Effects of High-Voltage Switching

on the EPRMMBS Coupling

Capacitor Voltage Transformer

(CCVT) Calibration System

Standard Divider

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and

Electrical Engineering

Electrosystems Division

Washington, DC 20234

March 1 983

Soonsored in part by:

Electric Power Sesearch Institute

Palo Alto, California 94304

.NATIONAL BUREAU

OF STANDARDS

LIE RAF. y

NBSIR 83-2666

APR

fieri- a Ci EFFECTS OF HIGH-VOLTAGE SWITCHING

ON THE EPRI-NBS COUPLING

CAPACITOR VOLTAGE TRANSFORMER

(CCVT) CALIBRATION SYSTEM

STANDARD DIVIDER

David L. Hillhouse

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and

Electrical Engineering

Electrosystems Division

Washington, DC 20234

March 1 983

Sponsored in part by:

Electric Power Research Institute

Palo Alto, California 94304

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

NATIONAL BUREAU OF STANDARDS, Ernest Ambler. Director

8 1983

TABLE OF CONTENTS

Page

LIST OF TABLES iv

LIST OF FIGURES iv

Abstract 1

1. INTRODUCTION 1

2. FIRST WALTZ MILL TESTS 2

2.1 Waltz Mill Facility 2 2.2 General Test Procedure and Circuits 2 2.3 Detailed Test Procedure 9

2.4 Test Results 9

3. NBS TESTS 11

3.1 Grounding Switch Operation 11

3.1.1 Discussion of Tests 11 3.1.2 Summary 17

3.2 High-Voltage Switching 17

4. ELIMINATION OF TRAPPED CHARGE 20

4.1 Divider Modification 20

4.2 Tests of the Modified Divider 20

5. SECOND WALTZ MILL TESTS 22

5.1 Test Procedure and Circuits 22

5.2 Test Results 22

6. SUMMARY OF RESULTS 26

7. RELATION OF RESULTS TO THE GSU CCVT CALIBRATIONS 26

7.1 Introduction 26

7.2 Assumptions 26

7.3 The "V- Effect" 26

7.4 Applying the Probable Offset 28

7.5 Comparing Consecutive Switching Operations on the Same Phase . . 28

8. DISCUSSION AND SUMMARY 31

9. REFERENCES 32
LIST OF TABLES

Page

Table 1. Standard divider ratio deviation due to high-voltage

switching, first Waltz Mill tests (V = 300 kV) 12

LIST OF FIGURES

Page

Figure 1. Simplified diagram of the Waltz Mill EHV-UHV test circuit . . 3

Figure 2. Step 1, calibration of Waltz Mill 1500 kV compressed gas

standard capacitor, C$w 4

Figure 3. Step 2, measurement of the NBS standard divider ratio,

PSD, using the results of step 1 5

Figure 4. Step 3, measurement of the Waltz Mill 1100 kV monitoring

CCVT ratio, Pc , using the results of step 1 6

Figure 5. Step 4, comparison of the NBS standard divider ratio and

the Waltz Mill monitoring CCVT ratio, using the simplified

system's voltage comparator 7

Figure 6 . Basic circuit of the NBS capacitive standard divider .... 10

Figure 7. Circuit for NBS grounding switch operation tests of

one-module standard divider 13

Figure 8 . Trapped dc voltage histogram, one-module divider 15

Figure 9. Trapped dc voltage histogram, one-module divider,

second manufacturer 16

Figure 10. Standard divider ratio offset for dc voltage (1) trapped

by grounding switch operation; ( 2 ) applied by dc power supply;

and (3) for ac voltage applied to low-side capacitor C 42

(regression fit, power curve, Apsq/p5q

= aV b

) 18

Figure 11. Ratio offset histogram, one-module divider 19

Figure 12. Divider modification to eliminate trapped dc voltage .... 21

Figure 13. Step 4', comparison of the NBS standard divider ratio and

the Waltz Mill monitoring CCVT ratio, using the current

comparator 23

Figure 14. Shot-to-shot deviation of standard divider ratio,

50 disconnect switch operations, average of direct

and indirect measurements 25

iv
LIST OF FIGURES (cont.)

Page

Figure 15. "Revised" history of X1X3 RCF and y, May 1979 to

December 1980, GSU calibrations 27

Figure 16. GSU "revised" March 1980 and December 1980 results

adjusted upward by 0.2% (1979 results assumed correct) ... 29

Figure 17. Calculated vs measured changes in RCF and y, for

one phase, from MUB to HSE, GSU, December 1980 30

v

EFFECTS OF HIGH-VOLTAGE SWITCHING ON THE EPRI-NBS

COUPLING CAPACITOR VOLTAGE TRANSFORMER (CCVT)

CALIBRATION SYSTEM STANDARD DIVIDER

David L. Hillhouse

Abstract

This report presents the results of tests of the effects of high-voltage

switching on the EPRI-NBS CCVT calibration system's capacitive standard divider,

completing an investigation stemming from the results of three calibrations at a Gulf States Utilities substation.

Initial tests consisted of full-scale (300 k V ) switching operations at

EPRI's Waltz Mill test facility, during which the divider exhibited significant

ratio offsets (average = +0.2%, maximum = nearly +0.5%). Tests were continued

at the National Bureau of Standards (NBS), where it was determined that

operation of a grounding switch, installed to protect the divider low side

during high-voltage switching, caused ratio offset by trapping charge on the

divider's low side capacitor. This resulted in residual dc voltage which

changed the value of the divider's low-side capacitance. The addition of a bleeder resistor eliminated the problem in the laboratory. With the bleeder

resistor in place, the Waltz Mill tests were repeated. Fifty high-voltage

switching operations indicated a negligible shot -to-shot variation

(average = -lppm, a = 80 ppm).

No obvious correlation was found between the ratio offsets described above

and the results of CCVT calibrations performed while the effect may have been

present in the divider. Experience indicates that a significant proportion of

calibrated CCVTs is outside metering tolerance. Long-term simultaneous

monitoring of a sizeable number of CCVTs is suggested.

Key words: calibration; capacitive divider; CCVT; error sources; high-voltage

measurements; high-voltage switching; ratio offset; trapped charge; Waltz Mill

tests.

1. INTRODUCTION

The work reported here represents the completion of an investigation,

co-sponsored by the Electric Power Research Institute (EPRI), into some unexplained results obtained from three calibrations at a Gulf States Utilities

(GStJ) substation [l].l These results consisted of changes in ratio and phase

angle between the first two sets of calibration values which 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. Seeking to clarify this, the

third set of measurements was undertaken.

^Numbers in brackets refer to the literature references listed at the end

of this report.

1
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. The analysis did suggest

the possibility that high-voltage switching^ might affect the divider ratio.

Since the NBS laboratories do not have the capability to perform fullvoltage switching on the 500 kV (300 kV to neutral) divider, such tests were

done at the EPRI Waltz Mill Underground Transmission Test Facility, Ruffs

Dale, PA. The initial tests, in October 1981, were followed up with further

tests at NBS and a return to Waltz Mill in April 1982.

2. FIRST WALTZ MILL TESTS

2.1 Waltz Mill Facility

A simplified circuit diagram [2] of the Waltz Mill facility is shown in

figure 1. Briefly, this facility can supply 60 Hz voltage to 1100 kV, in a step-up from utility-furnished 138 kV. The 1100 kV bus contains motor-driven

disconnect switches, allowing simulation of the switching environment encountered

by the standard divider in an operating substation. Voltage ratios can be

measured very accurately using a 1500 kV compressed gas standard capacitor

which is connected permanently to the 1100 k V bus. The voltage can also be

an 1100 kV CCVT.

2.2 General Test Procedure and Circuits

monitored via

The high

fol 1 owing pri i

Step 1 .

Step 2.

Step 3.

Step 4.

The four

Calibration of the Waltz Mill 1500 kV compressed gas standard

capacitor, C$w (fig. 2), using NBS Standard Capacitor, C$y,

and the current comparator bridge;

the current comparator bridge (fig. 3);

(fig. 4); and

Comparison of the standard divider and CCVT ratios using the

simplified system voltage comparator [3-5] to cross check

steps 2 and 3 (fig. 5).

C$

w

as the on-line reference for the bus voltage. Step

Step 1 establishes

2 measures p^q

directly, using accurately known standards C$m and C$w. Step 3 obtains the

ratio Pq of the monitoring CCVT in the same manner, and step 4 compares Pq

^Closing a motor-driven disconnect switch in order to connect the divider

to the 500 kV bus or opening it to disconnect the divider from the bus.

2
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the

Waltz

Mill

EHV-UHV

test

circuit
Waltz

Mill

Bus-100

kV 4

Figure

?..

Step

1,

calibration

of

Waltz

Mill

1S00

kV

compressed

gas

standard

capacitor,

05^
Disconnect

Waltz

Mill

Bus-300

kV

Switch 5

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3.

Step

2,

measurement

of

the

NBS

standard

divider

ratio,

PSP,

using

the

results

of

step 1
Disconnect 6

Figure

4.

Step

3,

measurement

of

the

Waltz

Mill

1100

kV

monitoring
Wailz

Mill

Bus-300

kV

— o>

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Step

4,

comparison

of

the

NBS

standard

divider

ratio

and
and Combining steps 3 and 4 yields psd indirectly. This indirect

value should agree with the direct value from step 2. Combining this with

the repetition of step 2 at the end of the sequence keeps the entire measurement

in control by forming a closed set, i.e., steps 2, 3, 4, 2. The complete

set was obtained initially, and after most disconnect switch operations.

In step 1 (fig. 2), the current comparator reads the ratio C^j/C sw

= M

2 W /p2W . The value of C$y, determined earlier in an identical comparison with a low

voltage reference standard (C 5 M

= 1000.00 pF), is then used to calculate

C 5 W. The value of 0$^ (103.155 pF) was determined twice -- at the beginning

of the five days of tests and at the end. The two values agreed to within

50 ppm (parts per million).

In step 2 (fig. 3), standard divider ratio P SD is determi ned di rectly

from the current comparator multiplier and ratio product readout (^ 3 ^3 ^/) and the previously known value of C$m and C$w» In step 3 (fig. 4), the

ratio Pq of the CCVT is determined in an identical manner.

In step 4 (fig. 5), the voltage comparator reads out p qV p$d = Pc 4

where is the CCVT ratio p q, modified by the ratio of the 120:277

low-voltage standard transformer (precisely, by 0.43344), and is the

ratio setting of the comparator. From this the indicated ratio of the divider

can be computed as 0.43344 Pq/Pq^. The standard transformer is required

in order to bring the nominal 5000 : 1 ratio of the CCVT to within the range

(1.11111:1) of the voltage comparator. For full discussion of this limit on the voltage comparator, see [5],

The usual procedures for a field calibration were followed, i.e., the

divider was calibrated in the NBS laboratory at 100 kV, and re-cal i brated at

100 kV upon arrival at the site, using the power supply aboard the truck [4].

This calibration was repeated during the week, at the end of the tests, and

again afterward in the NBS laboratories. The spread of all of these values

was approximately ±0.02% from the initial Waltz Mill value, which was taken

as arbitrary reference.

A change in divider ratio attributable to high-voltage switching was considered to be improbable for the following reasons: ( 1 ) field calibration

of highly accurate inductive voltage transformers (VTs) had agreed with NBS

calibration of the same devices in the factory to about 0.01% ([3], p. 8-15);

(2) in a calibration at another substation shortly before the third GSU tests,

NBS results for two CCVTs recently returned from the factory agreed with

factory calibration results to within ±0.02%. In order to obtain a reasonable

estimate of the limiting probability of the occurrence of a high-voltage

switching effect within the limits of available testing time, up to 50 switching

operations were planned. It can be shown that 50 tries with no occurrence

of an event indicate a 99.5% confidence 3 that the event occurs at most

one time in 10, and an 82% confidence that it occurs at most one time in 30.

3 P' = 1-(1-P) n

, where ?' is the above percentage, P is the probability

of occurrence guarded against, and n is the number of operations.

8
2.3 Detailed Test Procedure

After the preliminary calibrations alluded to earlier, the following

switching procedure was planned:

1. Close the divider low-side grounding switch S3, to protect the

auxiliary divider (see fig. 6, and discussion below);

2. Turn on the Waltz Mill power, raise voltage to 300 kV

(500 kV 1 i ne-to-1 i ne)

;

3. Open grounding switch S3;

4. Obtain a reference value of p^q, via steps 2 through 4 (sec. 2.2);

5. Close grounding switch S3;

6. Open, then close, the motor-driven disconnect switch on the

300 kV line; and

7. Repeat (3) through (6) above, up to 50 times.

The function of grounding switch S3 in (1), (3), and (5) above will be

discussed here, since it is a key element in this investigation, as will be seen later. Refer to figure 6, which shows the basic circuit of the divider.

The purpose of S3 is to protect the auxiliary divider from surge voltages

while the divider is being connected to or disconnected from the high-voltage

bus. When S3 is closed, it completes a low-inductance path around the low side

of the standard divider via a copper strap 40 mm wide by 2 mm thick. This

configuration was incorporated into the system in 1979, after some auxiliary

divider capacitors were damaged during switching.

2.4 Test Results

Contrary to expectations, some divider ratio offset was produced with

nearly every high-voltage switching operation. These offsets ranged up to

0.32%. After a number of such switching operations on the three-module divider

at 300 kV, other tests were performed. These included switching at lower

voltages, switching of single-module dividers at 100 kV, and operation of

the low-side grounding switch (S3, fig. 6) with the divider remaining energized

at 300 kV. Ratio shifts were found under all the conditions listed above.

A summary of these results at 300 kV is shown in table 1, both for

high-voltage switching (which includes grounding switch operation) and for

grounding switch operation only, together with average values and average

(+) and (-) values. Note that two-thirds of the values, including almost

all of those greater than 0.1%, are (+). The data taken at Waltz Mill

did not reveal the source of this effect, so further investigation was undertaken later at NBS.

As reported earlier, the before-and-after measurements of 0$^, the

Waltz Mill 1500 kV standard capacitor, agreed to within 50 ppm. In addition, the divider ratio Pjq, as obtained directly via measurement

step 2, was compared with the indirect value obtained via steps 3 and 4 for

9
V^Std.

Transformer

or

Line

Voltage

C .T

10

Figure

6.

Basic

circuit

of

the

NBS

capacitive

standard

divider
about 20 data points. The two sets of values agreed to about 0.01%, with

a standard deviation of about ±0.02%. This indicates that the Waltz Mill

measurements were probably under control to better than 0.06% (three

standard deviations).

3. NBS TESTS

Available voltage and space limited NBS testing to 50 kV on a one-module

divider for high-voltage switching simulating the substation disconnect

switch, and to 100 kV for grounding switch operation. This represented 50%

and 100% of rated voltage stress, respectively. However, the Waltz Mill

results indicated that these stress levels were high enough to produce the

unexplained ratio effects. This permitted laboratory investigation of the

high-voltage switching effect instead of far more expensive and inconvenient

full-scale field tests at Waltz Mill.

3.1 Grounding Switch Operation

3.1.1 Discussion of Tests

Because a correlation between grounding switch operation and divider

ratio offset was evident at Waltz Mill (table 1), a peculiar surge behavior

of the divider low side (fig. 6 ) was suspected, either in capacitor C 42 or

in the auxiliary divider. The test circuit for investigating this is shown

in figure 7. The procedure consisted of applying 100 kV, balancing the

current comparator, closing grounding switch S3 for a few seconds, opening

S3, then rebalancing the current comparator and measuring any change, Ap

so>

in P $n .

As at Waltz Mill, almost every grounding switch operation produced a significant ratio offset. After approximately 20 operations, the auxiliary

divider was disconnected at terminal 4' (see fig. 7), and low-voltage

standard replaced with 20 kV standard Cj, as shown dotted on the

figure. The effect was unchanged, indicating that the problem was not in

the auxiliary divider.

At this time, a residual dc voltage was discovered on C 42 after completing

a measurement. A different dc voltage, associated with a different divider ratio

offset, was then found each time . 4 Removing the dc voltage by discharging

C 42 removed the ratio offset. Thus it became evident that a major cause of

divider ratio offset was simply the voltage dependence of the capacitance of C 42 .

The mechanism for charge trapping, resulting in the residual voltage

above, can be explained by reference to figure 7. If switch S3 is opened

while ac voltage is applied to the divider, whatever voltage is present on

C 42 when the switching arc is extinguished remains there, subject to available

discharge paths. Since these paths are all capacitive, even with terminal 1 safety-grcunded, the voltage can decay only through the very high leakage

4This ratio offset was always positive, although both positive and negative

voltages were found.

11
Table 1. Standard divider ratio deviation due to

high-voltage switching, first Waltz Mill tests

(V = 300 kV)

Disconnect switch operation Grounding switch operation, only

Operati on Ac

sn/ p sn Operation Ap

sd/

psd

No. (percent) No. (percent)

1 0.18 1 0.20

2 0.20 2 0.19

3 0.06 3 0.26

4 0.06 4 0.20

5 -0.06 5 0.16

6 0.21 6 -0.01

7 -0.06 7 -0.02

8 0.23 8 0.11

9 -0.07 9 0.19

10 0.20 10 0.08

11 -0.02 11 0.16

12 -0.13 12 0.30

13 0.32 13 0.22

14 0.10 14 0.27

IS -0.08 15 0.10

16 -0.04 16 -0.07

17 0.13

Average 0.08 0.15

Average (+) 0.17 0.19

Average (-) -0.07 -0.03

Overall Average 0.11

Overal

1

Average ( + ) 0.18

Overal

1

Average (-) -0.06

12
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grounding

switch

operation

tests

of

one-module

standard

divider
resistances of these paths. This was confirmed by leaving the trapped

dc voltage on and tracking its decay. An equivalent time constant of the order

of three days was found. The decay was not a simple exponential, but was considerably faster during the first few hours (equivalent T = 25 hours). This

suggests a nonlinear, perhaps voltage-dependent, leakage resistance. A time

constant of three days implies a leakage resistance of about 1.7 x 10*1 n, and of 25 hours, about 6 x 10^ n.

In order to relate the above observations to the Waltz Mill tests, refer

to section 2.3. Note that the Waltz Mill routine (identical in this respect

to the routine followed in a regular CCVT field calibration) always included

opening the grounding switch with voltage on the divider. Therefore, trapped

charge would be expected each time the divider was connected to the high-voltage

bus. This appears to explain, at least qualitatively, the Waltz Mill results.

Furthermore, since the grounding switch is always closed before removing the

divider from the line during any full -voltage test, this voltage is always

removed and would therefore never be found during normal substation testing.

In relating the measured dc voltages to the ac voltage applied to C 42 , a purely capacitive circuit was postulated. It can be shown that in such a network the residual dc voltage should equal the peak value of the interrupted

ac wave. A few data points sufficed to show that this voltage was distributed

all the way from essentially zero to the peak value (as table 1 already

imp! i es )

.

In order to obtain the distribution and average value of this residual

voltage, 150 grounding switch operations were performed at 100 kV on a one-module divider, for which voltage stress was the same as at 300 kV on the

full divider. Figure 8 shows a histogram of the magnitude of the residual

dc voltage for these tests. The mean is 910 V, or about 89% of the average

value of the ac voltage (1020 V) applied to C 42 at 100 kV on the divider . 3

If the dc voltage were randomly distributed, its mean value would equal the

average value of the sine wave. Therefore, the residual voltage does not

appear to be truly randomly distributed.

A smaller number of grounding switch operations (51 values) was performed

on a very similar one-module divider manufactured by a different company.

A histogram of the magnitude of the residual dc voltage for this divider is

shown in figure 9. The mean value is higher and closer to that for the

average value of a sine wave. The distribution is considerably different

also, with a significantly larger proportion of values near the peak. Since

the purpose of these tests was to verify the existence of the effect and to

delineate it, not to investigate the physics of it, the reasons for the

unexpected distributions and for the differences between the two modules

were not investigated.

"T for a one-module divider = 88 , so that is approximately (10 5 /88)/2 = 1610 V.

'^avg

=

( 2 /tt )

V

peak - 1^20 V*

14
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Values

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=

89%

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Value

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voltage

histogram,

one-inodule

divider
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In addition to the grounding switch operations, ratio offset was also

affected by applying positive and negative dc voltages with a dc power supply,

and ac voltage up to the highest dc voltage trapped during the grounding switch

operations (=1600 V rms). The results are summarized in figure 10, which

shows power curves, Ap

$|}/ psd = aV » fitted to the data. Both the dc and

ac voltage data formed very smooth curves, as their R

2 (coefficient of

determination) values indicate. For a perfect fit, R^ = 1. Greater scatter

was present in the grounding switch operation data, as reflected in the

smaller R 2 value, but even here the maximum scatter of Ap

sp/

Psn was ^ GSS

than ±0.05% from the fitted curve. Most important, note that the power

coefficient b equals approximately two in all cases. This conforms to the

square-law increase in capacitance expected due to decrease in the distance

between the plates under the action of electrostatic forces, when the capacitor

is operating well above resonance [6], For this case, C = C 0 (1 + aV 2),

which C

Q

is the capacitance at zero voltage, and aV 2 corresponds to aV° above,

with b = 2. Since all CCVT capacitors are of paper-oi 1 -foi 1 construction,

they form non-rigid mechanical systems, whose resonant frequency should be

well below 60 Hz. The corresponding curves for the other manufacturer ' s one-module divider were almost identical to those in figure 10.

3.1.2 Summary

Figure 10 indicates that ratio errors approaching +0.5% (there are no negative errors) can be introduced into the standard divider at 500 kV by the

operation of the grounding switch. Figure 11 shows a histogram for Ap

$o

/

PSD

comparable to that shown for voltage in figure 8. The tabulation on figure 11 shows a generally decreasing distribution as the ratio offset increases. If

this distribution holds in the field, the probable offset is approximately

0.17%. The probability of either very large (>0.4%) or very small (<0.05%)

offsets is relatively small. Since at least three switching operations

are performed during a CCVT calibration series (one per phase), it seems

to be highly probable that an offset at least equal to 0.17% would be

introduced at least once per calibration (p = 1-0 .

5

3 = 0.875).

After completing the grounding switch operation tests, it was believed

that trapped charge was the major cause for the ratio offsets seen at Waltz

Mill. However, some doubt remained because of some unexplained data. Note

from table 1 that ten negative values were obtained. It can be argued

plausibly that those smaller than -0.06% (four values) are within measurement

uncertainty. The other six values, particularly disconnect switch operations

Nos. 12 and 15, remain. They suggest the possibility that some other mechanism

also operates during high-voltage switching. This was Investigated, as described below.

3.2 High-Voltage Switching

As indicated earlier, available voltage and space allowed high-voltage

switching to be performed only up to 50 kV, on a one-module divider. This

is only half the voltage stress encountered in a 500 kV CCVT field calibration.

The test circuit for high-voltage switching was the same as for the grounding

switch operations (fig. 7), with dotted capacitor Cj in place, the auxiliary

divider disconnected, and the resonant power supply replaced by a testing

transformer. The disconnect switch consisted of a hinged aluminum rod

extending from the transformer corona ring to the divider corona ring.

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19

Figure

11.

Ratio

offset

histogram,

one-module

divider
The switch was operated manually by a rope-and-pul 1 ey arrangement. In a procedure similar to that at Waltz Mill, the divider ratio was measured, the

disconnect switch opened for a few seconds, then reclosed, and the divider

ratio remeasured.

After eliminating the anomaly discussed in the following paragraph,

approximately 140 switching operations were performed at 50 kV with no ratio

offsets. Based on the earlier discussion of failure probability (see footnote 3),

140 failure-free operations indicate at least a 99.9999% confidence that the

failure rate is <0.1 (1 in 10), 75.5% that it is <0.01. Therefore, it appeared

that high-voltage switching did not cause ratio offset, at least up to half voltage,

or 50 kV/module.

The following observations should be taken into account in any situation in

which measurements with circuits such as this are made in conjunction with highvoltage switching. Five of the first 20 switching operations produced significant

ratio offsets. Investigation traced these to the same residual dc voltage on

C 42 as had been found for grounding switch operations. However, the grounding

switch had not been operated. Further investigation showed that removal of the

20 kV standard capacitor (Cj, fig. 7) eliminated the problem. The only reasonable

mechanism seemed to be flashover of that capacitor, which would act in the same manner as operating the grounding switch. This would, in turn, require excessive

voltage in the capacitor loop. Reduction of a long ground lead to a very short

length eliminated the problem, with Cj remaining in the circuit. Evidently,

switching surge voltages, even at 50 kV, induced ground loop voltages large

enough to spark over a 20 kV capacitor.

4. ELIMINATION OF TRAPPED CHARGE

4.1 Divider Modification

Elimination of the trapped charge in the divider is accomplished as shown

in figure 12. It consists of the addition of bleeder resistor R 42 across C 42 . A 10.5 Mq value was chosen, using stock metal film resistors. This produces

a 16-second time constant (C 42 + C^d = 1.52 yF) for bleeding the trapped

charge away. Then the maximum possible ratio offset of 0.5% will be reduced

to 0 . 01 % in one minute, a negligible time in terms of the usual several

hours on-line. R 42 introduces a phase shift of 1 / ojR (

C

42 + C/\g) = 0.17 mrad

into the divider. This, in turn, is compensated for by 2-Mft resistor R^,

placed across C ^3 in the high side of the auxiliary divider.

4.2 Tests of the Modified Divider

A total of 52 grounding switch operations, performed as described in section

3.1.1, produced a mean ratio drift of +12 ppm, with a standard deviation of 60 ppm.

This gives at least a 65% probability that a significant offset will be seen no more than 2% of the time.

20
o

1m o

21

Figure

12.

Divider

modification

to

eliminate

trapped

dc

voltage
5. SECOND WALTZ MILL TESTS

As noted earlier, high-voltage switching was carried out in the laboratory

only up to SO k V on a one-module divider, or to half the voltage stress

encountered in the field. Since it did not seem prudent to extrapolate these

results to full voltage, a second series of tests was carried out at Waltz Mill.

5.1 Test Procedure and Circuits

The measurement procedure was the same as for the first tests (sec. 2.2),

except for the fourth step. The procedure consisted of the following

principal steps:

Step 1. Calibration of the Waltz Mill 1500 kV compressed gas standard

capacitor C$w (fig. 2)

;

Step 2. Measurement of the standard divider ratio, using C sw and the

current comparator bridge (fig. 3);

Step 3. Repeat of step 2 for the Waltz Mill 1100 kV CCVT (fig. 4); and

Step 4'. Comparison of the standard divider and CCVT ratios using the

current comparator bridge to cross check steps 2 and 3

(fig. 13).

Step 4' was changed from that for the first Waltz Mill tests (step 4) so that the current comparator bridge could be used throughout. This, plus an improved switching sequence, increased the speed and convenience with which

the large number of switching operations (50) were performed and monitored.

The test procedure discussion in section 2.2 applies here, except that in

step 4' (fig. 13), the ratio P c/

P SD 1S measurec* directly, without the intervening 120:277 V transformer. In other words, step 1 determined the value of C$w»

the Waltz Mill standard capacitor (five measurements over the four-day test

period yielded a mean value of 103.165 pF, which agreed with the value for the

first Waltz Mill tests to within 0.01%). Steps 2 and 3 determined P^g and p q,

respectively; step 4' compared p cg and pg; and the combination of steps 3 and

4' yielded an indirect value of P^g for comparison with the direct value

found in step 2. A closed set consisting of steps 2, 3, 4', and 2 was obtained

initially and after each disconnect switch operation.

The detailed procedure was again as outlined in section 2.3. A total of

50 disconnect switch operations were performed, plus a 6-hour heat run at 300 kV

on the final day of testing.

5.2 Test Results

In the measurement sequence outlined above, step 2, which is the direct

measurement of P^g, was presumed to be the basic, or primary, measurement, with

steps 3 and 4' serving as a backup. However, analysis of the data showed the

fol 1 owi ng:

22
Waltz

Mill

Bus-300

kV

23

Step

4',

comparison

of

the

NBS

standard

divider

ratio

and

the

Waltz

Mill

monitoring

CCVT

ratio,

using

the

current

comparator
1. The repetition of step 2 at the end of the sequence revealed a highly consistent upward drift of PSD from the initial measurement.

The mean value of this drift was about 0.02%, with a standard

deviation of about 0.01%. The first value was usually obtained

approximately 30-^5 seconds after switching; the second, essentially

stable value, 3-6 minutes afterward.

2. The monitoring CCVT proved to be extremely stable, having a mean

drift of less than 10 ppm against 0$^, with a standard deviation

of only 70 ppm. Due to the drift in the direct measurement (step 2)

discussed above, indirect measurement can therefore be given equal

weight with the direct measurement. Furthermore,

3. The indirect value, no matter when taken, usually agreed closely

with the first direct reading, instead of falling somewhere between

the first and second direct values as expected (mean difference,

23 ppm, with a standard deviation of 0.01%, vs 218 ppm, 0.01%).

The above, apparently a peculiarity associated with the direct measurement,

was discovered only after careful analysis of all the data. Examination of

the data from the first Waltz Mill tests yields some evidence that this

effect may have been present then, but was obscured by the much larger effect

due to trapped charge.

At any rate, because of the close agreement between the direct-firstreading and the indirect data, the average of those two sets of values was used in assessing the effects of high-voltage switching on P SD * As shown

in figure 14, there appears to be little or no net effect

(mean = -1 ppm, standard deviation = 80 ppm).

Figure 14, which shows the deviations in the measured divider ratios, may

be explained as follows. The zero percent line represents, for each of the

50 disconnect switch operations, the next preceding shot as reference. Thus,

taking two of the deviations as examples, the deviation of operation No. 10

from operation No. 9 is +0.016%; the corresponding deviation of operation

No. 11 from operation No. 10 is -0.011%.

Further analysis has also revealed a fairly consistent downward shift in

divider ratio of several hundredths of a percent, associated solely with going

from truck power to Waltz Mill power. This operation involved disconnecting

the truck from the Waltz Mill bus and moving it from about 8 m away to about

30 m away, plus changing the power source. It was also found that this

shift occurs only at 300 kV for the direct measurement, but is already present

at 100 kV for the indirect measurement. Past experience suggests that this

is not an inherent characteristic of the system.

It should probably be concluded that the high-voltage switching effect

is negligible relative to other measurement uncertainties once trapped charge

is eliminated. However, pending further investigation of the apparent

anomalies discussed above, it seems prudent to enlarge the estimated

uncertainties assigned to the standard divider from the present 3a value

of 0.03% [1] to 3 a = 0.06%. This still leaves these uncertainties well

within the value of 0.1% presently stated for CCVT calibrations.

24
Shot-to-Shot

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and

indirect

measurements
The heat run results were not entirely conclusive, possibly because they were done outside on a clear, chilly, windy day (8-ll°C, average 12 mph wind).

Earlier heat runs were made in the stable environments of indoor 1 aboratori es

.

However, the overall trend was the same, indicating a decrease in of the

order of 0.03%, occurring in 2-3 hours.

6. SUMMARY OF RESULTS

High-voltage switching, per se, does not have a significant effect upon

the accuracy of the CCVT calibration system's capacitive divider. Opening

the grounding switch after connection to the line can trap charge on the low

side, producing an error proportional to the square of the resulting residual

dc voltage. This error has a maximum value of about 0.5%, and a probable

value of nearly 0.2%, for a 500 kV divider. Addition of a bleeder resistor

around the divider low side eliminates the error.

7. RELATION OF RESULTS TO THE GSU CCVT CALIBRATIONS

7.1

Introduction

As stated earlier, three sets of measurements at GSU triggered the

investigation described in [1], When that investigation did not explain some unexpected results obtained at GSU, one remaining possibility was some effect

from high-voltage switching. This, in turn, generated the tests described in

this report.

The grounding switch procedure was instituted in January 1980. Therefore,

the first calibration, performed in May 1979, was not affected.

7.2

Assumptions

In the following discussion, it will be assumed that the only effect

present is a positive ratio error in the standard divider, ranging from zero

to 0.5% for a 500 kV divider with a probable value of approximately 0.2%.

This error causes an equal negative ratio error in the CCVT.

7.3

The "V-Effect"

In the so-called "V-Effect," some results, particularly for C-phase

(fig. 15, [1], reproduced here also as fig. 15) showed the second ratio

value, or bottom of the "V," to be an average of about 0.3% lower than the

other two values. This is the right order of magnitude and, according to

figure 10, about 25% probable. It could explain the C-phase results if it is

remembered that the offset was absent in 1979, then assumed that a 0.3%

offset was present in March 1980, followed by a zero offset in December 1980.

It appears from figure 11 that the above sequence has a probability of about

0.32 x 0.16, or 5%. Furthermore, a similar effect is present on one A-phase

value (AW), but absent on the others. This not only involves two completely

different switching operations than for C-phase, but the effect is not present

in the other two A-phase values, as it would be if trapped dc voltage were the only perturbation . Finally, the trapped voltage effect, acting alone,

dictates that all 1980 ratio values must be equal to or less than the 1979

26
RCF

Metering

Parallelogram

In Out

O 5/79 4 5

# 3/80 3 6

12/80 4 5

5/79 3/80 12/80

A* ^ M

B <t> 0-t> •—> —

C <j> O—^ —

M

Figure 15. "Revised" history of X1X3 RCF and y, May 1979 to

December 1980, GSU calibrations

27
values (on the average, about 0.2% less). Figure 15 shows this not to be so. Thus, there is no obvious correlation between the trapped voltage effect

and the "V-effect."

7.4 Applying the Probable Offset

Assuming the probable +0.2% standard divider ratio offset (i.e., -0.2%

apparent CCVT ratio offset) to be present at all times, the effect was "corrected for" by shifting all 1980 values by +0.2%. This is shown in

figure 16. Comparing the tabulated "In" and "Out" values^ with those on figure 15, a slight "improvement" is seen for March 1980 and no change for

December 1980. Even applying the maximum permissible offset of 0.5%

(admittedly highly improbable for all of six consecutive switching operations)

made no significant improvement. Other combinations of adjustment were tried,

with similar results. Again, there is no obvious correlation with the

trapped voltage effect.

7.5 Comparing Consecutive Switching Operations on the Same Phase

Since charge trapping has been shown to be a random event, it would be

expected that two consecutive switching operations on the same phase should

produce two sets of measured ratio values differing by the same amount for

all the CCVTs connected to that phase. In the normal course of events, two

such switching operations on the same phase would be separated by the period

of months, or even years, between two calibration trips. One exception occurred

at GSU in December 1980, when it became necessary to go on one phase twice,

on two consecutive days. In this case, even though actual ratio offsets are unknown, it would seem probable (fig. 11) that the offsets would be different.

This difference should then show up as a fixed shift in magnitude and direction

for all comparable ratio values taken on the two days, assuming short-term

stability of other variables in the measurement.

Since the first set of measurements was taken in the control house and

the second at the make-up boxes (MUBs), a comparison of the data required

the calculation of lead drop between control house and MUB due to burden

current. This calculation was made, using utility-furnished burden and

lead impedance data, and applied to the control house data.

The results are shown in figure 17. The vectors represent the differences

between measurements on each tap. Not only are they widely different in

magnitude, but two vectors are opposite in direction to the others. Once

again there is no obvious correlation with ratio offset due to trapped charge.

6"In" is defined conservatively as being within the metering parallelogram

plus the NBS uncertainty.

28
RCF

Figure 16. GSU "revised" March 1980 and December 1980 results

adjusted upward by 0.2% (1979 results assumed correct)

29
ARCF(AF)

X103

12 --

10 —

WY2 / 6 —

FX2\ */GX2

FXU /gXI Ay-mrad - 4 -2

1 1 1 0 2 4 6 8

1 1 1 1

1 1 1 A - Average

1 1 t 1

• - XI Tap

X's" Reference— RCF and y at MUB

X - X2 Tap

0 - Y2 Tap ~ 2 1

F - Franklin

G - Gypsy

W - Webre

o— o Arrow Points From _ 4 _

Calculated to Measured Values

Figure 17. Calculated vs measured changes in RCF and y, for

one phase, from MUB to HSE , GSU, December 1980

30
8. DISCUSSION AND SUMMARY

High-voltage switching seems to have no significant effect on the

capacitive standard divider's ratio, other than that from trapped charge

on the low side in the absence of a bleeder resistor. Attempts to relate

the dc voltage effect to the results of the GSU investigation [1] yielded

no obvious correlation.

Data from calibrations performed at two other utilities while this effect

was supposedly present were also analyzed. Not only was no obvious correlation

found in either case, but as mentioned earlier (sec. 2.2), some extremely

close agreement between NBS and factory results seemed contrai ndi cati ve of

any dc voltage effect. This was one of the reasons for not anticipating a high-voltage switching effect at the outset of the Waltz Mill tests.

In retrospect, it has been suggested that divider leakage resistances

may be highly variable under field conditions, perhaps quite often low enough

to quickly remove any trapped charge. This could be postulated as an explanation for the close agreements mentioned above, and for the absence of

any obvious correlation between GSU or other calibration results and a possible ratio offset due to trapped charge. However, such variable leakage

resistance would have to be 109 ^ 0 r less (t= 30 min) to be at all

effective. A leakage resistance of 10 9

ft seems improbably low. Furthermore,

it would manifest itself as easily detectable drift, amounting to several

tenths of a percent, during those measurements for which several tenths of a percent initial offset was present. Such large drifts have never been seen in

the field.

With or without a ratio error in the EPRI-NBS divider due to trapped

charge, a significant proportion of the affected CCVTs were outside of metering

tolerance. Many were well outside of it. Some marginal CCVTs might appear to

be better if this error could be assumed and then removed by correcting the

results. Others would actually appear to be worse.

Cumulative experience with CCVTs suggests that a long-term monitoring

program for these devices would be useful. The Waltz Mill facility appears to

be excellent for such a program. A dozen or so CCVTs from various sources (new,

used, very old, taken-out-of-servi ce , etc.) could be connected to a single bus.

Computer and other facilities already in place for long-term cable tests should

be readily adaptable to automated collection and analysis of data.

31
9. REFERENCES

[1] D. L. Hillhouse and D. A. Leep, Analysis of the calibration of metering

CCVTs in a utility substation, Nat. Bur. Stand. (U.S.), NBSIR 82-2360

(Oct. 1981).

[2] C. J. Baldwin and J. W. Bankoski, Waltz Mill facility tests underground

cables and also demonstrates feasibility of 1100 kV overhead systems,

Westinghouse Engineer, Nov. 1973.

[3] D. L. Hillhouse, 0. Petersons, and W. C. Sze, A prototype field calibration

system for coupling capacitor voltage transformers (CCVTs), EPRI EL-690,

Apr. 1978.

[4] D. L. Hillhouse, 0. Petersons, and W. C. Sze, A prototype system for on-site

calibration of coupling capacitor voltage transformers (CCVTs), IEEE Trans.

Power Appar. Syst., Vol . PAS-98, No. 3, May/June 1979, p. 1026.

[5] D. L. Hillhouse, 0. Petersons, and W. C. Sze, A simplified system for

calibration of CCVTs in the substation, Nat. Bur. Stand. (U.S.), Technical

Note 1155 (May 1982).

[6] N. L. Kusters and 0. Petersons, The voltage coefficients of precision

capacitors, IEEE Trans. Power Appar. Syst., Vol. PAS-82, No. 11, Nov. 1963,

p. 612.

32
NBS-114A (rev. 2-ec) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

PUBLICATION OR

REPORT NO.

NBSIR 83-2666

2. Performing Organ. Report No. 3. Publication Date March 1983

4. TITLE AND SUBTITLE

EFFECTS OF HIGH-VOLTAGE SWITCHING ON THE EPRI-NBS COUPLING CAPACITOR

VOLTAGE TRANSFORMER (CCVT) CALIBRATION SYSTEM STANDARD DIVIDER

5. AUTHOR(S)

David L. Hi 11 house

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

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

Sponsored in part by

Electric Power Research Institute

Palo Alto, California 94304

10.

SUPPLEMENTARY NOTES

!

| Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

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

bi bl iography or literature survey, mention it here)

This report presents the results of tests of the effects of high-voltage

switching on the EPRI-NBS CCVT calibration system's capacitive standard divider,

completing an investigation stemming from the results of three calibrations at a Gulf States Utilities substation.

Initial tests consisted of full-scale (300 kV) switching operations at EPRI's Waltz Mill test facility, during which the divider exhibited significant

ratio offsets (average = +0.2%, maximum = nearly +0.5%). Tests were continued

at the National Bureau of Standards (NBS), where it was determined that

operation of a grounding switch, installed to protect the divider low side

during high-voltage switching, caused ratio offset by trapping charge on the

divider's low side capacitor. This resulted in residual dc voltage which

changed the value of the divider's low-side capacitance. The addition of a bleeder resistor eliminated the problem in the laboratory. With the bleeder

resistor in place, the Waltz Mill tests were repeated. Fifty high-voltage

switching operations indicated a negligible shot -to-shot variation

(average = -lppm, a = 80 ppm).

No obvious correlation was found between the ratio offsets described above

and the results of CCVT calibrations performed while the effect may have been

present in the divider. Experience indicates that a significant proportion of calibrated CCVTs is outside metering tolerance. Long-term simultaneous

monitoring of a sizeable number of CCVTs is suggested.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) calibration; capacitive divider; CCVT; error sources; high-voltage measurements;

high-voltage switching; ratio offset; trapped charge; Waltz Mill tests.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

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