

CU Q.

+J >>

CO +J I o

=3

O o s- U_ Dco

CD

CD
Ft = p2p3p4 (1)

where

ptN

Yt

= (DF 2 + DF 3 + DF4 ) (2)

>

PtN = nominal CCVT ratio, e.g., 1800, 2500, 3750,

Ft = value by which pt|\| must be multiplied to obtain the

true CCVT ratio,

Yt = angle by which the secondary voltage leads the

primary voltage, mrad,

P2 = bridge ratio reading, measurement 2, etc., and

DF2 = bridge DF dial reading, mrad, measurement 2, etc.

Sources of uncertainty in the prototype system, in order of

importance, are the high-voltage capacitive transfer standard divider,

the high-voltage compressed gas standard capacitor, the current

comparator bridge, and the low-voltage reference capacitors. Most

of this uncertainty is believed to reside in the divider, stemming

from voltage dependence, temperature dependence, including self-heating,

and proximity effects. Detailed analysis was carried out in [4], as

a result of which it was concluded that a reasonable estimate of

uncertainties for the system would be ±0.05 percent for ratio and

±0.3 mrad for phase angle. This analysis was based on a 500-kV system.

Errors are expected to be larger for a higher voltage system.

3. DESCRIPTION OF THE SIMPLIFIED SYSTEM

As discussed in the preceding section, the prototype system has

five principal components: (1) a modular capacitive transfer standard

divider, (2) a high-voltage compressed gas standard capacitor, (3) a current comparator bridge, (4) a 100-kV resonant power supply, and

(5) its control console. The transfer standard divider is an essential

element and cannot be replaced, since it is the link to the EHV bus.

The 200-kV capacitor and the 100-kV resonant power supply can be

replaced if a lower divider calibration voltage (e.g., 10-20 kV) can be tolerated. This, in turn, requires that the transfer standard

divider have acceptable voltage dependence from the lower calibration

voltage to rated voltage, and that there be an acceptable moderate

voltage standard to replace the compressed gas capacitor. Elimination

of the expensive current comparator bridge requires an adequate but

simpler comparator. These requirements have been met in the simplified

system to be described now.
As developed, the simplifed system has three principal components:

(1) the modular transfer standard divider, (2) a combined moderate

voltage (14.4 kV) supply transformer and reference standard transformer,

and (3) a voltage comparator. These are connected as shown in figure 4, as used in a two-step calibration procedure.

3.1 Calibration Procedure

The nomenclature "C3" and "C4" used for the measurement numbers

below relates these calibration steps to the corresponding steps in

the prototype system. The "C" stands for voltage "comparator."

The subscripts "C3" and "C4" in the terms refer also to measurements

C3 and C4.

In measurement C3 (fig. 4(a)), the standard transformer T2 and the

transfer standard divider are connected in parallel to the supply

transformer Tl . The voltage comparator compares the two output

voltages Vj and V s. From the comparator outputs (ratio <xC 3 = |V s /Vj|

and phase angle yC 3

= y s

- yy) , the ratio and phase angle of the standard

divider are found as

Ps = PT/ ac3 (3)

^s = H + YC 3 (4)

The ratio p-t of the standard transformer is determined in a laboratory

calibration, using techniques outlined in [7] and [8].

In measurement C4 (fig. 4(b)), the calibrated transfer standard

divider is connected to the EHV line in parallel with the CCVT to be

tested, and the output voltages V^ and V' s compared. From the

comparator outputs, where aC 4

= |V' s /Vy|, and Yc4 = Ys " Yt»

the ratio correction factor Ft and phase angle Yt of the test

CCVT are found as

F

= PJ Vac4\ (5)

PtN/\ ac3/

Yt = n + ^c3 " ^c4 ( 6 )

where ptN "is the nominal ratio of the test piece, e.g., 2500:1 for a 500-kV metering tap. Details regarding the derivation of the

above equations will be found in appendix A-2.2.

10
CD

CD > »- >

13

O

w -n

"S « S - O Q. 2 CO ^ «J|_ "D

CO CO >

(75

>--

CO

CO

CO >

Hf-H>'

CO cO ±^ Q.

-^ oO

i—

coo

>-. coo

CO o 2

CDF

C CO

CD "D o *- ,— M—

03 s CO M— CO c CD *i oc CO CO

CD "O

CD "^

CO - O

Q 3 o w _, a co £^ O CL ^ CO ^ co^i-a O c c? CO CO < \ \( r 'HH" E

CD 1 \\ - A 4-> CO CO J- >

>> a>

00 4-

CO . In" c e l—

ki)

O T3 5»

>*- •r- JO „ +->+-> o

CO fO 3

S- 4- 4- -Q O O

CO o i— E E

<D

O

O

CJ "i— 'i—

+-> 4->

v "O t3 03 CD o

CD i- S- r- -Q -Q

COo

CO

-t—> age arat

O pi

if

cali call c o> Volt

Comp, c

CD E

<- i i_ : co CD co -a "*

CDE

CDc E

CD -O -r- o E >

fU 4-> •!— +-> CD Lj O} CD CU 3

CO > 1- < CO

CO

fD E -a E •r- a; s_ CD -u s_ *o s_ CO

CD

X CD 3-o 3

(J CO

C CO LU 4—' ^

•i- (O (O <o +J CD 4-> CD

.^-^

|| t „. s^ E

CO a. ^^ _c -—- -—•. - s— O fO XI > < » >

CO

v

— < > If * Id ^ 1 • V • I' 1 «3- 1 \

h- a>

1_

L > -3

C> oo

"co

en

CD

CD Q-O

CO c >o

CM

co o* c*6

11
3.2 System Components

3.2.1 Standard Divider

The modular divider is common to both the prototype and the

simplified systems. A detailed description of the original divider

is given in [4]. Much of that information is still valid, but the

auxiliary divider and some other details have been modified. Those

changes are documented below.

An elementary circuit diagram of the divider is shown in figure 5.

The high side (C-| 4) consists of a stack of 138-kV (80 kV to neutral)

CCVT capacitor modules, each having a capacitance of approximately

0.016 pF. The number of modules used depends on the line-to-line

voltage (two for 230 kV, three for 500 kV, etc.). The base module

has a special nominal 1-kV tap brought out on terminal 4 (C42). This

tap is not present on standard base modules, which are tapped at

either 10 or 20 kV, depending on the manufacturer. 3 The auxiliary

divider (Chi and C[_ ) reduces the terminal 4 voltage to the nominal

metering and relaying voltages found on CCVTs, i.e., approximately

115 and 66 V, respectively.

The divider has three switch-selectable ratios (SI, fig. 5),

nominally equal to 2500:1, 4500:1, and 2250:1 for a three-module,

500-kV configuration. The first two are standard ANSI/IEEE C57. 13-1978

ratios. The third is for future use at 765 kV, and would give

nominally the ANSI standard 3750:1 with a five-module divider.

Other ratios can be added if desired. The exact nominal ANSI ratios

are provided only for convenience in calculations and in spotting

gross errors, 4 since the actual divider ratio is determined

(measurement C4) during each calibration.

Switch S3 protects the auxiliary divider from switching surges

by shorting it while the divider is being connected to or disconnected from the bus. S2 is open during switching to prevent surges

from travelling down the signal cable toward the instrumentation.

Disposition of leads, including the deliberate addition of a small

amount of inductance, provides further protection. The auxiliary

divider is constructed of stable computer-grade capacitors. Detailed

circuit diagrams and further discussion are contained in appendix A-5.

3Note that such base modules must be specially ordered.

4Exception - since the voltage comparator has a maximum range of 1.1:1,

this puts some restrictions on the nominal ratio. For more detail, see sections 3.2.2 and 3.3.1.

12
J _ CD OH—

_ 03 CO O) _

CO C CO CO

CD

£_

TO

sr

>

u

Q.

O

03

O

o

o

en

*i

13
3.2.2 Voltage Comparator

The basic circuit diagram of the voltage comparator is shown in

figure 6. This is the circuit encompassed by the voltage comparator

block in figure 4. The comparator has three principal elements: the

inductive voltage divider (IVD) T4, the quadrature circuit, and the

detector circuit. T4 is a six-dial precision variable autotransformer

with a ratio range of to 1.11 .... In measurement C3, T4 compares

a portion (V/\ = a^Vy) of standard transformer voltage Vy with the

in-phase portion of standard divider voltage V s

, for determining p s in

eq (3). The quadrature circuit takes aC 3Vy, accurately subdivides

it, phase shifts the fraction by 90°, and reinjects it in series

with the in-phase portion via transformer T3. The range of the

quadrature circuit is ±(0-100) mrad. Balance between Vy and V s is obtained by adjusting T4 and quadrature voltage JYqVa* and is

Vs = «c3 V T H + J>c3) (?)

where jyC 3 is the proportion of aC 3Vy reinjected at 90° by T3.

In measurement C4, T4 compares a portion (V'/\ = a^t) of CCVT

voltage Vt with the in-phase portion of standard divider voltage V' s. Balance is obtained when V' s = ac4^t(l + JYc4)« The detector

circuit consists of a commercial tuned detector preceded by a twin-T

filter to eliminate harmonics (found necessary in order to operate

with this detector in the substation - see section 4.5.2 and

appendix A-4 for further discussion).

Note that since the IVD (T4) has an output-to-input ratio range

of to 1.11 ..., i.e., ac

= to 1.11 ..., V s

, figure 6, can be

no more than 1.11 ... times Vy, and standard divider ratios must

be selected so that a tap is available at no less than 1/1.11 ... times the ANSI ratio of the CCVT.

In addition to the functions described, transformers T3 and T5

also provide needed isolation between circuit components. T6 and

T7, shown pictorially, are coaxial chokes [9,10]. These are special

1:1 transformers placed in the incoming signal lines to eliminate

ground loop voltages, a serious source of error invariably encountered

in the substation. These devices and their function are described

in more detail in appendix A-6. The comparator is mounted in a single standard 19- inch (483-mm) cabinet, as shown in figure 7.

3.2.3 Standard Transformer

The standard transformer Tl is a commercial unit with a maximum

rating of 14.4 kV, having ratios from 1:1 to 120:1 (14400:120 V).

It is commonly used as a laboratory standard, adjusted at the factory

to within approximately ±0.01 percent and ±0.02 mrad of nominal, as confirmed by calibration in our laboratory. Its ratio is highly stable

and insensitive to its environment (<0.01 percent change from 5-30 °C) as confirmed by calibration in the substation using the prototype system.

14
CO CD O "O "O K S > w

W J

r O-f

C>-*

n 03 CD « O

O o

L_

Y

J CD r oe 1- L.-T3 O r o o t- C CO <u o *=§5 V CD C hU-i-«

( Q nj Q

hJ

"1

-><~H j

s-o

+->

J-fOQ.

E

o

o

<D

fU

+-> O

>

CD -C

4-> M-O

3

o

s_

CJa

ra CO

O)

CD

— ~ o w

15
Figure 7. Voltage comparator assembly.

16
3.2.4 Supply Transformer

The supply transformer is a companion unit of the same construction

and voltage rating as the standard transformer. The two are packaged

as a single two-module unit. In principle, any power transformer

rated at about 15 kV and 1 kVA could be used to power the system.

The assembled standard-supply transformer unit is shown in figure 8.

3.3 System Configuration

3.3.1 Electrical Configuration

The configurations of figures 4 and 6 are mandatory, i.e., the

standard divider is always connected to the "S" terminal of the

voltage comparator. This is necessary because the IVD (T4) input

impedance is somewhat low relative to the standard divider output

impedance (the order of 1 Mft vs 2 kn). This results in significant

(and voltage-variable) loading of the divider if T4 is connected

to it. On the other hand, T4 does not load standard transformer

T2 or the CCVT output significantly. This mandatory configuration

establishes the constraints on standard divider ratio discussed

earlier. In other words, if pj/p s >1.11 ..., the comparator

connection cannot be reversed to compensate for it.

3.3.2 Physical Configuration

The NBS system is "grafted" onto the prototype, i.e., the supplystandard transformer module (T1-T2) is installed permanently in the

prototype system truck, together with its low-voltage, variable

autotransformer power supply (see fig. 2). Interlocks prevent the

system from being energized when someone is in the power compartment.

The standard divider is, of course, part of the prototype system.

The voltage comparator is independent and can be set up anywhere

that 120-V, 60-Hz power is available. In the field, it is normally

placed in the back of a van or on the tailgate of a station wagon,

close to the calibration truck.

This configuration not only provides economy of hardware, but is

extremely convenient in testing the system. Testing consists primarily

of operating the simplified system, either in tandem or in parallel

with the prototype system in calibrating the same group of CCVTs.

A stand-alone system could easily be housed in a truck similar to

the NBS calibration truck. Alternatively, to avoid a dedicated truck,

all hardware except the modular divider could be installed in a small van. A non-dedicated truck could then be used to transport the divider

to the substation. Going a step further, the comparator and standard

transformer could be set up in the control house, avoiding even the

necessity for a dedicated van.

17
Figure 8. Supply-standard transformer module (T1-T2)

18
4. SYSTEM EVALUATION

4.1 Overview

The steps taken as part of system evaluation included

(1) redetermination cf the voltage dependence of the standard divider;

(2) calibration of the standard transformer T2; (3) calibration of the

voltage comparator, including the IVD (T4) and the quadrature circu: 4

" and, most important, (4) system tests, including actual CCVT calibrations

which compared the results with those obtained with the prototype system

at the same or proximate time.

Item (1) above was performed in a commercial laboratory, items

(2) and (3) at NBS. Item (4) was partially simulated at NBS, but

the true evaluation was carried out in utility substations. These

tests will now be discussed in detail.

4.2 Voltage Dependence, Transfer Standard Divider

Before the simplified system could even be breadboarded, it was necessary to determine whether the transfer standard divider could be

calibrated at the proposed 10-20 kV instead of at the 100 kV used for

the prototype, i.e., whether it had a tolerable and reproducible

voltage dependence over this wider voltage range. These tests were performed in a commercial high- voltage laboratory using the prototype

system as a portable calibration laboratory. A 500-kV laboratory

standard capacitor was calibrated with the NBS system, then used as the high-voltage standard in tests on the NBS divider.

The procedure was similar to that used in the initial factory

tests of the divider (see section 6-4 and appendix B, [4]). Several

series of tests were run in which the voltage was varied up and

down between 15 or 20 kV and 300 kV as rapidly as possible, pausing

at the end points for at least one minute to allow the ratio value to

stabilize. This gave much more reasonable and consistent results

than the earlier tests (see specifically, appendix B, fig. B-3,

[4]). These stabilized end points were generally repeatable

to 10 or 20 ppm. A total of 19 such cycles were run. Treating the

results as random, average total voltage dependence was -0.018 percent

between 15 and 300 kV, with a scatter (3a) of about 0.012 percent.

A series of voltage dependence curves was also run, with the same hysteresis-type results as were found in the factory tests (appendix B,

fig. B-2, [4]). Because more stabilizing time was allowed at the

end points than in earlier tests, the results were somewhat more

orderly, and tended, in general, to confirm the results of the

up-and-down tests above. The divider was thus demonstrated to be

suitable for use in the proposed simplified system.

19
4.3 Standard Transformer T2

This particular transformer is marketed as a laboratory standard,

claimed to be within ±0.01 percent of nominal on ratio and within

±0.02 mrad of zero phase angle (at zero burden) at the ratio of

interest (14400:120 =120:1). It was calibrated using the NBS laboratory

standard current comparator bridge [8], with the following results:

RCF (or F) = 0.99989, or ratio = 119.987:1, and y = 0.008 mrad.

Approximately 10 subsequent calibrations using the prototype system

(identical in principle, but less accurate than the laboratory bridge

above), including spot checks in the field, have yielded an average RCF

of 0.99989 (a = 6 x 10-5) and an average phase angle of 0.03 mrad

(a = 0.03 mrad). Since a sizable part of the above uncertainty is in

the prototype system itself, it appears that T2 can safely be used as a field standard, with uncertainties of about ±0.02 percent and

±0.1 mrad, subject only to periodic recalibration in the laboratory.

4.4 Voltage Comparator

4.4.1 Inductive Voltage Divider (IVD) T4

The IVD is the most critical element in the comparator, since it

compares the ratio^ of the standard divider with the ratio of the

standard transformer (measurement C3) or the test CCVT (measurement C4).

In calibrating the standard divider, the nominal IVD ratio aC 3 is

a small fraction of full scale (e.g., aC 3 = 120/2500 = 0.048 for the

500-kV divider configuration, or about 5 percent). Therefore, in order

that the IVD introduce no more than 0.01 percent uncertainty into the

measurement, its error must not exceed 5% x 0.01% = 5 ppm of full scale.

This particular IVD has been calibrated against NBS standards,

and found to have maximum corrections at any dial setting of 0.1 or less, of approximately 0.2 ppm of full scale for ratio and 3 yrad

for phase angle. This translates to less than 4 ppm and 60 yrad

for the example above. It is expected to maintain this accuracy

indefinitely, subject only to periodic reverification.

4.4.2 Quadrature Circuit

As described in detail in appendix A-3, the quadrature circuit

selects an accurately known portion of the in-phase output of the IVD,

phase shifts it, and further subdivides it for injection in series

with the in-phase output. Since the in-phase value is controlled

yery accurately by the setting of the T'D (see section 4.4.1),

the quadrature circuit can, in principle, be calibrated at any setting

of the IVD. The simplest and most convenient point is at the 1:1

setting (a c= 1 .0)

.

^Strictly speaking, only the in-phase portion, but the difference is

negligible at the \/ery small phase angles normally encountered here,

20
A simple circuit for making this calibration is shown in figure 9,

where C is an accurately known capacitor, and R is an accurately known

variable resistor. Voltage V2 is compared with voltage V] » R is

varied to obtain phase shift between Vi and V2 over the quadrature

range of 0-100 mrad. The calculated values of yc and ratio correction

factor F are then compared with values measured with the comparator

yc

= RuC radians (8)

1 . (9)

F

=

(Ra)C) 2 + 1

Because the phase angles of metering CCVTs should be less than

±4.6 mrad, and are seldom much more than ±10 mrad, accurate Yc

measurement is required only over the range of ±10 mrad. System

accuracy tolerance of 0.3 mrad is 3 percent of this, so that the

absolute accuracy required of the quadrature test circuit is quite

moderate. A simple calculation will show that if R and C in the

circuit of figure 9 are known to ±0.25 percent, yc may be calculated

to an inaccuracy of no more than ±0.1 mrad at 10 mrad. If measured yc is out of tolerance, it can be adjusted with a trimmer resistor (see

appendix A-3 S fig. 13)

.

If R in figure 9 is reduced to zero, a simple check of the

comparator at ac= 1 .0 and yc

= ±0.00 can be made. If this

check shows ac to be unity to a few ppm and yc

= ±0.00 to

10 yrad or so, it is unlikely that anything is seriously wrong with

the comparator. This can be valuable as a quick troubleshooting

procedure in the field.

4.5 System Tests

4.5.1 Laboratory Tests

The conclusive evaluation of the system consisted of its use in

conjunction with the prototype system to calibrate CCVTs in the

substation, and the comparison of'the two sets of results. Prior to

this, preliminary laboratory tests included simultaneous calibrations

of the standard divider (measurement C3, simplified system, and

measurement 3, prototype system) at 14.4 kV, and simultaneous

calibrations of simulated test pieces (measurements 4 and C4) at 100 kV.

These tests, performed for the most part with semi -packaged, preliminary

configurations of the system, demonstrated its general feasibility.

"Simultaneous" above means the current comparator bridge of the

prototype system and the voltage comparator of the simplified system

were connected in parallel and balanced sequentially (the paralleled

systems are not entirely independent of each other). Parallel operation

allowed the same stray capacitance (which has a significant effect

21
CDeno>

CD

i. • O 4-> 4- •!-

•r- S-

=3 •- u o

•r- CD U i- E +-> O "3

•i- S_ +j -o

fV «3

i. 3

.o cr

•i— r— s_ as o

cu s- i— fa cl a. E E

•rO

CO (J

CX>

CD

S22
on divider ra io) to be presented to the divider low side for both

measurements, thus avoiding the necessity for auxiliary capacitance

measurements in order to compare results. Difference in stray

capacitance is brought about by the presence of a standard capacitor

(C$m, fig. 3) and its strays during the current comparator bridge

measurement, differing input cable lengths for the two measurements

(the effect of which is of the order of 100 ppm/m) , and stray

capacitance in the detector transformer (T5, fig. 6) during voltage

comparator measurements.

In the "simultaneous" mode, laboratory results from the two

systems consistently correlated to better than 0.02 percent and

0.1 mrad. For further discussion of these preliminary tests, see [4],

section 9, and appendix H.

4.5.2 Field Tests

Two field tests of the simplified system were carried out in

conjunction with CCVT calibrations at the 500-kV Juniata Substation

of the Pennsylvania Power and Light Co. (PP&L), New Bl cornfield,

Pennsylvania. The two systems have since been compared extensively

in another field calibration.

The first field tests, in October 1979, involved a semi -packaged

version of tht final system and were intended to demonstrate field

feasibility. Initially, the system functioned properly for standard

divider calibration (measurement C3), but could not be balanced

beyond three places when on-line (measurement C4) . It was postulated

that excessive third and fifth harmonics in the substation voltage

caused the commercial detector to saturate. Substitution of an older, less harmonic-sensitive tube-type wave analyzer for this

detector temporarily solved this problem, Results obtained with the

simplified system were then found to agree with prototype system

results in simultaneous calibration to a few hundredths of a percent

and a few tenths of a milliradian. Field feasibility was thus

demonstrated in these tests.

In the interim between the first and second field tests, the

detector saturation problem alluded to above was investigated in the

laboratory. A circuit was set up to introduce excess harmonics into

the input of the detector. This produced the saturation effect

observed in the field, A 60-Hz bandpass filter was designed, consisting

of a twin-T network in the feedback loop of an operational amplifier.

Placing this filter ahead of the detector solved the problem. A detailed

discussion of the twin-T filter network is found in appendix A-4.

The second field test was performed in May 1980 at the same PP&L substation. Calibrations of 12 CCVTs allowed comparison of the

results from 30 measurements (more than one tap per device) with

prototype system results (measurements 4 and C4). These results are

23
plotted in figure 10, in which the prototype system values are taken

as reference (the origin). The mean difference between systems for

30 data points was approximately 230 ppm and 0.05 mrad s with standard

deviations of 190 ppm and 0.17 mrad, respectively.

The new system has since been used extensively in conjunction

with the prototype in field tests involving 24 CCVTs at two other

substations. The comparison of nearly 60 data points is plotted in

figure 11. Results are seen to be quite similar - a mean difference

between systems of approximately 180 ppm and -0.10 mrad, with standard

deviations of 170 ppm and 0.10 mrad. The figure 10 mean and the

combined mean are plotted in figure 11 for purposes of comparison.

4.5.3 Summary

As figures 10 and 11 show, field comparison with the prototype

system shows mean agreement to better than 0.025 percent (250 ppm) and

0.1 mrad. This is considerably better than the apparent uncertainty of

the comparisons (3ap = 0.05 percent, 3 a

y

= 0.4 mrad). Since the

prototype system is believed to have uncertainties of about ±0.1 percent

and ±0.3 mrad, ±0.2 percent and ±0.6 mrad is believed to be a reasonable assessment of the overall accuracy of the simplified system.

The above conclusion is reinforced by an error analysis based

on the components of the simplified system. This analysis, for

ratio correction factor only, is summarized in table 1, and shows an expected uncertainty of about ±0.06 percent, somewhat larger than the

intercomparison uncertainty above. This should not be surprising

since the major sources of uncertainty, the standard divider and the

detector null bias, are common to both systems. A similar analysis

for phase angle, not shown, shows the expected uncertainty to be

well within the above numbers for phase angle.

5. SUMMARY AND CONCLUSIONS

A simplified system for field calibration of CCVTs has been

developed and field tested. It is estimated to have an inaccuracy

less than ±0.1 percent and ±0.6 mrad for ratio correction factor (F)

and phase angle (y) s respectively, at 500 kV. This is comparable to

the prototype system against which it was evaluated, and which it could

replace.

The system has only three major components: a modular capacitive

transfer standard divider, a combined supply and standard transformer,

and a cabinet-mounted voltage comparator. By comparison, the

prototype system has five majo^ components. Its mass (wit! 500-kV

divider) is about 1600 kg, ver:;us 2250 kg for the prototype system.

24
AF-PP m

500- •

400-

•

••

••300-

•

•

•

•

•

•

e

• 200-

/

•

28 Data Points: •

Prototype System (Refe

1

1

i

100-

rence)—

[ Mean =

j

+ 230 ppm, + 0.05 mrad

of = 190 ppm

o

y = 0.17 mrad

1

f s

I 1 1 0.4 —0.3 —0.2 —0.1

1 0.1

l S 1 0.2 0.3 0.4

Ay (mrad) —100-

•

• • —200-

•

—300-

Figure 10. Deviation of simplified system results from

prototype system results, field calibration

of CCVTs, May 1980.

25
Prototype System (Reference)

j

1 . __)__

0.4 —0.3 —0.2

Ay (mrad)

Mean, all 55 Data Points

{+ 176 ppm, —0.10 mrad)

of = 170 ppm

o v = 0.10 mrad

Mean from figure 10

Combined Mean,

• Figures 10 and 11

0.2 —r0.3

H

0.4

—300-1-

Figure 11, Deviation of simplified system results from

prototype system results, field calibration

of CCVTS, October 1980.

26
±0.01 10-4 2 2 x 10-4

±0.005 0.25 x 10-4 3 0.75 x 10-4

±0.02 4 x 10-4 3 12 x 10-4

Table 1. Summary of error and fixed correction estimates,

simplified field calibration system for CCVTs

Fixed Uncertainties (2) n2x Sources of Errors Corrections n(l) n2 Mult Mult

Percent Percent

1

.

Standard Transformer

2. Voltage Comparator

3. Detector (Null Bias)

4. Modular Standard Divider:

a. Voltage Dependence

(15-300 kV) -0.02 ±0.02 4 x 10-4 1 4 x 10-4

b. Proximity Effects -0.01 ±0.015 2.25 x 10-4 1 2.25 x 10-4

c. Temperature Effects ±0.015 2.25 x 10-4 1 2.25 x 10-4

d. Miscellaneous ±0.015 2.25 x 10-4 3 6.75 x 10-4

5. Ground Voltages ±0.010 10-4 1 io-4

£(Fixed Corrections) -0.03

£n2 31 x 10-4

(Jn2)l/2 = 3a 0.056

Total Estimated Random

Error - Percent 0>056

•TOT 3o

(2) Number of times che component is used in the entire calibration

e.g., the voltage comparator is used in meas. C3, meas. C4, and meas. C3 repeat, or three times

.

27
Although, as explained earlier, the new system is "grafted" onto

the older system, this is a matter of operational convenience.

Standing alone, the system could dispense with the dedicated truck.

The standard transformer, voltage comparator, and smaller accessories

could be transported in, and operated from, a medium-sized van (not

necessarily dedicated). The disassembled modular divider would

require a medium- si zed, non-dedicated truck. With proper attention

to weather, this could be an open truck, although it would be necessary

to protect the porcelain modules from flying stones and other road

hazards. If dedicated vehicles can be dispensed with, the cost of

replicating such a system should be substantially less than for the

prototype.

The calibration process itself is simpler, involving two measure- ment steps instead of three. It also appears feasible to make the

logistics much simpler, e.g., the standard transformer and voltage

comparator might be set up in the control house, freeing the calibration

van and permitting easy access to the CCVT test points.

Two disadvantages, neither very serious, appear to be associated

with the simplified system. The first is that the standard divider

modules cannot be tested at 100 kV (measurement 1, fig. 3). This is a procedure of somewhat marginal utility, although it affords some protection against catastrophic failure of the divider when it is

connected to the bus. Periodic over-voltage testing in the laboratory

should be almost as useful. The second disadvantage is that the

standard divider is calibrated at only 14.4 kV (measurement C3) versus 100 kV (measurement 3) for the prototype. This results in a 7:1 reduction in detector sensitivity. The lowered sensitivity

increases the vulnerability to noise (see, particularly item 3,

table 1), and puts more stress on the necessity to evaluate the

voltage dependence of the standard divider (section 4.2). Standard

transformers are available at ratings up to 35 kV. Their use would

increase sensitivity above by 2.5:1, and should be considered in any

reproduction of the present system.

28
6. REFERENCES

[1] Sweetana, A., Jr., and Flugum, R. W. A New Metering Accuracy

Capacitive Potential Device, IEEE Transactions on Power

Apparatus and Systems, Vol. PAS-85 (May 1966), 499-510.

[2] Povey, E. H. Accuracy Tests on Installed Capacitance-Coupled

Potential Devices, Conference Paper #C74-375-2, PES Summer

Power Meeting, Anaheim, California (July 1974).

[3] Phillips, C. E. Field Testing of 500 kV Metering Accuracy

Coupling Capacitor Voltage Transformers. Presented at the

Pacific Coast Electric Association Engineering and Operating

Conference, San Francisco (March 1974).

[4] Hillhouse, D. L., Petersons, 0., and Sze, W. C. A Prototype

Field Calibration System for Coupling Capacitor Voltage

Transformers (CCVTs), EPRI Final Report No. EL-690 (April 1978).

[5] Hillhouse, D. !_., Petersons, 0., and Sze, W. C. A Prototype System

for On-Site Calibration of Coupling Capacitor Voltage Transformers

(CCVTs), IEEE Transactions on Power Apparatus and Systems,

Vol. PAS-98 (May/June 1979), 1026-1036.

[6] Hillhouse, D. L. , and Peterson, A. E. A 300 kV Compressed Gas

Standard Capacitor with Negligible Voltage Dependence, IEEE

Transactions on Instrumentation and Measurement, Vol. IM-22,

No. 4 (December 1973), 408-416.

[7] Kusters, N. L. , and Petersons, 0. A Transformer Ratio-Arm Bridge

for High-Voltage Capacitance Measurements, IEEE Transactions

on Communications and Electronics, Vol. 82 (November 1963),

606-611.

[8] Petersons, 0., and Anderson, W. E. A Wide-Range High Voltage

Capacitance Bridge With One PPM Accuracy, IEEE Transactions

on Instrumentation and Measurement, Vol. IM-24, No. 4

(December 1975), 336-344.

[9] Homan, D. N. Application of Coaxial Chokes to AC Bridge

Circuits, Nat. Bur. Stand. (U.S.), J. Res., Engr. & Inst.,

Vol. 72C, No. 2 (April-June 1968).

[10] Riley, J. C. Coaxers - What They Are and How to Use Them,

Paper #69-616, 1969 ISA Annual Conference and Exhibit

(October 27-30, 1969).

[11] Hillhouse, D. L. Guide for Safe Operating Procedures at High

Voltage Substations By NBS and Utility Staff During the Field

Calibration of Coupling Capacitor Voltage Transformers (CCVTs),

Nat. Bur. Stand. (U.S.), NBSIR 81-2192 (February 1981).

29
APPENDIX A

SIMPLIFIED SYSTEM COMPONENT DETAILS

1. INTRODUCTION

Elementary circuit diagrams of the complete system are shown in

figure 4; an elementary diagram of the standard divider is shown in

figure 5 S and of the voltage comparator in figure 6. These are combined for ease of reference in figure 12, in the configuration

for standard divider calibration (measurement C3, fig. 4(a)).

Transformer Tl applies 14.4 kV (V]) to T2 and to the standard divider.

2. DERIVATION OF SYSTEM EQUATIONS

2.1 Standard Divider Calibration

Balance for the in-phase quantity is obtained by adjusting the

IVD ratio setting aC 3, and for the quadrature quantity by adjusting

the jyC 3 output of the quadrature circuit. Balance is indicated

by a zero reading on the null detector. Then, from figure 12, the exact

equation of balance is

Vs = V A (1 + JT C 3) = «c3 VT (1 + JYc3) (Al) and (7)

where V/\ = aC 3^T» ^c3 ^ s tne calculated fraction of Vy\ operated on

by the quadrature circuit, and V s and Vj are the output voltages of the

standard divider and the standard transformer, respectively. The

quantity of interest in eq (Al ) is V s /Vj, which strictly speaking equals

ac3 (1 + J^c3) anc' nas a magnitude larger than aC 3. However,

for the small phase angles (y - 10 mrad) normally encountered in

CCVT calibration, it is easy to show that aC 3 equals the magnitude

of V

s /Vj to within (Yq3)^/2 (50 ppm for y^ = 10 mrad, less for

YC 3 <10 mrad). Therefore, we may state that

V S /V T aG 3 - (A2)

The quantity of interest in figure 12 is the ratio p s of the

standard divider. But p s

= V"|/V s

, and the standard transformer ratio

py = V"j/Vj. Substituting these values in (A2) and solving for p s , we obtain

p s = pj/aC 3 . (A3) and (3)

30
(V

+-> to>>

to o

ex. to

CD +J w-

<D

O

Q.

C E O o «- u +->

t(- s- O -Q

'r-

+-> i •r- ro

=3

O o

s- -o o £

03

O

00 <4-

C\J

CD

31
2.2 CCVT Calibration

Replacing standard transformer T2 in figure 12 with the test

CCVT, thereby substituting V'/\, aC 4, V^, p^, and Yc4 for the

corresponding terms above, identical reasoning produces

Ps = Pt/«c4 (A4)

where the subscript t denotes "test piece," and the subscript 4

denotes "measurement C4." Applying the ANSI definition for ratio

correction factor (F^ = Pt/PtN) » wnere PtN equals the nominal

ratio of the test CCVT, e.g., 2500:1 for a 500-kV metering tap

Ps = Ft PtN/ ac4 • ( A5 )

Equating (A3) and (A5) and solving for Ft,

f -£lYs^ . t VptN/\ac3/

(A6) and (5)

From (Al), the voltage injected by the quadrature circuit is

JYc3 Va = JYc3 ac3^T* The corresponding phase angle is

tan~l (yC

3 V^)/V^ = tan"^ yC 3 = yC 3 for small phase angles. y~3

is the algebraic difference of the phase angles of the standard divider

(ys) and the standard transformer (yj)« Therefore, Yq3 = y s

- yj> or

Ys = Y C 3 + n • (A7) and (4)

Noting that the standard divider stays on the same side of the

comparator for both measurements C3 and C4, identical reasoning

produces Ys = Yc4 + Yt* Substituting for Ys in (A7), we have

finally

Yt = YT + Yc3 " YC 4

• ( A8 ) and ( 5 )

32
3. VOLTAGE COMPARATOR QUADRATURE CIRCUIT

A detailed diagram of the quadrature circuit is shown in

figure 13. Quadrature voltage is generated as follows: input

resistor R] and amplifier Al ' s feedback resistor Rpp operate as

an active voltage divider, producing

Vh = -Va (Rdf/Ri) (A9)

which is a voltage 180° out of phase with V/\(= a^Vy) . R^p is an accurate, highly linear 10-turn helical potentiometer, connected as

a rheostat, and controlled by the quadrature balance dial. Its

range is 0-10 percent of V/\, equivalent to 0-100 mrad. Operational

amplifier A2, resistor R2, and capacitor Ci form a phase shifting

network to generate voltage

V = V M H m- (A10)

Substituting V^ from (A9)

RDpVx Cn (All)

V = jV , M

J A\ Ri/\ R2

R2 can be adjusted to make |V^| = |'VhI» i.e., | R2 1 = | Xq -j | - Rg

through R-| 2 are precision resistors forming a four-ratio voltage

divider providing quadrature ranges of 10, 20, 50, and 100 mrad.

After passing through buffer amplifier A3 and surge buffer resistor Rg,

this voltage is injected into 1:1 quadrature transformer T3, which

introduces it into the balance circuit. Rdf/^I = Yc3> anc' by

selecting the maximum value of Rdf/^I to be equal to 0.1, the

quadrature dial is made to read directly in milliradians.

Precision components (0.1 percent or better) are starred. Components

not discussed comprise conventional operational amplifier configurations

for stabilization, dc blocking, surge protection, etc. The operational

amplifiers are off-the-shelf items. Other similar amplifiers could

be used. The entire quadrature circuit (except the dc power supply

and Rqp) plus detector transformer T5 is contained in a single

moderate-sized electronic chassis. The dc power supply is mounted

outside this chassis in order to reduce 60-Hz pickup by the low-level

signal leads inside. T3 and T5 are of NBS construction, wound

on high permeability cores, and double screened for complete

isolation.

33
vw-||

en o W ^ rogxO £

X 9 £ *9 * *9 ^

< ^AAr*-AMr-^-AAAr-^-AAAr-| 1

1

wv—1|

* ,- * CM

C I_ &°

Err

o • U CC

o u.

CD Q

cp o

° z

T3 . CD o

(13 Q

g >

o m

S.+1 E to" < Qi_ • o E

+J CD

fC +-> s- 00

fO >> Q_ lO E

O "O

(J a>

>r_

<U 4-

CT> •r— fa r—

+-> Q-

! E

o r— > (/) f- A o +-> •!- E =5 03 O

i- So-> •r—

<T3 u

•r— T3 0J

J- •a =3

a> -i-> r— fO

"r— 1_ ra -a

+-> ro cu 3 D CT

• CO

1—

a>

S-rs

CT>

34
4. VOLTAGE COMPARATOR DETECTOR CIRCUIT

From figure 12, note that the detector circuit consists of

detector transformer T5, in series with the twin-T filter and a commercial tuned detector. As mentioned earlier, T5 is mounted in

the quadrature circuit chassis.

Figure 14 shows the detailed circuit of the filter. It consists

of a twin-T circuit in the feedback loop of operational amplifier Al

The twin-T consists of two T-networks: a high-pass filter (C] , C2» and

R3) and a low-pass filter (R-|, R2, and C3). Connected as shown,

they form a notch filter (maximum impedance at frequency f , and

tending toward zero on either side of f ) . Thus when the twin-T

circuit is placed in the feedback loop in parallel with Rf, amplifier

gain approaches (-Rf/R-j n

) at f , and zero at frequencies far from

f , and the entire circuit functions as a band-pass filter.

If the components are selected so that C] = C2 = 0.5 C3, and

R] = R2 = 2R3, it can be shown that

f =

1 (A12)

2tt R-| Ci

and that for the values shown in figure 14, f = 60 Hz. The equivalent

impedance of the filter in parallel with Rf is

Z

_ 2R](1 + jooRiCi) (A13)

3 1 -. (coR C )2

1 1

and the overall gain of the filter circuit is

V /V.

in

= -f

RfY u \ .

< A14 > VRin/\Z6 + Rf,/

From eq (A12), note that cogR-j Ci = 1. Then from eq (A13), at

f , la = °°, from which eq (A14) tells us that circuit gain at

6 R.j n equals the approximate equivalent delta impedance of the Rg, Rg ; and R-|] (gain potentiometer) T-network, ignoring R-|q«

35
E

coO . >0>Oh

OO OOO

oooo

CO Q. C

3 a.

en ^ C CD

- !_° £_f=j I

E

S- 0) o +-> 4— to

1_ CO

r- <D

5 CO -M

#\ 4- s_ O o

+J E u

03 CU

S- +-> CD O)

tj -o

•i—

T3 £-O

(-> +J

•ifC 3 t.

C_> 03

S~ CL

•r— E

u o

o

"O

Qj <D i— CD

•i

<TJ to +J

+-> r—

cu O Q >

O)

SrsCD

'"*' C\J ° xo 6v

i- CD

36
f is (-Rf/R-j n ), as stated earlier. As f (i.e., u) approaches zero,

la approaches 2R] ; as f increases beyond f , Zg approaches zero. The latter condition is the important one, since it is 60-Hz

harmonics (specifically the 3rd and the 5th) that must be suppressed.

The efficiency with which this is accomplished, i.e., the

sharpness of the 60-Hz bandpass, is determined by the matching of

the six twin-T components. As a practical matter, the capacitors

are first matched as closely as possible, as are the resistors.

Then final adjustment is performed using trimmer resistors R4 and R5.

Perfect tuning need not be obtained because of the inherently narrow bandwidth of the filter, plus the fact that the frequencies to be

suppressed are far away from 60 Hz (180 and 300 Hz). The adjusted

circuit of figure 14 has a maximum gain somewhat less than unity

(gain is not critical, since the detector is highly sensitive at the

signal levels encountered), and a bandwidth of about 4 Hz, so that

180 and 300 Hz are down about 30 and 35 dB, respectively. More

important than tuning to peak output at 60 Hz is adjustment to make

certain the net feedback current of the twin-T filter is not positive,

or if it is, that it remains less than the negative feedback current

through Rf. Failure to do this will result in operational amplifier

instability and saturation.

As implied by the preceding discussion, the six twin-T components

should be quite stable and closely matched, and have small temperature

coefficients. The other components are non-critical. Gain potentiometer

R-j 1 is the only front panel control. The power supply is energized when

the comparator is plugged in.

The twin-T filter, except for its dc power supply, is mounted in

its own chassis. The power supply is external, to reduce 60-Hz

interference in the filter circuit.

5. CAPACITIVE TRANSFER STANDARD DIVIDER

The circuit diagram of the capacitive transfer standard divider,

with the auxiliary divider shown in detail, is presented in figure 15.

Most of the other divider elements are discussed in detail in [4].

The auxiliary divider reduces the nominal 1 kV present at

terminal 4 of the main divider to form nominal ratios of 2500:1 and

4500:1 (for a three-module, 500-kV divider), and 3750:1 (for a fivemodule, 765-kV divider - 2250:1 for a three-module divider). It is

composed of computer grade, metallized polycarbonate capacitors,

selected for their high stability and small voltage and temperature

dependence. The three-microfarad (3 yF) capacitors are rated at

200 V dc. All other individual capacitors are rated at 400 V dc.

They are grouped in various series, parallel or series-parallel

combinations, in order to obtain the desired net values using readily

available sizes and voltage ratings. For example, C n 2 contains

37
c

a>to

co

CDo

c

co5

COQ.

CO O

ajo

z.

CD-C

O) o

co CO g

c/) a.

CO QCcy)

h-o o o *~-> m CO r>- en

cr <r> CM

o

ctJ 4-> a; o

enc

I I HHHf

o l'V^\A/V^

C <D II

" ! —lies

o--

1

4—II"

en QJ.E

co co

ca 3

CO OI

38
series-paralleled 1.0, 0.2, and 0.5 yF units, so configured in order

to obtain an 800-V rating for Ch2« Similarly (not shown in fig. 15),

C n 3 through Chg are each made up of four 1.0 yF, 400-V units in

series-parallel, to obtain an 800-V rating.

The entire divider has a dc voltage rating of 8400 V based on the individual capacitors. This is about six times the steady-state

requirement (1 kV x /% )

, but is necessary in order for the divider

to withstand the surges generated when it is switched on and off the

substation bus. These transients, at frequencies of the order of

0.5 MHz, can have values up to 10 or more times the steady-state peak

voltage.

At surge frequencies, the series inductance of leads and components

largely determines voltage distribution. The design objective, therefore,

is to make the inductance of the auxiliary divider as low as possible

relative to that of the stack and its series leads. To this end,

switch S3 brings terminal 4 of the main divider to ground by a short,

direct path. For additional protection, part of the lead between

terminals 4 and 4A is coiled to form an inductor, Lsu (= 8 yH)

.

This further subdivides any residual surge voltage appearing at

terminal 4. The 60-Hz impedance of Lsu is negligible relative to

the auxiliary divider impedance. Low-voltage step response tests have

shown that this configuration reduces the surge voltage across the

auxiliary divider by a factor of four to six. This brings it within

the dc design values.

The auxiliary divider is contained in an electronic chassis,

mounted on the door of the main divider base housing. Terminal 4A

and the output terminals are brought out of the chassis on high-voltage

bushings. Individual capacitors are mounted on terminal boards

supported on high-voltage standoffs. Layout is arranged so as to

reduce effective series inductance as much as possible.

6. COAXIAL CHOKES (GROUND VOLTAGE COMPENSATION)

Ground voltage is the difference in potential between two

separated points in the substation ground grid. Its typical range

is 0.1-0.6 V. If not accounted for, this represents 0.08 to

0.5 percent, or 0.8 to 5 mrad of error based on 115 V. The sources

of ground voltage may be clarified by reference to figure 16, which

shows the CCVT calibration circuit in the substation. For reasons

of safety, there are three grounds in this measurement system (Gl

,

G2, and G3 - G4 is associated with the CCVT). These are connected to

the substation ground grid. Currents flowing in the ground grid due,

e.g., to unbalanced phase voltages, generate voltages igiZgi, ^ g2^g2

>

and ig3Zg3 between these ground points. In addition, magnetic

coupling from bus current I or from ground currents, represented

39
•f— 4-

<D -C

+-> co

CD

cr> fa

+-> • i— CO > =>o -aE

rs 4- o o

i_en co

4- T- O +-> fO c s_

•i- _Q OVr- o

<£>

CT)

40
by Bl , B2, and B3, may induce voltages in these ground loops.

The summations of these voltages can be represented by ground

voltages, Vg-| , Vg2, and Vg3. Considering the NBS system only, and

assuming for the moment that there are no coaxial chokes, V'gi is

impressed across the standard divider cable shield, and generates an error voltge, V e ] = Z sn i s hi, directly in series with the voltage to

be measured (V2). A similar effect occurs in the control house loop.

The ground voltage effect and its compensation by a coaxial

choke can be seen in figure 17. Figure 17(a) shows the ground loop

circuit in the absence of the choke. Error voltage, V e= Vg, as stated below the circuit diagram, since the cable center conductor

is connected to Vg through an impedance approaching infinity (i^ = 0).

Figure 17(b) shows the same circuit with the coaxial choke in

place. The choke is a 1:1 transformer, formed by winding a number

of turns of the signal cable on a high permeability toroidal core. Assume that its magnetizing impedance is Z [_; the winding resistance

and leakage reactance add to Z s h« Thus any voltage, V c h, appearing

across the shield conductor of the choke, also appears in the center

conductor portion (diminished only slightly by the combined effect

of Zsh and the finite value of Z[_), and in opposition. There is

a net error voltage, but since the choke can readily be designed so that 1\_ > 50 Z s h» V' e is easily reduced to 1 or 2 percent of Vg. Thus

if Vg, uncompensated, is 0.5 percent of measured voltage, Vg,

compensated, is < 0.5/50 = 0.01 percent.

Since the coaxial chokes are designed specifically to eliminate

Vg as an error source, Vg can appear only if these chokes fail to

function. Only three such failure modes are admissible: (1) core saturation, (2) grounded cable shield, and (3) open cable shield.

(1) Core Saturation - This occurs when Vg exceeds the choke

design voltage. It can be dismissed here, since the chokes were

designed and tested for ground voltages up to 2 V, whereas the

largest Vg we have seen in practice has been less than 1 V.

(2) Grounded Cable Shield - Referring to figure 17(b), a signal

cable shield shorted to ground ahead of the choke, in the direction

of i sn flow, effectively bypasses the choke, returning the circuit

to figure 17(a). This can occur, e.g., if an uninsulated cable joint

touches wet earth, or if the shield insulation at the truck entrance

breaks down.

(3) Open Cable Shield - Referring again to figure 17(b), an open

cable shield means Z sn + «. From the associated equation, note that

in this case V' e > Vg. An open shield is the limit of a sub-category - bad shield connections. In the latter case, Z s h need only approach

or exceed Zl (typically 50 to 100 n) for most of the compensation

41
Bridge

\-— Center \ Conductor

-e

Truck

Ground

CCVT

Control House (or Std. Divider)

Ground

V„ = 'st^Sh + hZi ~ V

g

a. Without Coaxial Choke:

ChokeBridge

V

"(f

©~" I\AAAAA/

ij-0

CCVT

V'rV ( ^h

e 9 \zSh + z,

Vg(ZSh /Z

L>' SinCeZL» ZSh

b. With Coaxial Choke:

Figure 17. Function of coaxial chokes in eliminating

ground voltages.

42
to be lost. This condition can occur if a shield is broken or intermittent, or if there is a poor connection to neutral in the

control house.

The field procedure for ascertaining that the coaxial chokes are functioning properly is illustrated in figure 18, as applied to the

truck-control house loop. It involves two simple voltage measurements

(DVM, fig. 18), as outlined in the following steps:

(1) Make the voltage measurement shown in the figure (points A-A

connected). In a properly designed choke, 1\_ » Z sn , so that most of V, appears across 1\_ and is transformed 1:1 into the center conductor,

so that

VDVM = V

g (Z sh /Z L ) = V e

- (A15)

where V e is the residual error voltage, and is normally <10 mV if the

choke is functioning.

(2) In the same circuit, repeat step 1 with the shield broken,

e.g., at A-A. Now, no current flows through the choke, the center

conductor becomes a drop lead, and the DVM is effectively across G2-G3

Therefore,

V DVM " V g

. (A16)

If the conditions stated in eqs (A15) and (A16) are satisfied, the

coaxial choke is functioning correctly.

43
o o

-f—

CD

CO

DC c 3 o o O O X o

CD CD ill

l- QO

-M

<v> ai a> -t-> o cu c -^: a> o

-a

CD

j^ f— E

<u

CD

X

fa

O as o

CD a S • a> '»—

00 a. a> E c

O) 3

CO CD o

aso s

•r— o a) OJ

-a j*; ^J OJ o a CL c o E CJ

CD -a CD & X C CO

05 ^z o o CO CJ

II II

rvi r%j ivi

44
APPENDIX B

A SAMPLE CCVT CALIBRATION PROCEDURE

1. CALIBRATE TRANSFER STANDARD DIVIDER (MEASUREMENT C3)

1.1 Connect the circuit as shown in figure 19.

1.2 Turn on the dc power supply for the voltage comparator

quadrature circuit.

1.3 Turn on the detector (battery powered), adjust to low gain.

1.4 Turn on the twin-T filter preceding the detector (ac powered).

1.5 Turn on the variable autotransformer power supply for the

standard transformer, inject 10-20 V.

1.6 Obtain a preliminary comparator balance by alternately

adjusting first the IVD and then the quadrature circuit for

detector minimum (increase detector gain as balance is

approached).

Note: Before full balance is obtained tune the detector for maximum

output (60 Hz).

1.7 Once a preliminary balance is obtained, increase the input

voltage to 120 V (yielding an output of 14 400 V, since

the standard transformer ratio is 120:1).

1.8 Obtain a final detector balance, using the technique described in

1.6 (final balance consists of six places on the IVD).

1.9 Record the result. For example, hypothetical values might be

ac3 = IVD ratio = 0.047621,

yC 3

= phase angle = 2.60 mrad.

2. CALIBRATE CCVT (MEASUREMENT C4)

2.1 After the standard divider has been connected to the bus,

reconnect the circuit as shown in figure 20.

45
-a

s_fD DC

03

+-> CO

S-<u

<+-toc

T3

i4-Oo

fa -Q

fOU

>>E

Q-O fa

05 c o 2

CDF

C 03

CD -o o »- ii M—

cd 5 (/) M— TO c CD +2

rrCO CO >>o

CM

CO ^_

COo

+-> E

ty

S= • cu J- S- CU

=3 T3

CO -I— fO >

dl -r- s: -o

CT)

ll

a)

s-en

46
4 * 1 / 1 f

f l *f V III U

1

»- "D

v+i- CD ^- ^

05 CD

co "o "o 6

5 c > fi5 05 —

£- -^ Q I— CO • i—

:>o

CO C/D 6 O

13 c

DQ

o

CD

"3 O)

-Q

05 i— fC +-

O C_> O o S,"5

1 > 4—

»

o

CD Hi'

o

-Mc

CD X

CD Q -^ o

E

CD

irsCO c o CD o • o

03 C\J -»—

»

CD CO

-Q h- ( ) O)

3 U. C/D

CDQ

> •"O

j

I 1 1 ( Ih * 1 \ II'

47
2.2 Repeat steps 1.3 thru 1.9, above (skip 1.7 since transformer

T2 is not used). Hypothetical values might be

aC 4

= IVD ratio = 1 .00007,

YC 4

= phase angle = -7.47 mrad.

3. CALCULATE RESULTS

The ANSI Standard values reported and used are ratio correction

factor Ft and phase angle yj; of the test piece, where F^ is

the number by which the nominal ratio of the test piece must be

multiplied to obtain its true ratio, and y^ is by convention the

angle by which the secondary (low) voltage leads the primary (high)

voltage.

3.1 Standard Divider Ratio (p s ):

Ps = PT/ ac3

where pj = ratio, standard transformer (nominally 120:1),

e.g., 119.990.

Then, using the hypothetical numbers from (1.9),

p s = 119.990/0.047621 = 2519.69.

3.2 Ratio Correction Factor of Test CCVT:

F _ t \

p tN/\ ac3/ *

Using hypothetical data from 2.2 and assuming that

PtN = 2500:1, and p T

= 119.990,

Ft

= (119.990/2500) x (1.00007/0.047621) = 1.00795

48
3.3 Phase Angle of Test CCVT:

\lery simply, from the conventional definition,

Yt

= Yc3 + Tc4 = (2.60 - 7.47) 10" 3 = -4.87 mrad.

Thus a report on this CCVT would show Ft = 1.00795, and

Yt = -4.87 mrad, modified by small corrections to the standard

divider for voltage dependence, proximity effects, etc. Typically

this might be -0.03 percent and +0.2 mrad, respectively, modifying

the above values to

F t

= 1.00765,

Yt = -4.67 mrad.

Note that this hypothetical test piece is out of tolerance on ratio correction factor and marginal on phase angle for metering

application, where the corresponding limits are 1.003 and -4.66 mrad,

respectively.

4. PRECAUTIONS

4.1 Ground voltage tests - Make certain these are run on the standard

divider and test piece at least once each time the divider goes

on line. Preferably, they should be checked more often, or even monitored. See appendix A, section 6, for further discussion.

4.2 Signal cables - Use the same standard divider signal cable for

both measurements C3 and C4. This cable forms part of C] (fig. 5)

and changes the divider ratio by approximately 100 ppm/m of cable

length on the 2500:1 tap.

5. SUBSTATION SAFETY

A safety protocol for the prototype system, followed in the

substation by NBS, is presented in detail in [11]. Most of the

material there is applicable to the simplified system.

6. MISCELLANEOUS

The complete (500-kV) standard divider mass is about 1150 kg

(2500 lb) and its height is 5.5 m (18 ft). This much lifting capacity

and height must be available for moving the divider. In modular form,

the greatest weight and height are 500 kg and 2 m, respectively.

49
Because it is desirable to set up the comparator and standard

transformer reasonably well away from energized buses, signal cable

runs of 200 m or more will be common. Heavy duty coaxial cables

with UHF connectors, in lengths of 15 m (50 ft) and 30 m (100 ft) are easiest to store and handle, unless a machine-operated reel is available.

Cable joints should be insulated from ground (see discussions on ground

voltages).

50
NBS-114A [rev. 2-8C) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO. NBS TN 1155

2. Performing Organ. Report No. 3. Publication Date May 1982

4. TITLE AND SUBTITLE

A SIMPLIFIED SYSTEM FOR CALIBRATION OF CCVTs IN THE SUBSTATION

5. AUTHOR(S)

David L. Hillhouse, Oskars Petersons, and Wilbur C. Sze

6. PERFORMING ORGANIZATION (If joint or other than NBS. see instructions)

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8. Type of Report & Period Covered

Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

Sponsored in part by

Pennsylvania Power and Light Company

Two North Ninth Street

Allentown, PA 18101

10. SUPPLEMENTARY NOTES

J Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a si gnificant

bibliography * v y or literature survey.y - , . . . . . „ ,„„.,,. v Coupling capacitor voltage transformers (CCVTs) are widely used for the revenue metering of energy exchanged

between utilities at EHV (345-500 kV) interties. These devices are installed permanently in substations, and must

be calibrated there. Allowable error is ±0.3 percent and ±4.6 mrad (milliradians)

.

NBS developed, and has had in operation for several years, a prototype system for field calibration of these CCVTs. This prototype system is more accurate, more complicated, more

bulky, and more costly than is essential for this application.

This report describes a simplified, lighter, and less

y CCVT calibration system, newly developed and field tested by NBS. The principal elements of this system are a portable reference standard transformer and moderate voltage

power supply (14.4 kV) , a modular capacitive transfer standard divider, and a voltage comparator. Results obtained with this SVStem sys aar-PP agree with thp nrntnti/np tn within +d_fn norront anH

±0.1 mrad.

a vuitdye tumpdrdLur. Results ODtainea witn tr with the prototype to within ±0.03 percent and

The prototype system is installed permanently in a dedicated calibration truck. The new system could operate

with a non-dedicated truck to transport the disassembled modular

divider, and a van to transport the rest of the components and

to serve as a field laboratory.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

CCVTs; EHV revenue metering; energy metering; field calibration;

500 kV; 500 kV substation measurements; metering accuracy CCVTs.

13. AVAILABILITY

[XJ Unlimited

~J For Official Distribution. Do Not Release to NTIS n Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402.

[Y

-1 Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

57

15. Price

USCOMM-DC 6043-P80



NBS TECHNICAL PUBLICATIONS

PERIODICALS

JOURNAL OF RESEARCH—The Journal of Research of the National Bureau of Standards reports NBS research and develop- ment in those disciplines of the physical and engineering sciences in which the Bureau is active. These include physics, chemistry,

engineering, mathematics, and computer sciences. Papers cover a broad range of subjects, with major emphasis on measurement

methodology and the basic technology underlying standardization. Also included from time to lime are survey articles on topics

closely related to the Bureau's technical and scientific programs. As a special service to subscribers each issue contains complete

citations to all recent Bureau publications in both NBS and non- NBS media. Issued six times a year. Annual subscription: domestic

$18; foreign $22.50. Single copy, $4.25 domestic; $5.35 foreign.

NONPERIODICALS

Monographs—Major contributions to the technical literature on

various subjects related to the Bureau's scientific and technical activities. Handbooks—Recommended codes of engineering and industrial practice (including safety codes) developed in cooperation with in- terested industries, professional organizations, and regulatory

bodies. Special Publications—Include proceedings of conferences sponsored by NBS, NBS annual reports, and other special publications

appropriate to this grouping such as wall charts, pocket cards, and

bibliographies. Applied Mathematics Series—Mathematical tables, manuals, and

studies of special interest to physicists, engineers, chemists,

biologists, mathematicians, computer programmers, and others engaged in scientific and technical work.

National Standard Reference Data Series—Provides quantitative data on the physical and chemical properties of materials, compiled from the world's literature and critically evaluated. Developed under a worldwide program coordinated by NBS under

the authority of the National Standard Data Act (Public Law

90-396).

NOTE: The principal publication outlet for the foregoing data is the Journal of Physical and Chemical Reference Data (JPCRD)

published quarterly for NBS by he American Chemical Society

(ACS) and the American Institute of Physics (AIP). Subscriptions,

reprints, and supplements available from ACS, I 155 Sixteenth St., NW, Washington, DC 20056.

Building Science Series—Disseminates technical information developed at the Bureau on building materials, components,

systems, and whole structures. The series presents research results,

test methods, and performance criteria related to the structural and

environmental functions and the durability and safety characteristics of building elements and systems.

Technical Notes—Studies or reports which are complete in themselves but restrictive in their treatment of a subject. Analogous to monographs but not so comprehensive in scope or definitive in treatment of the subject area. Often serve as a vehicle for final reports of work performed at NBS under the sponsorship of other government agencies.

Voluntary Product Standards—Developed under procedures

published by the Department of Commerce in Part 10, Title 15, of

the Code of Federal Regulations. The standards establish nationally recognized requirements for products, and provide all concerned interests with a basis for common understanding of the

characteristics of the products. NBS administers this program as a supplement to the activities of the private sector standardizing

organizations. Consumer Information Series—Practical information, based on N BS research and experience, covering areas of interest to the con- sumer. Easily understandable language and illustrations provide

useful background knowledge for shopping in today's tech- nological marketplace. Order the above NBS publications from: Superintendent of Documents, Government Printing Office, Washington. DC 20402. Order the following NBS publications—FIPS and NBSlR's—from

the National Technical Information Services, Springfield, VA 22161

.

Federal Information Processing Standards Publications (FIPS

PUB)—Publications in this series collectively constitute the Federal Information Processing Standards Register. The Register

serves as the official source of information in the Federal Govern- ment regarding standards issued by NBS pursuant to the Federal Property and Administrative Services Act of 1949 as amended,

Public Law 89-306 (79 Stat. 1127), and as implemented by Executive Order 11717(38 FR 1 23 1 5, dated May II, 1973) and Part 6

of Title 15 CFR (Code of Federal Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or

final reports on work performed by NBS for outside sponsors

(both government and non-government). In general, initial dis- tribution is handled by the sponsor; public distribution is by the National Technical Information Services, Springfield, VA 22161,

in paper copy or microfiche form.
U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Washington. DC 20234 POSTAGE AND FEES PAID US DEPARTMENT OF COMMERCE

COM-215

OFFICIAL BUSINESS

Penalty for Private Use. $300

THIRD CLASS