Disclosure to Promote the Right To Information

Whereas the Parliament of India has set out to provide a practical regime of right to information for citizens to secure access to information under the control of public authorities, in order to promote transparency and accountability in the working of every public authority, and whereas the attached publication of the Bureau of Indian Standards is of particular interest to the public, particularly disadvantaged communities and those engaged in the pursuit of education and knowledge, the attached public safety standard is made available to promote the timely dissemination of this information in an accurate manner to the public.

इंटरनेट मानक

“!ान $ एक न' भारत का +नम-ण”Satyanarayan Gangaram Pitroda

“Invent a New India Using Knowledge”

“प0रा1 को छोड न' 5 तरफ”Jawaharlal Nehru

“Step Out From the Old to the New”

“जान1 का अ+धकार, जी1 का अ+धकार”Mazdoor Kisan Shakti Sangathan

“The Right to Information, The Right to Live”

“!ान एक ऐसा खजाना > जो कभी च0राया नहB जा सकता है”Bhartṛhari—Nītiśatakam

“Knowledge is such a treasure which cannot be stolen”

“Invent a New India Using Knowledge”

है”ह”ह

IS 8478 (1977): Application guide for on-load tap changers[ETD 16: Transformers]

IS : 8478.1977

Indian Standard APPLICATION GUIDE FOR ON-LOAD TAP-CHANGERS

[ Second Reprint JANUARY 1991 )

ULX 621.314.214.332

0 Copyright 1978

BUREAU OF INDIAN STANDARDS MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG

NEWDELHIllOOU2

Gr3 January 1978

IS : 8478 . 1977

Indian Standard APPLICATION GUIDE FOR ON-LOAD TAP-CHANGERS

Transformers Sectional Committee, ETDC 16

Chairman SHKI U. K. PATWAHDHAN

Prayog Electricals Pvt Ltd, Bombay

Members SHRI S. AMMEERJAN

ReprerenCing

SHRI N. S. S. AROK~~WAMV Bharat Heavy Electricals Ltd (R&D Unit), Bhopal Tamil Nadu Electricity Board, Madras

SHRI M. K. SUNDARARAJAN (Alternate) SWRI B. G. BHAKEY

DR B. N. JAYARAM (Alternate) Kirloskar Electric Co Ltd, Bangalore

SHRI A. V. BHEE~IARAO SHRI J. S. IYER (A&mate)

Gujarat Electricity Board, Vadodara

SHRI S. D. CHOTRANEY Bombay Electric Supply and Transport Under- taking, Bombay

SHRI Y. K. PALVANKAR (Alternate) DIRECTOR (TRANSMISSION) Central Electricity Authority, New Delhi

DEPUTY DIRECTOR (TRANSMISSION) (Altemate) SHRI T. K. GHOSE Calcutta Electric Supply Corporation Ltd, Calcutta

SHRI P. K. BI~ATTACHARJEE (Alternate) JOINT DIRECTOR (SUB-STATION) Research, Designs and Standards Organization

DEPUTY DIRECTOR STANDARDS (Ministry of Railways), Lucknow

(ELECTRICAL) (Alternate) SIIRI J. K. KHANNA Directorate General of Supplies and Disposals

SHRI K. L. GARB (Alternate) (Inspection Wing), New Delhi

SIIRI B. S. KOCHAR Rural Electrilication Corporation Ltd, New Delhi SFIRI R. D. JAIN (Alte,mtcj

SIIRI J. R. MAHAJAN Indian Electrical Manufacturer’s Association,

SHRK P. K. PHILIP (Altej/late) Bombay

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

SHRI R. CHANDRAMOULI (Alterna’?) SHRI D. V. NARKE Bharat Heavy Electricals Ltd, Bhopal

SHRI ISHWAK CIIANDRA (A&mate I)

SIIRI SHRI PREM CIIAND (Alternate II), I. S. PATEL Hindustan Brown Bovcri Ltd, Bombay

(Continued on page 2)

I 0 Coprright 1978

BUREAU Oe’ INDIAN STANDARDS This publication is protected under the Zndiun Copyright Act (XXV 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 : 8478 - 1977

(Continued from page 1)

Members Refwesenting National Electrical Industries Ltd, Bombay SHRI V. N. PRAZILAD

SHRI A. G. GURJAR (Alternate) SHRI K. N. RAMASWAMY Directorate General of Technical Development,

New Delhi SHRI S. K. PALHAN (Alternate)

SHRI CHANDRA K. ROHATCI SHRI D. P. SAHGAL

SHRI A. R. SALVI (Alternate) SHRI I. C. SANGAR

SHRI R. C. KHANNA (Alternate) SHRI K. G. SHANMUKHAPPA

SHRI P. S. RAMAN (Alternate) SHRI.M. A. SHARIFF

SHRI B. C. ALVA (Alternate) SUPERINTENDING ENGINEER (OPERA-

TION) SUPERINTENDING E N G I N E E K

Pradip Lamp Works, Patna Siemens India Ltd, Bombay

Delhi Electric Supply Undertaking, New Delhi

NGEF Ltd, Bangalore

Karnataka Electricity Board, Bangalore

Andhra Pradesh State Electricity Department, (Electricity Projects and Board), Hyderabad

TECHNICAL (PROJECTS) (Alternate) SHRI C. R. VARIER Crommon Greaves Ltd, Bombay

SHP.I S. V. MANERIKAR (Alternate) * SI~RI T. RAJARMAN, Director General, IS1 (Ex-o#cio Member)

Deputy Director (Elec tech)

Secretary SHRI VIJAI

Deputy Director (Elec tech), IS1

Panel for On-Load Tap-Changers, ETDC 16 : P5

Cowener SHRI D. V. NARKE Bharat Heavy Elcctricals Ltd, Bhopal

Members SHRI S. C. AGARWAL (Alternate to

SHRI D. V. NARKE)‘ SHRI R. C. CHANDRAMOULI Tatd Hvdro-Electric Power Supply Co Ltd,

Bombay __

SHRI S. V. MANEIUKAR Crompton Greaves Ltd, Bombay SHRIV.N.PRAHLAD National Electrical Industries Ltd, Bombay

SHRI A. G. GURJAR (Alternate) SHRI c. s. N. RAJU NGEF Ltd, Bangalore

2

IS : 847% - 1977

APPLICATION GUIDE FOR ON-LOAD TAP-CHANGERS

0. FOREWORD

0.1 This Indian Standard was adopted by the Indian Standards Institution on 24 May 1977, after the draft finalized by the Transformers Sectional Committee had been approved by the Electrotechnical Division Council.

0.2 In the preparation of this standard assistance has been derived from IEC DOC: 14 (Central Office) 9, ‘Draft application guide for on-load tap- changers’, issued by the International Electrotechnical Commission.

0.3 For the purpose of deciding whether a particular requirement of this standard is complied with, the final value, observed or calculated, expressing the result of a test, shall be rounded off in accordance with IS : 2- 1960”. The number of significant places retained in the rounded off value shodd he

the same as that of the specified value in this standard.

1. SCOPE

1.1 This application guide is intended to assist in the selection of suitable on-load tap-changers for use in conjunction with the tapped windings of transformers or reactors (referred to as transformers in the following text).

NOTE 1 -All the tap-changers referred to shall be presumed to comply with the requirements specified in IS: 8468-1977t.

NOTI% 2 -The provisions of this application guide are not mandatory and only re- present advice to the tap-changer manufacturers and purchasers. The responsibility for the correct application of the fully assembled tap-changer in connection with the tram- former is with the manufacturer of the transformer.

2. SELECTION OF A TAP-CHANGER

2.1 General - Since the on-load tap-changer represents only a small part of the total cost of the equipment in which it is used, it should be freely chosen to suit the ~equipment. However, account should be taken of the available standard types of tap-changers.

2.2 Insulation Level - The following values occurring on all tapping positions of the transformer should be checked against the tap-changer manufacturers’ declared values in accordance with IS : 8468-1977t :

a) Normal power frequency operating voltages appearing on the tap-changer in service,

*Rules for rouhding off numerical values (revbd). tspecification for on-load tap-changers.

3

IS : 8478 - 1977

1~) Power frequency voltages appearing on the tap-changer during tests on the transformer, and

c) Impulse voltages appearing on the tap-changer during tests on the transformer or in service.

NOTE - With some winding arrangements the voltages appearing on thr transformer can be abnormally high, for cxamplc, star-point tappings in auto-transformers, line-end tappings, and booster transformer arrangements.

‘I‘hrsc voltages can br aKectcd considerably by the choice of linear, coarse/fine or re- vrrsing lapping arrangcnxnts. Variable flux voltage variation (VFVV) and combined voltage variation (cbVV) as defined in IS: 2026(Part IV)-1977* also can affect the voltages appearing on various parts of the tap-changer.

2.3 Current - ‘I’hc tap-changer should satisfy the conditions specified in 2.3.1 to 2.3.3.

2.3.1 Rated Through-Current - The rated through-current of the tap- changer as defined in IS : 8468-1977t should not be less than that resulting from the highest value of tapping current of the tapped winding of the transformer [sue 4.1 of IS : 2026 (Part I)-1977$].

2.3.2 Overload Current - When tap-changers are fitted on transformers which are subjected to overload conditions in accordance with IS : 6600- 19724, they shall generally be restricted to the occasional overload conditions stated in 4.1 of IS : 2026 (Part I)-1977: unless specifically ordered for other overload conditions by the transformer m’anufacturer.

The above requirements are met:

a)

b)

if the maximum rated through-current of the tap-changer is at least 1.2 times the highest tapping current of the transformer, or if the tap-changer manufacturer is able to demonstrate that the temperature rise limits given in 8.5 of IS : 8468-1977t are not exceeded when the contacts carry 1.2 times the maxinium rated through-current. (The values of current to apply for the other tests specified in 8 of IS : 8468-19771_ remain based on the value of the maximum rated through-current.)

The number of tap-changer for each occasional overload period is limited to the number of operations corresponding to one half of one complete operating cycle and the peak temperature rise of the transition resistors does not exceed 300°C for oil-environment tap-changers.

Where tap-changers are subject to overload conditions not in accordance with the limitations stated in IS : 2026 (Part I)-1977: with regard to IS : 6600-19729, the tap-changer manufacturer should be consulted giving current magnitude and duration so that the correct tap-changer can be recommended for the duty required.

. *Specification for power transformers: Part IV Terminal markings, tappings and connec-

tions (first r&ion). tspecification for on-load tap-changers. $3pecification for power transformers: Part I General (Jirst r&&n). §Guide for loading of oil immersed transformers.

4

PS : 8478 - 1977

2.3.3 Short-Circuit Current - The short-circuit current of the tap;changer as given in 8.7 of IS : 8468-19.77* ghould not be less than that resulting from the overcurrent of the associated transformer as given in IS : 2026 (Part I)-1977t.

NOTE - Particular care should be taken to check this current on low impedance and booster transformers. of tap-changer.

In some instances, the fault-current value could dictate the choice

2.4 Breaking Capacity -The highest tapping current and the voltage per step of the transformer shall be within the values of rated throdgh- current and relevant rated step voltage declared by the tap-changer manu- facturer for the particular tap-changer.

For values outside of those declared, the tap-changer manufacturer should be consulted.

For application to individual transformers with different currents and step voltages, the transition impedance shall be designed so that the switched current and recovery voltage in the tap-changer do not exceed those covered by the type tests.

NOTE - In certain applications, such as furnace transformers, the tap-changer may be called upon, if required, to operate during periods of momentary overloads of two to three times the transformer continuous maximum rating. The diverter switch or selector switch should be chosen so that it is capable of meeting this condition.

Where appropriate, the effect of variable flux voltage variation and combined voltage variation on the step voltage, and in consequence on the breaking capacity should be considered.

2.5 Number of Tapping Positions - The number of’inherent tap posi- tions of the tap-changers is generally standardized with various manu- facturers’ equipment. The selection of the number of service tap positions shou!d preferably be made within that range.

As the extent of the tapping range increases, the voltages to be catered for also increase and care shall be taken to avoid excessive voltages over the tapping range when operating or testing at minimum winding positions. The effect can be very marked on furnace and rectifier transformers feeding electrolytic plant where wide tapping ranges tit-e often necessary and the tap-changer is in the constant voltage winding (variable flux voltage variation).

2.6 Discharge Problems with Changeover Selectors - It should be noted that, under certain conditions, tap-changers with changeover selectors can allow momentary disconnection of the tapped winding. In such cases, discharges between the opening and closing contacts can occur during the operation of the changeover selector, and in order to avoid difficulties with regard to the dielectric stress and the formation ~of gases which could arise when the tap-changer operates under high voltage to earth, s

a ecial pre-

cautions shall be applied. There are many different metho s of over-

*Specification for on-load tap-changers, TSpecification for power transformers: Part I General (first revision).

5

IS : 8478 - 1977

coming the problem, the use of two-way changeover selectors, control resistors, or capacitive control between the main winding and the tapped portion of the winding are some examples.

2.7 Mechanical Life - The mechanical duty may need consideration if the expected number of operations per annum exceeds 20 000, for example, this can occur on transformers for use on rolling mills, electrolytic plant or furnace supplies.

2.8 Motor-Drive Mechanism - If the motor-drive mechanism is pur- chased from a manufacturer other than the manufacturer of the tap- changer then it is the purchaser’s responsibility to ensure that the motor- drive mechanism is suitable for all its necessary duties.

2..9 Pressure and Vacuum Tests - Where applicable, the tap-changer when fully assembled has to withstand all the pressure and vacuum tests of its associated transformer. For these, the relevant instructions of the tap- changer manufacturer have to be taken into account.

2.10 Low Temperature Conditions - Should the tap selectors, diverter switches or selector switches be located in separate containers outside the main transformer tanks, in air, and the ambient temperature be lower than -5”C, it is advisable to specify the quality of the insulating and/or lubricating oils.

Should’the tap selectors, diverter switches or selector switches be located inside the main transformer tank and the oil temperature can be lower than -5°C during operation, the tap-changer manufacturer shall be con- sulted, taking the quality ofthe main transformer oil into consideration.

If necessary, automatically controlled heating devices could be provided or, alternatively, means of preventing tap-change operation at abnormally low temperatures may be considered.

2.11 Continuous Operation - If the tap-changer is required to operate continuously the temperature conditions may need to be checked and the tap-changer manufacturer should be consulted.

3. LOCATION OF OIL-IMMERSED TAP-CHANGER COMPONtiNTS

3.1 Tap Selectors - Unless otherwise agreed between thr manufacturer and the purchaser, the tap selectors may be located within the main trans- former oil.

3.2 Diverter and Selector Switches - To prevent contamination of the main transformer oil, oil-immersed diverter or selector switches should be in a separate container which may be located inside or’outside the main transformer oil. When located in the main transformer oil, it is not necessary for the container to be absolutely oil-tight, providing that precautions are

IS : 8478 - 1977

taken to avoid any mixing of the switch oil with the transformer oil and that the oil in the container can be changed easily without lowering the oil in the main transformer tank. Where appropriate, arrangements should bc made for it to be made clear in the transformer maintenance instructions that the main transformer oil should not be drained until all of the oil leas been removed from the diverter switch container.

If an oil-tight container is required it should bc spcciliccl by the p~~rchascr.

4. FIELD SERVICE

4.1 Safety of Operation

4.1.1 Such protective devices as may be considered necessary ~h~ul~l be connected in accordance with the manufacturers instructions.

4.1.2 In order to minimize switching under excessive overload or .&ort- circuit conditions, it is recommended that, in the case of motor control, a protective device should be fitted to prevent, ar if initiated to interrupt, an operation of the motor-drive mechanism when the transformer load exceeds the agreed value. The protective device may be in the form of a contactor in the motor-drive mechanism.

NATE - In the case of manual control, protective devices are not considered necessary as it is not normal practice to tap-change manually during periods of overload and the probability of coincidence of a tap-change operation under short-circuit conditions is negligible.

4.2 Parallel Operation - In the case of parallel operation of transformers with-tapped winding, care should be taken by the manufacturer and the user of the transformer to ensure that the currents circulating between the transformers’are limited to an acceptable value.

4.3 Contact Erosion and Oil Contamination - Tap-changers have expendable items in their construction and the manufacturers figures should be noted for maintenance periods in terms of time and in numbers of opera- tions. Generally, expected contact life for a particular tap-changer is given at rated through-current. If the load current of the transformer is less than this value, contact life is increased.

The number of operations before oil-change is necessary, is conditional on the oil being in good condition originally and maintained in a dry state.

For good utilization of the tap-changer, it is necessary to follow the mai - tenance instruction book of the tap-changer manufacturer which 3 gives the above as a function of:

general y

a) rated through-current, and b) the service duty of the transformer.

5. INFORMATION REQUIRED WITH ENQUIRY OR ORDER

5.1 In order that the correct tap-changer can be supplied, the following information should be given by the transformer manufacturer.

7

KS : 8478 - 1977

5.1.1 Xatity otld General Bata

a) Relevant Indian Standard. b) Number of tap-changers required. c) Single-phase or polyphase units. d) Number of phases in system. cj Frequency. fj J<atcd power, in I<VA, of apparatus to which the tap-changer is to

IK connected. g) Rated voltage of the winding to which the tap-changer is to be

connected. 11 j Winding connections. j) Tapping range required given as percent above and below the

rated voltage of the winding [see IS : 2026 (Part IV)-1977”]. lr) Number of service tap positions ~required, the numbering of these

positions and their identification with reference to the transformer lappings.

m) Tapping arrangement, that is, linear, reversing or coarse/fine. n) Position of tappings in winding, for exampJe, line end, middle, star-

point. p) Highest tapping current of the winding to which the tap-changer

is to be connected [see IS: 2026 (Part IV)-1977*]. q) Maximum value and duration of short circuit current passing

through the tap-changer. r) Phase voltage per step (if the step voltage varies over the range,

give full details together with associated currents). sj On star-point tap-changers, if one neutral terminal or three separate

neutral terminal4 required. t) The power frequency voltage appearing between the opening and

closing contacts of the changeover selector (see 2.6).

NOTE - The power frequency voltage has two componet+, that is! an inductive compo- nent and a capacitive component, during the time the windmg is dlsconnected.

5.1.2 hulation I;evel - The following impulse voltage and power frequency voltage test value should be given:

a) Highest voltage between the extreme tappings and, where applica- ble, the highest voltage between the ends of the coarse tapping winding section and the fine tapping section.

b) Highest voItage between the most onerously stressed tapping and earth.

c) Highest voltage between tapping of adjacent phases.

*Specification for power transformers: Part IV Terminal markings, tappings and con- nections (@t revision).

8

IS : 8478 - 1977

d) Highest voltage between the diverter switch and earth. ej Highest voltage between phases of the diverter switch.. f) Highest voltage betkeen open diverter switch contacts. NOTE - If applicable, partial discharge and switching impulse levels should be given.

5.2 Pressure, Vacuum and Temperature Requirements

a) Maximum working pressure when oil-filled. b) Maximum pressure during oil-tests on the apparatus. c) Maximum vacuum to be applied. d) Type of processing, maximum temperature, vacuum and duration

if the tap-changer is fitted before this operation. e) Temperatures in s,>ecial environments, for example, noise enclo-

sures, etc. f) Minimum ~operating temperatures and details of any special low

temperature requirements if below -5°C.

5.3 Special a) Details of periods of overload, value and duration (see 2.3.2 and

_IS : 6600-1972*). b) Details of duty of apparatus, for example, distribution transformer,

arc furnace tranSfOrJnCr, etc. c) Details of transport of transformer. d) Any special reqltircments of the specification.

5.4 Fittings - Items not covered by the tap-changer manufacturer’s standard, for cxamplc, valves, should bc spccikd.

5.5 Motor-Drive Mechanism- ‘ro enable the correct control devices to be included in the Jnotor-drive mechanism the fullest possible details of the control scheme should be given, including any of the following basic control functions if required, together with the type of device necessary for the function.

Local electrical control and indication. Remote electrical control and indication. Automatic control and indication, with or without line drop compensation. Parallel control of two or more transformers. Supervisory control and indication.

c

In the case of remote and supervisory control and indication, the approximate distance between the tap-changer and the control point should be stated. Auxiliary supply details for electrical motor and control equip- ment, that is, normal voltage, maximum and minimum voltage limits ifnot within the standard limits given in 9.2 of IS : 8468-l 977t, ac or dc. If ac, frequency, number of phases and availability of neutral.

*Guide for loading of oil-immersed transformer+ ~Specilicntion for on-load tap-changers.

9

BUREAU OF INDIAN STANDARDS

Headquarters:

Manak Bhavan, 9 Bahadur Shah Zafar Marg, NEW DELHI 110002

Telephones: 331 01 31, 331 13 75 Telegrams: Manaksanstha ( Common to all Offices )

Regional Offices: Telephone Central : Manak Bhavan, 9 Bahadur Shah Zafar

NEW DELHI 110002 Marg, I 331 01 31

331 1375 ’ *Eastern : l/l 4 C. I. T. Scheme VII M, V. I, P. Road, 36 24 99

Maniktola. CALCUTTA 700054 Northern : SC0 445-446, Sector 35-C, I 21643

CHANDIGARH 160036 3 1641 41 24 42

Southern : C. I. T. Campus, MADRAS 600113 ( 41 25 19 41 2916

tWestern : Manakalaya, E9 MIDC, Marol, Andheri ( East ), 6 32 92 95 BOMBAY 400093

Branch Offices:

‘Pushpak’, Nurmohamed Shaikh Marg, Khanpur, I 2 63 48 AHMADABAD 380001 49 2 63

+,Peenya Industrial Area 1st Stage, Bangalore Tumkur Road I 38 49 55 BANGALORE 560058 38 49 56

Gangotri Complex, 5th Floor, Bhadbhada Road, T. T. Nagar, 667 16 BHOPAL 462003

Plot No. 82/83. Lewis Road, BHUBANESHWAR 751002 5 36 27 53/S. Ward No. 29, R.G. Barua Road, 5th Byelane, 3 31 77

GUWAHATI 781003 5-B-56C L. N. Gupta Marg ( Nampally Station Road ),

HYDERABAD 500001 23 1083

R14 Yudhister Marg, C Scheme, JAIPUR 302005 {

63471 6 98 32

117/418 B Sarvodaya Nagar, KANPUR 208005 I :: “e’: 3; Patliputra Industrial Estate, PATNA 800013 6 23 05 T.C. No. 14/l 421. University P.O.. Palayam 16 21 04

TRIVANDRUM 695035 16 2-1 17

inspection Offices ( With Sale Point ):

Pushpanjali. First Floor, 205-A West High Court Road, 2 51 71 Shankar Nagar Square, NAGPUR 440010

Institution of Engineers ( India ), Building, 1332 Shivaji-Nagar, 5 24 35 PUNE 411005

*Sales Office in Calcutta is at 6 Chowringhre Approach, P. 0. Princsp 27 68 00 Street, Calcutta 700072

tSales Office in Bombav is at Novelty Chambers. Grant Road. 89 66 28 Bom’bav 400007

tsaks~office in Bangalore is at Unity Building, Narasimharaia Square; 22 36 71 Bangalore 560002

Reprography Unit, BIS, New Delhi, India