Analysis of the Calibration of

fVletering CCVTs in A Utility

Substation

u

rr-f

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Eiectrosystems Division

Washington, DC 20234

October 1981

Final Report

Sponsored in part by:

Electric Power Research institute

P. O. Box 10412

Paio Aito, California 94313

•ATIOHAL bukbau

or STANDABU8

tlBRART

MAY 2 5 1982

NBSIR 81-2360

ANALYSIS OF THE CALIBRATION OF

METERING CCVTs IN A UTILITY

SUBSTATION

d,

David L Hillhouse and David A. Leap

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Electrosystems Division

Washington, DC 20234

October 1981

Final Report

This report contains results from a limited study. The data and analysis

presented here will be combined with additional measurements to produce a report of a full and complete investigation which is to be published at a later date.

Sponsored in part by;

Electric Power Research Institute

P. 0. Box 10412

Palo Alto, California 94313

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

TABLE OF CONTE^fTS

Page

LIST OF TABLES iv

LIST OF FIGURES iv

Abstract 1

1. IMTRODUCTION 1

2. DETAILED REVIEW OF RESULTS 3

2.1 History 3

2.2 December 1980 - More Detailed Results 5

3. ANALYSIS OF RESULTS 12

3.1 The "Triangle Effect" 12

3.2 Estimating the Unknown Voltage 17

3.3 Conclusion 24

4. EVALUATION OF KNOWN SOURCES OF ERROR 26

4.1 The NBS Calibration System 26

4.2 The Substation 37

4.2.1 Multiple Grounds on Neutrals 37

4.2.2 Electrostatic or Magnetic Coupling to Leads. 37

4.2.3 Burden Changes 39

4.2.4 Temperature Coefficients of Leads 43

4.2.5 Zero-Sequence Currents in the Neutral ... 43

4.2.6 Bad Temporary Connection of Cable Shield

in Control House 45

4.3 The CCVTs 46

5. SUMMARY OF RESULTS 46

6. REFERENCES ..... 47
LIST OF TABLES

Page

Table 1. Comparable Values From the Three Willow

Glen Calibrations 19

Table 2. Correction and Error Summary - MBS Calibration

System 27

Table 3. Total Burdens on Calibrated CCVTs at Willow Glen. . . ^0

LIST OF FIGURES

Page

Figure 1. History of XI X3 RCF and y, May 1979

to March 1980 4

Figure 2. History of XI X3 RCF and y, May 1979 to Dec. 1980. 6

Figure 3. Effect of burdens on XI X3 RCF and y, Dec. 1980. . 8

Figure 4. Effect of burdens on X2X3 RCF and y, Dec. 1980. . 9

Figure 5. Effect of burdens on Y2Y3 RCF and y, Dec. 1980. . 10

Figure 6. Intercompari son of the XI X3 RCF and y of pairs

of CCVTs on the same phase, Dec. 1980 11

Figure 7. Changes of control house metering RCF and y,

May 1979 to March 1980 13

Figure 8. Changes of control house metering RCF and y.

May 1979 to Dec. 1980 14

Figure 9. Changes of control house metering RCF and y,

March 1980 to Dec. 1980 15

Figure 10. Changes of control house metering and

relaying RCF and y, March 1980 to Dec. 1980 ... 16

Figure 11. Differences in relaying tap (X2X3) and

metering tap (X1X3) changes ( aF 2

-<aF-i ) = aF-] 2>

May 1979 to March 1980 20

Figure 12. Differences in relaying tap (X2X3) and

metering tap (X1X3) changes ( AF2-iiFi ) = ^i 2»

March 1980 to Dec. 1980 21

iv
LIST OF FIGURES (cont.)

Page

Figure 13. Changes of control house metering PCF and y, May 1979 to March 1980, after removing estimated

adjustment voltages 22

Figure 14. Changes of control house RCF and y, another

utility, Nov. 1979 to May 1980 23

Figure 15. "Revised" history of XI X3 RCF and y.

May 1979 to Dec. 1980 25

Figure 16. Circuit diagram, single uncompensated voltage,

Vg, added in the ground lead between the control

house and the MBS bridge 29

Figure 17. Vector diagram, single uncompensated voltage,

Vg, added in the ground lead between the control

house and the NBS bridge 30

Figure 18. Origin of ground voltages in the field

calibration of CCVTs 32

Figure 19. Function of coaxial chokes in eliminating

ground voltages 33

Figure 20. Coaxial choke test 35

Figure 21. Disposition of CCVT leads, makeup box to

control house 38

Figure 22. Circuit diagram, burden added to A-phase

relaying winding only. Note— B- and C-phase

relaying windings not shown 41

Figure 23. Vector diagram, burden added to A-phase

relaying winding only 42

Figure 24. Circuit diagram, balanced voltages and

impedances, for study of the effect of unbalancing

one phase voltage 44

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ANALYSIS OF THE CALIBRATION OF METERING CCVTs IN A UTILITY SUBSTATION

David L. Hillhouse and David A. Leep

This report presents the results of an investigation of unexpected variations in nine 500 kV

metering CCVTs, tested for Gulf States Utilities at

Baton Rouge, LA. These measurements were performed on three occasions - May 1979, March 1980, and December 1980.

On the first two occasions, six out of nine CCVTs

were out of tolerance; on the third, four out of nine.

More important, the changes between the first two

occasions seemed to be correlated by phase, i.e., most

of the devices on a given phase shifted in the same

general direction and by similar amounts. When analysis

failed to provide an explanation for this, the third set

of measurements was undertaken.

Analysis of the three sets of data produced some evidence of a bias voltage in the 1979 data. Investigation of all olausible known sources of error in the

MBS system, the substation, and the CCVTs themselves

failed to produce a probable source for such a bias

voltage.

No evidence of consistent malfunction of the MBS

system was found. Even allowing for a possible bias in one set of data, a majority of the CCVTs were still outside of metering tolerance. Continuous monitoring of a statistically significant number of operating CCVTs

should be considered.

Keywords: calibration; CCVT; EHV substations; error

sources; high voltage measurements; revenue metering.

1. INTRODUCTION

This report documents an investigation of some peculiar results

obtained during measurements of metering coupling capacitor voltage

transformers (CCVTs) at Gulf States Utilities' (GSU) Willow Glen

Substation, near Baton Rouge, LA. The work was an extension of EPRI

Project RP-134-1. The measurements under discussion were performed

using the field calibration system for CCVTs developed during the

above research project. This system is described in [1]^ and

documented in detail in [2].

"'Numbers in brackets refer to the literature references listed at the

end of this report.

1
Willow Glen is a typical low profile substation, situated next

to a generating station on the bank of the Mississippi River, about

15 miles below Baton Rouge. These measurements were performed in

the 500 kV section of the substation. Three lines (Franklin, Gypsy,

and Webre) meet at this substation. Each line has three metering

class CCVTs connected to it, for a total of nine for the substation.

Measurements were performed on the metering tap (X1X3) and two

relaying taps (X2X3, Y2Y3) of each CCVT, for a total of 27 per

calibration trip.

A total of three sets of measurements were performed at Willow

Glen on three occasions - May 1979, March 1980, and December 1980.

Sumnari zi ng:

May 1979 - Referring to metering values only^ (X1X3 tap),

six out of nine devices were found to be out of tolerance to within

NBS measurement uncertainty, the worst being >1 percent in ratio

and >9 mrad in phase angle from nominal. These values were measured

in the control house for the connected burdens.

March 1 980 - In view of the very poor showing of the CCVTs in

May 1979, and because of the large dollar value of the energy passing

through the substation, GSU invited NBS to repeat the measurements,

in order to see how stable the CCVTs were, and whether any more had

drifted out of tolerance. Once again, six devices were out of

tolerance, including two out of the three which had been in tolerance

in May, 1979. More important, it was noted during the measurements,

and confirmed by preliminary analysis immediately afterward, that

the changes from May 1979 to March 1980 appeared to be correlated by

phase (A, B, C). Rain, which continued for the remainder of the

available time at GSU, prevented repeating the measurements. Detailed

analysis after returning to NBS confirmed the initial observations,

but yielded no explanation for them.

December 1980 - These measurements were made in an attempt to

find an explanation for the peculiarities discussed above. A much

more detailed and comprehensive series of measurements was made than

for the first two trips. For example, whereas the earlier measurements

were performed only in the control house and on the normal working

(connected) burdens, and used only the NBS prototype system's current

comparator bridge, this third set used both the prototype and a new simplified system [1,2], and included zero-burden and connected-burden

measurements at the CCVT makeup boxes (MUBs) and numerous intercomparisons both between CCVTs on the same phase, and between taps

^Refers to ANSI accuracy requirements for the XI X3 output tap of a revenue metering device — 0.3 percent for ratio correction factor

(RCF) and 4.6 milliradians (mrad) for phase angle (t). See

reference [3], p. 21.

2
on the same CCVT. Results for connected burden at the control house

showed five CCVTs to be within metering tolerance (two from 1979, two

from March 1980) and four out of tolerance.

The results from all three measurement sets are compared and

analyzed in detail in the following sections. All plausible sources of

error known to us are reviewed in a search for possible explanations for

the observed results.

2. DETAILED REVIEW OF RESULTS

2.1 History

The history of the metering tap (X1X3) results from May 1979 to

March 1980 is shown in figure 1. The ratio correction factor (RCF)^

and phase angle (y) values are plotted in x-y coordinates for each

CCVT in conformance with the conventional presentation in ANSI Standard

C56. 13-78 [3]. In that standard, RCF is defined as the quantity by

which the nominal ratio (e.g., 300 kV:120 V, or 2500:1) of the test

piece must be multiplied to give the true ratio. The phase angle, y,

is defined as the angle by which the output voltage of the test piece

leads the input voltage. The ANSI paral lelogram, which delineates the

error limits of the metering device, is superimposed on the graph, as

is the estimated NBS measurement uncertainty.

In this figure and in figures 2-5 and 15, A-, B-, and C-phase

values are shown as solid, long-dashed and short-dashed lines,

respectively. Open and solid circles represent May 1979 and March 1980

data, respectively. Arrows on the lines connecting the data points

indicate the progression of time. Only one CCVT is connected to each

phase of each line. Therefore, the simple legends unambiguously

identify each CCVT by phase and line. For example, "AF" designates

the CCVT on A-phase, Franklin Line, "CW" the CCVT on C-phase, Webre

Line, etc.

Expanding upon the brief summary in the Introduction, figure 1 shows that in May 1979 three devices (AG, BW, and CF) were in

tolerance (inside the paral lelogram) . In March 1980, only two (BF, BW)

were in, and one (AW) was marginal, i.e., out only by approximately

our uncertainty of ±0.1 percent and ±0.3 mrad. The 1979 extremes

were BG, at F = 0.9892, and CW, at y

= +9.2 mrad. It should be noted

that in 1979 GSU adjusted*^ AF, AW, and CW. Figure 1 results for these

devices were obtained after these adjustments. Adjustment improved the

accuracy of AW, degraded that of CW, and left AF neither better nor worse. It did not bring any of them into tolerance.

^In this report, RCF 5 F in equations, for conciseness.

^Values for AF, AW, and CW before adjustment were (1.0073, -0.4 mrad),

(1.0043, +9.5 mrad), and (0.9947, +5.9 mrad), respecti vely

.

3

Figure 1 shows the apparent phase-by-phase correlation of the

May 1979 - March 1980 changes, i.e., all A-phase and C-phase values

moved generally downward, and all B-phase values generally upward.

Phase angle changes were somewhat less correlated. It was this

appearance of correlated changes which generated the investigation

reported here.

Figure 2 extends the metering tap history by having the

December 1980 results added to the figure 1 plot (closed squares

represent December 1980 data). The only notable changes are the

apparent reversal of some of the earlier trends. This will be

discussed in more detail later. As mentioned earlier, five devices

were in tolerance, four were out.

2.2 December 1980 - More Detailed Results

Because these measurements were planned specifically to look for

explanations for the seemingly unusual differences in earlier data,

they were considerably more extensive in their scope and in the equipment and personnel applied to them. Earlier measurements had been

performed only for connected burdens in the control house, using the

NBS-EPRI prototype system and its current comparator bridge. These

latest measurements included the above, plus connected burden measurements and zero burden measurements at the makeup boxes (f^Bs) at the

base of the CCVTs. They also included the use of a voltage comparator

of comparable accuracy (see "Simplified System," section 9, ref. [2])

for numerous intercomparisons of ratios between CCVTs on the same phase,

intercomparisons between taps on the same CCVT, and for spot checking

some of the ratio measurements made by the current comparator bridge.

Most of the i ntercomparisons were performed in the control house.

In almost all cases, these i ntercompari sons were also closely correlated

in time with ratio measurements on the same phase using the bridge.

CCVTs are adjusted and calibrated initially by the manufacturer

at zero burden (open circuit). They are installed on the substation

bus at the end of leads from the control house which may be 200 or more meters long. From there the CCVTs may drive burdens of 100 VA

(volt-amperes) or more. Because of voltage drops due to the internal

impedance of the CCVT and to lead impedance, the ratio in the control

house will be significantly different from the zero burden ratio of

the CCVT if the connected burden exceeds a few volt-amperes.

In order to obtain information on the effects of burden and of

lead impedance, zero burden and connected burden measurements were

performed at the MUBs and the results compared with connected burden

^VJith short cable runs, and a minimum of ground voltage and

other interference problems.

5
RCF

Figure 2. History of XI X3 RCF and y, May 1979 to Dec. 1980

6
values in the control house. Figure 3 shows the results of these

measurements. Nomenclature is the same as for earlier figures. The

lines connecting points on the graph show the progression from zero burden-MUB to connected burden-WB to connected burden-control

house. In all cases, the connected burden referred to was the total

burden for all windings.

Most of the results show the trends toward increasing RCF which

would be expected for predominantly resistive, lagging power-factor

burdens, as these were. The two exceptions (BF, CW) can be at least

partially accounted for by neutral currents due to unbalanced burdens.

The extremely large change in AW is probably due to high device

impedance, caused by detuning. The CCVT is a tuned device, in which

the high equivalent impedance of its capacitor stack (magnitude

approximately 16 000 Q, for a 500 kV CCVT, or 2.5 referred to the

CCVT output terminals) is approximately tuned out by a series inductor.

This reduces the output impedance for a properly adjusted CCVT to a small value (typically of the order 0.3-0. 5 \i at 40-50°, inductive,

referred to the CCVT output). During final adjustment, the desired

phase angle (y) is obtained by small changes in the tuning inductor.

If a large change iny is required, the circuit becomes detuned,

significantly increasing its impedance. Burden current flowing

through this larger impedance produces a greater than normal change

in output voltage.

Now recall from the discussion of figure 1 that AW was adjusted

in 1979, with a resultant large change in y. This means that

large changes were made in the tuning reactor during this adjustment.

To test the detuning hypothesis, an equivalent circuit was drawn up,

containing the impedance values typical for a CCVT of the AW type.

This circuit was assumed to be connected to lead resistance and

connected burden impedance approximating that of the actual CCVT.

Values of tuning inductance were assumed, and the resultant voltage

drop calculated and compared with the magnitude of the actual AW

voltage drop in figure 3, inferred as follows: aV = 120 aF = 120

(0.9990-0.9825) = 1.98 volts. This calculation showed that

inductance values well within the range of tuning can produce voltage

drops of the size encountered here.

Mote also that eight out of nine CCVTs are outside metering

tolerance, even at zero burden. The usual factory practice is to

set them at approximately (F = 0.997, V = +2.0 mrad) for zero burden.

Figures 4 and 5 show, for the relaying taps (X2X3, Y2Y3), the

same data as figure 3 showed for the metering taps. They display

the same general trends as figure 3.

In figure 6 are summarized the results of intercomparisons

among the metering taps of the CCVTs on each phase ‘in December 1980,

Coding is the same as for previous figures. The nomenclature is to

7
Figure 3. Effect of burdens on XiX3 RCF and y, Dec. 1980

8
RCF

Figure 4. Effect of burdens on X2X3 RCF and y, Dec. 1980

9
Figure 5. Effect of burdens on Y2Y3 RCF and y, Dec. 1980

10
ARCF

(xIO^)

Figure 6. Intercomparison of the XI X3 RCF and y of pairs of CCVTs

on the same phase, Dec. 1980

n
be read as follows: A(W/F), e.g., is the intercomparison of the

A-phase devices on the Webre and Franklin lines, with Franklin

(the denominator) as the reference, or origin, on the graph. Thus

AW differs from AF in RCF and y by approximately +0,0057 and +3.0 mrad,

respectively. Where small numbers appear alongside the data, they

indicate the date (December 8 through 12, 1980) on which the measurement occurred. The circled dots represent the same values derived

from two ratio measurements with the current comparator bridge.

The quality of closure of three i ntercompari sons (two degrees

of freedom—therefore, one is redundant) can be grasped by inspecting

the quadrilateral formed by connecting the means of each of the

three sets of values and the origin. This quadrilateral should

form a paral lelogram, as is demonstrated for the C-phase values.

Perhaps more significant are the ratio measurement results (circled

dots), all but one of which fall in with i ntercompari sons on the

same date to well within our measurement uncertainty. Since each of

these values required two ratio measurements, indeoendent from the

i ntercomparisons in several ways, they give confidence that the

ratio measurements themselves were not subject to significant random

error.

The spread of intercompari son points taken on different days

can be interpreted as a measure of the short-term instability of the

CCVTs due to temperature changes or other causes. Most such families

are grouped rather tightly. The most notable exceptions are B(F/G)

and B(W/G), which show shifts of RCF greater than 0.1 percent in two

days (combining the two results indicates the shift was in G, since

B(F/W) is tightly grouped).

3. ANALYSIS OF RESULTS

3.1 The "Triangle Effect"

In the discussion of the changes shown in figure 1, an apparent

correlation by phase was pointed out. This is brought out much more

clearly by replotting the same values as differences (fig. 7). In this figure, the May 1979 metering RCF and y values (the first, or open-circle values in fig. 1) are taken as reference, i.e., 1979

becomes the origin, and the changes from May 1979 to March 1980 the

plotted points. Here the correlation by phase shows up quite plainly

in the segregation of values, particularly for B and C phases. If

the means of the sets of phase values are connected, a figure approaching

an isosceles triangle is generated. A plot of the same kind for the

May 1979 to December 1980 differences (fig. 8) shows a similar triangle.

However, in a March to December 1980 difference plot (fig. 9), the

triangle has disappeared. There is almost no phase angle discrepancy

in figure 9. With the exception of phase C, the segregation by

phase is much less, and the changes are smaller. In figure 10, the

12
Figure 7. Changes of control house .'netaring RCF and y, ^ay 1979

to March 1980

13
Figure 3. Changes of control house metering RCF and y. May 1979

to Dec. 1980

14
• “ Mean

Figure 9. Changes of control house metering RCF and inarch 1980

to Dec. 1980

15
Figure 10. Changes of control house .metering and relaying RCF and y,

March 1980 to Dec. 1980

16
changes for the relaying taps from March to December 1980 have been

added to figure 9. They reinforce the inference that the "triangle

effect" has disappeared.

The presence of the "triangle effect" in both sets of data

involving May 1979 (figs. 7 and 8 ), and its absence in the March to

December 1980 set (fig. 9) suggests a bias in the 1979 data, i.e.,

the presence throughout that test of a voltage or voltages, affecting

all three phases, not present in the other two tests. Fortunately,

the available data provide a tool for estimating the required

magnitude and phase of such a voltage, as discussed in section 3.2.

3.2 Estimating the Unknown Voltage

In most of the measurements, the X1X3, X2X3, and Y2Y3 taps of

the CCVT were calibrated. For ratio changes due to the CCVT's

capacitive divider (e.g., temperature, shorted roll) and tuning

reactor or anything else on the primary side of the intermediate

voltage transformer, the relaying taps should "track" the metering

taps. In particular, the X2X3 tap should "track" the XI X3 tap to

other hand, changes due

side (e.g., burden or the CCVT or test system

metering taps

within a few hundredths of a percent. On the

to common effects on the output, or secondary

lead impedance changes, a spurious voltage in

ground circuits) will affect the relaying and

differently, because of the difference in ratio, hence in voltage

levels. For example, an in-phase 1-volt change applied to the

metering tap changes its ratio (or RCF) by 1/120, whereas the same voltage changes the ratio of the relaying tap on the same phase by

approximately 1.8/120, or by a factor of 1.8 more.^ With this in

mind, the differences in the changes of the XI X3 and X2X3 tap ratio

correction factors from May 1979 to March 1980 were determined as

£iF

] 2 ^2 ~ ^1 ( 1 )

where

AF

2

- (RCFgQ,

AFi = (RCFgg,

and (F, y) are the

^^8o) - (RCFyg,

^80 ) ‘ (RCFyg,

coordinates on

''^yg) for the X2X3 tap

'‘' 79 ) ^or the X1X3 tap,

an "x-y" plane.

^1.8 is based on 4500:1 for relaying, 2500:1 for metering.

17
The results are plotted in figure 11, using the data in table 1.

When this is done, e.g*, using the A-phase, Gypsy data^ it is found

that AF

2 = (-6.9 X 10"'^, +1.8 mrad), aE-| = (-3.5 x lO""^, +0.7 mrad).

aF -|2 is then calculated as (-3.4 x 10“^, +1.1 mrad), and plotted

as G-square, figure 11.

Connecting the means of the sets of phase quantities forms a near-perfect three-phase vector system (all vectors within ±5® of

120® separation). Two possible hypotheses are suggested: (1) a more or less fixed voltage (e.g., a voltage due to a ground loop,

commonly referred to as a ground voltage) approximately in phase

with A-phase; and (2) a voltage drop produced by approximately the

same burden change in each A-phase device. These hypotheses will be

dealt with when sources of error are discussed (section 4).

Figure 12 shows the results of the same analysis for changes

from March to December 1980. They show almost perfect agreement

between the XI X3 and X2X3 values (within measurement uncertainty).

This indicates that if a spurious effect was present at any time, it

should be assigned to the 1979 measurements. Assuming for the moment

that the March 1980 values are correct, the data in figure 11 can be

used to extract the "true" metering tap values for the 1979 measurements.

This is accomplished by simple application of analytic geometry to

the ratio of the tap-to-tap differences. If aF-j and AF 2 as calculated above are plotted on a (aF, Ay) graph, they define a straight line, which also passes through the "perfect tracking" point,

i.e., the point representing the change in both the metering and the

relaying ratios from May 1979 to March 1980 if the postulated extraneous

voltage is removed. Furthermore, relative to the "perfect tracking"

point, jAF 2 l = 1.8|aF]1 along this straight line. Since the

coordinates of aF] and AF 2 are known, the distance aF] to the

"perfect tracking" point can be found by simple proportions as (aF]- AF 2 )/ (1.8 - 1), or aF]2/0‘8. When this is done using

the A-Gypsy values for aF] and AF 2 calculated above, the

"perfect tracking" point is found to lie at (0.75 x lO”^, -0.7 mrad).

This point is A-Gypsy (G-square) in figure 13.

Figure 13 shows the results obtained when this process is applied

to all the May 1979 data in figure 7. The changes are smaller and

the "triangle effect" has essentially disappeared. There are still

sizeable RCF changes and evidence of segregation by phase (especially

C-phase). The pattern is now quite similar to that for March to

December 1980 (fig. 9), i.e., sizeable RCF shifts, very small phase

angle shifts. Furthermore, these patterns (figs. 9 and 13) are generally similar to those found at another utility's twice-calibrated

substation. Compare figure 14 with figures 9 and 13. Figure 14

shows changes in X2X3 tap ratios (X1X3 taps were not measured in

that substation) from November 1979 to May 1980. Evidence for phase

segregation, especially A-phase, is strong (statistical analysis has

indicated that the probability for random scatter of figure 14 data

is less than 5%). As at GSU , this apparent segregation has yet to

be explained.

18
Table 1 . Comparable Val ues From the Three

Wi 1 1 ow Glen Calibrations

Li ne Phase Tap May 1979 March 1980 Dec. 1980

RCF6 RCF X RCF Y

Franklin A ^1 0.9930 +1 .0 0.9918 0.6 0.9932 +0.9

Frankl i

n

A ^2 0.9940 +0.2 0.9909 0.9 0.9924 1 .0

Frankl i

n

A Y2 1.0019 +1 .9 0.9978 2.0 1 .0005 2.5

Frankl in B Xl 0.9992 -3.3 1 .0009 -2.3 1 .0010 -1 .9

Franklin B 0.9969 -3.3 0.9999 -1.7 1.0003 -1 .0

Frankl i

n

B Y2 1.0068 -0.4 1 .0101 ^1 .8 1 .0097 2.2

Frankl i

n

C Xl 0.9976 2.1 0.9951 0.4 0.9981 0.4

Franklin C ^2 0.9973 3.5 0.9953 0.4 0.9996 0.3

Frankl in C Y2 1.0026 8.2 1 .0009 4.8 1 .0045 5 .5

Gypsy A ^1 0.9988 +0.8 0.9953 1 .5 0.9954 2.2

Gypsy A X2 1.0018 -0.6 0.9949 1.2 0.9962 1 .7

Gypsy A Y2 1.0087 -1 .2 1 .0012 1.2 1 .0023 1 .3

Gypsy B ^1 0.9891 -0.6 0.9913 0.4 0.9925 1 .0

Gypsy B 0.9876 -2.4^ 0.9911 -0.3 0.9925 0.6

Gypsy B n - - 1 .0000 4.2 1 .0012 i.7

Gypsy C Xl 0.9958 3.8 0.9935 1 .0 0,9959 1.7

Gypsy C X? 0.9951 4.4 0.9935 0.6 0.9959 1 .3

Gypsy r Y2 1.0018 9.2 1.0002 5.4 1 .0045 7.4

Webre A Xl 1.0028 2.1 0.9966 3.4 0.9988 3.3

Web re A ^2 1 .0097 2.2 1.0006 4.3 1 .0027 4.7

Webre A Y2 1.0081 -8.2 1 .0004 -5.5 1 .0019 -5.0

Webre B Xl 0.9988 -0.3 1 .0006 1.5 1 .0012 1 .9

Webre 3 X2 0.9975 +0.2 1 .0005 2.9 1 .0010 3.5

Webre 3 ^2 1.0054 +1.4 1 .0084 3.9 1 .0092 4.7

Webre C Xl 0.9977 9.1 0.9947 4.5 0.9977 5.4

Webre C X2 0.99^4 9.0 - - 0.9946 3.2

Webre C X2 1.0063 13.8 1 .0044 3.5 1 .0079 9.3

^The AF, AW, and CW CCVTs were adjust ed after initial measurement.

However, only the metering taps (XI) were measured after adjustment.

The adjusted metering values and the initial relaying values ( X2 and

Y2), appropri ately noted, were reported. For purposes of this analys

the relaying values were adjusted by the same amounts as the reported

differences in the initial and adjusted values ^or the correspondi ng

metering windings. For this reason, the six values will not agree

with those reported for the 1979 tests.

^Reported as +3.4 mrad. Error discovered during this analysis.

19
Figure 11. Differences in relaying tap (X2X3) and metering tap

(X1X3) changes ( AF2

- aF] ) = aF-i 2 » May 1979

to March 1980

20
Figure 12. Differences in relaying tap (X2X3) and metering tap

(X1X3) changes ( AF 2 -AF] )=aF] 2 , March 1980

to Dec. 1 980

21
Figure 13. Changes of control house metering RCF and y, May 1579

to March 1980, after removing estimated adjustment voltages

22
Figure 14. Changes of control house RCF and y, another utility,

Mov. 1979 to May 1980

23
Finally, figure 15 shows the "revised" history of the GSU XI X3

ratios after the adjustments deduced above have been applied to

figure 2. Note that the adjustment produces results which are more

closely grouped, especially as to phase, and are nearer the metering

parallel ogram.

The evidence for this "revised" history is not entirely conclusive,

and it is no longer possible to test the hypothesis any further. If

the hypothesis is correct, however, it is possible that figure 15

produces a better picture of changes occurring in the CCYTs

themselves between May 1979 and December 1980 than does figure 2.

Note from figure 15 that although a number of the CCVTs are well

out of metering tolerance, seven out of nine appear at least to have

had the same values (iO.1%) in December 1980 as in May 1979. Two

devices (A-phase, Franklin, and B-phase, Gypsy), appear to have

increased by 0.3 percent and 0.2 percent, respectively. This would not

be unexpected, based on our experience with CCVTs.

The March 1980 results from five of the nine CCVTs agree with

the December 1980 and the adjusted May 1979 results to within our

stated measurement uncertainties. This tends further to confirm that

at least five of the devices may have been stable throughout the

entire 19-month period (although three of the five were outside of

metering tolerance). One device (A-phase Webre) appeared to be

0.2 percent less in March 1980 than both its earlier and later values.

This is perhaps unusual behavior, but not a cause for concern in

view of the fact that the other A-phase CCVTs did not exhibit this

"V effect."

The other three March 1980 results present a somewhat different

picture. All three devices are on C-phase; all appear to have been

about 0.3 percent lower than in December 1980 and May 1979 (adjusted

values), as were both corresponding relaying values for each device.

This apparent "V effect" arouses enough suspicion to justify some further tests on the stability of our standard divider, even though

previous tests and extensive experience have given us good reason to

rely on it. These tests will be carried out as soon as possible.

This will be discussed further in section 4.

3.3 Conclusion

Detailed analysis of the data from all three sets of measurements

has yielded some evidence for the presence of an unexplained bias in

the 1979 measurements. Furthermore, this bias voltage appears to

have been of the order of 0.3 volts (mean value of 10 estimates),

relatively constant (a = 0.11 volt), and very roughly 180° out of

phase with the A-phase voltage in the substation. Therefore, it may

be prudent not to ascribe all the apparent changes in CCVTs between

May 1979 and the later measurements to the CCVTs themselves.

24
RCF

Figure 15. "Revised" history of XI X3 RCF and y, May 1979 to Dec. 1980

25
4. EVALUATION OF KNOWN SOURCES OF ERROR

Errors in the calibration of CCVTs can come from three general

sources: (1) the MBS calibration system; (2) the substation; and

(3) the CCVTs themselves. Insofar as is possible, the following

discussion will be divided according to these sources. Since in

this investigation we are looking for effects which could account

for perturbations of the size of the apparent correlated changes

between May 1979 and March 1980, i.e., 0.2 to 0.5 percent for ratio

and 1 to 3 mrad for phase angle, the bulk of the discussion will be

devoted to sources which could most logically cause errors of that

magnitude.

4.1 The NBS Calibration System

The comprehended fixed corrections and error sources in the NBS

prototype calibration system are summarized in table 2. Items 1 through 5 and item 7-a are more or less subject to investigation

and assignment of estimated limits of systematic and random components.

Items 6 and 7-b are either indeterminate or not subject to a priori

evaluation.

All the items in the first group above except 1-d, 4-b,

4-c, and 7-a were evaluated and reported upon in great detail in

reference [2]. That work was reviewed in the course of this investigation. The results still appear to be valid. The estimates of the

standard divider temperature and proximity effects continue to be

borne out by field results - the former by many observations of

divider ratio before and after periods on-line, the latter by regular

shorted divider tests near the 500 kV bus (see especially, p. C-5

and figure C-2, reference [2], for the theory involved). Typical

shorted divider voltage measurements yield 10-20 mV or 0.008 to

0.016 percent of 120 V. The largest value of residual ground voltage

yet measured was 25 mV, at GSU in December 1980. Some further tests

of the divider's stability, and of its voltage, temperature, and

proximity dependence are planned, in view of the "V effects" discussed

in section 3.2. The remaining items in the first group will now be

discussed.

1-d. Magnetic Coupling to Stack

An error can be produced by voltage magnetically induced in the

divider capacitor string by currents in the buses or the ground mat.

These currents are at most a kilo-ampere or so at a distance of 5-10 m, generating a volt or less out of hundreds of kilovolts. Uncertainty

due to magnetic coupling is probably quite a bit less than the value

assigned to it.

26
Table 2. Correction and Error Summary - NBS Calibration System

Source

Percent

Correction

Estimated

Percent ^ Uncertainty

1. Standard Divider;

a. Voltage Dependence -0.02 0.010

b. Temperature Effects -0.02/hr 0.015

c. Proximity Effects -0.01 0.010

d. Magnetic Coupling to Stack 0 0.001

2. Compressed Gas Standard Capacitor 0

+ 0.007

3. Reference Standard Capacitor 0 0.001

4. Bridge:

a. Ultimate Accuracy 0 0.002

b. Zero Offset (Residual P.U.) 0 ± 0.010

c. Detector Noise 0 ± 0.003

5. Miscellaneous (Undiscovered) 0 0.015

6. Operator Error _ _

7-a. Ground Voltages (Choke-Compensated) 0 ± 0.010

Arithmetic Sum of Errors ("Worst--case"

)

r 0.084

Quadrature Sum of Errors ± 0.030

1 cr • Ground Voltages (Uncompensated) 0 ± (0.1 to 0.5)

^At the 3a confidence level

27
4-b. Bridge Zero Offset

Error can be produced by magnetic pickup in the current comparator

bridge. It is apprehended by balancing the bridge, then reducing

bridge voltage to zero and observing detector deflection. This effect

may vary from 0.005 percent to 0.04 percent of the ratio being measured,

with typical values being 0.01 to 0.02 percent. Most of this error

is believed to have been compensated for, either by offsetting the

bridge to the residual pickup value, or by determining the offset and

applying a correction.

4-c. Detector Noise

Detector noise is a "nuisance" perturbation. It is brought

about by low voltage in the battery-powered bridge detector. It

manifests itself in a noisy (unstable) detector null, making balance

difficult in the fifth (10"5) place. It is normally noticed and

corrected well before the stated uncertainty level is reached.

6. Ooerator Error

Operator error has been listed in table 2 because it has been

broached as a possible problem. However, it cannot be given a quantitative value. Our system of double-checking and read-back of

data, plus on-the-spot preliminary calculations, and the use of

operators accustomed to dealing with very exacting measurements,

greatly minimizes operator error. In any event, such an error would

be likely to manifest itself as an individual outlying value, not as the series of apparently systematic perturbations being investigated.

7. Ground Voltage _ _

An uncompensated ground voltage appears to be the only source

of error in the NBS system which could possibly account for the

unexplained voltage postulated to be present in the 1979 measurements.

Its typical range in the substation is from 0.1 to 0.6 volts. This

represents 0.08 to 0.5 percent or 0.8 to 5 mrad, which encompasses

the range of discrepancies being investigated. Furthermore, its

"signature" is similar to that produced in the 1979 vs. March 1980

data (fig. 11) by failure of the XI X3 and X2X3 taps to track each

other.

But consider the effect of injecting a single voltage (Vg)

into the NBS system in this manner, as indicated by the circuit of

figure 16. Figure 17 shows the vector diagram resulting from figure

16 (with the size of Vg greatly exaggerated). Note that the vector

change in voltage is identical in all three phases, but that the

in-phase and quadrature changes, which ultimately determine the

changes in ratio correction factor (aF) and phase angle (Ay),

are different for each ohase. It can be shown that aF for each

phase is proportional to the magnitude of Vg/Vp^ase Jiiultiplied by the

28
Metering

r

E —

03 r- 3

oo CO «3 03 no 03 ^

a 03

vs

(=

oo c

w

O o 03 O ha

CT

29

Circuit

diagram,

single

uncompensated

voltage,

V^,

added

in

the

ground

lead

between

the

control

house

and

the

MBS

bridge
30
cosine of the angle between Vg and Vp^gsg A-phase, 9g + 120°

for B-phase, 9g + 240° for C-phase), Similarly, Ay is proportional

to the magnitude of q/'^ phase multiplied by the sine of the angle

between Vg and V phase*

A detailed analysis based on these relationships shows the

"signature" of a single injected voltage, in terms of the aRCF

and Ay of figure 11, to be three vectors identical in magnitude

and separated by 120°. In figure 11, the magnitude of the A-phase

vector is approximately 1.8 times greater than the magnitudes of

the B- and C-phase vectors. The "signatures" do not match.

However, the difference could be attributed to variations in the

unknown injected voltage during the measurement. Our experience

has shown that ground voltage can vary by the order of 2:1 over a few hours time. Therefore, the ground voltage hypothesis cannot

be dismissed entirely.

In any event, because of the importance of ground voltage

differences, compensation for it in the form of coaxial chokes was designed into the original system. But first, let us consider the

sources of ground voltage errors. Refer to figure 18, which is a semi-pictorial representation of the CCVT calibration circuit in the

substation. For safety reasons, there are three instrumentation

grounds in this 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 "iglZg], ig2Zg2> ^nd ig 3 Zg 3 , respectively,

between these ground points. In addition, magnetic coupling from

bus current I or from ground currents, represented by Bl, B2, and

B3, may induce additional voltages in these ground loops. The

summations of these voltages can be represented by ground voltages,

Vgi, Vg2 , and Vg3

. Considering the NBS system only and assuming

for the moment that there are no coaxial chokes, Vg] is impressed

across the standard divider cable shield, and generates an error

voltage, Vg] s "ishl^shl* directly in series with the voltage

to be measured (V2)* A similar effect occurs in the control house

1 oop.

The ground voltage effect, and its compensation by a coaxial

choke, can be seen more clearly in figure 19. Figure 19-a shows the

ground loop circuit in the absence of the choke. Error voltage

Vg = Vg as stated, since the cable center conductor is connected

to Vg through an impedance approaching infinity (ij^ e 0).

Figure 19-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 through a high permeability toroidal

core. Assume that its magnetizing impedance is Z^; the resistance

and leakage reactance add to Zsh* Thus any voltage, Vg^, appearing

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

conductor (diminished only slightly by the combined effect of the

31
500

kV

Bus

32

Figure

18.

Origin

of

ground

voltages

in

the

field

calibration

of

CCVTs
Bridge CCVT

Truck Control House (or Std. Divider)

Ground Ground ~ 'Sh^h ^ ~

a. Without Coaxial Choke:

Bridge CCVT

- Vg(Z3f^/Zj_) , Since >> Z^j^

b. With Coaxial Choke:

Figure 19. Function of coaxial chokes in eliminating ground voltages

33
presence of Zsh ^nd the finite value of Zi_) and in opposition.

This effectively cancels the ground voltage error. There is a net

error voltage, but since the choke is designed so that Z|_ > 50 Zsh»

Vg is reduced to 1 or 2 percent of Yg. Thus, if Vg, uncompensated

(item 7b, table 1) is 0.5%, Vq, compensated (item /a, table 2)

is < 0.5/50 = 0.01%.

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 Vg up to two volts, whereas the largest Vg

yet seen in practice has been less than one volt.

(2) Grounded Cable Shield - Referring to figure 19-b, a signal

cable shield shorted to ground (ahead of the choke in terms of the

direction of i'sh) effectively bypasses the choke, returning the

circuit to figure 19-a. This can occur, for example, 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 19-b, an open

cable shield means •> ®. From the associated equation, note

that in this case Vg Vg. An open shield is the limit of a subcategory -- bad shield connections. In such cases, Zg^ need only

approach or exceed Z|_ (typically 50-100 a) for most of the compensation to be lost. This condition can occur if a cable shield is

broken or intermittent, or if there is a poor connection to neutral

in the control house. The latter seems more likely, since this

connection is made with clip leads (since control house connections

are made by utility personnel, this error source is properly associated

with the substation, not the NBS system, and will be discussed in

section 4.2).

Before further discussion of any of these ground voltage compensation failure modes, it is appropriate to look at the field procedure

for detecting and correcting such a failure. This procedure involves

two simple voltage measurements, as outlined below, using the circuit

shown in figure 20. The procedure, as applied to the truck-to-control

house loop (Vg2 ) is:

1. Make the voltage measurement shown in the figure (points

A-A connected). Since in a properly designed choke, Z|_ » Zj^,

most of Vg appears across Z[_ and is transformed 1:1 into the center

conductor, so that

34
Test

Bridge

Choke

Piece

Plugboard ^

Cable

CUUc

03

"O

03

03U

03 a

03oo

-adoo

03

03 o

C3

03d

dO

cc o ^ c

03

-a

03

O

OL ~d E o

-a .2

03 ^

COo

CO o

03

ivj rvi fvi

35

Figure

20.

Coaxial

choke

test
VdVM = ^g2 (Zsh/Zl) = Vg = 0 ( 2 )

where Ve 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 directly

across G 2

- G 3

. Therefore,

^DVM = Vg2 . (3)

Note 1

:

The effect of opening the link A-A can be obtained by

disconnecting the shield from G 3 at B in the control house.

Note 2: A less convenient alternative is to disconnect the test

piece cable at the truck and measure Vg 2 via either the center

conductor or shield (equivalent to step 2 above), then reconnect the

test piece cable and repeat step 1 above. This method was in use in

1979 and in March 1980.

This procedure detects any of the three failure modes, so that

they can be corrected. Thus the only possibility for ground voltage

to introduce error would seem to be for compensation to be intermittent,

i.e., present during the above tests and absent during the affected

measurements. It seems highly improbable that this condition would

persist throughout an entire calibration (May 1979) and be absent

throughout two others. It is arguable that such an occurrence was more likely in 1979 and March 1980 than in December 1980, since (1)

a less convenient procedure for choke testing was used earlier (see

Note 2, above), and (2) the test was performed fewer times (spot

checks), whereas, in December 1980, it was performed at least once after each significant circuit change, and more thoroughly documented.

In any event, the large changes occurred between 1979 and March 1980,

unaccompanied by a change in this test procedure.

In suimiary, none of the known sources of error in the NBS calibration system seemslikely to account for the changes under

investigation here. The quantifiable error sources in table 2 have

been summed, both for "worst-case" and for normal random uncertainty.

Since even the "worst-case" total uncertainty (± 0.08%) is much

less than the changes being sought, these error sources can safely

be dismissed. Even ground voltage, the best candidate because of

its size and the similarity of its "signature," to the actual "signature"

of figure 11 , would seem to be an improbable source because of system

design and test procedures.

36
4.2 The Substation

Error sources which might be associated with the substation

(exclusive of the CCVTs themselves) include: (a) multiple grounds on neutrals; (b) electrostatic or magnetic coupling to leads; (c) burden

changes; (d) temperature coefficients of leads; (e) zero-sequence

currents in the neutral; and (f) bad temporary connections to test

piece terminals in the control house during calibrations. Since the

substation is a complex system not within the general area of NBS

expertise, no claims are made for the completeness of this list.

The items listed will be discussed in order.

4.2.1 Multiple Grounds on Neutrals

Multiple grounds on the neutrals produce errors for exactly the

same reasons as discussed earlier for the NBS system. This is avoided

in the CCYT-to-control house circuit (fig. 18, G3-G4 ground loop)

by running the signal leads as twisted pairs inside a grounded sheath,

and leaving the CCVT end of the neutral ungrounded (X3 in fig. 18).

The first step minimizes the effect of B3, the second eliminates the

ohmic voltage, ig3 Zg3

. If the neutral should be grounded more than

once (e.g., wiring error, or short to the sheath), an active loop,

exactly analogous to the G1G2 loop discussed earlier, is created and

an error voltage equal to Vg3 or some fraction thereof is introduced.

Ohmmeter measurements from neutral to ground with both X3 points

ungrounded demonstrated that no such multiple grounds existed in

December 1980. These measurements were not made during earlier

tests.

4.2.2 Electrostatic or Magnetic Coupling to Leads

Electrostatic coupling is most unlikely if standard practices

are followed. In any event, there seems to be no reasonable way in

which it could have changed between calibrations.

Magnetic coupling or, more precisely, changes in magnetic coupling

can occur, since the magnitude and direction of bus currents and/or

ground grid currents are, in general, different at different times.

However, since it is standard practice to run leads twisted

inside a multiply grounded shield or, more precisely, as multiconductor

cables (fig. 21), with the conductor bundle twisted inside a multiply

grounded conduit, it is difficult to imagine an effective loop large

enough to produce significant coupling. If for some reason (fig. 21)

the X and Y neutrals were interchanged, a much larger loop would be

created, but even this would probably be too small to be significant.

As an extreme hypothetical case, assume that twisted leads, or even misplaced neutrals in figure 21, are separated so as to form a loop

0.3 m X 100 m (!' X 300'). From simple considerations, it can be

calculated that 1 kA in a bus 10 m (30') above, or 10 A ground current

0.6 m (2') away would produce only Vg = 0.25 volts. Coupling in

the actual configuration should be many times smaller.

37
8-Conductor Cable (#8 Wires)

Ph Leads

A X1, X2

B XI, X2

C XI, X2

N X3

S Spare

4-Conductor Cable (#8 Wires)

Ph Leads

A Y2

B Y2

C Y2

N Y3

Figure 21. Disposition of CCVT leeds, inakeup box to control house
It has also been proposed that cross coupling between the

neutral and phase wires, or among phases in intimate proximity inside

the same cable, might produce a significant effect. A calculation

based on two #8 wires 0.5 mm on centers, a length of 200 m (600 ft ),

and a Z-burden current (in one wire only) of 1.6 A, yields = 0.12

volts for untwisted wire. Therefore, this coupling source should be

totally negligible for a twisted wire.

4.2.3 Burden Changes

A burden change betv/een calibrations could easily produce

discrepancies of the order being investigated here. Burden data furnished

by GSU after the May 1979 tests disagree significantly for all nine CCVTs

with burden data actually measured during the December 1980 tests (table 3).

Therefore, the possibility of an undocumented burden change has to be

considered. It can be shown that a burden change as small as 20 VA

resistive in the relaying circuit on A-phase produces a "signature" on the

differences between the changes in the XI X3 and the X2X3 tap ratios which

is formally similar to that extracted for the actual system (fig. 11).

The unbalanced effect is brought about by unbalanced burden currents from

the three phases, returning on a common neutral, and by extra drop in the

A-phase relaying leads (assuming that the burden change is in the A-phase

relaying circuit). The same change could of course also be produced by an equivalent combination of burden changes in all three phases.

An analysis, formally identical to that performed in section 4.7

for a single ground voltage injected into the neutral, can be carried

out to determine what the "signature" of a burden change in one phase

would be. The circuit and the vector diagram for a burden, Zb, added

in the A-phase relaying winding only, are shown in figures 22 and 23,

respectively. For simplicity all other taps are assumed to be

unburdened, and all lead impedances to be equal (Z-j). Vq(= ira^l)

figure 23 is formally identical to Vg in figure 16. Note that the only

difference in the vector diagrams of figures 16 and 23 is that Vq adds

twice (2 Vg) to the relaying voltage in A-phase.

Carrying out the same analysis as before, the "signature" of the

burden change postulated in terms of the aRCF and Ay of figure 11 again yields three vectors at 120® to each other as expected. But the

magnitude of the A-phase vector is approximately 3.25 times greater

than the magnitude of the B- and C-phase vectors, whereas the magnitude

of the figure 11, A-phase vector is only 1.3 times greater, nearly a 2:1

discrepancy.^ Once again, the "signatures" do not match.

^The detailed analysis proves tri gonometrical ly what could be inferred

intuitively--Va = 1.8 therefore, any common influence on ratio

(hence RCF, y) is 1.8 times greater on the relaying tap than on the

metering tap. Vg appears twice in the relaying circuit so that its total

effect is 3.6 times greater. The difference is (3.6-1) = 2.6. By

identical reasoning for the other phases, the difference is (1.8-1) = 0.8,

so that 2. 6/0. 8 = 3.25.

39
Table 3. Total Burdens on Calibrated CCVTs at

Willow Glen

VA VA

(Furni shed ( Measured

by GSU) by GSU)

Li ne Phase 1979 Dec. 1980

Frank! i

n

A 37 /-57 50 /-27

Franklin 3 108 57

F rankl i

n

C 86 7^ 57 737

Gypsy A 83 /-56 49 /-29

Gypsy B 104 7^ 58 7^

Gypsy C 86 /-45 46 /-52

Webre A 83 /-56 133 /-41

Web re B 104 /-37 125 /-54

Webre C 86 93 737

40
Metering

—

cac > E O 0)

41

Figure

22,

Circuit

diagram,

burden

added

to

A-phase

relaying

winding

only.

Note--Band

C-phase

relaying

windings

not

shown
Figure 23. Vector diagram, burden added to A-phase relaying winding

only

42
In addition, in order to obtain the figure 1 grouping by phase,

a similar A-phase burden change would have to occur on all three lines.

Thus, like other plausible explanations for the subject discrepancies,

this one also seems improbable.

4.2.4 Temperature Coefficients of Leads

The long runs (200-300 m) of #8 copper wire involved contribute

significant voltage drops (order of 0.8% for 100 VA burden). However,

since the total lead drop is less than three times the unknown voltage

under investigation, temperature effects are negligible. Simple

calculations show that no imaginable temperature change could explain

the changes sought.

4.2.5 Zero-Sequence Currents in the Neutral

In a perfectly balanced four-wire three-phase circuit (e.g.,

three CCVTs with the same burdens and line voltages) no net neutral

current flows. If the impedances or the voltages become unbalanced,

neutral current will flow. The unbalanced impedance case has already

been dealt with (burden change, one phase), and it was shown that net

neutral current perturbs the CCVT ratios in all three phases. In practice, line voltages can also become unbalanced. The effect can

be determined by perturbing one of the source voltages, e.g., V^, in

the circuit of figure 24.

In figure 24, which shows the metering windings only, for simplicity,

assume that all three phase voltages have the same magnitude, that all

three phases have the same burden, Z3 , and that all lead impedances are

equal (= Z] ) and much smaller than Z3

. Writing and solving the mesh

equations for V 3 , V 5 , and V^., and taking the appropriate partial derivatives with respect to V^, it can be shown that

3Fa/3Va = Zi/(V3Z3)

where Fg = ratio correction factor, phase A. Similarly

(4)

3Fb/3Va = Zl/(VbZ8 ) 120° (5)

3Fc/3Va = Zi/(VcZb) -120° ( 6 )

Assuming some rather extreme values for voltage unbalance, burden,

and lead impedance, let AVg/Vg = +5%, Z3

= 70 n (Z- burden), and

Z] = 1 and find that

43
N

1—>

O

o

w

ccc

£

0)

44

Figure

24.

Circuit

diagram,

balanced

voltages

and

impedances,

for

study

of

the

effect

of

unbalancing

one

phase

voltage
( 7 AF 2 + 0.05 (_L) = 0.07% ) 3 70

aF = 0.07% (cos 120<’ + j sin 120'’) (8)

b

aF = 0.07% (cos 120'’ - j sin 120'’) (8A)

c

so that aF^ = 0.035%, Arb =0.35 /J mrad, etc. Thus even

very large voltage unbalances operating on large burdens and

large lead impedances produce very small effects. More reasonable

values might be AVg = 2%, Z] =0.6 ohm, Z3 = 100 ohms, for

which AFg = 0.024%. Therefore, reasonable zero-sequence currents

could not produce the effects seen here.

4.2.6 Bad Temporary Connection of Cable Shield

in Control House

This error source was mentioned briefly in conjunction with the

discussion of ground voltage errors due to open signal cable shields

in the NBS system. Although a bad shield connection could occur

elsewhere, it is less likely. All the other shield connections are "permanent," i.e., are made with UHF screw-connection fittings, and

made once during the calibration of a phase, whereas the control

house shield connection is moved at least once for every test piece.

Also, whereas a bad connection to the center conductor will be apparent

immediately at the bridge, a bad or open shield connection will not.

Because this connection is moved, a proper shield connection as shown

by a coaxial choke test (fig. 13), can be lost when the connection is

moved during calibration.

Bad shield connection can take the following forms: (1) failure

to make the shield connection; (2) clamping the clip lead on wire

insulation or terminal board insulation; or, most likely, (3) high

contact resistance - due to corrosion or to the panel hardware having

been sprayed with an electrically insulating corrosion inhibitor.

As pointed out earlier, such bad contacts do not have to be very

"bad" (50-100 is sufficient). Contact resistances in the kilohm

range are common for corroded contacts. Furthermore, since the

contact voltage is less than one volt, even a very thin oxide or sprayed plastic film can produce an insulating contact. Contributing

to the possibility of making bad shield connections are the conditions

under which these connections are made - cramped, poorly lighted

locations, and closely adjacent terminals.

45
Bad shield connection has been offered as a possible explanation

because it can produce spurious voltages of the proper magnitude and

signature. However, it does not seem likely that bad shield connections I could have been made consistently throughout a whole series of measurements.

|

j In summary, as for the NBS calibration system, it seems improbable

that any of the individual substation error sources considered caused i the apparent discrepancies in question. Even burden change or bad I shield connections would have required a very unusual concentration ' of errors and/or bad luck in order to have been the culprits. j

I 4.3 The CCVTs I

The remaining possible source of the apparent discrepancies is

| the CCVTs themselves. NBS experience has shown a significant proportion I of these devices to be out of metering tolerance. Thera is also

evidence from other sources [4-6] that CCVTs are subject to seasonal

and even diurnal cycling of 0.2 percent or more, probably temperatureinduced. Regression analysis by one utility [7] of a sizeable amount

of data on a dozen CCVTs showed a mean temperature dependence of about

-90 ppm/°C, but with a spread of 5:1 in this dependence.

A common failure mode is the shorting of an individual capacitor

in the modular stack. This produces an RCF decrease of about

0.4 percent for a 500 kV CCVT if it occurs on the high side of the

capacitive divider. The data indicate that ratio decreases of this

general magnitude did occur in some of the GSU devices between the

first and second measurements. However, if such shorts occurred in

the GSU devices, they seem to have "healed" later (see the discussion

of fig. 15, section 3.2). Such "healing" seems improbable. Some of

the GSU CCVTs are more than 10 years old. Perhaps old CCVTs are subject to minor failures of unspecified or unexpected origins.

For example, in the December 1980 i ntercompari sons at GSU, there was

at least one instance in which a relative shift of >0.1 percent

occurred between two CCVTs in a period of 10-15 minutes. The only

known relevant event during that time was line switching to place

the NBS divider on the 500 kV bus. Although it seems improbable, it

has been suggested that some phenomenon related to a system fault

which affected one phase more than the other two may have caused

values in that phase's CCVTs to shift together, or that line

switching affects the value of the NBS divider or the CCVT (see the

note on the title page). I

5. SUMMARY OF RESULTS

From the evidence available, it appears likely that a voltage

of the order of 0.3 volts, approximately 180® out of phase with

A-phase, may have been present during the May 1979 measurements.

Since the March and December 1980 results are in much better agreement,

it is postulated that this voltage was not present during the last

46
two sets of measurements. This postulate is reinforced by the fact

that adjusting the 1979 data for the effect of this voltage makes

those data agree much more closely with the later measurements, and

to fall with them into a general pattern observed at another

substation (significant shifts between calibrations, almost solely

in RCF).

An exhaustive search for the apparent discrepancies, including

extremely careful and much more extensive measurements in

December 1980, has not yielded a specific cause for them. Two suspects,

uncompensated ground voltage and a moderate burden change (20 VA) on the A-phase devices, were singled out for special attention because

they have about the right "signature" and magnitude. However, the

combination of circumstances required to indict either of these two

seems improbable. It is believed that no reasonable combination of

other known possible error sources could produce differences of

the magnitude seen here. Because of the seeming improbability of

all the other causes explored, even the possibility that the CCVTs

themselves changed randomly in a seemingly non-random manner cannot

be dismissed.

Some conclusions can be drawn. First, in all three of these

measurement sets, many of the CCVTs were out of metering tolerance,

some by large amounts. Second, further investigation is called for,

including the effects of high voltage switching on the NBS divider,

and possibly some form of continuous monitoring of a statistically

significant number of operating CCVTs.

6. REFERENCES

[1] A Prototype System for On-Site Calibration of Coupling Capacitor

Voltage Transformers (CCVTs), D. L. Hillhouse, 0. Petersons,

and W. C. Sze, PAS Transactions, Vol . PAS-98, No. 3, May/June 1979,

p. 1026,

[2] A Prototype Field Calibration System for Coupling Capacitor

Voltage Transformers (CCVTs) EPRI EL-690, April 1978.

[3] Requirements for Instrument Transformers , ANSI/IEEE C56.13-1978.

[4] Stability of Capacitor Voltage Transformers for Revenue Metering,

K. M. Mitchell, Ontario Hydro Research Quarterly, Vol. 24, No. 2,

1972, pp. 15-19. ~

[5] Tests Monitor Accuracy of EHV Metering Devices, Electrical World,

Jan. 29, 1970, pp. 40-41.

47
[6] Field Testing of 500 kV Metering Accuracy Coupling Capacitor Voltage

Transformers , C. E. Phillips, Pacific Coast Electric Association

Engineering and Operating Conference, San Francisco, 1974.

[7] In-Service Performance of Capacitive Potential Devices Used for

Revenue Metering, T. A. Dahlke, Minutes of the Meter and Service

Committee, AEIC-EEI, Appendix 0, Sept. 1970.

48
NBS-n4A iREv. 2 «« 0 ) U.S. OEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3. NBSIR 81-2360

Publication Date October 1981

Analysis of the Calibration of Metering CCVTs in a Utility Substation

5. AUTHOR(S)

D. L. Hillhouse and D. A. Leep

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE S. Type of Report & Period Covereo

WASHINGTON. D.C. 20234

Final

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

Electric Power Research Institute National Bureau of Standards

P.O. Box 10412 Viashington, D.C. 20234

Palo Alto, California 94313

10.

SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, Is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant in formation. If document Includes a significant

bibliography or literature survey, mention it here)

This report presents the results of an investigation of unexpected variations in nine

500 kV metering CCVTs, tested for Gulf States Utilities at Baton Rouge, LA. These

measurements were performed on three occasions - May 1979, March 1980, and

December 1980.

On the first two occasions, six out of nine CCVTs were out of tolerance; on the third,

four out of nine. More important, the changes between the first two occasions seemed

to be correlated by phase, i.e., most of the devices on a given ohase shifted in the same general direction and by similar amounts. When analysis failed to provide an explanation for this, the third set of measurements was undertaken.

Analysis of the three sets of data produced some evidence of a bias voltage in the

1979 data. Investigation of all plausible known sources of error in the NBS system,

the substation, and the CCVTs themselves failed to produce a probable source for such

a bias voltage.

No evidence of consistent malfunction of the NBS system was found. Even allowing

for a possible bias in one set of data, a majority of the CCVTs were still outside ct

metering tolerance. Continuous monitoring of a statistically significant number of

operating CCVTs should be considered.

12.

KEY WORDS (Six to twelve entries; alphabet) cal order; capitalize only proper names; and separate <ey words by semicolons)

calibration; CCVT; EHV substations; error sources; high voltage measurements;

revenue metering.

I

13. availability

U nlimited

I

i For Official Distribution. Do Not Release to NT1S

'

i Order From Superintendent of Documents, U.S. Government Printing Office, Washington, O.C. 20402.

I XX Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

printed PAGES

30

15. Price

S9 .00

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