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भारतीय मानक IS 1180 (Part 3) : 2021

Indian Standard

बाहय/आंतरिक-रंग तरल इम्मेर्सड वितरण

ट्रांसफार्मर तक 2 500 kVA,
33 kV — विशिष्टि
भाग 3 प्राकृतिक/सश्
ं लेषित कार्बनिक
एस्टर तरल निमिज्जित

Outdoor/Indoor Type Liquid

Immersed Distribution Transformers
Up to and Including 2 500 kVA,
33 kV — Specification
Part 3 Natural/Synthetic Organic
Ester Liquid Immersed

ICS 29.180

© BIS 2021

भारतीय मानक ब्रयू ो

B U R E A U O F I N D I A N S TA N D A R D S
मानक भवन, 9 बहादरु शाह ज़फर मार्ग, नई िदल्ली – 110002
MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG
NEW DELHI-110002
         www.bis.gov.in  
www.standardsbis.in

March 2021  Price Group 9

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Transformers Sectional Committee, ETD 16

FOREWORD
This Indian Standard was adopted by the Bureau of Indian Standards, after the draft finalized by the Transformers
Sectional Committee, had been approved by the Electrotechnical Division Council.
This standard specifies the requirements of natural/synthetic organic ester liquid immersed distribution
transformers. This standard is a part of IS 1180 series on distribution transformers. Other standards in this series
are:
Part 1 Mineral oil immersed
IS 1180 (Part 1) : 2014 specifies requirements for mineral oil immersed outdoor/indoor type three phase distribution
transformers up to 2 500 kVA, 33 kV and single phase distribution transformers up to 100 kVA, 33 kV. A note
under 9.1 of IS 1180 (Part 1) allowed use of other insulating liquids namely natural ester, synthetic organic ester
subject to agreement between the user and the supplier.
However, it was also mentioned in the foreword of IS 1180 (Part 1) that when sufficient experience of ester liquids
would be available, separate standard on distribution transformers filled with natural/synthetic organic esters
shall be brought out. In the intervening period, several distribution transformers have been filled/retro filled with
esters and a few power transformers have also been developed using esters. A need is now felt to have a separate
standard on natural/synthetic organic ester liquid immersed distribution transformers.
The main advantage of ester liquid is its higher fire point (above 300 °C) compared to that of mineral oil. The use
of ester liquid with fire point above 300 °C has increased in several countries due to fire safety and environmental
considerations.
Being ‘K’ class liquid having fire point above 300 °C, this liquid can work on higher temperature rises with
compatible high temperature insulation material. This standard also explores such possibility based on
IS 2026 (Part 14) ‘Liquid-immersed power transformers using high temperature insulation materials’.
This standard also permits use of conventional insulation with ester liquids and temperature rises for the same
have also been specified.
This standard specifies five energy efficiency levels: level 1, level 2, level 3, level 4 and level 5. Maximum total
losses for each level (at 75 ºC) have been kept same as given in IS 1180 (Part 1). In due course of time with
improvements in technology and materials, higher levels of energy efficient transformers shall be progressively
used.
Pad mounted distribution transformers are popular in other countries. They are self-protected and obviate the
need of ring main unit as used in prefabricated compact substations up to 33 kV. Such pad mounted distribution
transformers are also suggested for use in the country up to 33 kV filled with ester liquids. When sufficient
experience builds a separate standard in IS 1180 series can be developed for Pad mounted transformers.
At present, there is no IEC standard available on ester liquid immersed distribution transformers. However,
considerable assistance has been taken from following standards while preparing this standard:
IS 16659 : 2017 Fluids for electrotechnical applications — Unused natural esters for
transformers and similar electrical equipment
IS 16081 : 2013 Insulating Liquids — Specification for unused synthetic organic esters for
electrical purposes
IS 2026 (Part 14) : 2018 Power transformers (Part 14) Liquid immersed power transformers using
high-temperature insulation materials

IS 1180 (Part 1) : 2014 is a necessary adjunct to this standard.

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 accordance with
IS 2 : 1960 ‘Rules for rounding off numerical values ( revised )’. The number of significant places retained in the
rounded off value should be the same as that of the specified value in this standard.
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IS 1180 (Part 3) : 2021

Indian Standard
OUTDOOR/INDOOR TYPE LIQUID IMMERSED
DISTRIBUTION TRANSFORMERS UP TO AND
INCLUDING 2500 kVA, 33 kV — SPECIFICATION
PART 3 NATURAL/SYNTHETIC ORGANIC ESTER LIQUID IMMERSED

1 SCOPE 2 REFERNCES
This standard specifies the requirements and tests The standards listed in Annex A contain provisions
including standard loss levels of natural/synthetic which, through reference in this text, constitute
organic ester liquid immersed, natural air-cooled, provisions of this standard. All standards are subject
outdoor/indoor type, double-wound distribution to revision, and parties to agreements based on this
transformers for use in power distribution systems with standard are encouraged to investigate the possibility
nominal system voltages up to and including 33 kV and of applying the most recent editions of the standards
of following types and ratings: listed in Annex A.
a) Three phase ratings up to and including 200 kVA Annex B gives a list of IEC standards, IEEE standards,
both non-sealed and sealed type; CIGRE brochures etc. available on ester liquids.
b) Three phase ratings higher than 200 kVA, up to
and including 2 500 kVA both non-sealed type and 3 TERMINOLOGY
sealed type; and For the purpose of this standard, the following terms
c) Single phase ratings up to and including 100 kVA and definitions shall apply in addition to those given in
sealed type. IS 1885 (Part 38).
This standard permits use of natural esters as well as 3.1 Distribution Transformer — A distribution
synthetic organic esters in both non-sealed and sealed transformer is a transformer that provides the final
type of transformers. However, in case of non-sealed voltage transformation by stepping voltages down
type transformers immersed in natural ester, suitable within a distribution circuit or from a distribution
liquid preservation system such as airbags shall be used circuit to an end user or application.
in the conservator to prevent direct contact of natural
NOTE — The distribution circuit voltages are 3.3 kV, 6.6 kV,
ester with the atmosphere. 11 kV, 22 kV and 33 kV in the country. The power supply
NOTES for the end users is 415 V, 3 Phase (240 V, 1 Phase), 50 Hz.
1 The following types of transformers are not covered under Transformers with primary voltages of 3.3, 6.6, 11, 22 or
the scope of this standard: 33 kV and secondary voltage of 433 V, 3 Phase
a) Inverter duty transformers; (and 250 V single Phase) are called Distribution Transformers.
b) Traction transformers; The maximum rating of these transformers for the purpose of
c) Instrument transformers; this standard is considered up to 2 500 kVA, 3 Phase.
d) Transformers for static converters; 3.2 Non-Sealed Type Transformer — A transformer
e) Starting transformers; which has a breather for breathing out and breathing
f) Testing transformers; in and/or a conservator with expansion and contraction
g) Welding transformers; of liquid with temperature. The transformer tank body
h) Earthing transformers; and cover are bolted/clamped/welded type. The tank
j) Mining transformers; can also be of corrugated construction.
k) Transformers for solar, wind power application; Transformers equipped with suitable liquid preservation
m) Transformers for railways (locomotive and other system such as airbags, which prevents direct contact
   applications);
of atmosphere with the liquid in the conservator,
n) Furnace transformers;
are covered under the category of non-sealed type
p) Rectifier transformers; transformers.
q) Dual ratio in primary/secondary windings transformers;
  and 3.3 Sealed Type Transformer — A transformer which
r) Transformers for Static VAR compensator. is non-breathing, that is, so sealed that normally there is
2 Forindoor type distribution transformers, relevant provisions no significant interchange between its contents and the
of Central Electricity Authority (CEA) Regulations, if any, external atmosphere. No conservator is provided. Such
shall be applicable.

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IS 1180 (Part 3) : 2021

a transformer may or may not have a cushion of inert Synthetic organic esters are suitable for non-sealed and
gas (for example, nitrogen, IS 1747). sealed transformers without any preservation system.
Sealed transformers fall into two categories: NOTE — For guidance on use and maintenance of synthetic
esters, IS 16081 and IS 16099 may be referred.
a) Transformers in which the total volume of
liquid together with inert gas/nitrogen or any 3.5 Pad Mounted Transformer — An outdoor
combination thereof, remains constant over the transformer utilized as part of underground distribution
temperature range. system with enclosed compartment(s) for high voltage
b) Transformers in which the total volume of liquid, and low voltage cables entering from below and
inert gas/nitrogen or any combination thereof, mounted on a foundation pad.
varies over the temperature range and this variation The pad mounted transformer generally covers two
is accommodated by a sealed flexible container bushings and terminal arrangements for radial feed
(corrugated tank) or a flexible membrane systems. It consists of a tank with high voltage and
Sealed type transformers usually have a low voltage cable terminating compartments separated
bolted/clamped/welded cover construction. by a barrier of metal or other rigid material. These
compartments are located side by side on one side of
3.4 ‘K’ Class Insulating Liquids — According to the transformer tank. The transformer shall be of sealed
IS 13503, liquids with fire point above 300 °C are construction.
classified as K class liquids. Synthetic organic ester,
natural ester and silicon liquids come under this 4 SERVICE CONDITIONS
category. The provisions of IS 2026 (Part 1) shall apply.
Percentage of biodegradability of silicone liquid is low NOTE — In case of indoor type transformers and transformers
(around 5 percent). For the purpose of this standard, installed in an enclosure, suitable ventilation, if required,
only synthetic esters and natural esters which are 80 to shall be provided to maintain service conditions as per
100 percent biodegradable are considered. IS 2026 (Part 1).

3.4.1 Natural Esters — Vegetable oils obtained from 5 GENERAL

seeds and oils obtained from other suitable biological Technical parameters including standard loss levels of
materials and comprised of triglycerides are called three categories of distribution transformers are given
natural esters. in 6, 7 and 8.
Suitable chemical substances called additives are Other requirements as described in 9 to 22 are applicable
deliberately added to natural ester insulating liquids in for all types and ratings of distribution transformers.
order to improve certain characteristics, for example
pour point, viscosity, foaming and oxidation stability. 6 TECHNICAL PARAMETERS OF THREE
NOTE — Examples of additives include antioxidants, PHASE DISTRIBUTION TRANSFORMERS UP
pour-point depressants, refining process improver, coloure etc. TO AND INCLUDING 200 KVA (NON-SEALED
Natural esters are recommended only for transformers AND SEALED TYPE)
that are not open to the atmosphere (for example,
sealed transformers, non-sealed transformers with 6.1 Ratings
suitable liquid preservation system which prevents The standard ratings shall be as per Table 1.
direct contact of atmosphere with the liquid in the
conservator) because these liquids are prone to rapid 6.2 Rated Frequency
oxidation. The rated frequency shall be 50 Hz.
NOTE — For guidance on use and maintenance of natural
Table 1 Standard Ratings
esters, IS 16659 and IS 16899 may be referred.
( Clause 6.1 )
3.4.2 Synthetic Organic Esters — By definition, an
ester is a reaction product from the combination of Sl No. Nominal System Standard Ratings (kVA)
an acid and an alcohol. Synthetic organic esters are Voltage
manufactured from carefully selected raw materials to (1) (2) (3)
give a product that is tailored to specific application of i) Up to and including *6.3, *10, 16, *20, 25, *40, 63,
transformers blended with additives to improve certain 11 kV 100, 160 and 200
characteristics, for example, pour point, viscosity, ii) Above 11 kV up to *6.3, *10, 16, *20, 25, *40, 63,
foaming, oxidation stability making it suitable for the and including 22 kV 100, 160 and 200
breathing system where the liquid has free access to iii) Above 22 kV up to *6.3, *10, 16, *20, 25, *40, 63,
oxygen from air. and including 33 kV 100, 160 and 200
NOTE — * ratings are non-preferred.

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IS 1180 (Part 3) : 2021

6.3 Nominal System Voltage Winding tapped — HV

Nominal system voltage shall be chosen from the Number of tap — 4
following: positions
High Voltage (HV) — 3.3, 6.6, 11, 22 and 33 kV Voltage variation — + 2.5 percent to − 5
Low Voltage (LV) — 415 V percent of HV in steps
of 2.5 percent
6.4 Basic Insulation Level (BIL)
Minimum basic insulation level shall be as given in 6.7.3 Off circuit tap-changing arrangement shall
Table 2. be either by means of links or by means of an
Table 2 Minimum Basic Insulation Level externally-operated switch with mechanical locking
device and a position indicator. Arrangement for
( Clause 6.4 ) pad-locking shall be provided.
Sl No. Nominal System Voltage Minimum BIL 6.7.4 Provision of any other tapping range and tapping
(kV) (kVP)
step is subject to agreement between the user and the
(1) (2) (3) supplier.
i) 3.3 40
6.8 Losses and Impedance Values
ii) 6.6 60
iii) 11 75 6.8.1 Losses — Multiple Rating with Regard to Energy
iv) 22 125
Efficiency
v) 33 170 6.8.1.1 For transformers of HV voltage up to 11 kV,
the total losses (no-load + load losses at 75 °C) at
NOTE — Insulation coordination of all relevant fittings 50 percent of rated load and total losses at 100 percent
and accessories corresponding to higher BIL values shall be
ensured.
of rated load shall not exceed the maximum total loss
values given in Table 3.
6.5 No-Load Voltage Ratios 6.8.1.2 For transformers having voltage class above
The no-load voltage ratios shall be as follows: 11 kV and up to and including 22 kV, the permissible
total loss values shall not exceed by 5 percent of the
3  300/433-250, 6  600/433-250, 11  000/433-250, maximum total loss values mentioned in Table 3.
22 000/433-250 and 33 000/433-250 V
NOTE — Secondary voltage may be selected as 415-240 V, 6.8.1.3 For transformers having voltage class above
subject to agreement between the user and the supplier. 22 kV and up to and including 33 kV, the permissible
total loss values shall not exceed by 7 ½ percent of the
6.6 Winding Connections and Phase Displacement maximum total loss values mentioned in Table 3.
The primary winding shall be connected in delta and NOTE — Maximum total losses at 85 °C are under
the secondary winding in star [vector symbol, Dyn 11 consideration.
(see IS 2026 Part 1)], so as to produce, a positive phase
displacement of 30° from the primary to the secondary 6.8.2 Impedance
vectors of the same phase. The neutral of the secondary The recommended impedance at 75 °C for different
winding shall be brought out to a separate insulated ratings is as per Table 3.
terminal.
6.9 Permissible Flux Density and Over Fluxing
Alternatively, [Dyn1, see IS 2026 (Part 1)] can also
be specified. If system and application requirements 6.9.1 The maximum flux density in any part of the
demand different vector groups, the same can also be core and yoke at rated voltage and frequency shall be
adopted. such that the flux density with +12.5 percent combined
voltage and frequency variation from rated voltage and
6.7 Tapping Range and Tapping Methods frequency shall not exceed 1.9 Tesla.
6.7.1 No taps are normally required to be provided up NOTE — The design calculations in support of flux density
to 100 kVA rating, unless specifically specified by the shall be furnished by manufacturer.
user. 6.9.2 No load current up to 200 kVA shall not exceed
3 percent of full load current and will be measured
6.7.2 The standard tapping range, when taps are
by energizing the transformer at rated voltage and
provided above 100 kVA rating shall be as follows:
frequency. Increase of 12.5 percent of rated voltage
shall not increase the no load current by 6 percent
maximum of full load current.

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IS 1180 (Part 3) : 2021

Table 3 Maximum Total Losses Upto 11 kV Class Transformers

( Clauses 6.8.1.1, 6.8.1.2, 6.8.1.3 and 6.8.2 )
Sl Rating Impedance Maximum Total Loss (W)
No. (kVA) (Percent)

Energy Efficiency Energy Efficiency Energy Efficiency Energy Efficiency Energy Efficiency
Level 1 Level 2 Level 3 Level 4 Level 5

50 100 50 100 50 100 50 100 50 100

Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent
Load Load Load Load Load Load Load Load Load Load
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
i) 16 4.5 135 440 120 400 108 364 97 331 87 301
ii) 25 4.5 190 635 175 595 158 541 142 493 128 448
iii) 63 4.5 340 1140 300 1050 270 956 243 870 219 791
iv) 100 4.5 475 1650 435 1500 392 1365 352 1242 317 1130
v) 160 4.5 670 1950 570 1700 513 1547 462 1408 416 1281
vi) 200 4.5 780 2300 670 2100 603 1911 543 1739 488 1582

NOTE — For non-preferred ratings of Table 1, maximum losses are subject to agreement between the user and the supplier.

6.10 Limits of Temperature-rise 7 TECHNICAL PARAMETERS OF THREE

PHASE DISTRIBUTION TRANSFORMERS
6.10.1 The type of cooling shall be type KNAN as per HIGHER THAN 200 KVA UP TO AND
IS 2026 (Part 2). INCLUDING 2 500 KVA (NON-SEALED AND
6.10.2 The following types of dielectric systems are SEALED TYPE)
considered:
7.1 Ratings
a) Type ‘A’ — Conventional kraft paper insulation
The standard ratings shall be as per Table 4.
on winding conductors and all other insulating
materials of conventional type (e.g. radial/axial
Table 4 Standard Ratings
spacers, cylinders and barriers, static end ring if
any, moulded angle ring/cap if any, shall be of ( Clause 7.1 )
conventional pressboard) immersed in ester liquid.
Sl No. Nominal System Standard Ratings
b) Type ‘B’ — Thermally Upgraded Paper (TUP) Voltage (kVA)
[see IS 2026 (Part 7) and IS 2026 (Part 14)] or (1) (2) (3)
class 130 °C/140 °C paper on winding conductors
i) Up to and including 250, 315, 400, 500, 630,
or enamel coating of minimum 130 °C class and 11 kV 800, 1 000, 1 250, 1 600,
all other insulating materials of conventional type, 2 000 and 2 500
immersed in ester liquid. ii) Above 11 kV up to and 250, 315, 400, 500, 630,
including 22 kV 800, 1 000, 1 250, 1 600,
6.10.3 The permissible temperature-rise shall not
2 000 and 2 500
exceed the limits as given below:
iii) Above 22 kV up to and 250, 315, 400, 500, 630,
Dielectric Top Liquid Average Winding including 33 kV 800, 1 000, 1 250, 1 600,
System temperature rise temperature rise 2 000 and 2 500

(1) (2) (3) 7.2 Rated Frequency

Type A 40 °C 45 °C The rated frequency shall be 50 Hz.
Type B 50 °C 55 °C

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IS 1180 (Part 3) : 2021

7.3 Nominal System Voltage Winding tapped — HV

Nominal system voltage shall be chosen from the Number of tap positions — 7
following: Voltage variations — + 5 percent to − 10
HV — 3.3, 6.6, 11, 22 and 33 kV percent in steps of
2.5 percent for variation
LV — 415 V
of HV
7.4 Basic Insulation Level (BIL)
7.7.2 Off circuit tap-changing arrangement shall
The minimum basic insulation level (BIL) shall be as be either by means of links or by means of an
given in Table 5. externally-operated switch with mechanical locking
Table 5 Minimum Basic Insulation Level device and a position indicator. Arrangement for
pad-locking shall be provided.
( Clause 7.4 )
Sl No. Nominal System Voltage Minimum Basic
7.7.3 For ratings 500 kVA and above, on load tap
(kV) Insulation Level changers may be provided for variation of HV voltage
(kVP) from + 5 percent to – 15 percent in steps of 2.5 percent.
(1) (2) (3) NOTE — Wherever the requirement of OLTC is envisaged,
i) 3.3 40 OLTC with a proven design and testing should be used in order
to ensure the suitability and prevention of liquid exposure to
ii) 6.6 60
the atmosphere.
iii) 11 75
iv) 22 125 7.7.4 Provision of any other tapping range and tapping
v) 33 170
step is subject to agreement between user and the
supplier.
NOTE — Insulation coordination of all relevant fittings and accessories
corresponding to higher BIL values shall be ensured. 7.8 Losses and Impedance Values

7.5 No-Load Voltage Ratios 7.8.1 Losses — Multiple rating with regard to Energy
Efficiency
The no-load voltage ratios shall be as follows:
3 300/433-250, 6 600/433-250, 11 000/433-250, 7.8.1.1 For transformers of HV voltage up to 11 kV,
22 000/433-250 and 33 000/433-250 V the total losses (no-load + load losses at 75 °C) at
50 percent of rated load and total losses at 100 percent
NOTE — Secondary voltage may be selected as 415-240 V,
subject to agreement between user and supplier.
of rated load shall not exceed the maximum total loss
values given in Table 6.
7.6 Winding Connections and Phase Displacement
7.8.1.2 For transformers having voltage class above
The primary winding shall be connected in delta 11 kV and up to and including 22 kV, the permissible
and the secondary winding in star [vector symbol, total loss values shall not exceed by 5 percent of the
Dyn 11 (see IS 2026 Part 1)], so as to produce, a maximum total loss values mentioned in Table 6.
positive phase displacement of 30° from the primary to
the secondary vectors of the same phase. The neutral 7.8.1.3 For transformers having voltage class above
of the secondary winding shall be brought out to a 22 kV and up to and including 33 kV, the permissible
separate insulated terminal. total loss values shall not exceed by 7 ½ percent of the
Alternatively [Dyn1 (see IS 2026 Part 1)] can also maximum total loss values mentioned in Table 6.
be specified. If system and application requirements NOTE — Maximum total losses at 85 °C are under
demand different vector groups, the same can also be consideration.
adopted. 7.8.2 Impedance
7.7 Tapping Ranges and Tapping Methods The recommended percent impedance at 75 °C for
different ratings is as per Table 6.
7.7.1 The standard tapping ranges, when taps are
provided, shall be as follows:

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IS 1180 (Part 3) : 2021

Table 6 Maximum Total Losses up to 11 kV Class Transformer

( Clauses 7.8.1.1, 7.8.1.2, 7.8.1.3 and 7.8.2 )
Sl No. Rating Impedance Maximum Total Loss (W)
(kVA) (Percent)

Energy Efficiency Energy Efficiency Energy Efficiency Energy Efficiency Energy Efficiency
Level 1 Level 2 Level 3 Level 4 Level 5
50 100 50 100 50 100 50 100 50 100
Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent
Load Load Load Load Load Load Load Load Load Load
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
i) 250 4.50 980 2 930 920 2 700 864 2 488 811 2 293 761 2 113
ii) 315 4.50 1 025 3 100 955 2 750 890 2 440 829 2 164 772 1 920
iii) 400 4.50 1 225 3 450 1 150 3 330 1 080 3 214 1 013 3 102 951 2 994
iv) 500 4.50 1 510 4 300 1 430 4 100 1 354 3 909 1 282 3 727 1 215 3 554
v) 630 4.50 1 860 5 300 1 745 4 850 1 637 4 438 1 536 4 061 1 441 3 717
vi) 800 5.00 2 287 6 403 2 147 5 838 2 015 5 323 1 892 4 853 1 776 4 425
vii) 1 000 5.00 2 790 7 700 2 620 7 000 2 460 6 364 2 310 5 785 2 170 5 259
viii) 1 250 5.00 3 300 9 200 3 220 8 400 3 142 7 670 3 066 7 003 2 991 6 394
ix) 1 600 6.25 4 200 11 800 3 970 11 300 3 753 10 821 3 547 10 363 3 353 9 924
x) 2 000 6.25 5 050 15 000 4 790 14 100 4543 13 254 4309 12 459 4 088 11 711
xi) 2 500 6.25 6 150 18 500 5 900 17 500 5660 16 554 5430 15 659 5 209 14 813

7.9 Permissible Flux Density and Overfluxing Dielectric Top Liquid Average Winding
7.9.1 The maximum flux density in any part of the System Temperature Rise Temperature Rise
core and yoke at rated voltage and frequency shall be (1) (2) (3)
such that the flux density with + 12.5 percent combined
voltage and frequency variation from rated voltage and TYPE A 45 °C 50 °C
frequency does not exceed 1.9 Tesla. TYPE B 55 °C 60 °C
NOTE — The design calculations in support of flux density
shall be furnished by the manufacturer. 8 TECHNICAL PARAMETERS OF SINGLE
PHASE DISTRIBUTION TRANSFORMERS
7.9.2 No load current shall not exceed 2 percent of the UP TO AND INCLUDING 100 KVA
full load current and shall be measured by energizing (SEALED TYPE)
the transformer at rated voltage and frequency. Increase
of 12.5 percent of rated voltage shall not increase the 8.1 Ratings
no load current by 5 percent of full load current.
The standard ratings shall be as per Table 7.
7.10 Limits of Temperature Rise Table 7 Standard Ratings
7.10.1 The type of cooling shall be KNAN as per ( Clause 8.1 )
IS 2026 (Part 2).
Sl Nominal System Voltage Standard Ratings (kVA)
No.
7.10.2 The following types of dielectric systems are
considered: (1) (2) (3)

a) Type A (see 6.10.2); and i) Up to and including 11 kV 5, 10, 16, 25, *50, *75 and
*100
b) Type B (see 6.10.2).
ii) Above 11 kV up to and 5, 10, 16, 25, *50, *75 and
7.10.3 The permissible temperature-rise shall not including 22 kV *100
exceed the limits as given below: iii) Above 22 kV up to and 5, 10, 16, 25, *50, *75 and
including 33 kV *100

*ratings are non-preferred.

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IS 1180 (Part 3) : 2021

8.2 Rated Frequency 8.7 Tap Changing Arrangement

The rated frequency shall be 50 Hz.
Taps are not required.
8.3 Nominal System Voltage 8.8 Losses and Impedance Values
Nominal system voltage shall be chosen from the 8.8.1 Losses — Multiple Rating with Regard to Energy
following: Efficiency
HV — 3.3, 6.6, 11, 22 and 33 kV
8.8.1.1 For transformer of HV voltage up to 11 kV,
LV — 415 V (240 V, 1 phase) the total losses (no load + load losses at 75 °C) at
8.4 Basic Insulation Level (BIL) 50 percent of rated load and total losses at 100 percent
of rated load shall not exceed the maximum total loss
Minimum basic insulation level shall be as given in values given in Table 9.
Table 8.
Table 8 Minimum Basic Insulation Level 8.8.1.2 For transformers having voltage class above
11 kV and up to and including 22 kV, the permissible
( Clause 8.4 ) total loss values shall not exceed by 7 ½ percent of the
Sl No. Nominal System Voltage Minimum BIL maximum total loss values mentioned in Table 9.
(kV) (kVp)
8.8.1.3 For transformers having voltage class above
(1) (2) (3)
22 kV and up to and including 33 kV, the permissible
i) 3.3 40 total loss values shall not exceed by 10 percent of the
ii) 6.6 60 maximum total loss values mentioned in Table 9.
iii) 11 75 NOTE — Maximum total losses at 85 °C are under
iv) 22 125 consideration.

v) 33 170 8.8.2 Impedance

NOTE — Insulation coordination of all relevant fittings and accessories The recommended percent impedance at 75 °C for
corresponding to higher BIL values shall be ensured. different ratings is as per Table 9.
8.5 No Load Voltage Ratio 8.9 Permissible Flux Density and Overfluxing
The no-load voltage ratios shall be as follows: 8.9.1 The maximum flux density in any part of the
core and yoke at rated voltage and frequency shall be
3 300/√3/250 V, 3 300/250 V
such that the flux density with + 12.5 percent combined
6 600/√3/250 V, 6 600/250 V voltage and frequency variation from rated voltage and
11 000/√3/250 V, 11 000/250 V frequency does not exceed 1.9 Tesla.
NOTE — The design calculations in support of flux density
22 000/√3/250 V, 22 000/250 V
shall be furnished by the manufacturer.
33 000/√3/250 V, 33 000/250 V
8.9.2 No load current shall not exceed 3 percent of full
NOTE — Secondary voltage may be selected as 415-240 V, load current and will be measured by energizing the
subject to agreement between the user and the supplier. transformer at rated voltage and frequency. Increase
8.6 Number of Phases and Polarity of 12.5 percent of rated voltage shall not increase the
no-load current by 6 percent maximum of full load
Number of phases shall be one (single phase). current.
Polarity: Additive or subtractive

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IS 1180 (Part 3) : 2021

Table 9 Maximum Total Losses of Single Phase Transformers Up to 11 kV

( Clauses 8.8.1.1, 8.8.1.2, 8.8.1.3 and 8.8.2 )
Sl Rating Impedance Maximum Total Loss (W)
No. (kVA) (Percent)

Energy Efficiency Energy Efficiency Energy Efficiency Energy Efficiency Energy Efficiency
Level 1 Level 2 Level 3 Level 4 Level 5
50 100 50 100 50 100 50 100 50 100
Percent Percent Percent Percent Percent Percent Percent Percent Percent Percent
Load Load Load Load Load Load Load Load Load Load
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
i) 5 2.50 35 95 30 75 27 68 24 62 21 57
ii) 10 4.00 60 170 55 150 50 135 45 122 40 112
iii) 16 4.00 82 224 63 190 58 175 54 164 50 145
iv) 25 4.00 110 300 95 260 88 240 80 225 74 210
v) 50 4.00 210 590 190 520 177 480 160 451 148 420
vi) 75 4.00 310 880 285 780 265 720 242 670 223 625
vii) 100 4.00 410 1 140 375 1 030 350 964 320 900 299 842

8.10 Limits of Temperature Rise 10 TERMINAL ARRANGEMENT

8.10.1 The type of cooling shall be KNAN as per 10.1 For Three Phase Transformers
IS 2026 (Part 2).
10.1.1 The transformers shall be fitted on high
8.10.2 The following types of dielectric systems are voltage and low voltage sides with outdoor type
considered: bushings of appropriate voltage and current ratings.
a) Type A (see 6.10.2) The high voltage bushings (3 Nos.) shall conform to
IS/IEC 60137. The low voltage bushings (4 Nos.) shall
b) Type B (see 6.10.2)
conform to IS 7421. Alternatively, the low voltage
8.10.3 The permissible temperature-rise shall not side may be made suitable for adoption of PVC/XLPE
exceed the limits as given below: cables of suitable size.
Dielectric Top Liquid Average Winding 10.1.2 If required by the user, a suitable cable-end
System Temperature Rise Temperature box may be provided on the high voltage and/or low
Rise voltage side. Alternatively bus duct arrangement may
(1) (2) (3) be provided on low voltage side by agreement between
the user and the supplier.
TYPE A 40 °C 45 °C
NOTE — Porcelain/epoxy/silicon rubber bushing may also be
TYPE B 50 °C 55 °C used in the cable box subject to agreement between the user
and the supplier.
9 STANDARD MATERIALS
10.1.3 In case of sealed type transformer, the terminal
9.1 Major material used in the transformer shall arrangements shall be such that it shall be possible to
conform to the following Indian Standards: replace the bushings (external) without opening the
cover and also without affecting the sealing of the
a) Cold rolled grain oriented electrical steel — transformer.
IS 3024;
The bushing shall conform to relevant Part/Section of
b) Amorphous core material — IS 16585; IS 3347 depending on the voltage class.
c) Copper/aluminium conductor — IS 191, IS 1897, NOTE — Any other suitable arrangement can be used subject
IS 7404, IS 12444, IS 13730/IS 6162 series as to agreement between the user and the supplier.
given in Annex A;
10.1.4 Gaskets shall be made of synthetic rubber or
d) Kraft paper — IS 9335 series as given in Annex A;
synthetic rubberized cork resistant to hot transformer
e) Press board — IS 1576; ester liquid.
f) Synthetic organic ester — IS 16081; and
g) Natural ester — IS 16659.
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IS 1180 (Part 3) : 2021

10.1.5 The dimensions of bushings of the following Table 10 External (Air) Clearances between
voltage classes shall conform to the following Indian Bushings Mounted on Transformers
Standards mentioned against them: ( Clause 11 )
Voltage For Porcelain Parts For Metal Nominal System Phase to Phase Phase to earth
Class Parts Voltage clearance clearance
(mm) (mm)
(1) (2) (3)
(1) (2) (3)
Up to 1.0 IS 3347 IS 3347
Up to 1.1 kV 75 40
kV bushings (Part 1/Sec 1) (Part 1/Sec2)
11 kV 255 140
3.6 kV IS 3347 IS 347
bushings (Part 2/Sec 1) (Part 2/Sec 2) 22 kV 330 230
33 kV 350 320
12 kV IS 3347 IS 3347
bushings (Part 3/Sec 1) (Part 3/Sec 2) 11.1 For transformers with air filled cable-end
24 kV IS 3347 IS 3347 box/connection chamber, the phase-to-phase and
bushings (Part 4/Sec 1) (Part 4/sec 2) phase-to-earth clearance shall be as per Table 11.
36 kV IS 3347 IS 3347 Table 11 Air Clearances in Cable Box
bushings (Part 5/Sec 1) (Part 5/Sec 2) ( Clause 11.1 )
52 kV IS 3347 IS 3347 Nominal System Phase to Phase Phase to Earth
bushings (Part 8/Sec 1) (Part 8/Sec 2) Voltage Clearance in Clearance in
NOTES mm mm
1 For heavily polluted atmosphere, dimensions of bushings (1) (2) (3)
shall conform to IS 8603 or IS 8603 (Part 4) depending on the Up to 1.1 kV 25 20
voltage class.
11 kV 130 80
2 Cast resin or polymer bushing can also be used with
performance requirements as per IS/IEC 60137 and IS 7421. 22 kV 240 140
3 Epoxy bushings can also be used with performance 33 kV 350 220
requirements as per agreement between the user and the
supplier. 12 CONNECTORS (APPLICABLE FOR BARE
10.2 For Single Phase Transformers BUSHING TERMINATIONS ONLY)

For 3.3/√3, 6.6/√3, 11/√3, 22/√3, and 33/√3 kV Wherever specified, suitable bimetallic connectors
transformers, neutral end of the HV winding shall be (clamp type) shall be provided on both HV and LV side
brought out to neutral through 1.0 kV bushing. Neutral in order to ensure sound and robust connection.
terminal shall be connected to tank by a tinned copper
13 MARKING
strip of adequate size.
For 3.3, 6.6, 11, 22, 33 kV transformers, two HV 13.1 Rating Plate
bushings shall be used for termination of both ends of Each transformer shall be provided with rating plate
HV winding. made of anodized aluminum/stainless steel material
The HV bushings shall be fixed to the top cover or side securely fixed on the outer body, easily accessible,
walls and the LV bushings of 1.0 kV class shall be fixed showing the information given in Fig. 1 for 3 phase
to the transformer tank on sides or on the top cover. transformers and Fig. 2 for single phase transformers.
The entries on the rating plate shall be indelibly marked
10.3 Marking and Relative Positions of Terminals for example, by etching, engraving or stamping.
Appropriate characters in accordance with
IS 2026 (Part 1) shall be indelibly marked upon or 13.2 Terminal Marking Plate
adjacent to terminals. Each transformer shall be provided with a terminal
marking plate in accordance with Fig. 3 to Fig. 5
11 MINIMUM CLEARANCES IN AIR whichever is applicable.
The minimum phase-to-phase and phase-to-earth 13.3 The rating and terminal marking plates may
external clearances for LV and HV bushings shall be be combined into one plate at the option of the
as per Table 10. manufacturer.

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IS 1180 (Part 3) : 2021

NOTE — Dimensions of Rating Plate, Terminal Marking 14.2 The under base of all transformers beyond
Plate and Combined Rating and Terminal Plate can be changed 200 kVA may be as per Fig. 7 to make them suitable for
subject to agreement between the user and the supplier.
mounting on rollers.
13.4 BIS Certification Marking
14.3 Suitable pole mounting arrangement may be
The distribution transformer conforming to the alternatively provided for 3 phase transformers upto
requirements of this standard may be certified as per the 500 kVA, subject to agreement between the user and
conformity assessment schemes under the provisions of the supplier.
the Bureau of Indian Standards Act, 2016 and the Rules
and Regulations framed thereunder, and the distribution 14.4 Single phase transformers are pole mounted type
transformer may be marked with the Standard Mark. and shall be provided with two mounting lugs suitable
for fixing the transformer to a single pole by means of
14 MOUNTING ARRANGEMENT two bolts of 20 mm diameter.
Both mounting lugs are made with steel of minimum 5
14.1 The under-base of all three phase transformers upto
mm thickness.
200 kVA ratings shall be provided with two channels of
minimum size 75 mm × 40 mm as shown in Fig. 6 to NOTE — For single phase transformers above 25 kVA, base
channels may be provided as per agreement between the user
make them suitable for fixing to a platform or plinth.
and the supplier.

105
95
4 HOLES, Ø3.6
3
DISTRIBUTION TRANSFORMER
MANUFACTURER'S NAME
3 PHASE TRANSFORMER
ENERGY EFFICIENCY LEVEL
STANDARD IS 1180 (PART 3)
MAX. TOTAL LOSSES
AT 50% RATED LOAD W
KVA
MAX. TOTAL LOSSES
HV AT 100% RATED LOAD W
VOLTS AT
NO LOAD TYPE OF COOLING
LV KNAN
ESTER
HV
LIQUID °c
BIL
TEMP RISE
LV
WDG °c
HV 95 105
AMPERES MASS OF ESTER LIQUID Kg
LV
TOTAL MASS Kg
FREQUENCY Hz 50

Dyn 11
VOL OF ESTER LIQUID I
VECTOR GROUP

MONTH & YEAR OF MFG.

IMPEDANCE VOLT %

TAPPINGS OFF CKT. / ON LOAD SERIAL NO.

FOR HV VARIATION
IN _
STEPS FROM % TO + %
CUSTOMER

ORDER NUMBER

MADE IN INDIA

All dimensions in mm
3

FIG. 1 RATING PLATE FOR THREE PHASE TRANSFORMERS

Fig. 1 Rating Plate for 3 Phase Transformers
105
95
4 HOLES, Ø3.6
10
3

DISTRIBUTION TRANSFORMER
MANUFACTURER'S NAME

1 PHASE TRANSFORMER
ENERGY EFFICIENCY LEVEL
 MADE IN INDIA
Free Standard provided by BIS via BSB
All Edge Private
dimensions in mm Limited to Wilson Power and Distribution
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3

IS 1180 (Part 3) : 2021

FIG. 1 RATING PLATE FOR THREE PHASE TRANSFORMERS

105
95
4 HOLES, Ø3.6
3

DISTRIBUTION TRANSFORMER
MANUFACTURER'S NAME

1 PHASE TRANSFORMER ENERGY EFFICIENCY LEVEL

STANDARD IS 1180 (PART 3) MAX. TOTAL LOSSES
AT 50% RATED LOAD W
KVA MAX. TOTAL LOSSES
AT 100% RATED LOAD W

VOLTS AT HV
TYPE OF COOLING KNAN
NO LOAD
LV ESTER
HV LIQUID °c
TEMP RISE
BIL
LV WDG °c
95 105
HV MASS OF ESTER LIQUID Kg
AMPERES
LV
TOTAL MASS Kg

FREQUENCY HZ 50
VOL OF ESTER LIQUID I
VECTOR GROUP 1 - PHASE MONTH & YEAR OF MFG.

IMPEDANCE VOLT % SERIAL NO.

CUSTOMER

ORDER NUMBER

MADE IN INDIA

All dimensions in mm
3
FIG. 2 RATING PLATE FOR SINGLE PHASE TRANSFORMERS
Fig.2 Rating Plate for Single Phase Transformers

All dimensions in mm
Fig. 3 Terminal Marking Plate for 3 Phase Transformers without Taps

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IS 1180 (Part 3) : 2021

All dimensions in mm
Fig. 4 Terminal Marking Plate for 3 Phase Transformers with Taps

Fig. 5 Terminal Marking Plate for Single Phase Transformers

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IS 1180 (Part 3) : 2021

All dimensions in mm
NOTE — Any other mounting dimensions are subject to agreement between the user and the supplier.

Fig. 6 Mounting Dimension of Transformers up to 200 KVA

Rating (kVA) X Y
2 500 1200 800
2 000 900 800
1 600 900 800
1 250 800 800
1 000 800 800
800 800 800
630 800 800
500 700 450
400 700 450
315 600 450
250 500 415

All dimensions in mm

NOTES
1 Bidirectional rollers can also be used as per mutual agreement between the user and the supplier.
2 Any other mounting dimensions are subjected to agreement between the user and the supplier.

Fig. 7 Mounting Dimensions of Transformers Beyond 200 KVA

14.5 For pad mounted transformers other constructional design in case of welded construction shall be such that
features and fixing details are subject to agreement it is possible to remove the weld and reweld the tank at
between the user and the supplier. least two times.
NOTE — Minimum thickness of corrugations shall be 1.0 mm.
15 TRANSFORMER TANK
15.1.2 The transformer tank shall be of adequate
15.1 Construction mechanical strength to withstand positive and negative
pressures built up inside the tank while the transformer
15.1.1 For non-sealed or sealed type transformer, is in operation.
transformer tank can be of plain tank configuration
with/without radiator fins or cooling tubes. The tank 15.1.3 All welding operations shall be carried out by
can also be made of corrugated panels of adequate qualified welders.
thickness, also used for cooling. The transformer tank
covers shall be bolted/clamped alternatively welded 15.1.4 The tank design shall be such that the core and
with tank rim so as to make a leak proof joint. The curb windings can be lifted freely.

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IS 1180 (Part 3) : 2021

15.1.5 For round shape single phase sealed type thermo setting powder paint or one coat of epoxy primer
transformers, the circular base plate edges of the followed by two coats of polyurethane base paint shall
tank shall be folded upward for at least 25 mm, to be used. Table 12 shall be referred to for paint thickness
have sufficient overlap with vertical sidewall of the for normal to medium corrosive atmosphere. For highly
transformer. polluted atmosphere and special application external
paint work shall be subject to agreement between the
15.2 Pressure and Vacuum Requirements user and the transformer manufacturer.
15.2.1 In case of transformers up to 200 kVA, the plain Table 12 Paint scheme for Distribution
tank shall be capable of withstanding a pressure of Transformers
80 kPa and a vacuum of 250 mm of mercury. Limiting ( Clause 15.5 )
values of the deflections are specified in 21.5.1.
Sl Paint Type Area No. of Total
For transformers above 200 kVA, the plain tank shall No. to be Coats Dry Film
be capable of withstanding a pressure of 80 kPa and a Painted Thickness
vacuum of 500 mm of mercury. Limiting values of the Min.
deflections are specified in 21.5.2. (microns)

For single phase transformers up to and including (1) (2) (3) (4) (5)
100 kVA, the plain tank shall be capable of withstanding i) Thermo setting powder Inside 01 30
a pressure of 100 kPa and a vacuum of 760 mm of paint Outside 01 60
mercury. Limiting values of deflections are specified in ii) Liquid Paint
21.5.3.1.
a) Epoxy (primer) Outside 01 30
NOTE — Permanent deflection is not applicable for round
tanks. b) Polyurethane Outside 02 25 each
(Finish coat)
15.2.2 For three phase transformers up to c) Hot liquid resistant Inside 01 35/10
2 500 kVA, transformer tanks with corrugations shall paint/Varnish
be designed for a pressure of 15 kPa measured at the
top of the tank with no leakage. NOTE — It is recommended to choose paint shade of the transformer
as green 218 following IS 5.
For single phase transformers up to 100 kVA,
transformer tanks with corrugations shall be designed 16 CONSERVATOR FOR NON-SEALED TYPE
for a pressure of 15 kPa measured at the top of the tank TRANSFORMERS
with no leakage.
16.1 Transformers of ratings 63 kVA and above with
15.2.3 For three phase sealed type transformers with plain tank construction, the provision of conservator
cover welded to the curb of the tank shall be of sound is mandatory. For sealed type transformers with or
and robust construction so as to withstand pressure of without inert gas cushion, conservator is not required.
80 kPa without any deformation.
16.2 When a conservator is provided, liquid gauge
15.2.4 For single phase transformers of sealed type and the plain or de-hydrating breathing device shall be
construction, when the space on the top liquid is filled fixed to the conservator which shall also be provided
with dry air or inert gas, the inert gas plus liquid volume with a drain plug and a filling hole (1¼” normal size
inside the tank shall be such that even under extreme thread) with cover. The capacity of a conservator tank
operating conditions, the pressure generated inside the shall be designed keeping in view the total quantity
tank does not exceed 0.4 kg/cm2 positive or negative. of liquid and its contraction and expansion due to
temperature variations. In addition, the cover of main
15.3 All bolts/nuts/washers exposed to atmosphere
tank shall be provided with an air release plug to enable
shall be as follows:
air trapped within to be released,unless the conservator
a) Size 12 mm or below — stainless steel. is so located as to eliminate the possibility of air being
b) Above 12 mm — steel with suitable finish like trapped within the main tank.
electro galvanized with passivation or hot dip In case of transformers immersed in natural ester
galvanized. liquid, care shall be taken to prevent the liquid
15.4 Gaskets wherever used shall conform to getting exposed to atmosphere by the use of an airbag
Type III as per IS 11149/Type C as per IS 4253 (Part 2) (which is connected to a breather with silica gel) or
and shall be compatible with high contact temperature suitable liquid preservation system.
i.e. thermal class of 130 °C. 16.3 The inside diameter of the pipe connecting the
15.5 Inside of tank shall be painted with varnish or conservator to the main tank should be 25 to 50 mm and
liquid resistant paint. For external surfaces one coat of it should be projected into the conservator so that its end

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IS 1180 (Part 3) : 2021

is at least 20 mm above the bottom of the conservator k) Pressure relief device or explosion vent [for sealed
so as to create a sump for collection of impurities. The type transformers (for all ratings) and non-sealed
minimum ester liquid level corresponding to − 5 °C type transformers (for ratings above 200 kVA)];
should be above the sump level. m) One filter valve on the upper side of the tank
(for transformers above 200 kVA);
17 ABILITY OF TRANSFORMERS TO
WITHSTAND EXTERNAL SHORT CIRCUIT n) HV side neutral grounding strip (where one of the
HV bushing terminal is connected to earth);
The performance of transformer under external p) LV earthing arrangement for single phase
short-circuit conditions shall be in accordance with transformers;
IS 2026 (Part 5).
q) Buchholz relay for transformers above 1 000 kVA;
18 EFFICIENCY AND REGULATION and
r) Arcing horns for HT side (one number per phase)
When statements of efficiency and regulations are
NOTES
required they shall be based on specified loading at
the rated kVA and unity power factor and computed in 1 For cable box/bus duct arrangement, arcing horns are not
required.
accordance with Annex C and Annex D respectively.
2 Providing arcing horn is optional in case lightning arrester is
NOTE — Efficiency and regulations at other power factors as provided.
agreed between the user and supplier shall also be computed.
20.2 Optional Fittings
19 TOLERANCES
The following shall be available as additional fittings at
The tolerance on electrical performance excluding the option of the user wherever specified:
losses shall be as given in IS 2026 (Part 1). a) Dehydrating breather in lieu of plain breathing
device for transformers up to 200 kVA;
20 FITTINGS
b) Filter valve for transformers up to 200 kVA;
20.1 Standard Fittings NOTE — Valve size shall be as per agreement between the
user and the supplier.
The following standard fittings shall be provided:
c) Suitable rating lightning arrestors for HT side (one
a) Two earthing terminals with the earthing number per phase);
symbol ╧;
d) Bird guard;
b) Liquid level gauge indicating level at minimum,
e) Terminal connectors;
30 °C and maximum operating temperature.
NOTES
f) Liquid temperature indicator and winding
temperature indicators for transformers above
1 Minimum and maximum positions correspond to the
operating temperature of – 5 °C and 105 °C respectively
200 kVA;
(for non-sealed type transformer). g) Jacking pads (for transformer above 1 600 kVA);
2 Only minimum position corresponding to the operating h) Buchholz relay (for transformers above 200 kVA);
temperature of 30 °C (for sealed type transformers).
j) Magnetic ester liquid level gauge (for transformer
c) Air release device (for non-sealed type above 1 600 kVA) with low liquid level alarm
transformers, where natural esters are not used); contact;
d) Rating and terminal marking plates; k) Non return valve (for conducting pressure test);
e) Dehydrating breather (connecting the air bags m) Pressure relief device or explosion vent (up to
if present) shall be provided for non-sealed type 200 kVA for non-sealed type transformers);
transformers. n) Protection relay for sealed type transformers for
f) Drain-cum-sampling valve preferably steel with internal parameters that is pressure, temperature,
plug for three phase transformers (for transformers liquid level and gas detection;
above 500 kVA); p) 4 No’s Anti-Theft stainless steel Fasteners with
NOTE— Valve size shall be as per agreement between the user breakaway nut shall be provided at top cover
and the supplier. (up to 200 kVA);
g) Thermometer pocket with cap; NOTE — IS 3639 describes some of the fittings and
h) Ester liquid/Nitrogen/Air filling hole having accessories.
(1¼” nominal size thread) with cover (for sealed q) Unidirectional flat rollers (for transformers above
type transformers without conservator); 200 kVA); and
j) Lifting lugs for the complete transformer as well r) Drain-cum-sampling valve preferably steel with
as for core and winding assembly; plug for three phase transformers (for transformers
up to 500 kVA).

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IS 1180 (Part 3) : 2021

NOTE— Valve size shall be as per agreement between the user a) Determination of sound levels [IS 2026 (Part 10)];
and the supplier.
b) Short-circuit withstand test [IS 2026 (Part 5)]
21 TESTS (above 200 kVA);
NOTE — Routine tests before and after short circuit test shall
21.1 General be conducted as per IS 2026 (Part 1).

All routine, type and special tests as described in c) No load current at 112.5 percent voltage
21.2 to 21.4 shall be performed as per relevant parts of (see 6.9.2, 7.9.2, 8.9.2);
IS 2026. Pressure and ester liquid leakage test shall be d) Paint adhesion test — The test is performed as
conducted as per 21.5. per ASTM D3359 (Standard Test Methods for
measuring adhesion by tape test).
21.2 Routine Tests (to be conducted on all units)
e) BDV and Moisture content of liquid in the
The following shall constitute the routine tests: transformer (IS 16081, IS 16099, IS 16899 or
a) Measurement of winding resistance IS 16659, whichever is applicable).
[IS 2026 (Part 1)]. NOTE — Tests at d) and e) may be carried out on more than
b) Measurement of voltage ratio and check of phase one unit subject to agreement between user and supplier.
displacement [IS 2026 (Part 1)]. 21.5 Pressure and Ester Liquid Leakage Test
c) Measurement of short-circuit impedance
(principal tapping, when applicable) and load 21.5.1 For Transformers up to 200 kVA
loss at 50 percent and 100 percent load
21.5.1.1 Pressure test (type test)
[IS 2026 (Part 1)].
d) Measurement of no-load loss and current For non-sealed and sealed type transformers, the
[IS 2026 (Part 1)]. transformer tank shall be subjected to air pressure of
80 kPa for 30 min (25 kPa for 30 min for corrugated
e) Measurement of insulation resistance
tanks) and vacuum of 250 mm of mercury for 30 min.
[IS 2026 (Part 1)].
There should not be air leakage at any point.
f) Induced over-voltage withstand test
[IS 2026 (Part 3)]. The permanent deflection of flat plates, after
pressure/vacuum has been released, shall not exceed
g) Separate-source voltage withstand test the values given below.
[IS 2026 (Part 3)].
NOTE — For single phase transformer with 3.3/√3, 6.6/√3, Length of Plate Deflection
11/√3 or 22/√3 or 33/√3 kV and with 1.0 kV neutral bushing, this
Up to 750 mm 5.0 mm
test shall be conducted at test voltage of neutral (3 kV rms for
1 min). 751 to 1 250 mm 6.5 mm
h) Pressure test (see 21.5). 1 251 to 1 750 mm 8.0 mm
j) Ester liquid leakage test (see 21.5).
NOTES
21.3 Type Tests (to be conducted on one unit) 1 Permanent deflection is not applicable for corrugations.
2 Vaccum is not applicable for corrugations.
The following shall constitute the type tests:
a) Lightning impulse test [IS 2026 (Part 3)]. 21.5.1.2 Pressure (routine test)
b) Temperature-rise test [IS 2026 (Part 2)]. a) Non-sealed type transformers (plain tanks)
NOTES The transformer with bolted cover shall be tested
1 For Type A dielectric system, maximum total losses at 75 °C at an air pressure of 35 kPa above atmosphere
(as given in 6.8.1, 7.8.1 and 8.8.1) shall be fed. pressure maintained inside the tank for 10 min.
2 For Type B dielectric system, maximum total losses at 85 °C There should be no leakage at any point.
(measured during routine test and corrected to 85 °C) shall be
b) Corrugated tanks
fed.
c) Short-circuit withstand test [IS 2026 (Part 5)] The corrugated transformer tank shall be tested for
(up to 200 kVA) and air pressure of 15 kPa above atmosphere pressure
maintained inside the tank for 10 min. There
NOTE — Routine tests before and after short circuit test shall
be conducted as per IS 2026 (Part 1).
should be no leakage at any point.
d) Pressure test (see 21.5). c) Sealed type transformers
The transformer with welded cover shall be tested
21.4 Special Tests (to be conducted on one unit) at an air pressure of 80 kPa above atmosphere
The following constitutes the special tests which may pressure maintained inside the tank for 10 min.
be carried out subject to mutual agreement between the There should be no leakage at any point.
user and the supplier:

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IS 1180 (Part 3) : 2021

21.5.1.3 Ester liquid leakage test (routine test) There should be no leakage at any point. Tank with
The assembled transformer for non-sealed and sealed corrugations shall be tested for Ester liquid leakage test
type with all fittings including bushings in position at a pressure of 15 kPa measured at the top of the tank
shall be tested at a pressure equivalent to twice the for 6 h. There should be no leakage at any point.
normal head measured at the base of the tank for 8 h. 21.5.3 For Single Phase Distribution Transformers up
There should be no leakage at any point. Tank with to Including 100 kVA.
corrugations shall be tested for ester liquid leakage test
at a pressure of 15 kPa measured at the top of the tank 21.5.3.1 Pressure test (type test)
for 6 h. There should be no leakage at any point. For transformers up to and including 100 kVA
21.5.2 For Transformers Above 200 kVA and up to The transformer tank shall be subjected to air
Including 2500 kVA. pressure of 100 kPa for 30 min (15 kPa for 30 min for
corrugated tanks) and vacuum of 760 mm of mercury
21.5.2.1 Pressure test (type test) for 30 min. There should be no air leakage at any
For non-sealed and sealed type transformers, the point. The permanent deflection of flat plates, after
transformer tank shall be subjected to air pressure of pressure/vacuum has been released, shall not exceed
80 kPa for 30 min (25 kPa for 30 minutes for the values given below:
corrugated tanks) and vacuum of 500 mm of mercury
for 30 min. There should not be air leakage at any Length of Plate Deflection
point. The permanent deflection of flat plate, after Up to 750 mm 5 mm
pressure/vacuum has been released, shall not exceed 751 mm to 1 250 mm 6.5 mm
the values given below.
1 251 mm to 1 750 mm 8.0 mm
Length of Plate Deflection
NOTES
Up to 750 mm 5.0 mm 1 Permanent deflection is not applicable for round tanks.
751 mm to 1 250 mm 6.5 mm 2 Permanent deflection is not applicable for corrugations.
1 251 mm to 1 750 mm 8.0 mm 3 Vaccum is not applicable for corrugations.

Above 1 751 mm 9.0 mm 21.5.3.2 Pressure (routine test)

NOTES
For transformers up to and including 100 kVA
1 Permanent deflection is not applicable for corrugations. The transformer tank shall be tested at a pressure of
2 Vaccum is not applicable for corrugations. 35 kPa for 10 min (15 kPa for 10 min for corrugated
tanks). There should be no leakage at any point.
21.5.2.2 Pressure test (routine test)
a) Plain tanks 21.5.3.3 Ester liquid leakage test (routine test)
The transformer tank with welded/bolted cover For transformers up to and including 100 kVA
shall be tested at a pressure of 35 kPa above The assembled transformer with all fittings including
atmosphere pressure maintained inside the tank bushings in position, shall be tested at a pressure
for 10 min. There should be no leakage at any equivalent to twice the normal head measured at the
point. base of the tank for 6 h. There should be no leakage
b) Corrugated tanks at any point. Tank with corrugations shall be tested
The corrugated transformer tank shall be tested for for ester liquid leakage test at a pressure of 15 kPa
air pressure of 15 kPa above atmosphere pressure measured at the top of the tank for 6 h. There should be
maintained inside the tank for 10 min. There no leakage at any point.
should be no leakage at any point.
22 INFORMATION REQUIRED WITH
21.5.2.3 Ester liquid leakage test (routine test) ENQUIRY AND ORDER
The assembled transformer for non-sealed and sealed 22.1 The information to be supplied by the manufacturer
type with all fittings including bushing in position with enquiry and order to the purchaser shall be in
shall be tested at a pressure equivalent to twice the accordance with Annex E.
normal head measured at the base of the tank for 8 h.

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IS 1180 (Part 3) : 2021

ANNEX A
( Clause 2 )
LIST OF REFERRED INDIAN STANDARDS

IS No. Title IS No. Title

5 : 2007 Colours forready mixed paints 3347 Dimensions for porcelain

andenamels (sixth revision) transformer bushings for use in
lightly polluted atmospheres
191 : 2007 Copper — Specification
(fourth revision) (Part 1/Sec 1) : Up to and including 1 kV, Section 1
1979 Porcelain parts (first revision)
554 : 1999 Pipe threads where pressure tight
joints are required on the threads — (Part 1/Sec 2) : Up to and including 1 kV – Section
Dimensions, tolerances and 1979 2 Metal parts (first revision)
designation (fourth revision) (Part 2/Sec 1) : 3.6 kV bushings, Section 1
1576 : 1992 Solid pressboard for electrical 1979 Porcelain parts (first revision)
purpose (first revision) (Part 2/Sec 2) : 3.6 kV bushings, Section 2 Metal
1608 Metallic materials — Tensile 1979 parts (first revision)
testing (Part 3/Sec 1) : 17.5 kV bushings, Section 1
(Part 1) : 2018 Method of test at room temperature 1988 Porcelain parts (second revision)
(fourth revision) (Part 3/Sec 2) : 17.5 kV bushings, Section 2 Metal
(Part 3) : 2018 Method of test at low temperature 1982 parts (first revision)
1747 : 1972 Nitrogen (first revision) (Part 4/Sec 1) : 24 kV bushings, Section1 Porcelain
1988 parts (second revision)
1885 Electrotechnical vocabulary
(Part 4/Sec 2) : 24 kV bushings, Section 2 Metal
(Part 38) : Power transformers and reactors 1982 parts (first revision)
1993 (second revision)
(Part 5/Sec 1) : 36 kV bushings, Section 1
1897 : 2008 Copper strip for electrical 1979 Porcelain parts (second revision)
purpose — Specification
(third revision) (Part 5/Sec 2) : 36 kV bushings, Section 2 Metal
1979 parts (first revision)
2026 Power transformers
(Part 8/Sec 1) : 52 kV Bushings, Section 1
(Part 1) : 2011 General (second revision) 1988 Porcelainparts
(Part 2) : 2010 Temperature rise (first revision) (Part 8/Sec 2) : 52 kV bushings, Section 2 Metal
(Part 3) : 2018 Insulation levels, dielectric tests 1992 parts
and external clearances in air 3639 : 1966 Fittings and accessories forpower
(fourth revision) transformers (under revision)
(Part 5) : 2011 Ability to withstand short circuit 4253 Cork composition sheet:
(first revision)
(Part 2) : 2008 Cork and rubber (second revision)
(Part 7) : 2009 Loading guide for oil-immersed
power transformers 6162 Paper-covered aluminum
conductors
(Part 8) : 2009 Application guide
(Part 1) : 1971 Round conductors
(Part 10) : Determination of sound levels
2009 (Part 2) : 1971 Rectangular conductors
(Part 14) : Liquid immersed power 7404 Paper covered copper
2018 transformers using conductors — Specification
high-temperature insulation (Part 1) : 1991 Round conductors (first revision)
materials 7421 : 1988 Porcelain bushings for alternating
3024 : 2015 Grain oriented electrical steel voltages up to and including
sheets and strip (third revision) 1 000 V (first revision)

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IS 1180 (Part 3) : 2021

IS No. Title IS No. Title

8603 : 2008 Dimensions for porcelain (Part 0/Sec 1) : General requirements, Section
transformer bushings for use in 2018 1 Enamelled round copper wire
heavily polluted atmospheres (second revision)
12/17.5 kV, 24 kV and 36 kV (Part 0/Sec 2) : General requirements, Section 2
(first revision) 2018 Enamelled rectangular copper wire
8603 (Part 4) : Dimensions for porcelain (second revision)
2003 transformer bushings for use in (Part 0/Sec 3) : General requirements, Section 3
heavily polluted atmospheres: 2012 Enameled round aluminum wire
Part 4 52 kV Bushings (first revision)
8999 : 2003 Gauging practice for pipe threads (Part 17) : Polyvinyl acetal enameled
where pressure tight joints are 2014 rectangular copper wire, Class 105
required on the threads (first revision)
9335 Cellulosic papers for electrical (Part 27) : Paper tape covered rectangular
purposes 2018 copper wire (first revision)
(Part1) : 1979 Definitions and general 16081 : 2013 Insulating liquids — Specification
requirements for unused synthetic organic esters
(Part 2) : 1998 Part 2 Methods of test for electrical purposes
(first revision) 16659 : 2017 Fluids for electrotechnical
(Part 3/Sec 1) : Specifications for individual applications — Unused natural
1984 materials, Section 1 General esters for transformers and similar
purposes electrical paper electrical equipment
(Part 3/Sec 3) : Specifications for individual 13503 : 2013 Classification of insulating liquids
1984 materials, Section 3 Crepe paper (first revision)
(Part 3/Sec 5) : Specifications for individual 16099 : 2013 Synthetic organic esters for
1985 materials, Section 5 Special papers electrical purposes — Guide for
11149 : 1984 Specification for rubber gaskets maintenance of transformer esters
in equipment
12444 : 2020 Copper wire rods for electrical
applications — Specification IS 16899 : Guide for acceptance and
(first revision) 2019 maintenance of natural esterfluids
in transformers
13730 Specification for particular types of
winding wires: IS/IEC 60137 : Insulated bushings for alternative
2017 voltages above 1 000 volts

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IS 1180 (Part 3) : 2021

ANNEX B
( Clause 2 )
LIST OF INTERNATIONAL STANDARDS
1. Cigre Brochure 443 (Working Group D1.32), 920 Volts and Below; Low Voltage, 480 Volts
DGA in Non-Mineral oil and Load Tap changers and Below.
and improved DGA diagnosis criteria. 5. IEEE Std. C57.12.28, IEEE standard for
2. Cigre Brochure 436, WG A2.35, October 2010, Pad-mounted Equipment — Enclosure integrity.
Experiences in service with new insulating 6. IEEE Std. C57.12.29, IEEE standard for
liquids. Pad — Mounted Equipment — Enclosure
3. ASTM D 6871-03, Standard specification for integrity for Coastal Environment.
natural (vegetable oil) ester liquids in electrical 7. IS 16785 : 2018/IEEE Std. C57.155 : 2014,
apparatus. IEEE guide for interpretation of gases generated
4. IEEE/ANSI C57.12.22 - American in Natural Esters and Synthetic organic Esters
National Standard for Pad-Mounted, immersed transformers.
Compartmental-Type, Self-Cooled, 8. IEC 62975 : 2021 — Natural esters —
Three-Phase Distribution Transformers Guidelines for maintenance and use in
With High-Voltage Bushings, 2500 kVA electrical
electrical equipment.
equipment.
and Smaller; High Voltage, 34 500 GrdY/19

ANNEX C
( Clause 18 )
METHOD OF DECLARING EFFICIENCY

C-1 EFFICIENCY Total losses comprise:

a) No-load loss, which is considered to be constant at
C-1.1 The efficiency to be declared is the ratio of the
all loads; and
output in kW to the input in kW and calculated
as under. b) Load loss, which varies with load.
The total loss, on load is the sum of (a) and (b).
Output Input – Total Losses
Efficiency (η) = =
Input Input

ANNEX D
( Clause 18 )
CALCULATION OF INHERENT VOLTAGE REGULATION

D-1 INHERENT VOLTAGE REGULATION Isc = current measured in winding excited

Ezsc = voltage measured across winding excited
D-1.1 The inherent voltage regulation from (impedance voltage);
no-load to a load of any assumed value and power
factor may be computed from the impedance voltage Psc = watts measured across winding excited
and corresponding load loss measured with rated 2
 PSC 
current in the winding [see also IS 2026 (Part 8)]. 2
Exsc = reactance voltage = E ZSC −  
Let  I SC 
I = rated current in winding excited; P =PSC corrected to 75 oC, and from current Isc to
I;
E = rated voltage of winding excited;

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IS 1180 (Part 3) : 2021

I (Ex % cos φ − Er % sin φ) 2

Ex = E XSC × Er % cos φ + Ex % sin φ +
I sc 200
P
Er= D-1.4 For any assumed load other than rated load
I and unity power factor, the percentage regulation is
D-1.2 For rated load at unity power factor, the
approximately equal to:
percentage regulation is approximately equal to
( n. Ex % )
2

(E x %) 2 n. Er % +
Er % + 200
200
Ex% = 100 Ex/E; D-1.5 For any assumed load other than rated load
and at any power factor cosφ, the percentage regulation
Er% = 100 Er/E
is approximately equal to:
n = Ia/I; and
n.Er % cos φ + n. Ex % sin φ +
Ia =  current in the winding excited during
the short circuit tests corresponding to that (n. Ex % cos φ − n. Er % sin φ) 2
obtained when loading at the assumed load 200
on the output side and with rated voltage on
the input side. D-1.6 The above formulae are sufficiently accurate for
transformers covered by this specification.
D-1.3 For rated load any power factor cosφ, the
percentage regulation is approximately equal to:

ANNEX E
( Clause 22.1 )
INFORMATION REQUIRED WITH ENQUIRY AND ORDER

E-1 NORMAL INFORMATION r) Neutral terminals, if required (for each winding)

and their insulation level to earth;
The following information should be given in all cases:
a) Particulars of the specification to be complied s) Special requirements of installation, assembly,
with; transport and handling;
b) Application of Transformer for example normal t) Fittings required and an indication of the side from
Distribution Transformer, Solar duty, wind which meters, rating plates, liquid-level indicator,
application, Motor starting etc. etc. may be readable and
c) Single or three phase unit; u) Natural ester liquid or synthetic organic ester
liquid.
d) Number of phases in system;
e) Frequency; E-2 SPECIAL INFORMATION
f) Indoor or outdoor type; The following additional information may be required
g) Type of cooling (KNAN); to be given:
h) Rated power (in kVA) a) If a lightning impulse voltage test is required,
j) Rated voltages (for each winding); whether or not the test is to include chopped
waves [see IS 2026 (Part 3)];
k) State if tappings are required and if on-load or
off-circuit tap-changers, or links are required. b) Impedance voltage at rated current, if specific
value is required;
m) Highest voltage for equipment (for each winding);
c) Altitude above mean sea-level, if in excess of
n) Method of system earthing (for each winding);
1 000 m;
p) Insulation level (for each winding), power
d) Whether transformers will be subjected to frequent
frequency test level/impulse level;
overcurrent, for example, furnace transformers
q) Connection symbol; and traction feeding transformers;

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IS 1180 (Part 3) : 2021

e) Any other exceptional service conditions; j) Type of mounting for example pole mounted,
f) Whether noise level measurement is to be carried ground mounted etc.; and
out; k) Any other appropriate information, including
g) Vacuum withstand of the transformer tank, if a reference to any special tests not referred to above
specific value is required; which may be required.
h) Type of Tap-changer controls required (if OLTC is
provided);

ANNEX F
( Clauses 21.2 (j), 21.5.1.3, 21.5.2.3 and 21.5.3.3 )
ADDITIONAL INFORMATION ON LEAKAGE TEST

F-1 Calculation of Gauge pressure during ester liquid a) The depth of static head at bottom gauge position
leakage test from Normal static head shall be the height from highest liquid level to
Hydrostatic pressure in liquid is given by: base of tank; and
b) The depth of static head at top of the tank shall be
P = ρgh
the height from highest liquid level in conservator
where up to tank top gauge location.
P = Pressure at a point (Pa); In case the conservator is not provided, (for example,
ρ = Density of liquid (kg/m3) (Ref. relevant single phase transformers and small 3 phase transformer
standard or supplier’s test certificate); below 63 kVA) and pressure gauge is mounted on
g = acceleration due to gravity (9.81 m/s2); and cover of tank, a pressure equivalent to one static head
(tank height in this case) shall be applied since as per
h = height of liquid column at a particular point requirement test pressure is equivalent to twice the
(m) (measured from top). static head.
F-2 As per 21.5.1.3, 21.5.2.3 and 21.5.3.3, the amount F-4 Sample calculation of pressure for a transformer
of pressure application during the leakage test on
having ester liquid level of 1000 mm:
assembled transformer for non-sealed and sealed type
transformers with all fittings including bushing in Density of ester liquid, ρ = 1.0 g/cc = 1000 kg/m3
position is summarized below: Ester liquid level in the T/F, h = 1000 mm = 1 m
a) Tank without corrugations — Pressure equivalent Hence, normal head pressure = (1000 × 9.81 × 1)
to twice the normal head measured/calculated at Pa = 9810 Pa ≈ 9.81 kPa
base of tank for 8 h (for 3 phase transformer) and
6 h (for 1 phase transformers) Twice the normal head pressure = 2 × 9.81
kPa = 19.62 kPa
b) Tank with corrugations — 15 kPa measured at top
of the tank for 6 h for both 3 phase and 1 phase Hence, Pressure to be measured in the gauge is,
transformer. a) 19.62 kPa, if gauge is fixed at base of tank; and
F-3 If position of pressure gauge is not specified, b) 9.81 kPa, if gauge is fixed at top.
based on facility available, the pressure gauge can be
mounted near the base of the tank or near the top cover
(or on the cover):

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Bureau of Indian Standards

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Review of Indian Standards

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periodically; a standard along with amendments is reaffirmed when such review indicates that no changes are
needed; if the review indicates that changes are needed, it is taken up for revision. Users of Indian Standards
should ascertain that they are in possession of the latest amendments or edition by referring to the latest issue of
‘BIS Catalogue’ and ‘Standards: Monthly Additions’.
This Indian Standard has been developed from Doc No.: ETD 16 (6824).

Amendments Issued Since Publication

Amend No. Date of Issue Text Affected

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