( Reaffirmed 1996 )

Indian Standard
SPECIFICATION FOR
DRY-TYPE POWER TRANSFORMERS

Transformers Sectional Committee, ETDC 16

Chairman Representing
SHRI C. R. VARIER Crompton Greaves Ltd, Bombay
Members
SHRI S. V. MANERIKAR ( Alternate to
Shri C. R. Varier )
SHRI R. S. ARORA Directorate General of Supplies & Disposals
( Inspection Wing ), New Delhi
SHRI D. R. CHANDRAN ( Alternate )
SHRI P. K. BHATTACHARYA Calcutta Electric Supply Corporation Ltd,
Calcutta
SHRI B. C. GHOSII ( Alternate )
DIRECTOR ( SUBSTATIONS ) Central Electricity Authority, New Delhi
DEPUTY DIRECTOR ( SUBSTA-
TIONS ) ( Alternate )
SARI G. L. DUA Rural Blectrification Corporation Ltd, New
Delhi
SHRI S. C. KHURANA ( Alternate )
SHRI A. R. DWAILI(ANATH Madhya Pradesh Electricity Board, Jabalpur
SHRI N. N. BHAI~~AVA ( Alternate )
SHRI S. D. GUPTA UP State Electricity Board, Lucknow
S~nr SATISH KUMAR (Alternate )
SHRI P. J. INoLB Central Power Research Institute, Bangalore
SENIOI~ DEPUTY DIRECTOR
( STDS ) (Alternate )
JOINT DIRECTOR TI ( SUBSTATION ) Research, Designs and Standards Organization,
Lucknow
DEPUTY DIRECTOR STDS
( ELEC )-C2 ( Alternate )
SHRI R. S. KHODE Bharat Bijlee Ltd, Bombay
SERI R. G. PARDH~NANI ( Alternate)
SHRI D. B. MEHTA Tata Hydro-Electric Power Supply Company
Ltd, Bombay
SRRI R. C~ANDRAMOULI (Alternate)
SHRI D. V. NAI~KE General Electric Company of India Ltd,
Allahabad
SHRI B. A. SUBRAMANYAM ( Alternate )
( Co&sued on page 2 )

0 Copyright 1985
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 permissionof the
nublisher shall be deemed to be an infringement of copyright under the said Act.
18 : 11171- l!HH

( Continued from puga 1 )

Members Representing
SERI B. K. PATEL Gujarat Electricity Board, Vadodara
S~<RIC. D. PATEL ( Alternate)
SHRI Y. K. PALVANEAR Bombay Electric Supply and Transport Underta-
king, Bombay
SHRI J. L. RODRI~UES ( Altcrnafc )
DR G. M. P~ADKE Indian Electrical Manufacturers’ Association,
Bombay
SARI P. K. PHILIP ( Alternate )
DR K. V. N. RAO Electrical Research and Development Associa-
tion, Bombay
SHRI N. J. BUCH ( Alternate )
SHRI CBANDRA K. ROHATQI Pradip Lamp Works, Patna
SHRI P. S. SAWHNEY Delhi Electric Supply Undertaking, New Delhi
SHRI S. P. A~QARWAL ( Alternate )
SHRI R. K. SEH~AL Bombay Suburban Electric Supply Ltd, Bombay
SHRI H. S. NATARAJAN ( Afternate )
SHRI K. G. SHAN~~UKHAPPA NGEF Ltd, Bangalore
SHRI MOHAMBIED YOUWJF~ (Alternate )
SUPERINTENDINQ E N Q I N E E B Andhra Pradesh State Electricit Department
TECIXNICAL ( PROJECTS) ( Electricity Projects and Boar(r ), Hyderabad
SUPERINTENDINQ ENGINEER
( GRID OPERATIONS) ( Altcrnats )
SIIRI I. C. TAYAL Bharat Heavy Electricals Ltd, New Delhi
SHRI R. S. SIIAH ( Alternate I )
SHRI D. P. GUPTA ( Alternate II )
DR VAIUL AHUED Directorate General of Technical Development,
New Delhi
SHRI S. K. PAIHAN ( Alternate )
SHRI T. V. VIDYARATNA RAJ Kirloskar Electric Company Ltd, Bangalore
SHT~I1’. RAN~ARAJAN ( Alternate )
SHRI VISH~AJIT M. ME~TA Indian Transformer Manufacturers’ Association,
New Delhi
SHRI S. P. SACHDEV, Director General, IS1 ( Ex-omo Mambo )
Director ( Elec tech )

SHRI K. M. BHATIA
Joint Director ( Elec tech ), IS1

2
 IS:11173 -1985
(

Indkzn Standard
SPECIFICATION FOR
DRY-TYPE POWER TRANSFORMERS

0. FOREWORD
0.1 This Indian Standard was adopted by the Indian Standards Institution
on 15 May 1985, after the draft finalized by the Transformers Sectional
Committee had been approved by the Electrotechnical Division Council.
0.2 Supply voltages have been increased in recent years to meet the
growing demand for electricity. IS : 2026* ( issued in four parts ) covers
every aspect of power transformers including high ratings to keep abreast
of technological progress. Since dry-type power transformers are not made
for such high voltages and ratings as oil-immersed transformers, a large
portion of IS : 2026* serves no purpose to manufacturers and the users of
dry-type transformers. Thus, for convenience sake, this separate and
concise standard has been prepared to set out specific requirements for dry-
type power transformers.
0.3 This standard is to be read in conjunction with IS : 2026*, since
a reference has been made to the technical data of that standard, where
appropriate.
0.4 With the publication of this standard, the requirements of IS : 2026*,
apply to dry-type transformers only in so far as they are referred to in this
standard.
0.5 This standard is primarily based on IEC Publication 726 ( 1982 ) ‘Dry-
type power transformers’ issued by the International Electrotechnical
Commission.
0.6 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 or analysis, shall be rounded off in accord-
ance 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 power transformers:

Part 1 General (jr& r&ion ).
Part 2 Temperature-rise ( jirst ret&ion ).
Part 3 Insulation levels and dielectric tests ( second revision ).
Part 4 Terminal markings tappings and connections (first r&ion ).
tRules for rounding off numerical valuer ( revised ).

3
IS : r1171- 19ii5

SECTION 1 GENERAL

1. SCOPE

1.1 This standard applies to dry-type power transformers ( including auto-

transformers ) having values of highest voltage for equipment ( urrr ) up to
and including 36 kV.

1.2 The following small and special dry-type transformers are not covered
by this standard :

4 Single-phase transformers rated at less than 1 kVA and polyphase

transformers rated at less than 5 kVA,
b) Instrument transformers ( see IS : 2705* and IS : 31561_ ),
Cl Transformers for static convertors,
4 Starting transformers,
e) Testing transformers,
f ) Traction transformers mounted on rolling stock,
8) Flameproof and mining transformers,
h) Welding transformers ( see IS : 1851-19751 ),
j) Voltage regulating transformers, and
k) Small power transformers in which safety is a special considera-
tion.

2. TERMINOLOGY
2.0 For the purpose of this standard, the following definitions, in addition
to those given in IS : 1885 ( Part 38 )-1977s shall apply.
2.1 DryiTransformers - A transformer in which the core and windings
are not immersed in an insulating liquid.
NOTE - The cooling of a dry-type transformer may be natural, natural/forced
or forced circulation.

*Specification for current transformers :

Part 1 General requirements (first revision ).
Part 2 Measuring current transformers ( jirst revision ).
Part 3 Protective current transformers (jfirst revision ).
Part 4 Protective current transformers for special purpose applications (first
w&ion ) .
tSpe.cification for voltage transformers :
Part 1 General requirements (Jirsl revision ).
Part 2 Measuring voltage transformers (jirst rruision ).
Part 3 Protective voltage transformers ( jirst revision ).
Part 4 Capacitor voltage transformers (Jirst revision ).
fSpecification for single pperator type are welding transformers ( sccand revision ).
$Electrotechnical vocabulary : Part 38 Transformers (first revision ).

4
 IS: Ill?1 -1985

2.1.1 Encapsulated- Winding Dry-Type Transformers - A dry-type transfor-

mer having one or more windings encapsulated with solid insulation.
2.1.2 Non-Encajsulated- Winding Dry- Tyje Transformer - A dry-type
transformer having none of the windings encapsulated with solid insula-
tion.
2.2 Terms Relating to the Containment of a Dry-Type Transfer;
mer
2.2.1 Sealed Dry-Tybe Transformer - An air or gas-immersed dry-type
transformer within a sealed protective enclosure so constructed that there
can be no interchange between its contents and the external atmosphere,
that is, the transformer is non-breathing.
NOTE- Thegas in a gas-immersed transformer remains in a gaseous state
throughout the range of transformer operations.

2.2.2 Totally Enclosed Dry-Type Transformer - An air immersed dry-type

transformer within a protective enclosure so constructed that the ambient
air does not circulate to cool the core and windings, but which can
breathe to atmosphere.
2.2.3 Enclosed Dry-Type Transformer - A dry-type transformer within a
protective enclosure so constructed that the ambient air may circulate so
as to cool the core and windings directly.

2.2.4 Non-Enclosed Dry- Type Transformer - A dry-type transformer

without a protective enclosure in which the core and windings are cooled
by ambient air.

3. SERVICE CONDITIONS
3.1 Normal ‘Service Conditions - This standard gives detailed require-
ments for transformers for use under the following conditions :
a) Altitude - A height above mean sea-level not exceeding 1 000 m.
NOTE- For greater altitudes, see 3.2.

b) Reference ambient temperature

1) Maximum ambient air temperature 50°C
2) Maximum daily average ambient air temperature 40%
3) Maximum yearly weighted average ambient temperature 32°C
4) Minimum ambient air temperature -5°C
NOTE - See also 3.2.

Cl Wave-shape of supply uoltage - A supply voltage of which the wave-

shape is approximately sinusoidal.

4 Symmetry of polyphase supPly voltages - For polyphase transformers,

supply voltages which are approximately symmetrical.

5
IS : 11171- 1985

3.2 Provision for Unusual Service Conditions

3.2.1 The purchaser shall specify in his enquiry any service conditions
not covered by the normal service conditions in 3.1 ( SCGAppendix A ).

3.2.2 Supplementary requirements, within defined limits, for the rating

and testing of transformers designed for other than normal service condi-
tions listed in 3.1, such as high temperature of cooling air or altitude
above 1 000 m are given in 18.2, 10.3 and 11.2.
3.2.3 For temperature conditions outside the limits covered by the sup-
plementary requirements and special operating conditions, for example
restricted cooling air circulation, the temperature rise is to be subject to
agreement between the manufacturer and the purchaser.

4. TERMINAL MARKINGS, TAPPINGS AND CONNECTIONS

4.1 Relevant provisions,of IS : 2026 ( Part 4 )-1977* shall be applicable,
but where tappings are required the preferred. tapping range is + 5 per-
cent in 2-5 percent steps, that is, +5, +2*5, -2.5 and -5 percent by
means of off-circuit tap-changing links or tap switch.

5. ABILITY TO WITHSTAND SHORT-CIRCUIT

5.4 The relevant provisions of IS : 2026 ( Part 1 )-1977t apply to the
transformers covered by this standard.

6. INFORMATION REQUIRED WITH ENQUIRY AND ORDER

6.1 The technical information that the purchaser is required to supply
with enquiry and order is given in Appendix B.

SECTION 2 RATING
7. RATING
7.0 General - The manufacturer shall assign ratings to the transformer,
which shall be marked on the rating plate (see 8 ). These ratings shall be
such that the transformer can deliver its rated current under steady
loading conditions without exceeding the limits of temperature-rise speci-
fied in Section 3, assuming that the applied voltage is equal to the rated
voltage and that the supply is at rated frequency.

7.1 Rated Power ( kVA ) -The rated kVA assigned shall take into
account service conditions corresponding to those specified in 3 and shall
be related to the product of rated voltage, rated current and the appro-
priate phase factor given in Table 1.
*Specification for power transformers : Part 4 Terminal markings, tappings and
connections (Jirst revision).
$Specilication for power transformers : Part 1 General (first revision).

6
 TABLE 1 PHASE FACTORS
( Clause 7.1 )
NUMBEE OF PRASE~ PHASE FACTOR

1
3

NOTE - The rated power corresponds to continuous duty; nevertheless, dry-

type transformers complying with this standard can be occasionally overloaded to
some extent under certain conditions. Guidance on overloads will be either added
in this standard or provided in a separate standard sometimes later.

7.2 Power ( kVA ) Ratings - Provisions of 4.2 of IS : 2026 ( Part 1 )-

1977* shall apply.
7.3 Rated Frequency - The standard frequency shall be 50 Hz with a
tolerance of f 3 percent.
NOTE - Nevertheless, transformers made for other frequencies shall be conside-
red to comply with this standard provided they do so in all other relevant respects.
7.4 Operation at Other than Rated Voltage and Frequency -
Provisions of 4.3 of IS : 2026 ( Part 1 )-1977” shall apply.
8. RATING PLATES
8.1 Each transformer shall be provided with a rating plate of weather-
proof material, fitted in a visible position, showing the items indicated
below. The entries on the plate shall be indelibly marked ( for example,
by etching, engraving or sta&pi,ng ) .

a>Kind of transformer ( see definitions given in 2 );

b) Number of this standard, Ref IS : 11171-1985;
cl Manufacturer’s name;
4 Manufacturer’s serial number;
e) Year of manufacture;
f 1 Temperature class of insulation ( Insulation system temperature )
and maximum permissible temperature rise for all windings, or
for each individual winding, if applicable ( see 10.1 );
g> Number of phases;
h) Rated power ( kVA ) for each kind of cooling;
3 Rated frequency;
k) Rated voltages, including tapping voltages, if any;
ml Rated currents for each kind of cooling;
4 Connection symbol;
*Specification for power transformers : Part 1 General (jirstrevision ).

7
IS 011171 - 1985

P) Impedance voltage at rated current [determined according to

16.4 of IS : 2026 ( Part 1 )-1977*];
cl) Type of cooling;
r) Type of filling
f
s) Operating pressure range i
t) Maximum and minimum absolute 1
> applicable only to
pressures for which the enclosure is
1 sealed units
designed
I
u) Pressure and temperature of filling 1
medium at the time of sealing j
w) Total mass; and
x) Insulation levels
NOTE - The rated withstand voltages for all windings shall appear on the
rating plate. The principles of the standard notation are illustrated in 3 of
IS : 2026 ( Part 3 )-1981t.

8.2 Dry-type power transformers may also be marked with ISI Certifica-
tion Mark.

NOTE -The use of the IS1 Certification Mark is governed by the provisions of
the Indian Standards Institution ( Certification Marks) Act and the Rules and
Regulations made thereunder. The IS1 Mark on products covered by an Indian
Standard conveys the assurance that they have been produced to comply witb the
requirements of that standard under a well-defined system of inspection, testing and
quality control which is devised and supervised by IS1 and operated by the
producer. IS1 marked products are also continuously checked by IS1 for conformity
to that standard as a further safeguard. Details of conditions, under which a licence
for the use of the IS1 Certification Mark may be granted to manufacturers or
processors, may be obtained from the Indian Standards Institution.

SECTION 3 COOLING METHODS AND

TEMPERATURE-RISE

9. IDENTIFICATION ACCORDING TO COOLING METHOD

9.1 Identification Symbols - Transformers shall be identified actor d-

ing to the cooling method employed. Letter symbols for use in connection
with each cooling method shall be as given in Table 2.

*Specification for power transformers : Part 1 General (Jirsl revision ).

tSpecilication for power transformers : Part 3 Insulation levels and dielectric tests
( second revision ).

8
 Isrlll7l-r98!!

TABLE 2 LETTER SYh¶BOLS

( Cfausr 9.1 )
.
KIND OB COOLINGMEDIUM Symbol
Air A
Gas G
KIND OB CIBOULAT~GN
Natural N
Forced F

9.2 Arrangement of Symbols - Transformers without protective enclo-

sures or within enclosures through which the cooling air can circulate are
identified by two symbols only for the cooling medium ( air ) which is in
contact with the windings or the surface’ coating of the windings. All other
transformers shall be identified by four symbols for each cooling method
for which a rating is assigned by the manufacturer.
9.2.1 The order in which the symbols are used shall be as given in
Table 3. Oblique strokes shall be used to separate the group of symbols
for different cooling methods for the same piece of equipment.

TABLE 3 ORDER OF SYMBOLS

COOSINGMEDIUM IN CONTACTWITH COOLINGMEDIUM IN CONTACTWITH TEE
THE WINDINQS EXTERNALCOOLINGSYSTEM
1st Letter 2nd Letter 3rd Letter 4th Letter
Kindm;g;ding Kind of Kind of cooling Kind of
circulation medium circulation

9.2.2 For example, the cooling method of a transformer without a

protectiveenclosure or within an enclosure through which the cooling air
can circulate and with natural air cooling is designated by :
AN
9.2.3 For a transformer within a protective enclosure through which the
cooling air cannot circulate, with natural air cooling inside and outside the
enclosure, the designation is :
ANAN
9.2.4 For a transformer in a sealed enclosure with natural nitrogen
cooling inside and alternatives of natural or forced air cooling outside the
cnchure, the designation is :
GNAN/ GNAF
10. TEMPERATURE-RISE LIMITS
10.1 Normal Temperature-Rise Limits - The temperature rises of
the windings, core and metal parts of transformers designed for operation

9
Is t’lll7l - 1985

at altitudes not exceeding those given in 3.1 (a) and with external cooling
air temperature as described in 3.1 (b) shall not exceed the limits specified
in Table 4 when tested in accordance with 17.
NOTE - Insulating materials may be used separately or in a combination provi-
ding that, in any application, each system will not be continuously subjected to a
temperature in excess of that for which it is suitable when operating under rated
conditions. Furthermore, the electrical and mechanical properties of the insulated
winding must not be impaired by the application of the hot-spot temperature per-
mitted for the specrfic insulation system.

TABLE 4 TEMPERATURE-RISE LIMITS

( Cluu~cs 10.1 and 10.3 )
SL PART TEMPERATURE TEMPERATURE-
No. CLASS OP RISE
INSOLATION*

(1) (2) (3) (4)

YI
i) Windings (Temperature-rise A 50
” measured by resistance method ) E 65
B 70

I!I 1::
C 140
ii) Core, metallic parts and The temperature shall in
adjacent materials no case reach a value
that will damage the
core itself, other parts. or
adjacent materials
*In accordance with IS : 1271-1958 ‘ Classification of insulating material for electri-
cal machinery and apparatus in relation t,o their thermal stability in service. ’

10.2 Reduced Temperature-Rises for Transformers Designed foti

High Cooling Air Temperatures or Special Air Cooling Con&;
tions - If the transformer is designed for service where the temperature
of the cooling air exceeds one of the maximum values of 3.1 by. no more
than lO”C, .the allowable temperature-rises for the windings shall be
reduced as follows :
a) By 5°C if the excess temperature is less than or equal to 5°C; and
b) By 10°C if the excess temperature is greater than 5°C and less
-_ . -,,than or equal.to 10°C.
Where the excess temperature exceeds one of the values specified
in 3.1 by more than 1O”C, the allowable temperature-rises require an agree-
ment between the manufacturer and the purchaser.
Any site conditions which may either impose restrictions on the
cooling air or produce high ambient air temperatures should be stated by
the ipurchaser.

10
 IS : 11171 - 198!!

19.3~ Reduced Temperature-Rises for Transformers Designed for

I&i& Altitudes - Unless otherwise agreed between the manufacturer
and.the purchaser, for transformers designed for operation at an altitude
greater than 1 000 m but tested at normal altitudes, the limits of tempera-
turerise given in Table 4 are reduced by the following amounts for each
500 m by which the intended working altitude exceeds I 000 m :
a) Natural-air-cooled transformers 2-5 percent
b) Forced-air-cooled transformers 5 percent
NOTE - If transformers which are designed for operation below 1 000 m are
tested at altitudes above 1 000 m, the measured temperature rises are to be reduced
by the above-mentioned amounts for each 500 m by which the test altitude exceed
1000 m.

SECTION 4 INSULATION LEVELS

11. INSULATION LEVELS

ilk General - When transformers are intended for general power
distribution in public or industrial systems, the insulation levels shall be
asgiven~in Table 5, List 1 or 2. The choice between Lists 1 and 2 should
be made by considering the degree of exposure to lightning and switching
overvoltages, the type of system neutral earthing and, where applicable,
the type of overvoltage protective device. Guidance may be obtained from
13:.2165-1977*.
However, for application in particular systems where the insulation
requirements are lower than in general and where the transformers are not
required to be tested with lightning impulse withstand voltages, the power
f&quency testvoltage applied may be below the value specified in Table 5
subject to agreement between the manufacturer and the purchaser. No
‘I
definite figures are recommended here.
.*_ .,
_.. -Y----~-
~ ~

., TABLE 5 INSULATION LEVELS

I&HEST VOLTAUEFOR RATED SHORTDURATION RATED LI~HTNINU IMPULSE
EQUIPYENT Um POWER FREQUENCY WITHSTAND VOLTAGE
WITHSTAND VOLTAQE kV ( peak )
r---- *_--
kV ( rms) kV ( rms ) List 1 List 2
<l-l 3 -

_j, 3’6, 10 20 40
7.2 20 40 60
12 28 60 75
17’5 38 75 95
24 50 95 125
36 7il 145 170

*Insulation coordination ( second

revision ).

11
g t 11171-1985

11.2 Transformers for Use at High Altitudes - When the transbb

mers are specified for operation at altitudes between 1 000 m and 3 000 m
above mean sea-level, but tested at normal altitude, the rated short
duration power frequency withstand voltage shall be increased
by 6.25 percent for each 500 m by which the altitude of the installation
exceeds 1 000 m.
NOTE - The above does not apply to sealed dry-type transformers but rpecjal
consideration should be given to bushings.

SECTION 5 TESTS

12. GENERAL REQUIREMENTS FOR TESTS

12.1 Transformers shall be subjected to tests as specified in 13.
12.2 Tests shall be made at the manufacturer’s works, unless otherwise
agreed to between the manufacturer and the purchaser.
12.3 Dielectric tests in accordance with 14, 15 and 16 shall be made with
the transformer at approximately ambient temperature.
12.4 All external components and fittings that are likely to affect the
performance of the transformer during each test shall be in place.
12.5 Tapped windings shall be connected on their principal tapping,
unless otherwise given in relevant part of IS : 2026* or agreed between
the manufacturer and the purchaser.
12.6 The test basis for all characteristics other than insulation is the rated
condition, unless the test clause states otherwise.
12.7 The reference temperature to which the load loss, impedance voltage
and shorticircuit impedance are to be corrected shall be the maximum
winding temperature-rise as given in Table 4 plus 30°C.
NOTE- When a trsnsformer has windings of different insulation system temt
peratures ( class of insulation ), only one reference temperature is to be used, that
is, the one relating to the winding having the higher inrulation system temperature,

13. CLASSIFICATION OF TESTS

13.1 Type Tests - The following shall constitute the type tests :
a> Measurement of winding resistance [ 16.2 of IS : 2026 ( Part 1 >-
1977* 1,
b) Measurement of voltage ratio and check of voltage vector rela-
tionship [ 16.3 of IS : 2026 ( Part 1 )-1977* ] :

*Specification for power transformers :

Part 1 General ( jrst revision).
Part 2 Temperature-rise ( Jirsi r&ion ).
Part 3 Insulation levels and dielectric tests (s~ond revision).
Part 4 Terminal markings, tappings and connections (first rcvition ),

12
 IS : 11171- 1983

4 Measurement of impedance voltage ( principal tapping ), short-

circuit impedance and load loss [ 16.4 of IS : 2026 ( Part 1 )-
1977* 1;
4 Measurement of no-load loss and current [ 16.5 of IS : 2026
( Part 1 )-1977*];
4 Separate-source voltage withstand test ( 14 );
f> Induced overvoltage withstand test ( 15 );
d Lightning impulse test ( 16 ); and
h) Temperature-rise test ( 17 ).

13.2 Routine Tests - The following shall constitute routine tests :

a) Measurement of winding resistance [ 16.2 of IS : 2026 ( Part 1 )-
1977* 1;
b) Measurement of voltage ratio and check of voltage vector
relationship [ 16.3 of IS : 2026 ( Part 1 )-1977* 1;
‘c) Measurement of impedance of voltage ( principal tapping ),
short-circuit impedence and load loss [ 16.4 of IS : 2026 (Part I)-
197i”] ;
d) Measurement of no-load loss and. current [ 16.5 of IS : 2026
( Part 1 )-1977* ] ;
e) Separate-source voltage withstand test ( 14 ); and
f) Induced overvoltage withstand test ( 15 ).

13.3 Special Tests - The following tests may be carried out by mutual
agreement between the purchaser and the supplier :

a) Partial discharge measurement ( 18 );

b) Measurement of acoustic sound level ( 19 ); and
c) Short-circuit test ( 20 ).

14. SEPARATE-SOURCE VOLTAGE WITHSTAND TEST

14.1 The provisions of 10 of IS : 2026 ( Part 3 ) - 1981 t are applicable.
14.2 The test voltage shall be in accordance with Table 5 for the specified
insulation level of the transformer, if no other value has been agreed upon
by the manufacturer and the purchaser.

The full test voltage shall be applied for 60 s between the winding
under test and all the remaining windings, core, frame and transformer
enclosure, connected to earth.

*Specification for power transformers : Part 1 General (Jirsi reuision ).

+Specification for power transformers : Part 3 Insulation level and dielectric tests
( second revision ) .

13
IS : 11171 - 1985

15. INDUCED OVERVOLTAGE WITHSTAND TEST

15.1 The test shall be carried out in accordance with 11.1 and 11.2
of IS : 2026 ( Part 3 ) - 1981*.
15.2 The test voltage shall be twice the value corresponding to the rated
voltage across an untapped winding of the transformer.
The duration of the test at full volage shall be 60 s for any test
frequency up to and including twice the rated frequency.
When the test frequency exceeds twice the rated frequency, the
duration of the test shall be:
120 x rated frequency seconds
2
test frequency
but not less than 15 s.

16. LIGHTNING IMPULSE TEST

16.1 The test shall be carried out in accordance with 12 of IS : 2026
( Part 3 )-1981*.
16.2 The test voltage shall be in accordance with List 1 or List 2 of
Table 5 for the specified insulation level of the transformer.
The test impulse shall be a full standard lightning impulse:
l-2 & 30 percent/50 & 20 percent micro-seconds
The test voltage shall normally be of negative polarity. The test
sequence per line terminal shall be one calibration impulse at a voltage
between 50 and 75 percent of the full voltage followed by three impulses
at full voltage.
By agreement between the manufacturer and the purchaser at the
time of the enquiry, tests may be made with positive polarity, but in this
case sudden changes of polarity have to be avoided.
NOTE- In dry-type transformers, the lightning impulse test can give rise to
capacitive partial discharges in the air which do not endanger the insulation. These
partial discharges lead to changes in the current waveform, whilst the voltage
waveform varies only slightly or not at all.

17. TEMPERATURE-RISE TEST

17.1 Provisions of 4.2.1, 4.1~2, 4.1.3, 4.3, 4.9.1, 4.9‘2 and 4.9.3 of
IS : 2026 ( Part 2 )-I9777 are applicable, in addition to the following.
17.2 Methods of Loading - At the choice of the manufacturer, any of
the following methods may be applied.
-
*Specification for power transformers : Part 3 Insulation level and dielectric tests
( second revision ) .
tSpecilication for power transformer3 : Part 2 Temperature-rise (Jirst rctisioh ).

14
Y

IS:11171- 1985

17.2.1 Direct Loading Method* - One winding, preferably the inner

winding, of the transformer is excited at rated volatge with the other
connected to a suitable load such that rated currents flow in both
windings.

17.2.2 Back-to-Back Method* - This is the preferred method, applicable

when there are two similar transformers available.

Two transformers, one of which is the transformer under test, are

connected in parallel, and preferably the inner windings are excited at
the rated voltage of the transformer under test. By means of different
voltage ratios or an injected voltage, the rated current is made to flow in
the transformer under test.
NOTE - In view of the fact that the transtormers will be operated at two differ-
ent taps the winding temperature-rise of two transformer may be slightly different.
While interpreting the results, appropriate corrections to the temperature-rise should
be made in accordance with 17.3.

17.2.3 Simulated Load Method ( By Agreement Between /he Munufacturer and

Purchaser ) - This method is used for an enclosed or non-enclosed dry-
type unit with natural air cooling, when only the transformer under test is
available, or when there is a similar transformer available but there are
limitations on the test equipment.
Temperature tests are made by utilizing the rises obtained on two
tests, one with no-load loss only, and one with load losses only, that is,
short-circuit run.
The no-load test at rated voltage is continued until stead-state condi-
tions are obtained; individual winding temperature rises, AtI, are then
measured.
The short-circuit run with rated current flowing in one winding and
the other winding short-circuited is started immediately following the no-
load run, and continued until steady-state conditions are obtained when
the individual winding temperature rises, A BO, are measured.

The total winding rise, no’,, of each winding, with rated current in
the winding and normal excitation of the core, is calculated by the
following formula:

Afo = A&[ 1 + ( $$-) lea’]

0e8
NOTE - For other types of transformer, modifications to the formula may be
necessary.

*The duration of the test may be reduced by exciting the core for a period of time
( preferably not less than 12 h ) prior to the application of test current to the
windings.

15
iS : 11171- 1985

17.3 Winding Temperature-Rise Correction for Reduced Curr-

ent - When the input test current It is below the rated value of current
I,, but not less than 90 percent IN, of the temperature rises, A& of the
windings, shall be measured by the resistance method when steady-state
conditions have been reached, and corrected to rated load conditions,
aeN, by the formula:

A& = A4 [$I”
The value of q shall be taken as:

AN transformers : l-6, and

AF transformers : 1.8.

17.4 Determination of Constant Temperature Conditions - .The

ultimate temperature-rise is reached when the temperature rise becomes
constant; this is considered to have been achieved when the temperature-
rise does not vary by more than 2 percent uf the permissible temperature-
rise per hour or 2°C per hour, whichever is the smaller.

For the purpose of determining when constant temperature condi-

tions have been achieved, thermocouples, thermistors, RTDs or thermome-
ters shall be applied to the following surfaces:
a) Enclosed and non-enclosed units - Centre of top yoke and as close
as practicable to the innermost low-voltage winding conductors
at the top of the winding, the measurement being on the centre
leg of a three-phase unit.
b) Baled and totally enclosed units - Centre of top cover surface and
centre of one side of enclosure surface.

18. PARTIAL DISCHARGE MEASUREMENT

18.1 General - The partial discharge measurement is performed in

accordance with IS : 6209-1982* and with Appendix A of IS : 2026
( Part 3 )-19815_. It may be performed on all kinds of dry-type transfor-
mers, but is especially applicable to transformers having encapsulated
windings.

18.2 Basic Measuring Circuit ( Typical Only ) - A basic measuring

circuit for the partial discharge test is shown in Fig. 1 and 2.
-
*Methods for partial discharge measurements ( first revision ).
tSpecification for power transformers: Part 3 Insulation levels and dielectric tests
( secotui revision).

16
 IS : 11171 - 1985

WINDING WINDING

FIG. 1 BASIC MEASURING CIRCUIT FOR THE PARTIAL DISCHARGE

TEST FOR A SINGLE-PHASETRANSFORMER

‘3
LOW-VOLTAGE
WINDING HIGH-VOLTAGE
‘WINDING, DELTA
OR STAR CDNN-
FCTED

FIG. 2 BASIC MEASURING CIRCUIT FOR THE PARTIAL DISCHARGE

TEST FOR A THREE-PHASE TRANSFORMER

In the figures, a partial discharge-free capacitor, C ( having a

capacitance value large in comparison with the calibration generator capaci-
tance, C, ) in series with a detection impedance, xm, is connected to each
of the high-voltage terminals.

18.3 Calibration of Measuring Circuit - Attenuation of the disch-

arge pulses occurs both within the windings and in the measuring circuit.
Calibration is carried out as described in Appendix A of IS: 2026
( Part 3 )-1981*, by injecting simulated discharge pulses from a standard
discharge calibrator at the transformer terminals. The repetition rate of
calibration pulses should be of the order of twice the frequency of the .
excitation voltage.

*Specification for power transformer: Part 3 Insulation levels and dielectric test
( second revision ).
I@-rlll7d l 1985

18.4 Voltage Application - The partial discharge measurement shall

be carried out after all dielectric tests are completed. The low-voltage
winding shall be supplied from a three-phase or single-phase source,
depending on whether the transformer itself is three-phase or single-phase.
The voltage shall be as nearly as possible & sinewave form and of a
frequency suitably increased above the rated fr@queircy to avoid excessive
excitation current during the test. The procedure shall be as given
in 18.4.1 or 18.4.2.
NOTE- When the operating voltage of a transformer is wall below the voltage
stated for the highest voltage for equipment, lower values for the pre-stress voltage
and the measuring voltage may be selected by agreement between the manufacturer
and the purchaser.

18.4.1 Three-Phase Transformers

a) Windings intended for connection to systems which are solidly

earthed or earthed through a low impedance:
1.5 ir,
A line-to-earth pre-stress voltage ofI shall be indu-
1/ 3
ted for 30 s ( LJ,,, being the highest v&age for the equip
ment ), followed without interruption by a line-to-earth
voltage ofK>$ for 3 min, during which the partial

discharge shall be measured.

b) Windings intended for connection to systems which are isolated
or earthed through a considerable impedance (for example, an
arc-suppression coil ) :
A phase-to-phase pre-stress voltage of 1.5 U, shall be indu-
ced for 30 s, with one line terminal earthed, followed with-
out interruption by a phase-to-phase voltage of 1.1 U,,, for
3 min, during which the partial discharge shall be measured.
This test shall be repeated with another line terminal
earthed.
18.4.2 Single-Phase Transformers - Single-phase transformers for use on
three-phase systems shall be tested as for three-phase transfomers, bearing
in mind that the highest voltage for equipment, U,, is a phase-to-phase
voltage.
Furthermore, in the case of a single-phase transformer connected
phase-to-neutral and tested according to 18.4.1(b), repeating the test with
another line terminal earthed does not apply.
18.5 Partial Discharge Acceptance Levels - The acceptance level
for the partial discharge measured as above shall be subject to agreement
between the manufacturer and the purchaser.

18
 IS :11171-1965

In cases of dispute, investigational procedures as detailed in

Appendix A of IS : 2026 ( Part 3 )-1981* shall be applied.
19. MEASUREMENT OF ACOUSTIC SOUND LEVEL
19.1 The method of test and criteria for conformity shall be as agreed to
between the manufacturer and the purchaser.
20. SHORT-CIRCUIT TEST
20.1 This test shall be as specified in IS : 2026 ( Part 1 )-1977t.
NOTE- References to gas relays and tanks in IS : 2026 (Part 1 ) - 1977t are
not applicable to dry-type transformers.

21. TOLERANCES
21.1 Tolerances shall be as specified in Table 7 of IS : 2026 ( Part 1 ) -
1977t.

SECTION 6 ENCLOSURES

22. DEGREE OF PROTECTION BY ENCLOSURES

22.1 The design of an enclosure is dependent upon the location and
environmental conditions in which the transformer is installed.
NOTE- A separate standard on classification of degree of protection provided
by enclosures is under preparation. In the meantime, reference may be made to
IS : 2147-1962$.

APPENDIX A
( Clause 3.2.1 )

UNUSUAL SERVICE CONDITIONS

A-l. In addition to high or low ambient temperature and high altitude,

other environment conditions which may affect design and application can
exist and are not necessarily covered by this standard.
*Specification for power transformers: Part 3 Insulation levels and ‘dielectric
tests ( second revision ).
tspecification for power transformers: Part 1 General (,&St rsision. )
#Degree of protection provided by enclosures for low voltage switchgear and
controlgear.

19
t$ : 11171 - 1985

Examples of such conditions are damaging. fumes and vapours,

excessive and abrasive dust, steam, salt spray, excessive moisture, dripping
water.
NOTE - The seriousness of the effects of the conditions listed above varies
widely depending upon which design of dry-type transformer is involved. While
such conditions may have little or no effect on a sealed, dry-type transformer, they
may have serious effects on other kinds of dry-type txansfbrmer.
A-2. Enclosed or non-enclosed non-encapsulated-winding dry-type trans-
formers are normally designed for installation indoors in dry locations.
They will, however, operate successfully where the humidity is high, provil
ded precautions are taken to keep.them dry if they are shut down for long
periods.

APPENDIX B
( Clause 6.1 )

INFORMATION REQUIRED WITH ENQUIRY AND ORDER

B-l. RATING AND GENERAL DATA

B-l.1 Normal - The following information should be given in all cases:

4 Particulars of the specifications to be complied with;

b) Kind of transformer ( see definitions given in 2 );
4 Single or polyphase units;
4 Number of phases in system;
e) Frequency;
f) Information as to whether an enclosure is required or not.
If it is, the degree of protection of the enclosure ( see 22 ).
Information on the gas, if required, to fill the enclosure;

g) Method of cooling;

h) Indoor or outdoor installation;

3 Rated power ( in LVA);

k) Rated voltage ( for each winding );

4 State whether tappings are required, and if SO, whether off-

circuit tap-changers or links are required;

20
 4 Highest voltage for equipment, U, ( for each winding );

P) Insulation level, that is, values of test voltage at power frequency

and, if required, lightning impulse ,for which each winding is to
be designed [see IS: 2026 ( Part 3 )-1981* 1.
q) Connection symbol;
r) Impedance voltage at rated current, if specific value is required;
s) Neutral terminals, if required ( for each winding );
t) Method of system earthing ( for each winding );

u) Any peculiarities of installation, for example space limitations,

difficulty of maintenance, storage conditions;

v) Any other appropriate information, for example site conditions,

duty for which the transformer is required, system overvoltages;
and
w) Details of fittings required and an indication of the side from
which meters, rating plates, etc, should be readable.

B-l.2 Special - The following additional information may be required

to be given:

4 Whether a lightning impulse test is required;

b) Altitude above sea level if in excess of 1 000 m ( see 10.3 and
11.2 );

c) Temperature of cooling air if above or below values given in

3.1 ( b ) ( see 10.2 also );

d) Whether the transformer is to be directly connected to other

apparatus which may affect short-circuit conditions;

e) Whether unbalanced loading is anticipated and if so, details

thereof;

f) Details of intended regular cyclic overloading;

g) Connections required ex-works in the case of a transformer

having alternative winding connections; and

h) Which snecial tests ( see Section 5 ) are reouired. and for each of
these te’sts the relevant acceptance conditions ( levels, lim;ts,
etc ).

*Specification for power transformers: Part 3 Insulation levels and dielectric tests
(second revision ).

21
 B-2. PARALLEL OPERATION
B-2.1 If parallel operation with existing transformers is required, this
should be ‘stated and the following information given concerning the
existing trnsformers:

a) Rated power ( kVA );

‘b) Rated voltage ratio;
.I
1

Cl Voltage ratios corresponding to tappings other than the principal

tapping;
I 4 L,oad loss at rated current on the principal tapping, corrected to
the appropriate,reference temperature;
Impedance voltage at rated current ( on the principal tapping );
.,t . e)
.fi
Short-circuit impedances, at least on the extreme tappings, if the
tapping. range of the tapped winding exceeds f 5 percent; and
g) Diagram of connections, or connection symbol, or both.