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है”ह”ह

IS 5547 (1983): Application guide for capacitor voltagetransformers [ETD 34: Instrument Transformers]

fS : 5547 - 1983

Indian Standard

APPLICATION GUIDE F-OR CAPACITOR VOLTAGE TRANSFORMERS

(First Revision)

Instrument Transformers Sectional Committee, ETDC 34

Chairman

SHRI J. S. NEQI

Members

Representing

Jyoti Ltd, Vadodara

SHRI V. B. DESAI (Alternate to Shri J. S. Negi )

SHRI C. D. BA~UL Siemens India Ltd, Bombay SHRI S. M. KELKAR ( Alternate)

SHRI A. K. BARMAN The Calcutta Electric Supply Corporation Ltd, Calcutta

SHRI K. C. BHATTACHARYYA (Alternate) SHRI V. K. BATRA

SHRI V. N. SHARMA ( Alternate ) National Physical Laboratory ( CSIR ), New Delhi

SHRI A. C. BEDEKAR Madhya Pradesh Electricity Roard, Jabalpur SHILI J. J. DAR~WALA All India Instruments Manufacturers and Dealers’

Association, Bombay SHRI c. P. soou (Alternate I ) SHRI 0. P. PURI ( Alternate II )

SHRI P. S. DESHMUE~ Maharashtra State Electricity Board, Bombay SHRI S. G. KASHI ( Alternate )

DIRECTOR ( PROTECTION & INSTRU- Central Electricity Authority, New Delhi MENTATIC N )

DEPUTY DIRECTOR ( PROTECTION & INSTRUMENTATION ) ( Alternate )

SHEI N. D. GADGIL Gujarat Electricity Board, Vadodara SHRI K. L. GARG Inspection Wing, Directorate General of Supplies

& Disposals, New Delhi SHRI R. P. SEHGAL (Alternate )

SI~RI S. D. JINSIWALE Silkaans, Bombay SHEI S. R. ALURKAR (Alternate )

JOINT DIRECTOR ( TI )-2 Research Designs & Standards Organization ( Ministry of Railways ), Lucknow

DEPUTY DIRECTOR ( TI )-3 ( dlternate )

( Continued on page 2 )

0 Copyright 1983

INDIAN STANDARDS INSTITUTION

This publication is protected under the Indian Copyright Act ( XIV of 1957) and reproduction in whole or in part ,by any means except with written permission of the publisher shall be deemed to be an infringement of copyright under the said Act.

IS r5547 - 1983

( Continued from page 1 )

Members

SHRI R. N. KHARSHIN~KAR

Representing

SHRI R. C. BAJPAI ( Alternate ) Tata Consulting Engineers, Bombay

SRRI S. K. LAWBA Voltas Ltd, Thane SHRI E. J. MARABLESRWARWALLA The Bombay Electric Supply and Transport

SHRI K. C. M~RANRAJ ( Allernate ) Undertaking, Bombay

SHRI M. B. MEHTA Tata Hydro-Electric Power Supply Co Ltd, Bombay

SHRI S. DORAISWAMY ( Alternate ) SHRI V. V. MOO~I Crompton Greaves Ltd, Bombay

SHRI A. K. GOVIL ( Alternate ) SHRI R. NATARAJAN Central Public Works Department, New Delhi

SURVEYOR OF WORKS ( ELECTRICAL )-I ( Alternate )

SHRIN.NATH The English Electric Co of India Ltd, Madras SHRI R. SUBRAMANIAM ( Alternate )

SHRI P. U. PATWARUHAN Prayog Electricals Pvt Ltd, Bombay SHRI A. V. NARKE (Alternate )

SHRI 0. P. PURI Automatic Electric Ltd, Bombay SHRI S. V. KARKHANIS ( Alternate )

SHRI P. S. SATNAM Punjab State Electricity Board, Patiala SHRI B. S. SHARMA U. P. State Electricity Board, Lucknow SHRI D. SURYANARAYANA Bharat Heavy Electricals Ltd, New Delhi

SHRI A. V. SOMAN ( Alternate I ) SHRI H. S. VALIA (Alternate II )

SHRI N. THYAGARAJAN Tamil Nadu Electricity Board, Madras SHRI S. K. RAXASUBRAMANIAN

( Alternate ) SHRI J. M. UDIA Directorate General of Technical Development j

New Delhi SHRI L. GOPALA KRISHNA ( Aiternate)

SHRI B. K. VENKATESH Karnataka Electricity Board, Bangalore SHRI S. P. SACHDEV,

Director ( Elec tech ) Director General, IS1 ( Ex-ojicio Member )

Secretary

SHRI SUKH BIR SINGH Assistant Director ( Elec tech ), IS1

IS u5547 - 1983

Indian Standard

APPLICATION GUIDE FOR

CAPACITOR VOLTAGE TRANSFORMERS

( First Revision )

0. FOREWORD

0.1 This Indian Standard ( First Revision ) was adopted by the Indian Standards Institution on 28 April 1983, after the draft finalized by the Instrument Transformers Sectional Committee had been approved by the Electrotechnical Division Council.

0.2 Capacitor voltage transformers are being employed for line voltmeters, synchroscopes, protective relays tariff-meters, etc.

0.3 Although the performance of capacitor voltage transformer does not equal that of an electromagnetic voltage transformer due to dependence of its performance on the supply frequency, switching transients, magnitude of connected ,burden, etc, the capacitor voltage transformer is more econo- mical than an electromagnetic voltage transformer when the nominal system voltage increases above 66 kV.

-0.4 The trend of higher transmission voltages and the increasing use of power-line carrier for protection and telephony has accentuated the use of capacitor voltage transformers.

0.5 This standard is closely associated with IS : 3156 ( Part 4 )-1978* and hence should be read along with it.

0.6 This standard was first published in 1967. This revision has been under- taken to align it with IS : 3 156 ( Part 4 )-1978” and to incorporate the developments that have taken place since its first publication.

0.7 For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated expres- sing the result of a test, shall be rounded off in accordance with IS : 2- 196Ot. The number of significant places retained in the rounded off value should be the same as that of the specified value in this standard.

*Specification for voltage transformers : revision ).

Part 4 Capacitor voltage transformer (first

SRules for rounding off numerical values ( revised ). LG.

3

IS:5547 - 1983

1. SCOPE

1.1 This guide covers the application of capacitor voltage transformers for use with electrical measuring and protective devices.

2. TERMINOLOGY

2.0 For the purpose of this guide, the definitions given in IS : 1885( Part 28)- 1973* and 1-S : 3156 ( Part 1 )-1978t apply *

and IS : 3156 (Part 4)-1978$ shall

SECTION 1 GENERAL

3. INFORMATION TO BE GIVEN WII’H ENQUIRY AND ORDER

3.1 In addition to the information called in Section 1 of this standard, for capacitor voltage transformer, the following information is to be furnished:

a)

b) Cl

Whether the capacitor voltage transformer ( CVT ) is required to be used for carrier coupling;

If so, the carrier frequency range; and

For non-composite CVTs where the electromagnetic unit is manu- factured separately to match an existing high voltage coupling capacitor or a capacitor potential divider the nominal capacitance and dielectric dissipation factor of the capacitors and the inter- mediate voltage.

3.2 Reference Range of Frequency - The variation in the operating power frequency has significant influence on the accuracy of a capacitor voltage transformers. Normally a capacitor voltage transformer is tuned to yield the best accuracy at the rated frequency ( 50 Hz ) and to comply with the limits of an accuracy class as long as the operating frequency does not deviate beyond the reference ranges of frequency for measuring or protective purpose capacitor voltage transformer. ‘When the operating frequency deviates beyond the reference range of frequency, the accuracy limits are likely to be exceeded. Coincident influential factors are the power factor and the magnitude of connected burden. Where accuracy is important, as in tariff metering applications, it is desirable to obtain accuracy curves for the capacitor voltage transformer corresponding to the limiting operating frequencies. They are available from the manufacturers.

3.2.1 For capacitor voltage transformers meant for metering applications, the reference range of frequency as per IS : 3156 (Part 4)-1978t as 99percent

*Electrotechnical vocabulary : Part 28 Instrument transformers. tspecification for voltage transformers : Part 1 General requirements (first revision ). JSpecification for capacitor voltage transformers :

former ( Jirst revision ) . Part 4 Capacitor voltage trans-

<,i:

4

IS:5547- 1983

to 101 percent of the rated frequency. meant for protection applications,

For capacitor voltage transformers the reference range of frequency is

97 percent to 103 percent of the rated frequency. The above frequency variations are normally encountered in system operations. Should these system frequency limits are likely to be exceeded, this should be communic- ated to the manufacturer. The accuracy and output with the modified frequency range are to be negotiated between the supplier and the manu- facturer. It is to be understood that during system fault conditions the metering accuracy shall not be available.

3.2.2 A typical illustration showing the variation of accuracy of capacitor voltage transformers with operating frequency and power factor of the burden is given in Fig. 1.

FIG. 1 TYPICAL ILLUSTRATION OF VARIATION ck ACCURACY OF CAPACITOR VOLYAGE TRANSFORMERS WITH OPERATING

FREQUENCY AND POWER FACTOR OF THE BURDEN

5

.

IS : 5547 - I983

3.3 CAPACITOR VOLTAGE TRANSFORMER USED AS COUP- LING CAPACITOR FOR CARRIER COUPLING

3.3.1 For purpose of carrier coupling any device connected between the earth end of the intermediate voltage capacitor and the earth shall have a high impedance in comparison to the impedance of the intermediate voltage capacitor at the rated frequency.

Practical experience indicates that if the impedance of the inter- mediate voltage capacitor at rated frequency is of the order of 1 000 times the impedance of the carrier frequency coupling device, the influence on the operation of the capacitor voltage transformer is negligible.

3.3.2 The carrier frequency power loss in the electromagnetic portion shall not exceed 0.5 dB over the entire carrier frequency range. If loss is more than 0.5 dB, a carrier frequency choke is to be inserted between the inter- mediate voltage terminal and the electromagnetic unit of the capacitor voltage transformer. The value of the impedance of this choke shall be SO

chosen as to limit the loss to less than O-5 dB. NOTE - If the stray capacitance and stray conductors of the LV terminal of the

capacitor divider of the CVT is within the limits given in Appendix A, 8.7 and A-2.2 of IS : 9348-1979*. Coupling capacitor and capacitor potential divider, it may not be necessary to fit a carrier frequency choke for preventing loss of carrier frequency signal in the rlrctromagnetic portion.

3.4 FACTORS INFLUENCING THE CHOICE OF VALUE OF CAPACITANCE AND OUTPUT OF CAPACITOR VOLTAGE TRANSFORMERS

3.4.1 The maximum output from a capacitor voltage transformer is governed by the range of frequency over which the accuracy has to be maint- ained. The change in error with variation of frequency is mainly a change in phase when the burden is of unity power factor. The permissible rated output rnay be derived from the expression:

W = x ( Cl + c, ) vi2 0

where

w=

X=

Maximum permissible rated output in VA,

Factor depending upon the losses in the capacitors and the electromagnetic portion and the frequency,

Capacitance of primary voltage capacitor in farads,

Capacitance of intermediate voltage capacitor in farads,

Intermediate voltage in volts, and

Phase-angle error change in minutes per Hz.

*Co\lpling capacitor and capacitor divider.

6

ICS : 5547 i 198j

34.2 Therefore, for a given accuracy over a given frequency range the output can be increased by increasing the value of the capacitance while keeping the same intermediate voltage or vice-versa. The capacitance required may be reduced to the economic limits by a suitable selection of the intermediate voltage. On the other hand, when the capacitance values are fixed by other considerations for example carrier coupling require- ments, the rated output depends upon among other things, the permissible phase angle error changes per Hz.

,

3.4.3 Capacitor voltage transformers have certain design limitations because of the need to suppress ferroresonance oscillations a.s detailed in 4.1. These limitations also impose certain constraints on the output obtainable from capacitor voltage transformers.

3.4.4 Because of these inherent limitations, it is not possible to obtain a very high output and accuracy from capacitor voltage transformer. It would, therefore, be advisable if the rated burden specified with dual purpose capacitor voltage transformer is limited to the output exclusively required for metering. As the output for the operation of the protective relays, etc, is mostly required for short time duration and as the relaying accuracy is applicable with a different frequency range, for purpose of testing for metering accuracy the output necessary exclusively for metering application only may be considered. However, for protective application, the total simultaneous burden on both the protective and metering windings may be specified with the appropriate frequency range.

3.4.5 For capacitor voltage transformers with a protective winding meant to be used in a residual voltage connection ( Bank of 3 windings connected

in broken delta ) class 6P is quite adequate.

SECTION 2

LIMITATIONS OF CAPACITOR VOLTAGE TRANSFORMERS

4. FERRORESONANCE IN CAPACITOR VOLTAGE TRANSFOR- MERS

4.1 Capacitor voltage transformers are susceptible to ferroresonance oscill- ations under primary and secondary short circuit conditions. This pheno- menon occurs due to interchange of energy between the capacitor of the divider and non-linear inductance of the electromagnetic unit. It results in severe distortions of the output voltage and poor design can result in sustained sub-harmonic oscillations. The latter can be avoided at the design

7 4:L

stage by taking suitable precautions. Some of the usuaily adopted methods are given below. A combination of the methods may be used for the prevention of ferroresonance occurrence.

a)

b)

C)

4

e>

Maintaining the working flux density of the electromagnetic units at much lower levels as compared wiih the conventional voltage transformers.

Greater utilization of the linear position of the magnetization curve by using strip wound cores, thus avoiding local saturation effects.

Providing an air gap in the magnetic circuit to maintain the linearity of magnetizing inductance over a wide range of operating conditions.

Connecting a suitable damping resistance permanently across the secondary.

Depl.oying auxiliary tuning and damping networks in the electro- magnetic unit. In this case, it is necessary that additional precau- tions, are-taken to avoid introduction of additional transients in the process of damping ferroresonance effects.

5. RESPONSE OF CAPACITOR VOLTAGE TRANSFORMER TO PRIMARY VOLTAGE TRANSIENTS

5.1. Capacitor voltage transformers are subject to transient oscillations on the secondary voltage when there is a sudden change in the primary voltage. Such oscillations may effect operations of distance relays.

The operation of capacitor voltage transformers under such transient voltage conditions may be improved by keeping the burdens on the secondary side to as low a value as possible.

6. WSE OF CAPACITOR VOLTAGE TRANSFORMERS WITH SATURABLE BURDENS

6.1 Use of capacitor voltage transformers which are likely to saturate at the rated voltage, is to be avoided. Also, use of auxiliary voltage trans- formers in conjunction with the capacitor voltage transformers, is to be avoided. This is because saturable burdens are likely to cause ferroreson- ante oscihations on the secondary side of the CVT. It is preferable to order multiple secondary CVTs in place of auxiliary VTs.

7. CAPACITOR VOLTAGE TRANSFORMER CONNECTIONS

7.1 Capacitor voltage transformers shall not be operated with the secondary windings connected in closed delta because excessive current may circulate in closed delta.

8