IS 2026

"Standards are to the industry what culture is to the society" -Shri C. Rajagopalachari

Today, a product can not even been visualized without a standard. Every customer wants assurance of quality. Standards are common yardstick for the customers. Standards are evolved to meet a generally recognize demand, taking into account the interest of manufacturers and users and fulfilling the need of the economy.
A standard is a useful guide in all facets of a product-conception, design, manufacturing, testing, installation, operation, maintenance, etc.

Here we will be focusing only on Indian Standard 2026 "Specification for power transformer" , it is the governing standard on power transformers.
It was first published in 1962 and covered initially naturally cooled oil immersed transformers. By subsequent amendments forced cool transformers were included.

At present the requirements for power transformers are covered in five parts as below:-
PART I : GENERAL
PART II : TEMPERATURE RISE
PART III : INSULATION LEVELS, DIELECTRIC TEST AND TRANSFORMER BUSHING MINIMUM CLEARANCES IN AIR

PART IV : TERMINAL MARKINGS, TAPPINGS AND CONNECTIONS

PART V : ABILITY TO WITHSTAND SHORT CIRCUIT

PART VII : LOADING GUIDE FOR OIL IMMERSED POWER TRANSFORMERS

PART VIII : APPLICATION GUIDE

PART X : DETERMINATION OF SOUND LEVELS

IS 2026 (Part 1) : 2011 



conditions for shipment, storage and installation, such 
as weight or space limitations (see Annex B). 

Supplementary rules for rating and testing are given 
in other publications for: 

a) temperature rise and cooling in high ambient 
temperature or at high altitude: IS 2026 
(Part 2) for oil-immersed transformers, and 
IS 11171for dry-type transformers, and 

b) external insulation at high altitude: IS 2026 
(Part 3) for oil-immersed transformers, and 
IS 11171 for dry-type transformers. 

2 REFERENCES 

The standards given in Annex A are necessary adjuncts 
to this standard. 

3 DEFINITIONS 

For the purpose of this standard, the following 
definitions shall apply. 

3.1 General 

3.1.1 Power Transformer — A static piece of apparatus 
with two or more windings which, by electromagnetic 
induction, transforms a system of alternating voltage 
and current into another system of voltage and current 
usually of different values and at the same frequency 
for the purpose of transmitting electrical power. 

3.1.2 Auto-Transformer — A transformer in which at 
least two windings have a common part. 

NOTE — Where there is a need to express that a transformer is 
not auto-connected, use is made of terms sucli as separate 
winding transformer, or double-wound transformer 

3.1.3 Booster Transformer — A transformer of which 
one winding is intended to be connected in series with 
a circuit in order to alter its voltage and/or shift its 
phase. The other winding is an energizing winding. 

3.1.4 Oil-Immersed Type Transformer — A transformer 
of which the magnetic circuit and windings are 
immersed in oil. 

NOTE — For the purpose of this part any insulating liquid, 
mineral oil or other product, is regarded as oil. 

3.1.5 Dry-Type Transformer — A transformer of which 
the magnetic circuit and windings are not immersed in 
an insulating liquid. 

3.1.6 Oil Preservation System — The system in an oil- 
immersed transformer by which the thermal expansion 
of the oil is accommodated. Contact between the oil and 
external air may sometimes be diminished or prevented. 

3.2 Terminals and Neutral Point 

3.2.1 Terminal — A conducting element intended for 
connecting a winding to external conductors. 



3.2.2 Line Terminal — A terminal intended for 
connection to a line conductor of a network. 

3.2.3 Neutral Terminal 

a) For three-phase transformers and three-phase 
banks of single-phase transformers: 

The terminal or terminals connected to the 
common point (the neutral point) of a star- 
connected or zigzag connected winding. 

b) For single-phase transformers: 

The terminal intended for connection to a 
neutral point of a network. 

3.2.4 Neutral Point — The point of a symmetrical 
system of voltages which is normally at zero potential 

3.2.5 Corresponding Terminals — Terminals of 
different windings of a transformer, marked with the 
same letter or corresponding symbol. 

3.3 Windings 

3.3.1 Winding — The assembly of turns forming an 
electrical circuit associated with one of the voltages 
assigned to the transformer. 

NOTE — For a three-phase transformer, the 'winding' is the 
combination of the phase windings (see 3.3.3). 

3.3.2 Tapped Winding — A winding in which the 
effective number of turns can be changed in steps. 

3.3.3 Phase Winding — The assembly of turns forming 
one phase of a three-phase winding. 

NOTE — The term 'phase winding' should not be used for 
identifying the assembly of all coils on a specific leg. 

3.3.4 High-Voltage Winding — The winding having 
the highest rated voltage. 



3.3.5 Low-Voltage Winding - 
lowest rated voltage. 



- The winding having the 



NOTE — For a booster transformer, the winding having the 
lower rated voltage may be that having the higher insulation 
level. 

3.3.6 Intermediate-VoltageWinding — A winding of a 
multi-winding transformer having a rated voltage 
intermediate between the highest and lowest winding 
rated voltages. The winding which receives active 
power from the supply source in service is referred 
to as a 'primary winding', and that which delivers 
active power to a load as a 'secondary winding'. 
These terms have no significance as to which of the 
windings has the higher rated voltage and should 
not be used except in the context of direction of 
active power flow. A further winding in the 
transformer, usually with lower value of rated power 
than the secondary winding, is then often referred to 
as 'tertiary winding' {see also 3.3.8). 



IS 2026 (Part 1) : 2011 



3.3.7 Auxiliary Winding — A winding intended only 
for a small load compared with the rated power of the 
transformer. 

3.3.8 Stabilizing Winding — A supplementary delta- 
connected winding provided in a star-star-connected 
or star-zigzag-connected transformer to decrease its 
zero- sequence impedance (see 3.7.3). 

NOTE — A winding is referred to as a stabilizing winding 
only if it is not intended for three-phase connection to an 
external circuit. 

3.3.9 Common Winding — The common part of the 
windings of an auto-transformer. 

3.3.10 Series Winding — The part of the winding of 
an auto-transformer or the winding of a booster 
transformer which is intended to be connected in series 
with a circuit. 

3.3.11 Energizing Winding — The winding of a booster 
transformer which is intended to supply power to the 
series winding. 

3.4 Rating 

3.4.1 Rating — Those numerical values assigned to 
the quantities which define the operation of the trans- 
former in the conditions specified in IS 2026 (Part 2) 
and on which the manufacturer's guarantees and the 
tests are based. 

3.4.2 Rated Quantities — Quantities (voltage, current, 
etc), the numerical values of which define the rating. 

NOTES 

1 For transformers having tappings, rated quantities are related 
to the principal tapping (see 3.5.2), unless otherwise specified. 
Corresponding quantities with analogous meaning, related to 
other specific tappings, are called tapping quantities (see 
3.5.10). 

2 Voltages and currents are always expressed by their rm.s. 
values, unless otherwise specified. 

3.4.3 Rated Voltage of a Winding (U^) — The voltage 
assigned to be applied, or developed at no-load, 
between the terminals of an untapped winding, or of a 
tapped winding connected on the principal tapping 
{see 3.5.2). For a three-phase winding it is the voltage 
between line terminals. 

NOTES 

1 The rated voltages of all windings appear simultaneously at 
no-load when the voltage applied to one of them has its rated 
value. 

2 For single-phase transformers intended to be connected in 
star to form a three-phase bank, the rated voltage is indicated 
as phase-to-phase voltage, divided by V3, for example 
U, = 400^/3 kV. 

3 For the series winding of a three-phase booster transformer 
which is designed as an open winding (see 3.10.5) the rated 
voltage is indicated as if the winding were connected in star, 
for example J/, = 23V3 kV. 



3.4.4 Rated Voltage Ratio — The ratio of the rated 
voltage of a winding to the rated voltage of another 
winding associated with a lower or equal rated voltage. 

3.4.5 Rated Frequency (fj — The frequency at which 
the transformer is designed to operate. 

3.4.6 Rated Power (SJ — A conventional value of 
apparent power assigned to a winding which, together 
with the rated voltage of the winding, determines its 
rated current. 

NOTES 

1 Both windings of a two-winding transformer have the same 
rated power which by definition is the rated power of the whole 
transformer. 

2 For a multi-winding transformer, half the arithmetic sum of 
the rated power values of all windings (separate windings, not 
auto-connected) gives a rough estimate of its physical size as 
compared with a two-winding transformer. 

3.4.7 Rated Current (I J — The current flowing through 
a line terminal of a winding which is derived from rated 
power 5j and rated voltage C/,. for the winding. 

NOTES 

1 For a three-phase winding the rated current /^ is given by: 



2 For single-phase transformer windings intended to be 
connected in delta to form a three-phase bank the rated current 

is indicated as line current divided by V3 , for example: 
500 , 



^ 



3.5 Tappings 



3.5.1 Tapping — In a transformer having a tapped 
winding, a specific connection of that winding, 
representing a definite effective number of turns in the 
tapped winding and, consequently, a definite turns ratio 
between this winding and any other winding with fixed 
number of turns. 

NOTE — One of the tappings is the principal tapping, and 
other tappings are described in relation to the principal tapping 
by their respective tapping factors. See definitions of these 
terms below. 

3.5.2 Principal Tapping — The tapping to which the 
rated quantities are related. 

3.5.3 Tapping Factor (corresponding to a given 
tapping) 

The ratio: 






(tapping factor) or 



f/n 



100 rx (tapping factor expressed as a percentage) 



IS 2026 (Part 1) : 2011 



where 

{/,. = rated voltage of the winding (see 3.4.3); and 

t/j = vohage which would be developed at no- 
load at the terminals of the winding, at the 
tapping concerned, by applying rated 
voltage to an untapped winding. 

NOTE — This definition is not appropriate in relation to a 
series winding of a booster transformer (see 3.1.3), and in tliat 
case the percentage notation would be referred to the voltage 
of the energizing winding or of the winding of an associated 
system transformer. 

3.5.4 Plus Tapping — A tapping whose tapping factor 
is higher than 1 . 

3.5.5 Minus Tapping — A tapping whose tapping factor 
is lower than 1 . 

3.5.6 Tapping Step — The difference between the 
tapping factors, expressed as a percentage, of two 
adjacent tappings. 

3.5.7 Tapping Range — The variation range of the 
tapping factor, expressed as a percentage, compared 
with the value '100'. 

NOTE — If this factor ranges from 100 + a to 100 - fo, the 
tapping range is said to be; +a percent, - b percent or ± a 
percent, if a = b. 

3.5.8 Tapping Voltage Ratio (of a pair of windings) — 
The ratio which is equal to the rated voltage ratio: 

a) multiplied by the tapping factor of the tapped 
winding, if this is the high-voltage winding; 
and 

b) divided by the tapping factor of the tapped 
winding, if this is the low- voltage winding. 

NOTE — While the rated voltage ratio is, by definition, at 
least equal to 1 , the tapping voltage ratio can be lower than 1 
for certain tappings when the rated voltage ratio is close to 1. 

3.5.9 Tapping Duty — The numerical values assigned 
to the quantities, analogous to rated quantities, which 
refer to tappings other than the principal tapping. 

3.5.10 Tapping Quantities — Those quantities the 
numerical values of which define the tapping duty of a 
particular tapping (other than the principal tapping). 

The tapping quantities are: 

a) Tapping voltage (analogous to rated voltage, 
see 3.4.3); 

b) Tapping power (analogous to rated power, 
see 3.4.6); and 

c) Tapping current (analogous to rated current, 
see 3.4.7). 

NOTE — Tapping quantities exist for any winding in the 
transformer, not only for the tapped winding (see 5.2 and 5.3). 

3.5.11 Full-Power Tapping — A tapping whose tapping 
power is equal to the rated power. 



3.5.12 Reduced-Power Tapping — A tapping whose 
tapping power is lower than the rated power. 

3.5.13 On-load Tap-changer — A device for changing 
the tapping connections of a winding, suitable for 
operation while the transformer is energized or on 
load. 

3.6 Losses and No-load Current 

NOTE — The values are related to the principal tapping, unless 
another tapping is specifically stated. 

3.6.1 No-load Loss — The active power absorbed when 
rated voltage (tapping voltage) at rated frequency is 
applied to the terminals of one of the windings, the 
other winding or windings being open-circuited. 

3.6.2 No-load Current — The r.m.s. value of the current 
flowing through a line terminal of a winding when rated 
voltage (tapping voltage) is applied at rated frequency, 
the other winding or windings being open-circuited. 

NOTES 

1 For a three-phase transformer, the value is the arithmetic 
mean of the values of current in the three phases. 

2 The no-load current of a winding is often expressed as a 
percentage of the rated current of that winding. For a multi- 
winding transformer this percentage is refen'ed to the winding 
with the highest rated power. 

3.6.3 Load Loss — The absorbed active power at rated 
frequency and reference temperature {see 10.1), 
associated with a pair of windings when rated current 
(tapping current) is flowing through the line terminals 
of one of the windings, and the terminals of the other 
winding are short-circuited. Further windings, if 
existing, are open-circuited. 

NOTES 

1 For a two-winding transformer there is only one winding 
combination and one value of load loss. For a multi-winding 
transformer there are several values of load loss corresponding 
to the different two-winding combinations. A combined load 
loss figure for the complete transformer is referred to a specified 
winding load combination. In general, it is usually not 
accessible for direct measurement in testing. 

2 When the windings of the pair have different rated power 
values the load loss is referred to rated current in the winding 
with the lower rated power and the reference power should be 
mentioned. 



3.6.4 Total Losses - 
the load loss. 



The sum of the no-load loss and 



NOTE — The power consumption of the auxiliary plant is not 
included in the total losses and is stated separately. 

3.7 Short- Circuit Impedance and Voltage Drop 

3.7.1 Short-circuit impedance of a pair of windings — 
the equivalent series impedance Z = R -\- jX, in ohms, 
at rated frequency and reference temperature, across 
the terminals of one winding of a pair, when the 
terminals of the other winding are short-circuited and 



IS 2026 (Part 1) : 2011 



further windings, if existing, are open-circuited. For a 
three-phase transformer the impedance is expressed 
as phase impedance (equivalent star connection). 

In a transformer having a tapped winding, the short- 
circuit impedance is referred to a particular tapping. 
Unless otherwise specified the principal tapping 
applies. 

NOTE — This quantity may be expressed in relative, 
dimensionless form, as a fraction 7, of the reference impedance 
Zjjp of the same winding of the pair. In percentage notation: 



z=100 



where 



Zjj, = — (Formula valid for both three-phase and single- 
phase transformers), 

U = voltage (rated voltage or tapping voltage) of the 

winding to which Z and Z^^, belong, and 
S, = reference value of rated power. 

The relative value is also equal to the ratio between 
the applied voltage during a short-circuit measurement 
which causes the relevant rated current (or tapping 
current) to flow, and rated voltage (or tapping voltage). 
This applied voltage is referred to as the short-circuit 
voltage of the pair of windings. It is normally expressed 
as a percentage. 

3.7.2 Voltage Drop or Rise for a Specified Load 
Condition — The arithmetic difference between the 
no-load voltage of a winding and the voltage developed 
at the terminals of the same winding at a specified load 
and power factor, the voltage supplied to (one of) the 
other winding(s) being equal to, 

a) its rated value if the transformer is connected 
on the principal tapping (the no-load voltage 
of the former winding is then equal to its rated 
value); and 

b) the tapping voltage if the transformer is 
connected on another tapping. 

This difference is generally expressed as a percentage 
of the no-load voltage of the former winding. 

NOTE — For multi-winding transformers, the voltage drop or 
rise depends not only on the load and power factor of the 
winding itself, but also on the load and power factor of the 
other windings. 

3.7.3 Zero-Sequence Impedance (of a three-phase 
winding) — The impedance, expressed in ohms per 
phase at rated frequency, between the line terminals of 
a three-phase star-connected or zigzag-connected 
winding, connected together, and its neutral terminal. 

NOTES 

1 The zero-sequence impedance may have several values 
because it depends on how the terminals of the other winding 
or windings are connected and loaded. 

2 The zero-sequence impedance may be dependent on the value 



of the current and the temperature, particularly in transformers 
without any delta-connected winding. 

3 The zero-sequence impedance may also be expressed as a 
relative value in the same way as the (positive sequence) short- 
circuit impedance (see 3.7.1). 

3.8 Temperature Rise — The difference between the 
temperature of the part under consideration and the 
temperature of the external cooling medium. 

3.9 Insulation — For definitions relating to insulation, 
see IS 2026 (Part 3). 

3.10 Connections 

3.10.1 Star Connection (Y-connection) — The winding 
connection so arranged that each of the phase windings 
of a three-phase transformer, or of each of the windings 
for the same rated voltage of single-phase transformers 
associated in a three-phase bank, is connected to a 
common point (the neutral point) and the other end to 
its appropriate line terminal. 

3.10.2 Delta Connection (D-connection) — The 
winding connection so arranged that the phase 
windings of a three-phase transformer, or the windings 
for the same rated voltage of single-phase transformers 
associated in a three-phase bank, are connected in series 
to form a closed circuit. 

3.10.3 Open-Delta Connection — The winding 
connection in which the phase windings of a three- 
phase transformer, or the windings for the same rated 
voltage of single-phase transformers associated in a 
three-phase bank, are connected in series without 
closing one corner of the delta. 

3.10.4 Zigzag Connection (Z-connection) — The 
winding connection in which one end of each phase 
winding of a three-phase transformer is connected to a 
common point (neutral point), and each phase winding 
consists of two parts in which phase-displaced voltages 
are induced. 

NOTE — These two parts normally have the same number of 
turns. 

3.10.5 Open Windings — Phase windings of a three- 
phase transformer which are not interconnected within 
the transformer. 

3.10.6 Phase Displacement of a Three-Phase 
Winding — The angular difference between the phasors 
representing the voltages between the neutral point 
(real or imaginary) and the corresponding terminals 
of two windings, a positive-sequence voltage system 
being applied to the high-voltage terminals, following 
each other in alphabetical sequence if they are lettered, 
or in numerical sequence if they are numbered. The 
phasors are assumed to rotate in a counter-clockwise 
sense. 



IS 2026 (Part 1) : 2011 



NOTE — The high-voltage winding phasor is taken as 
reference, and the displacement for any other winding is 
conventionally expressed by the 'clock notation', that is, the 
hour indicated by the winding phasor when the H.V. winding 
phasor is at 12 O'clock (rising numbers indicate increasing 
phase lag). 

3.10.7 Connection Symbol — A conventional notation 
indicating the connections of the high-voltage, 
intermediate-voltage (if any), and low-voltage 
windings and their relative phase displacement(s) 
expressed as a combination of letters and clock-hour 
figure(s). 

3.11 Kinds of Tests 

3.11.1 Routine Test — A test to which each individual 
transformer is subjected 

3.11.2 Type Test — A test made on a transformer which 
is representative of other transformers, to demonstrate 
that these transformers comply with specified 
requirements not covered by routine tests. 

NOTE — A transformer is considered to be representative of 
others if it is fully identical in rating and construction, but the 
type test may also be considered valid if it is made on a 
transformer which has minor deviations of rating or other 
characteristics. These deviations should be subject to agreement 
between the manufacturer and the purchaser. 

3.11.3 Special Test — A test other than a type test or a 
routine test, agreed by the manufacturer and the 
purchaser. 

3.12 Meteorological Data with Respect to Cooling 

3.12.1 Monthly Average Temperature — Half the sum 
of the average of the daily maxima and the average of 
the daily minima during a particular month — over 
many years. 

3.12.2 Yearly Average Temperature — One-twelfth of 
the sum of the monthly average temperatures. 

4 RATING 

4.1 Rated Power 

The transformer shall have an assigned rated power 
for each winding which shall be marked on the rating 
plate. The rated power refers to continuous loading. 
This is a reference value for guarantees and tests 
concerning load losses and temperature rises. 

If different values of apparent power are assigned under 
different circumstances, for example, with different 
methods of cooling, the highest of these values is the 
rated power. 

A two- winding transformer has only one value of rated 
power, identical for both windings. 

When the transformer has rated voltage applied to a 
primary winding, and rated current flows through the 



terminals of a secondary winding, the transformer 
receives the relevant rated power for that pair of 
windings. 

The transformer shall be capable of carrying, in 
continuous service, the rated power [for a multi- 
winding transformer: the specified combination(s) of 
winding rated powers] under conditions listed in 4.2 
and without exceeding the temperature-rise limitations 
specified in IS 2026 (Part 2). 

NOTE — The interpretation of rated power according to this 
subclause implies that it is a value of apparent power input to 
the transformer — including its own absorption of active and 
reactive power. The apparent power that the transformer delivers 
to the circuit connected to the terminals of the secondary 
winding under rated loading differs from the rated power. The 
voltage across the secondary terminals differs from rated 
voltage by the voltage drop (or rise) in the transformer. 
Allowance for voltage drop, with regard to load power factor, 
is made in the specification of the rated voltage and the tapping 
range. 

This is different from the method used in transformer standards, 
where 'rated kVA' is 'the output that can be delivered at... rated 
secondary voltage...". According to that method, allowance for 
voltage drop has to be made in the design so that the necessary 
primary voltage can be applied to the transformer. 

4.2 Loading Cycle 

If specified in the enquiry or the contract, the 
transformer may, in addition to its rated power for 
continuous loading, be assigned a temporary load cycle 
which it shall be capable of performing under 
conditions specified in IS 2026 (Part 2). 

NOTE — This option is to be used in particular to give a basis 
for design and guarantees concerning temporary emergency 
loading of large power transformers. 

In the absence of such specification, guidance on 
loading of transformers complying with this part may 
be found in IS 6600. 

The bushings, tap-changers and other auxiliary 
equipment shall be selected so as not to restrict the 
loading capability of the transformer. 

NOTE — These requirements do not apply to special purpose 
transformers, some of which do not need loading capability 
above rated power. For others, special requirements will be 
specified. 

4.3 Preferred Values of Rated Power 

For transformers up to 10 MVA, values of rated power 
should preferably be taken from the RIO series given 
in IS 1076 (Part 1) 

(...100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 
1 000, etc) 

4.4 Operation at Higher than Rated Voltage and/or 
at Disturbed Frequency 

Methods for the specification of suitable rated voltage 



IS 2026 (Part 1) : 2011 



values and tapping range to cope with a set of loading 
cases (loading power and power factor, corresponding 
line-to-line service voltages) are described in as per 
requirement. 

Within the prescribed value of U^, a transformer shall 
be capable of continuous service without damage under 
conditions of 'overfluxing' where the ratio of voltage 
over frequency exceeds the corresponding ratio at rated 
voltage and rated frequency by no more than 5 percent. 

NOTE — (/„, is the highest voltage for equipment applicable 
to a transformer winding [see IS 2026 (Part 3)]. 

5 REQUIREMENTS FOR TRANSFORMERS 
HAVING A TAPPED WINDING 

5.1 General — Notation of Tapping Range 

The following clauses apply to transformers in which 
only one of the windings is a tapped winding. 

In a multi-winding transformer, the statements apply 
to the combination of the tapped winding with either 
of the untapped windings. 

In auto-connected transformers, tappings are 
sometimes arranged at the neutral which means that 
the effective number of turns is changed simultaneously 
in both windings. For such transformers, the tapping 
particulars are subject to agreement. The requirements 
of this clause should be used as far as applicable. 

Unless otherwise specified, the principal tapping is 
located in the middle of the tapping range. Other 
tappings are identified by their tapping factors. The 
number of tappings and the range of variation of the 
transformer ratio may be expressed in short notation 
by the deviations of the tapping factor percentages from 
the value 100 (for definitions of terms, see 3.5). 

Example: A transformer with a tapped 160 kV winding 
having altogether 21 tappings, symmetrically placed, 
is designated: 

(160 ± 10 X 1.5 percent)/66 kV 

If for some reason the tapping range is specified 
asymmetrically around the rated voltage, we may get; 



(l60!-',r)/66kV 



NOTE — This way of short notation is only a description of 
the arrangement of the tapped winding and does not imply 
actual variations of applied voltage on that winding in service. 
This is dealt with in 5.2 and 5.3. 

Regarding the full presentation on the nameplate of 
data related to individual tappings {see 7). 

Some tappings may be 'reduced-power tappings' due 
to restrictions in either tapping voltage or tapping 
current. The boundary tappings where such limitations 
appear are called 'maximum voltage tapping' and 
'maximum current tapping' (see Fig. 1). 



5.2 Tapping Voltage — Tapping Current Standard 
Categories of Tapping Voltage Variation Maximum 
Voltage Tapping 

The short notation of tapping range and tapping steps 
indicates the variation range of the ratio of the 
transformer. But the assigned values of tapping 
quantities are not fully defined by this alone. Additional 
information is necessary. This can be given either in 
tabular form with tapping power, tapping voltage and 
tapping current for each tapping, or as text, indicating 
'category of voltage variation' and possible limitations 
of the range within which the tappings are 'full-power 
tappings'. 

The extreme categories of tapping voltage variation are: 

a) constant flux voltage variation (CFVV); and 

b) variable flux voltage variation (VFVV). 

They are defined as follows: 

a) CFW 

The tapping voltage in any untapped winding 
is constant from tapping to tapping. The 
tapping voltages in the tapped winding are 
proportional to the tapping factors. 

b) VFW 

The tapping voltage in the tapped winding 
is constant from tapping to tapping. The 
tapping voltages in any untapped winding 
are inversely proportional to the tapping 
factor. 

c) CbVV (Combined voltage variation) 

In many applications and particularly with 
transformers having a large tapping range, a 
combination is specified using both principles 
applied to different parts of the range: 
combined voltage variation (CbVV). The 
change-over point is called 'maximum voltage 
tapping'. For this system the following 
applies: 

1 ) CFVV applies for tappings with tapping 
factors below the maximum voltage 
tapping factor. 

2) VFVV applies for tappings with tapping 
factors above the maximum voltage 
tapping factor. 

3) Graphic presentation of tapping voltage 
variation categories: 

CFVV (see Fig. 1 A) — VFVV 
(see Fig. IB) — CbVV (see Fig. IC). 



Symbols: 



UaJa 



Tapping voltage and tapping current in 
the tapped winding. 



IS 2026 (Part 1) : 2011 



t/g /b : Tapping voltage and tapping current in 
tiie untapped winding. 

5^3 : Tapping power. 

Abscissa : Tapping factor, percentage (indicating 
relative number of effective turns in 
tapped winding). 

1 : Indicates full-power tappings 

throughout the tapping range. 

2 : Indicates 'maximum- voltage tapping', 

'maximum current tapping' and range 
of reduced power tappings. 

The change-over point is shown in the plus tapping 



range. It constitutes both a maximum voltage tapping 
{UfJ and a maximum current tapping (/g constant, not 
rising above the change-over point). An additional, 
optional maximum current tapping (in the CFVV 
range) is also shown. 

5.3 Tapping Power. Full-Power Tappings — 
Reduced-Power Tappings 

All tappings shall be full-power tappings, except as 
specified below. 

In separate-winding transformers up to and 
including 2 500 kVA with a tapping range not 
exceeding ±5 percent the tapping current in the tapped 



100- 




100-- 



90 100 110 

Tapping factor 

Optional maximum current tapping shown 
1A Constant Flux Voltage Variation CFVV 



100 - 



\ 



\ 



\ 



\ 



\ 



100-- 



/ 



/ 



/ 



/ 



/ 



100 - 



u. 



\ 



\ 



\ 



\ 



\ u. 






. 1 'b 
y 

— 2 



1 L 




'AB 



90 100 110 

Tapping factor 

Optional maximum current tapping shown 
IB Variable Flux Voltage Variation VFVV 



Fig. 1 Tapping Voltage Variation (CoMf/nMet/) 



IS 2026 (Part 1) : 2011 



100 -- 



100 



100-- 




90 100 110 

Tapping factor 

1C Combined Voltage Variation CbVV 
Fig. 1 Tapping Voltage Variation 



winding shall be equal to rated current at all minus 
tappings. This means that the principal tapping is a 
'maximum current tapping'. 

In transformers with a tapping range wider 
than ±5 percent, restrictions may be specified on values 
of tapping voltage or tapping current which would 
otherwise rise considerably above the rated values. 
When such restrictions are specified, the tappings 
concerned will be 'reduced-power tappings'. This 
subclause describes such arrangements. 

When the tapping factor deviates from unity, the 
tapping current for full-power tappings may rise above 
rated current on one of the windings. As Fig. lA 
illustrates, this applies for minus tappings, on the 
tapped winding, under CFVV, and for plus tappings 
on the untapped winding under VFVV {see Fig. IB). 
In order to limit the corresponding reinforcement of 
the winding in question, it is possible to specify a 
maximum current tapping. From this tapping onwards 
the tapping current values for the winding are then 
specified to be constant. This means that the remaining 
tappings towards the extreme tapping are reduced- 
power tappings {see Fig. lA, IB and IC). 

Under CbVV, the 'maximum voltage tapping', the 
change-over point between CFVV and VFVV shall at 
the same time be a 'maximum current tapping' unless 



otherwise specified. This means that the untapped 
winding current stays constant up to the extreme plus 
tapping {see Fig. IC). 

5.4 Specification of Tappings in Enquiry and Order 

The following data are necessary to define the design 
of the transformer: 

a) Which winding shall be tapped; 

b) The number of steps and the tapping step (or 
the tapping range and number of steps). 
Unless otherwise specified it shall be assumed 
that the range is symmetrical around the 
principal tapping and that the tapping steps 
in the tapped winding are equal. If for some 
reason the design has unequal steps, this shall 
be indicated in the tender; 

c) The category of voltage variation and, if 
combined variation is applied, the change- 
over point ('maximum voltage tapping', 
see 5.2); and 

d) Whether maximum current limitation 
(reduced power tappings) shall apply, and if 
so, for which tappings. 

Instead of 5.4 (c) and 5.4 (d), tabulation of the same 
type as used on the rating plate may be used to 
advantage {see example in Annex C). 



IS 2026 (Part 1) : 2011 



The specification of these data may be accomplished 
in two different ways, 

a) either the user may specify all data from the 
beginning, in his enquiry; and 

b) alternatively, the user may submit a set of 
loading cases with values of active and 
reactive power (clearly indicating the 
direction of power flow), and corresponding 
on-load voltages. 

These cases should indicate the extreme values of 
voltage ratio under full and reduced power. Based on 
this information the manufacturer will then select the 
tapped winding and specify rated quantities and tapping 
quantities in his tender proposal. 

5.5 Specification of Short- Circuit Impedance 

Unless otherwise specified, the short-circuit impedance 
of a pair of windings is referred to the principal tapping 
(see 3.7.1). For transformers having a tapped winding 
with tapping range exceeding ±5 percent, impedance 
values are also to be given for the two extreme tappings. 
On such transformers these three values of impedance 
shall also be measured during the short-circuit test 
{see 10.4). 

When impedance values are given for several tappings, 
and particularly when the windings of the pair have 
dissimilar rated power values, it is recommended that 
the impedance values be submitted in ohms per phase, 
referred to either of the windings, rather than as 
percentage values. Percentage values may lead to 
confusion because of varying practices concerning 
reference values. Whenever percentage values are given 
it is advisable that the corresponding reference power 
and reference voltage values be explicitly indicated. 

A way of specifying short-circuit impedance values in 
the enquiry which leaves some degree of freedom in 
the design, is to indicate an acceptable range between 
upper and lower boundaries, across the whole tapping 
range. This may be done with the aid of a graph or a 
table. 

The boundaries shall be at least as far apart as to permit 
the double-sided tolerances of 9 to be applied on a 
median value between them. An example is shown in 
Annex D. The manufacturer shall select and guarantee 
impedance values for the principal tapping and for the 
extreme tappings which are between the boundaries. 
Measured values may deviate from guaranteed values 
within the tolerances according to 9, but shall not fall 
outside the boundaries, which are limits without 
tolerance. 

NOTE — The selection of an impedance value by the user is 
subject to conflicting demands: limitation of voltage drop 
versus limitation of overcurrent under system fault conditions. 



Economic optimization of the design, bearing in mind loss, 
leads towards a certain range of impedance values. Parallel 
operation with an existing transformer requires matching 
impedance. 

If an enquiry contains a specification of not only the impedance 
at the principal tapping but also its variation across the tapping 
range, this means a quite important restriction on the design 
(placing of windings in relation to each other). Such a detailed 
specification should therefore not be issued without good 
reason. 

5.6 Load Loss and Temperature Rise 

a) If the tapping range is within ±5 percent, and 
the rated power not above 2 500 kVA, load 
loss guarantees and temperature rise refer to 
the principal tapping only, and the temperature 
rise test is run on that tapping. 

b) If the tapping range exceeds ±5 percent or the 
rated power is above 2 500 kVA, it shall be 
stated for which tappings, in addition to the 
principal tapping, the load losses are to be 
guaranteed by the manufacturer. These load 
losses are referred to the relevant tapping 
current values. The temperature-rise limits are 
valid for all tappings, at the appropriate tapping 
power, tapping voltage and tapping current. 

A temperature-rise type test, if specified, shall be 
carried out on one tapping only. It will, unless otherwise 
agreed, be the 'maximum current tapping' (which is 
usually the tapping with the highest load loss). The 
total loss for the selected tapping is the test power for 
determination of oil temperature-rise during the 
temperature-rise test, and the tapping current for that 
tapping is the reference current for determination of 
winding temperature-rise above oil. For information 
about rules and tests regarding the temperature rise of 
oil-immersed transformers [see IS 2026 (Part 2)]. 

In principle, the temperature-rise type test shall 
demonstrate that the cooling equipment is sufficient 
for dissipation of maximum total loss on any tapping, 
and that the temperature-rise over ambient of any 
winding, at any tapping, does not exceed the specified 
maximum value. 

The second purpose normally requires the 'maximum 
current tapping' to be selected for the test. But the 
amount of total loss to be injected in order to determine 
maximum oil temperature-rise shall correspond to the 
highest value for any tapping, even if this is other than 
the tapping connected for the test [see also 5.2 in 
IS 2026 (Part 2)]. 

6 CONNECTIONAND PHASE DISPLACEMENT 
SYMBOLS FOR THREE-PHASE 

TRANSFORMERS 

The star, delta, or zigzag connection of a set of phase 
windings of a three-phase transformer or of windings 



10 



IS 2026 (Part 1) : 2011 



of the same voltage of single-phase transformers 
associated in a three-phase bank shall be indicated by 
the capital letters Y, D or Z for the high- voltage (HV) 
winding and small letters y, d or z for the intermediate 
and low-voltage (LV) windings. If the neutral point of 
a star-connected or zigzag-connected winding is 
brought out, the indication shall be YN (yn) or ZN (zn) 
respectively. 

Open windings in a three-phase transformer (that are 
not connected together in the transformer but have both 
ends of each phase winding brought out to terminals) 
are indicated as III (HV), or iii (intermediate or low- 
voltage windings). 

For an auto-connected pair of windings, the symbol of 
the lower voltage winding is replaced by 'auto', or 'a', 
for example, 'YNauto' or 'YNa' or 'YNaO', 'ZNall'. 

Letter symbols for the different windings of a 
transformer are noted in descending order of rated 
voltage. The winding connection letter for any 
intermediate and low-voltage winding is immediately 
followed by its phase displacement 'clock number' 
{see 3.10.6). Three examples are shown below and 
illustrated in Fig. 2. 

The existence of a stabilizing winding (a delta- 
connected winding which is not terminated for external 
three-phase loading) is indicated, after the symbols of 
loadable windings, with the symbol 'H-d'. 

If a transformer is specified with its winding connection 



changeable (series-parallel or Y-D), both connections 
will be noted, coupled with the corresponding rated 
voltages as indicated by the following examples: 



220(1 10)/10,5kV 
110/ll(6,35)kV 



YN(YN)dll 
YNyO(dll) 



Full information shall be given on the rating plate 
[see 7.2 (e)]. 

Examples of connections in general use, with 
connection diagrams, are shown in Annex E. 

Diagrams, with terminal markings, and with indication 
of built-in current transformers when used, may be 
presented on the rating plate together with the text 
information that is specified in 7. 

The following conventions of notation apply: 

The connection diagrams show the high-voltage 
winding above, and the low-voltage winding below. 
(The directions of induced voltages are indicated.) 

The high- voltage winding phasor diagram is oriented 
with phase I pointing at 12 O'clock. The phase I phasor 
of the low-voltage winding is oriented according to 
the induced voltage relation which results for the 
connection shown. 

The sense of rotation of the phasor diagrams is counter- 
clockwise, giving the sequence I — II — III. 

NOTE — This numbering is arbitrary. Terminal marking on 
the transformer follows national practice. 





Dynll YNynOdS 

Fig. 2 Illustration of 'Clock Number' Notation ■ 



YNadll 



■ Three Examples 



11 



IS 2026 (Part 1) : 2011 



Example 1 

A distribution transformer with high-voltage winding 
for 20 kV, delta-connected. The low- voltage winding 
is 400 V star-connected with neutral brought out. The 
LV winding lags the HV by 330°. 

Symbol: Dynll 

Example 2 

A three- winding transformer: 123 kV star with neutral 
brought out. 36 kV star with neutral brought out, in 
phase with the HV winding but not auto-connected. 
7.2 kV delta, lagging by 150°. 

Symbol: YNynOdS 

Example 3 

A group of three single-phase auto-transformers 



400 

V3/ 



'130 



kV with 22 kV tertiary windings. 



The auto-connected windings are connected in star, 
while the tertiary windings are connected in delta. The 
delta winding phasors lag the high-voltage winding 
phasors by 330°. 

Symbol: YNautodl 1 or YNadl 1 

The symbol would be the same for a three-phase auto- 
transformer with the same connection, internally. 

If the delta winding is not taken out to three line 
terminals but only provided as a stabilizing winding, 
the symbol would indicate this by a plus sign. No phase 
displacement notation would then apply for the 
stabilizing winding. 

Symbol: YNauto-i-d. 

7 RATING PLATES 

The transformer shall be provided with a rating plate 
of weatherproof material, fitted in a visible position, 
showing the appropriate items indicated below. The 
entries on the plate shall be indelibly marked. 

7.1 Information to be Given in All Cases 

a) Kind of transformer (for example transformer, 
auto-transformer, booster transformer, etc); 

b) Number of this standard; 

c) Manufacturer's name; 

d) Manufacturer's serial number; 

e) Year of manufacture; 

f) Number of phases; 

g) Rated power (in kVA or MVA). (For multi- 
winding transformers, the rated power of each 
winding should be given. The loading 



combinations should also be indicated unless 
the rated power of one of the windings is the 
sum of the rated powers of the other 
windings); 

h) Rated frequency (in Hz); 

j ) Rated voltages (in V or kV) and tapping range ; 

k) Rated currents (in A or kA); 

m) Connection symbol; 

n) Short-circuit impedance, measured value in 
percentage. For multi-winding transformers, 
several impedances for different two-winding 
combinations are to be given with the 
respective reference power values. For 
transformers having a tapped winding [see 
also 5.5 and 7.2(b)]; 

p) Type of cooling. (If the transformer has 
several assigned cooling methods, the 
respective power values may be expressed as 
percentages of rated power, for example 
ONAN/ONAF 70/100 percent); 

q) Total mass; and 

r) Mass of insulating oil. 

If the transformer has more than one set of ratings, 
depending upon different connections of windings 
which have been specifically allowed for in the design, 
the additional ratings shall all be given on the rating 
plate, or separate rating plates shall be fitted for each 
set. 

7.2 Additional Information to be Given when 
Applicable 

a) For transformers having one or more windings 
with 'highest voltage for equipment' t/^ equal 
to or above 3.6 kV: 

short notation of insulation levels (withstand 
voltages) as described in IS 2026 (Part 3). 

b) For transformers having a tapped winding, 
particulars about the tappings are as follows: 

1) for transformers having a tapping range 
not exceeding ±5 percent: tapping 
voltages on the tapped winding for all 
tappings. This applies in particular to 
distribution transformers; 

2) for transformers having a tapping range 
exceeding ±5 percent: a table stating 
tapping voltage, tapping current and 
tapping power for all tappings. In 
addition the short-circuit impedance 
values for the principal tapping and at 
least the extreme tappings shall be given, 
preferably in ohms per phase referred to 
a specific winding. 

c) Temperature-rises of top oil and windings (if 



12 



IS 2026 (Part 1) : 2011 



not normal values). When a transformer is 
specified for installation at high altitude, this 
shall be indicated, together with information 
on either the reduced temperature-rise figures 
valid under normal ambient conditions, or the 
reduced loading which will result in normal 
temperature rise at the high altitude (standard 
transformer with normal cooling capacity). 

d) Insulating liquid, if not mineral oil. 

e) Connection diagram (in cases where the 
connection symbol will not give complete 
information regarding the internal 
connections). If the connections can be 
changed inside the transformer, this shall be 
indicated on a separate plate or with duplicate 
rating plates. The connection fitted at the 
works shall be indicated. 

f) Transportation mass (for transformers 
exceeding 5 t total mass). 

g) Untanking mass (for transformers exceeding 
5 t total mass). 

h) Vacuum withstand capability of the tank and 
of the conservator 

In addition to the main rating plate with the information 
listed above, the transformer shall also carry plates with 
identification and characteristics of auxiliary 
equipment according to standards for such components 
(bushings, tap-changers, current transformers, special 
cooling equipment). 

8 MISCELLANEOUS REQUIREMENTS 

8.1 Dimensioning of Neutral Connection 

The neutral conductor and terminal of transformers 
intended to carry a load between phase and neutral 
(for example, distribution transformers) shall be 
dimensioned for the appropriate load current and earth- 
fault current. 

The neutral conductor and terminal of transformers not 
intended to carry load between phase and neutral shall 
be dimensioned for earth-fault current. 

8.2 Oil Preservation System 

For oil-immersed transformers the type of oil 
preservation system shall be specified in the enquiry 
and order The following types are distinguished: 

a) Freely breathing system or conservator 
system where there is free communication 
between the ambient air and an air-filled 
expansion space above the surface of the oil, 
in the tank or in a separate expansion vessel 
(conservator). A moisture-removing breather 
is usually fitted in the connection to the 
atmosphere. 



b) Diaphragm-type oil preservation system 
where an expansion volume of air at 
atmospheric pressure is provided above the 
oil but prevented from direct contact with the 
oil by a flexible diaphragm or bladder. 

c) Inert gas pressure system where an expansion 
space above the oil is filled with dry inert gas 
at slight over-pressure, being connected to 
either a pressure controlled source or an 
elastic bladder 

d) Sealed-tank system with gas cushion, in which 
a volume of gas above the oil surface in a stiff 
tank accommodates the oil expansion under 
variable pressure. 

e) Sealed, completely filled system in which the 
expansion of the oil is taken up by elastic 
movement of the permanently sealed, usually 
corrugated tank. 

8.3 Load Rejection on Generator Transformers 

Transformers intended to be connected directly to 
generators in such a way that they may be subjected to 
load rejection conditions shall be able to withstand 1.4 
times rated voltage for 5 s at the transformer terminals 
to which the generator is to be connected. 

9 TOLERANCES 

It is not always possible, particularly in large, multi- 
winding transformers with relatively low rated 
voltages, to accommodate turns ratios which 
correspond to specified rated voltage ratios with high 
accuracy. There are also other quantities which may 
not be accurately explored at the time of tender, or 
are subject to manufacturing and measuring 
uncertainty. 

Therefore tolerances are necessary on certain 
guaranteed values. 

Table 1 gives tolerances to be applied to certain rated 
quantities and to other quantities when they are the 
subject of manufacturer's guarantees referred to in this 
standard. Where a tolerance in one direction is omitted, 
there is no restriction on the value in that direction. 

A transformer is considered as complying with this 
part when the quantities subject to tolerances are not 
outside the tolerances given in Table 1 . 

10 TESTS 

10.1 General Requirements for Routine, Type and 
Special Tests 

Transformers shall be subjected to tests as specified 
below. 



13 



IS 2026 (Part 1) : 2011 



Table 1 Tolerances 

(Clause 9) 



SI 


Item 


Tolerance 


No. 






(1) 


(2) 


(3) 



i) a) Total losses 

b) Component losses (see Note 1 ) 

ii) a) Voltage ratio at no load on principal 

tapping for a specified first pair of 
windings 

b) Voltage ratio on other tappings, same pair 

c) Voltage ratio for further pairs 

iii) Short-circuit impedance for: 

a) separate-winding transformer with two 
windings, or 

b) a specified first pair of separate windings 
in a multi-winding transformer 

1) principal tapping 



2) any other tapping of the pair 



iv) Short-circuit impedance for: 

a) an auto-connected pair of winding, or 

b) a specified second pair of separate 
windings in a multi-winding transformer 

1) principal tapping 

2) any other tapping of the pair 

3) further pairs of windings 

v) No-load current 



+10 percent of the total losses 

+15 percent of each component loss, provided that the tolerance for total 

losses is not exceeded 

The lower of the following values: 

a) ±0.5 percent of declared ratio 

b) ±1/10 of the actual percentage impedance on the principal tapping 
To be agreed, but not less than the lesser of the values given in (a) and (b) 
above 

To be agreed, but not less than the lesser of the values given in (a) and (b) 



When the impedance value is >10 percent 
±7.5 percent of the declared value 
When the impedance value is <10 percent 
±10 percent of the declared value 
When the impedance value is >10 percent 
±10 percent of the declared value 
When the impedance value is <10 percent 
±15 percent of the declared value 



±10 percent of the declared value 

±15 percent of the declared value for that tapping 

To be agreed, but >15 percent 

+30 percent of the declared value 



NOTES 

1 The loss tolerances of multi-winding transformers apply to every pair of windings unless the guarantee states that they apply to a 
given load condition. 

2 For certain auto-transformers and booster transformers the smallness of their impedance justifies more liberal tolerance. Transformers 
having large tapping ranges, particularly if the range is asymmetrical, may also require special consideration. On the other hand, for 
example, when a transformer is to be combined with previously existing units, it may be justified to specify and agree on narrower 
impedance tolerances. Matters of special tolerances shall be brought to attention at the tender stage, and revised tolerances agreed 
upon between manufacturer and purchaser. 

3 'Declared value' should be understood as meaning the value declared by the manufacturer. 



Tests shall be made at any ambient temperature 
between 10 °C and 50 °C and with cooling water 
(if required) at any temperature not exceeding 30 °C. 

Tests shall be made at the manufacturer's works, unless 
otherwise agreed between the manufacturer and the 
purchaser. 

All external components and fittings that are likely to 
affect the performance of the transformer during the 
test shall be in place. 

Tapped windings shall be connected on their principal 
tapping, unless the relevant test clause requires 
otherwise or unless the manufacturer and the purchaser 
agree otherwise. 



The test basis for all characteristics other than 
insulation is the rated condition, unless the test clause 
states otherwise. 

All measuring systems used for the tests shall have 
certified, traceable accuracy and be subjected to 
periodic calibration, according to IS/ISO 9001. 

Where it is required that test results are to be corrected 
to a reference temperature, this shall be: 

a) for oil-immersed transformers; 75 °C; and 

b) for dry-type transformers: according to the 
general requirements for tests in IS 1 1 171. 

NOTE — Specific requirements on the accuracy and 
verification of the measuring systems are under 
consideration. 



14 



IS 2026 (Part 1) : 2011 



10.1.1 Routine Tests 

a) Measurement of winding resistance 
(see 10.2); 

b) Measurement of voltage ratio and check of 
phase displacement (see 10.3); 

c) Measurement of short-circuit impedance and 
load loss (see 10.4); 

d) Measurement of no-load loss and current 
(see 10.5); 

e) Dielectric routine tests IS 2026 (Part 3); and 

f) Tests on on-load tap-changers, where 
appropriate (see 10.8). 

10.1.2 Type Tests 

a) Temperature-rise test [see IS 2026 (Part 2)]; 
and 

b) Dielectric type tests [see IS 2026 (Part 3)]. 

10.1.3 Special Tests 

a) Dielectric special tests [see IS 2026 (Part 3)]; 

b) Determination of capacitances windings-to- 
earth, and between windings; 

c) Determination of transient voltage transfer 
characteristics; 

d) Measurement of zero-sequence impedance(s) 
on three-phase transformers (see 10.7); 

e) Short-circuit withstand test [see IS 2026 
(Parts)]; 

f) Determination of sound levels (see IS 1 3964) ; 

g) Measurement of the harmonics of the no-load 
current (see 10.6); 

h) Measurement of the power taken by the fan 
and oil pump motors; and 

j) Measurement of insulation resistance to earth 
of the windings, and/or measurement of 
dissipation factor (tan 5) of the insulation 
system capacitances. (These are reference 
values for comparison with later measurement 
in the field. No limitations for the values are 
given here.) 

If test methods are not prescribed in this standard, or 
if tests other than those listed above are specified in 
the contract, such test methods are subject to 
agreement. 

10.2 Measurement of Winding Resistance 

10.2.1 General 

The resistance of each winding, the terminals between 
which it is measured and the temperature of the 
windings shall be recorded. Direct current shall be used 
for the measurement. 



In all resistance measurements, care shall be taken that 
the effects of self-induction are minimized. 

10.2.2 Dry -Type Transformers 

Before measurement the transformer shall be at rest in 
a constant ambient temperature for at least 3 h. 

Winding resistance and winding temperature shall be 
measured at the same time. The winding temperature 
shall be measured by sensors placed at representative 
positions, preferably inside the set of windings, for 
example, in a duct between the high-voltage and low- 
voltage windings. 

10.2 Oil-Immersed Type Transformers 

After the transformer has been under oil without 
excitation for at least 3 h, the average oil temperature 
shall be determined and the temperature of the winding 
shall be deemed to be the same as the average oil 
temperature. The average oil temperature is taken as 
the mean of the top and bottom oil temperatures. 

In measuring the cold resistance for the purpose of 
temperature-rise determination, special efforts shall be 
made to determine the average winding temperature 
accurately. Thus, the difference in temperature between 
the top and bottom oil should be small. To obtain this 
result more rapidly, the oil may be circulated by a 
pump. 

10.3 Measurement of Voltage Ratio and Check of 
Phase Displacement 

The voltage ratio shall be measured on each tapping. 
The polarity of single-phase transformers and the 
connection symbol of three-phase transformers shall 
be checked. 

10.4 Measurement of Short-Circuit Impedance and 
Load Loss 

The short-circuit impedance and load loss for a pair of 
windings shall be measured at rated frequency with 
approximately sinusoidal voltage applied to the 
terminals of one winding, with the terminals of the 
other winding short-circuited, and with possible other 
windings open-circuited (For selection of tapping for 
the test, see 5.5 and 5.6). The supplied current should 
be equal to the relevant rated current (tapping current) 
but shall not be less than 50 percent thereof. The 
measurements shall be performed quickly so that 
temperature rises do not cause significant errors. The 
difference in temperature between the top oil and the 
bottom oil shall be small enough to enable the mean 
temperature to be determined accurately. If the 
cooling system is OF or OD, the pump may be used 
to mix the oil. 



15 



IS 2026 (Part 1) : 2011 



The measured value of load loss shall be multiplied 
with the square of the ratio of rated current (tapping 
current) to test current. The resulting figure shall then 
be corrected to reference temperature (see 10.1). The 
PR loss (R being dc resistance) is taken as varying 
directly with the winding resistance and all other losses 
inversely with the winding resistance. The 
measurement of winding resistance shall be made 
according to 10.2. The temperature correction 
procedure is detailed in Annex F. 

The short-circuit impedance is represented as reactance 
and ac resistance in series. The impedance is corrected 
to reference temperature assuming that the reactance 
is constant and that the ac resistance derived from the 
load loss varies as described above. 

On transformers having a tapped winding with tapping 
range exceeding ±5 percent, the short-circuit 
impedance shall be measured on the principal tapping 
and the two extreme tappings. 

On a three-winding transformer, measurements are 
performed on the three different two-winding 
combinations. The results are re-calculated, allocating 
impedances and losses to individual windings. Total 
losses for specified loading cases involving all these 
windings are determined accordingly. 

NOTES 

1 For transformers with two secondary windings having the 
same rated power and rated voltage and equal impedance to 
the primary (sometimes referred to as 'dual-secondary 
transformers'), it may be agreed to investigate the symmetrical 
loading case by an extra test with both secondary windings 
short-circuited simultaneously. 

2 The measurement of load loss on a large transformer requires 
considerable care and good measuring equipment because of 
the low power factor and the often large test cuiTents. Correction 
for measuring transformer errors and for resistance of the test 
connections should be applied unless they are obviously 
negligible. 

10.5 Measurement of No-load Loss and Current 

The no-load loss and the no-load current shall be 
measured on one of the windings at rated frequency 
and at a voltage corresponding to rated voltage if the 
test is performed on the principal tapping, or to the 
appropriate tapping voltage if the test is performed on 
another tapping. The remaining winding or windings 
shall be left open-circuited and any windings which 
can be connected in open delta shall have the delta 
closed. 

The transformer shall be approximately at factory 
ambient temperature. 

For a three-phase transformer the selection of the 
winding and the connection to the test power source 
shall be made to provide, as far as possible, symmetrical 



and sinusoidal voltages across the three wound limbs. 

The test voltage shall be adjusted according to a 
voltmeter responsive to mean value of voltage but 
scaled to read the r.m.s. voltage of a sinusoidal wave 
having the same mean value. The reading of this 
voltmeter is U'. 

At the same time, a voltmeter responsive to the r.m.s. 
value of voltage shall be connected in parallel with the 
mean- value voltmeter and its indicated voltage U shall 
be recorded. 

When a three-phase transformer is tested, the voltages 
shall be measured between line terminals, if a delta- 
connected winding is energized, and between phase 
and neutral terminals if a YN or ZN connected winding 
is energized. 

The test voltage wave shape is satisfactory if the 
readings U' and U are equal within 3 percent. 

The measured no-load loss is P^, and the corrected no 
load loss is taken as: 

U'-U 

(usually negative) 



U' 

If the difference between voltmeter readings is larger 
than 3 percent, the validity of the test is subject to 
agreement. 

The r.m.s. value of no-load current is measured at the 
same time as the loss. For a three-phase transformer, 
the mean value of readings in the three phases is taken. 

NOTES 

1 It is recognized that the most severe loading conditions for 
test voltage source accuracy are usually imposed by large 
single-phase transformers. 

2 In deciding the place of the no-load test in the complete 
test sequence, it should be borne in mind that no-load loss 
measurements performed before impulse tests and/or 
temperature rise tests are, in general, representative of the 
average loss level over long time in service. Measurements 
after other tests sometimes show higher values caused by 
spitting between laminate edges during the impulse tests, etc. 
Such measurements may be less representative of losses in 
service. 

10.6 Measurement of the Harmonics of the No-load 
Current 

The harmonics of the no-load current in the three 
phases are measured and the magnitude of the 
harmonics is expressed as a percentage of the 
fundamental component. 

10.7 Measurement of Zero-Sequence Impedance(s) 
on Three-Phase Transformers 

The zero-sequence impedance is measured at rated 



16 



IS 2026 (Part 1) : 2011 



frequency between the line terminals of a star- 
connected or zigzag-connected winding connected 
together, and its neutral terminal. It is expressed in 
ohms per phase and is given by 3 UII, where U is the 
test voltage and / is the test current. 



The test current per phase — shall be stated 

It shall be ensured that the current in the neutral 
connection is compatible with its current-carrying 
capability. 

In the case of a transformer with an additional delta- 
connected winding, the value of the test current shall 
be such that the current in the delta-connected winding 
is not excessive, taking into account the duration of 
application. 

If winding balancing ampere-turns are missing in the 
zero-sequence system, for example, in a star-star- 
connected transformer without delta winding, the 
applied voltage shall not exceed the phase-to-neutral 
voltage at normal operation. The current in the neutral 
and the duration of application should be limited to avoid 
excessive temperatures of metallic constructional parts. 

In the case of transformers having more than one star- 
connected winding with neutral terminal, the zero- 
sequence impedance is dependent upon the connection 
{see 3.7.3) and the tests to be made shall be subject to 
agreement between the manufacturer and the purchaser 

Auto-transformers with a neutral terminal intended to 
be permanently connected to earth shall be treated as 
normal transformers with two star-connected windings. 
Thereby, the series winding and the common winding 
together form one measuring circuit, and the common 
winding alone forms the other. The measurements are 
carried out with a current not exceeding the difference 
between the rated currents on the low-voltage side and 
the high- voltage side. 

NOTES 

1 In conditions where winding balancing ampere-turns are 
missing, the relation between voltage and current is generally 
not linear. In that case several measurements at different values 
of current may give useful information. 



2 The zero-sequence impedance is dependent upon the physical 
disposition of the windings and the magnetic parts and 
measurements on different windings may not, therefore, agree. 

10.8 Tests on On-load Tap-Changers 

10.8.1 Operation Test 

With the tap-changer fully assembled on the 
transformer the following sequence of operations shall 
be performed without failure: 

a) with the transformer un-energized, eight 
complete cycles of operation (a cycle of 
operation goes from one end of the tapping 
range to the other, and back again); 

b) with the transformer un-energized, and with 
the auxiliary voltage reduced to 85 percent of 
its rated value, one complete cycle of operation; 

c ) with the transformer energized at rated voltage 
and frequency at no load, one complete cycle 
of operation; and 

d) with one winding short-circuited and, as far 
as practicable, rated current in the tapped 
winding, 10 tap-change operations across the 
range of two steps on each side from where a 
coarse or reversing changeover selector 
operates, or otherwise from the middle 
tapping. 

10.8.2 Auxiliary Circuits Insulation Test 

After the tap-changer is assembled on the transformer, 
a power frequency test shall be applied to the auxiliary 
circuits as specified in IS 2026 (Part 3). 

11 ELECTROMAGNETIC COMPATIBILITY 

(EMC) 

Power transformer shall be considered as passive 
elements in respect to emission of, and immunity to, 
electromagnetic disturbances. 

NOTES 

1 Certain accessories may be susceptible to electromagnetic 
interference. 

2 Passive elements are not liable to cause electromagnetic 
disturbances and their performance is not liable to be affected 
by such disturbances. 



17 



IS 2026 (Part 1) : 2011 



ANNEX A 

(Clause 2) 

LIST OF REFERRED INDIAN STANDARDS 



IS No Title IS No 

1076 (Part 1): Preferred numbers: Part 1 Series of 5553 : 1989 

1985 preferred number (All parts) 

2026 Power transformers: 6600:1972 

(Part 2) : 2010 Temperature rise (first revision) 

(Part 3) : 2009 Insulation levels and dielectric tests 11171 : 1985 
(Part 4) : 1977 Terminal marking, tappings and 

connections 13964 : 1994 
(Part 5) : 20 1 1 Ability to withstand short circuit (first 

revision) IS/ISO 9001 : 

2000 



Title 
Reactors 

Guide for loading of oil immersed 

transformers 

Specification for dry-type power 

transformers 

Methods of measurement of 

transformer and reactor sound levels 

Quality management systems — 

Requirements 



ANNEX B 

(Clause 1.1.2) 

INFORMATION REQUIRED WITH ENQUIRY AND ORDER 



B-1 RATING AND GENERAL DATA 



B-1.1 Normal Information 

The following information shall be given in all cases: 



a) 



b) 



c) 
d) 
e) 
f) 



h) 
J) 



Particulars of the specifications to which the 
transformer shall comply; 
Kind of transformer, for example, separate 
winding transformer, auto-transformer or 
booster transformer; 
Single or three-phase unit; 
Number of phases in system; 
Frequency; 

Dry-type or oil-immersed type. If oil- 
immersed type, whether mineral oil or 
synthetic insulating liquid. If dry-type, degree 
of protection (see IS 11171); 

Indoor or outdoor type; 
Type of cooling; 

Rated power for each winding and, for tapping 
range exceeding ±5 percent, the specified 
maximum current tapping, if applicable. If the 
transformer is specified with alternative 
methods of cooling, the respective lower 
power values are to be stated together with 
the rated power (which refers to the most 
efficient cooling); 



k) Rated voltage for each winding; 
m) For a transformer with tappings: 

1 ) which winding is tapped, the number of 
tappings, and the tapping range or 
tapping step; 

2) whether 'off-circuit' or 'on-load' tap- 
changing is required; 

3) if the tapping range is more than ±5 
percent, the type of voltage variation, and 
the location of the maximum current 
tapping, if applicable, (see 5.4). 

n) Highest voltage for equipment (U^) for each 

winding [with respect to insulation [see 

IS 2026 (Part 3)]; 
p) Method of system earthing (for each 

winding); 
q) Insulation level [see IS 2026 (Part 3)] , for each 

winding; 
r) Connection symbol and neutral terminals, if 

required for any winding; 
s) Any peculiarities of installation, assembly, 

transport and handling. Restrictions on 

dimensions and mass; 
t) Details of auxiliary supply voltage (for fans 

and pumps, tap-changer, alarms, etc); 
u) Fittings required and an indication of the side 



18 



IS 2026 (Part 1) : 2011 



from which meters, rating plates, oil-level 
indicators, etc, shall be legible; 
v) Type of oil preservation system; and 
w) For multi-winding transformers, required 
power-loading combinations, stating, when 
necessary, the active and reactive outputs 
separately, especially in the case of multi- 
winding auto-transformers. 

B-1.2 Special Information 

The following additional information may need to be 
given: 

a) If a lightning impulse voltage test is required, 
whether or not the test is to include chopped 
waves [see IS 2026 (Part 3)]; 

b) Whether a stabilizing winding is required and, 
if so, the method of earthing; 

c) Short-circuit impedance, or impedance range 
(see Annex D). For multi-winding 
transformers, any impedances that are 
specified for particular pairs of windings 
(together with relevant reference ratings, if 
percentage values are given); 

d) Tolerances on voltage ratios and short-circuit 
impedances as left to agreement in Table 1 , 
or deviating from values given in the table; 

e) Whether a generator transformer is to be 
connected to the generator directly or through 
switchgear, and whether it will be subjected 
to load rejection conditions; 

f) Whether a transformer is to be connected 
directly or by a short length of overhead line 
to gas-insulated switchgear (GIS); 

g) Altitude above sea-level, if in excess 
of 1 000 m (3 300 ft); 

h) Special ambient temperature conditions or 
restrictions to circulation of cooling air; 

j) Expected seismic activity at the installation 
site which requires special consideration; 

k) Special installation space restrictions which 
may influence the insulation clearances and 
terminal locations on the transformer; 

m) Whether load current wave shape will be 
heavily distorted. Whether unbalanced three- 
phase loading is anticipated. In both cases. 



n) 



P) 



q) 

r) 



s) 



t) 



u) 



V) 



details to be given; 

Whether transformers will be subjected to 

frequent overcurrents, for example, furnace 

transformers and traction feeding transformers; 

Details of intended regular cyclic 

overloading other than covered by 4.2 (to 

enable the rating of the transformer auxiliary 

equipment to be established); 

Any other exceptional service conditions; 

If a transformer has alternative winding 

connections, how they should be changed, and 

which connection is required ex works; 

Short-circuit characteristics of the connected 

systems (expressed as short-circuit power or 

current, or system impedance data) and 

possible limitations affecting the transformer 

design [see IS 2026 (Part 5)]; 

Whether sound-level measurement is to be 

carried out (see IS 13964); 

Vacuum withstand of the transformer tank 

and, possibly, the conservator, if a specific 

value is required; and 

Any special tests not referred to above which 

may be required. 



B-2 PARALLEL OPERATION 

If parallel operation with existing transformers is 
required, this shall be stated and the following 
information on the existing transformers given: 



a) 
b) 
c) 

d) 



e) 



f) 



Rated power; 

Rated voltage ratio; 

Voltage ratios corresponding to tappings other 

than the principal tapping; 

Load loss at rated current on the principal 

tapping, corrected to the appropriate reference 

temperature; 

Short-circuit impedance on the principal 

tapping and at least on the extreme tappings, 

if the tapping range of the tapped winding 

exceeds ±5 percent; and 

Diagram of connections, or connection 

symbol, or both. 



NOTE — On multi-winding transformers, supplementary 
information will generally be required. 



19 



IS 2026 (Part 1) : 2011 



ANNEX C 

(Clause 5.4) 

EXAMPLES OF SPECIFICATIONS FOR TRANSFORMERS WITH TAPPINGS 



C-1 EXAMPLE 1 — CONSTANT FLUX VOLTAGE 
VARIATION 

Transformer having a 66 kV/20 kV three- 
phase 40 MVA rating and a ±10 percent tapping range 
on the 66 kV winding, with 1 1 tapping positions. Short 
notation: (66 ± 5 x 2 percent ) / 20 kV. 



Category of voltage variation 
Rated power 
Rated voltages 
Tapped winding 



Number of tapping positions 



CFVV 
40 MVA 
66 kV/20 kV 
66 kV 

(tapping range 
±10 percent ) 
11 



If this transformer shall have reduced power tappings, 
say, from tapping -6 percent, add: 



maximum current tapping 



: tapping -6 
percent 



The tapping current of the HV winding is then limited 
to 372 A from the tapping -6 percent to the extreme 
tapping -10 percent where tapping power is reduced 
to 38.3 MVA. 



C-2 EXAMPLE 2 
VARIATION 



VARIABLE FLUX VOLTAGE 



Transformer having a 66 kV/6 kV, three-phase 20 MVA 
rating and a +15 percent, -5 percent tapping range on 
the HV winding, but having a constant tapping voltage 
for the HV winding and a variable tapping voltage for 
the LV winding, between: 



6.32 kV to 



0.95 1.15 

Category of voltage variation 

Rated power 

Rated voltages 

Tapped winding 



:5.22kV 



Number of tapping positions : 
Tapping voltages of 6 kV winding : 



VFVV 

20 MVA 

66 kV/6 kV 

66 kV 

(tapping 

range -1-15 

percent,-5 

percent ) 

13 

6.32 kV 6 

kV, 5.22 kV 



If this transformer shall have reduced power tappings, 
add for example: 



maximum current tapping 



tapping +5 
percent 



The 'tapping current' of the untapped winding (LV) is 
then limited to 2 020 A from the tapping +5 percent to 
the extreme tapping -1-15 percent where the tapping 
power is reduced to 18.3 MVA. 



C-3 EXAMPLE 3 
VARIATION 



COMBINED VOLTAGE 



Transformer having a 160 kV/20 kV three- 
phase 40 MVA rating and a ±15 percent tapping range 
on the 160 kV winding. The changeover point 
(maximum voltage tapping), is at +6 percent, and there 
is also a maximum current tapping in the CFVV range 
at -9 percent: 

Tapped winding: 160 kV, range ± 10 x 1.5 percent. 



Tappings 






c 










.2 


1 


S 








^^? 


a<3 




.l^c.^ 








1 


1 


1 




















UuT 


4t 


/ht 


/bt 


7ht/ 






kV 


A 


A 


A 


/bt 


(1) 


(2) 


(3) 


(4) 


(5) 


(6) 


(7) 


1(-Hl5 


9/20 


169.6 


18.43 


125.6 


1 155 


36.86 


percent ) 














7 (+6 


8/48 


169.6 


20 


136.2 


1 155 


40 


percent) 














11(0 


8 


160 


20 


144.4 


1 155 


40 


percent) 














17 (-9 


7/28 


145.6 


20 


158.7 


1 155 


40 


percent) 














21 (-15 


6/80 


136 


20 


158.7 


1080 


37.4 


percent) 















NOTES 

1 On completing with data for intermediate tappings, the 
preceding table can be used on a rating plate. 

2 Compare this specification and a CFVV specification which 
would be: 

(160 ± 15 percent ) / 20 kV — 40 MVA 

The difference is that the HV tapping voltage, 
according to the example, does not exceed the 'system 
highest voltage' of the HV system, which is 170 kV 
(lEC standardized value). The quantity 'highest voltage 
for equipment' which characterizes the insulation of 
the winding, is also 170 kV [see IS 2026 (Part 3)]. 



20 



IS 2026 (Part 1) : 2011 



ANNEX D 

(Clauses 5.5 and B-l. 2) 

SHORT-CIRCUIT IMPEDANCE BY BOUNDARIES 



The upper boundary (Fig. 3) is a constant value of 
short-circuit impedance as a percentage, which is 
determined by the permissible voltage drop at a 
specified loading and at a specified power factor. 

The lower boundary is determined by permissible 



overcurrent on the secondary side during a through- 
fault. 

The dashed line is an example of a transformer short- 
circuit impedance curve which would satisfy this 
specification. 



30 



28 



26 - 



24 



o 

c 
n 

T3 

o. 



22 - 



18 



16 



14 



//////////// 



24.^ ^ — — — " 



'///// //////// /J_C^ ^ 



21.7 \ WW \\\\\ \\\\\\\\^\V^^V^ ' 



T- 





T 



27,8 



20,6 



_20 -15 -10 -5 +5 +10 

Tapping range % 

Fig. 3 Example of Specification of Short-Circuit Impedance by Boundaries 



21 



IS 2026 (Part 1) : 2011 

ANNEX E 

(Clause 6) 

THREE-PHASE TRANSFORMER CONNECTIONS 

Common Connections are given in Fig. 4 and other connections are given in Fig. 5, 6 and 7. 



I II III I 



YyO 



III II 



II Hi > 



ill A 



HI II 



m 

DdO 



mA 



m 



DzO 



m 




LU X 



Yd1 




A 



iij 



Dyl 



Yz1 



m 




m 



J 



m 




UJ 



Yds 



^A 



ill 



m<. 



Dy5 



Yz5 



m 



m 



a 




UJ 



Yy6 




Mi 



m 



my 



m 



Od6 



Dz6 





LU 



11 



Yd11 



[ttS A 



Hj 



m > 



Dyll 



Yzll 



Hi 







NOTE — It should be noted that these conventions differ from those previously used in Fig. 5 of IS 2026 (Part 5). 

Fig. 4 Common Connections 



22 



IS 2026 (Part 1) : 2011 



Additional Connections 



tlA 



Dd2 



m v 



M 




Dz2 



*^ 




m A 



m A 



Dd4 



Dz4 



II .1 (I 

WW A, 



m 



fl) 




w 



Yd7 



MA 



ID 



Dy7 



Yz7 




E> 



m y 



i]£] 



mi 




/ 



8 

Ma 



m A 



Dz8 



[O 




10 



ffi) A 



m 




DdIO 



Dz10 



tlv 



Mt,V^ 



t 



NOTE — It should be noted that these conventions differ from those previously used in Fig. 5 of IS 2026 (Part 4) 

Fig. 5 Additional Connections 
23 



IS 2026 (Part 1) : 2011 




II 
II 



III 



II 
II 



Fig. 6 Designation of Connections of Three-Phase Auto-transformers by Connection Symbols 

Auto-transformer YaO 



(i) ♦ {»)<> (I) « (ii) <> 




n I 






(i) 



(ii) 



L_ 



(i) (ii) 



(') 



(ii) 



III 



-t(i) (ii)' 

L 



L. 



.J L 



Fig. 7 Example of Three Single-Phase Transformers Connected to Form a Three-Phase Bank 

(Connection Symbol Yd5) 



24 



IS 2026 (Part 1) : 2011 



ANNEX F 

(Clause 10.4) 

TEMPERATURE CORRECTION OF LOAD LOSS 



F-1 LIST OF SYMBOLS 

Index 1 : Refers to measurement of 'cold winding 
resistance' (see 10.2). 

Index 2 : Indicates conditions during measurement 
of load loss (see 10.4). 

r : Indicates conditions at 'reference 

temperature' (see 10.1). 

R : Resistance. 

9 : Winding temperature, in °C. 

P : Load loss. 

/ : Specified load current for loss 

determination (rated current, tapping 
current, other specified value related to a 
particular loading case). 

P^ : Additional loss. 

The winding resistance measurement is made at a 
temperature 9,. The measured value is /?,. 

The load loss is measured with the winding at an 
average temperature 92. The measured loss referred to 
specified current /, is P^. This loss is composed of 
ohmic loss: fR^ and 'additional loss': f , 



„ 235 + a ^ 



At reference temperature 9^, the winding resistance is 
R,, the additional loss P..,, the whole load loss P,. 



235 + 0. ^ 
R^ = R, ■:^:^ — J- (copper) 



235 + a 



235 + a 
Pi 

°'235+a 



„ 225 + 0. , , . . , 

R = R, (aluminium) 

'225 + 01 

225 + a 



P. = ^2 



225+0 



For oil-immersed transformers with reference 
temperature 75 °C the formulae become as follows: 



„ 310 , 
R = R, (copper) 

'235 + 0, 



P,i 



235 + 0, 
310 

300 



R = R, — '^^^ — (aluminium) 
225 + 0, 



„ 225 + 0, , , . . , 
R, ^^^ — ^ (alumimum) 



^^ " "'225+0 



fR. 



P. = P.2 



225 + 0, 



300 
Finally: P,= /2/;,.+ P^ 



25 



Bureau of Indian Standards