Table 1 Tolerances (Clause 9) Sl No. Item Tolerance (1) (2) (3) i) a) Total losses b) Component losses (see Note 1) +10 percent of the total losses +15 percent of each component loss, provided that the tolerance for total losses is not exceeded ii) a) Voltage ratio at no load on principal tapping for a specified first pair of windings 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 b) Voltage ratio on other tappings, same pair To be agreed, but not less than the lesser of the values given in (a) and (b) above c) Voltage ratio for further pairs To be agreed, but not less than the lesser of the values given in (a) and (b) 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 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 

 2) any other tapping of the pair 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 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 ±10 percent of the declared value 2) any other tapping of the pair ±15 percent of the declared value for that tapping 3) further pairs of windings To be agreed, but ≥15 percent v) No-load current +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 11171. NOTE — Specific requirements on the accuracy and verification of the measuring systems are under consideration. 15 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-toearth, 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 (Part 5)]; f) Determination of sound levels (see IS 13964); 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 δ) 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 lowvoltage 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. 16 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 I2R 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 currents. 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 deltaconnected 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 Pm, and the corrected no load loss is taken as: Po = Pm (1 + d) d = U U U ′ − ′ (usually negative) 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 17 IS 2026 (Part 1) : 2011 frequency between the line terminals of a starconnected or zigzag-connected winding connected together, and its neutral terminal. It is expressed in ohms per phase and is given by 3 U/I, where U is the test voltage and I is the test current. The test current per phase 3 I 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 deltaconnected 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-starconnected 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 starconnected winding with neutral terminal, the zerosequence 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