INSPECTION PROCEDURES FOR POWER TRANSFORMERS STAGE INSPECTIONThe Stage Inspection will particularly include the following tests/checks besides the general routine tests to be conducted during manufacturing stages as per the manufacturer’s general practice. No deviation shall be acceptable. Physical inspection/checking of winding insulating material, core material and other accessories/fitting of transformer. Measurement of Core area, cross sectional area of winding(s), number of turns in each winding. Measurement of thickness of tank plates (Top, Bottom and sides) and to conduct pressure and Vacuum tests as per the laid down in CBIP manual for transformer tests to ensure the adequate strength of tank plates. Sample testing of Core Material for checking specific loss, magnetisation characteristics (i.e., B.H. Curve plot) and thickness. If Core material sample testing facility for checking of specific loss & magnetisation characteristics are not available at suppliers works then core sample is to be got tested at Govt. approved / recognised testing laboratory. Visual and dimensional check during assembly stage of core. Check for proper provisions of spacers and bracing to arrest the movement of core and winding assembly inside the tank. Check for providing Condenser rings on HV & LV Windings. I) / 1977 at 90%. 100% and 110 % rated Voltage and excitation losses at 90%. The tests shall be carried out in the presence of purchaser’s representative. finish. 415V RMS energising of HV and LV winding. Measurement of excitation current at low voltage (AC ) say 3 Phase. Checking of polarity and phase relation-ships for each winding. Regulation at rated load at unity.80 lagging power factor. the supplier should intimate the complete schedule of manufacturing programme of these transformers at least 15 days advance.To carry out the stage inspection. Measurements of turn ratio between HV – LV Windings at each gap. provision for all fittings. The voltage of measurement will be indicated in the test report. oil level etc. Positive Phase sequence impedance / short circuit impedance between HV-LV Windings on minimum. No extra charges for any of the tests shall be paid. the purchaser will not be able to come for stage inspection. Exciting Current and its harmonics as per IS: 2026(part. 0. Check complete transformer against approved outline drawing. Measurement of resistance of each winding. 100% and 110% of rated Voltage. Zero sequence impedance between HV – LV. In case.90 and 0. the supplier shall submit all the relevant documents generated during stage by stage manufacturing process for checking. maximum and normal taps.. ROUTINE TESTS Each completed transformer shall be subjected to following routine tests as per IS: 2026. . No deviation shall be acceptable. Measurement of insulation resistance. The lightning impulse withstand test along with chopped wave test: this test is intended to verify the impulse withstand strength of each line terminal to earth and other windings. Measurement of acoustic noise level test as per NEMA standards. RTCC & auxiliary circuits. Tests on transformer oil TYPE TESTS Transformers shall be subjected to the following type tests. Leakage / Pressure test on completely assembled transformer as per procedure laid down in CBIP manual. Load Losses shall be that corresponding to rated load on HV & LV Winding. The total losses shall comprise of the No-Load losses. load Losses and auxiliary losses at rated output duly converted at 75 degree C average winding temperature and shall also be indicated in the test report. Tests on “ON Load” tap Changer Functional tests on panels. . Magnetic balance test. Load Losses measured at rated frequency by applying a primary / secondary voltage sufficient to produce the rated current in the windings with the secondary / primary windings short circuited. Measurement of power taken by cooler fans. Routine di-electric tests Measurement of Capacitance and tan delta of transformer winding and each HV Bushing. and along the winding under test. d) If value obtained does not found to be satisfatory vacuum . Insulation Resistance Measurement Test a) Line wire of meggar is connected to the top of bushing of winding under test and earth wire is connected to the tank of the transformer. and between HV and LV Windings. marshalling boxes. The calibration details of the instruments used to conduct various tests shall be provided to the purchaser’s representative to verify the accuracy & validity of the equipments. TYPICAL TEST PROCEDURES (for reference only) Some of routine tests procedures are given below. LV and Earth. Short circuit tests The manufacturer shall furnish the typical type tests certificate for transformers of similar or higher rating. the procedures which are followed by manufacturer can be used. Temperature of the transformer oil is recorded at the top of the transformer by thermometer. These also can be followed after reviewing the procedures. shall be submitted for purchasers review and approval. Temperature rise test at minimum tap along with dissolved gas analysis test before and after heat run test. b) Meggar is switched "ON" and reading is taken after 15 sec or 60 sec. etc. Type test report of transformers and all components / accessories carried out on identical items such as bushings. 1. Each manufacturer has different test procedures. Also for additional tests. c) Reading is taken between HV and Earth. tap changers. if required. 2u. respective leads are connected together by suitable clip wire. Vector Group and Polarity Test a) All Wires are connected to all phases and neutral of HV and LV windings are brought out to terminal board. All readings are completed with the desired value specified by design department. 3. Reading on the Ratio meter is taken when Galvanometer of Ratio meter shows null indication by adjusting variable resistor of Ratio meter. b) 3 Phase 400 Volts supply is applied across HV Winding & . 2. All connections are checked thoroughly before commencing the test. Wires connected to all LV & HV Phases i. 1V. After completion of ratio on all three phases the tap is changed and above procedure is repeated for all taps. Ratio Test a) After cleaning the end of b) c) d) e) f) tapping lead. 1W are brought to change over switching arrangement for above test.is applied or filtration of oil is done depending upon the values to be achieved. The reading should be within tolerance specified in IS 2026.. 2w & 1U. After taking reading on one phase it is changed over to remaining phases with the help of change over switching arrangement. 2v.e. At terminal Board preferably "1U" shorted with "2U" phase each other. The test is done as per the procedure adopted before connections. average should be taken & Calculated by formula : Resistance per winding = 0. c) For Star & Delta connected winding measurement shall be made between pairs of line terminal and resistance per winding.Voltage measured between Voltage across different phases is measured as depending upon the vector group. b) To reduce the high inductive effect it is advisable to use a sufficiently High current to saturate the core.5 X Measured resistance. Measurement of Winding Resistance Test a) Wheat stone or Kelvin Bridge or Digital meter shall measure the resistance of each winding at principal tap depending on the Value of resistance. c) Voltage applied between 1U1V-1W = 1V-2V= 4. e) The top & bottom oil temperature shall be recorded & the temperature of the winding shall be taken as the average of the two readings. This will reduce the time required to get a stabilized reading. (Delta winding) d) Before recording cold resistance. (Star winding) Resistance per winding = 1. .S.5 X Measured resistance. 2026 & the value to be compared with testing detail given by design department. f) The measured resistance value shall be converted to the reference temperature of 75° C as per I. the transformer shall have been in oil without excitation or load for sufficient time to ensure that the winding is at the same temperature as the surrounding oil. Voltage across transformer is raised gradually by varying the voltage of intermediate transformer through dimmer stat. Measurement of No-Load Losses and No-Load Current Test a) All Connections are made as per Circuit Diagram. Generally voltage is fed from LV side keeping HV side open. No Load current is current is measured in all three phases by three ammeters. b) CT of WTI of the transformer is c) d) e) f) g) h) i) shorted and earthed. 6. Readings are taken at 90%. Final Losses are compared with the specified value. If any difference is there in the readings of peak voltmeter and RMS voltmeter the correction factor given in IS 2026 part 1 is applied and final losses are calculated.5. Selection of winding to be fed depends upon voltage class of the transformer. All Current Transformer (C. Here two wattmeter or 3 Wattmeter method is used for measurement of no load losses. Measurement of Short Circuit Impedance Voltage & Total Loss . Earthing to main tank is done properly. 100% & 110% of rated voltage or as per purchase order. OLTC is kept at nominal ratio tap position.T) connections are checked thoroughly before commencement of test.T) & Potential Transformer (P. Reading measured through wattmeter or power analyzer. The difference in temperature between the top oil and bottom oil shall be small enough to enable the average temperature to be determined with the required accuracy. sinusoidal supply to one winding with the other winding short circuited. The value of impedance voltage so derived shall be corrected to the reference temperature 75 Degree C. f) The measured value of the impedance voltage shall be corrected by multiplying it with ratio of reduced current to test current (if the measurement is done at reduced current than the rated current). e) The Measured value of the load loss shall be corrected by multiplying them by the square of the ratio of rated current or tapping current to test current (the measurement is done at reduced current than the rated current).a) All Connections are made as per the Circuit Diagram. d) Each Measurement is performed quickly & intervals between them are kept long enough to ensure that temperature rises do not cause significant errors. with the winding connected on the Normal tapping c) The Measurement may be made at any current between 25% & 100% but preferably not less than 50% of the rated current of under test winding. b) The Impedance Voltage / Short Circuit Impedance & Load Losses are measured at rated frequency by applying an approx. The value of impedance voltage so derived shall be corrected to the . Between Winding 2 & Winding 3. The other winding being open circuited. d) The test commences with voltage not greater than one third of the total test voltage. g) On three winding transformers the impedance voltage / short circuit impedance & the load loses are measured between windings taken in pairs as shown below : i. e) The frequency of the voltage applied should be suitably increased above the rated frequency. a) Three phase supply is given to one of the winding preferably LV Winding in case of higher voltage rating transformer. then period for the test is reduced as per the formulas given in IS 2026 i. If it is more than.e. Between Winding 1 & Winding 2+3. Induced Over Voltage Withstand Test As per IS 2026 the procedure for uniformly insulated transformers.reference temperature 75 Degree C. c) The time period for test is 60 Seconds. b) The applied voltage is double the rated voltage of the winding under test. ii. Time = 120X rated frequency in seconds / Test frequency. But not less than 15 Seconds. Between Winding 1 & Winding 3.. Between Winding 1 & Winding 2. f) After 60 Seconds being completed the voltage is . 7. iv. iii. e. j) The above procedure i. d) The test Voltage is measured with Voltmeter. Short Circuit Test . HV & LV of the transformer under test are shorted with suitable wire..e. i) After 60 Seconds the voltage is reduced to zero rapidly and then switched off. 8. b) LV Winding and tank are earthed properly.e. h) The test is treated as successful if there is no breakdown during the period of 60 Seconds.. It is kept for 60 Seconds.e.. f) Voltage is increased gradually from Zero to the required Value with respect to the earth. c) All Connections are made as shown in circuit diagram is under test. Separate Source Voltage Withstand Test a) Both the windings i. g) As soon as the test voltage is reached. when HV Winding is under test. 9. e) All Connections are checked thoroughly before commencing the test. from "c" to "i" is repeated for other winding i. the stop watch is made "ON". HV Winding and tank earthed. However for both the types test voltage is taken as per IS 2026. k) The selection of test voltage for HV Winding depends upon the type of insulation used for HV Winding i. g) The test is said to be successful if there is no breakdown during the test. LV Winding.reduced rapidly to less than one third of the test value before switching off. uniform or nonuniform. 2 IS: 2026 (part-I)-1977.. .e. Zt .5 second with a tolerance of ± 10 percent. Routine tests and type tests are performed as far as applicable at the manufacturer place before the short circuit test. First peak value I of the asymmetrical test current is calculated as follows. Xt + Xs R is the sum of the resistance’s of the transformer and the system i.Rated power of the transformer in MVA Un . their duration and position of the tap changer. Uz/100 X (Un) ^2 /Sn in Ohms per Phase.11. Sn .55 (clause 16.I K√2 .Short Circuit impedance of the transformer referred to winding & the Tapping under consideration in ohms/phase. Where X is the sum of the resistance and the system i.e. Symmetric short circuit current (1) for 3 phase transformer. If it is not specified test will be treated on each limb of transformers with tapping are made in different position for categories 2 and 3 an agreement between purchaser & manufacturer is always needed with regards to the number of tests. LV bushing conservator. The transformer is fitted with HV.Symmetrical Short circuit current K√2 .Rated Voltage of the system (kV) S . For category I three phase transformer the total number of tests shall be nine that in the three tests on each limb. Us .Short Circuit test is carried out on the prototype. After the test some routine tests are repeated by CPRI physical inspection of transformer relay and Buchholz relay also carried out.8 √2 =2.Short Circuit apparent power of the system (MVA) Zs . Rt + Rs. i . I = U/ (Zt + Zs) in kA.related to X/R ratio.Impedance Voltage at rated current in % at reference temperature.(Us) ^2/s = system impedance is ohms/phase. Un -Rated Voltage of winding under test (kV) at normal tap. the duration of each test being 0. Unless otherwise specified the factor K√2 is limited to 1.Voltage at the particular tap of the winding.. “Zt” is the short circuit impedance if the transformer referred to winding the tapping under consideration. U . e.P. a) Short the LV Phases with neutral and short HV all phase by suitable wires b) Connect the Ch. g) Above procedure is repeated for following (a) GST mode. d) Switch On power supply to Capacitance tan bridge. Measurement Of Tan Delta Capacitance Test Ref: C. e) After adjustment of null point on galvanometer for capacitance measurement the detector switch is change to dissipation factor position.The transformers shall be opened for physical inspection of core coil assembly to see if there is any displacement. With the help of knob provided on meter for final reading f) The galvanometer is again readjusted to null point and then final reading is noted down.. With the help of Knob provided on meter for capacitance measurement. Terminal on capacitance tan bridge to HV Windings and Terminal to Ground / Earthing pad provided on transformer tank c) Set the knob on capacitance tan bridge over UST i.e. i. The reference ambient temperature is noted down.. 10. capacitance. Then adjust the null point on the galvanometer. capacitance between LV & HV.e. Thus adjust null point on galvanometer by keeping detector switch on "C" i.Mode .B.I. Capacitance between LV-HV + Capacitance between LV and ground keeping HV Grounded = C L-H + CL-G h) GST . capacitance measured between LV and Ground provided Ground for LV.. Temperature Rise Test a) All connection is made as per load loss connection b) The thermometers (at least three) are placed at different points around the transformer of a level approximately half way up the cooling surface. These losses are maintained constantly till the temperature rise of the transformer become steady i. All readings like . if transformer is more than 2500KVA. One thermometer is placed at the top of the transformer tank into the oil filled pocket.8 times top oil temperature rise. the average oil temperature rise is taken approximately as 0.a) Capacitance between HV-G by providing ground for LV. d) Either guaranteed or actual measured no load losses and load losses at 75°C depending upon the purchaser’s specification are fed from any one winding with other winding being short circuited. b) Capacitance between HV-LV + CHV-G by keeping ground for LV. Two thermometers are placed on the top and bottom of the radiators pipe or inlet and outlet of the radiator bank. c) For transformer having rated power up to 2500 KVA with natural oil circulation. thermal equilibrium is reached.e. After taking the above capacitance measurement the reading are verified as follows CH-L (UST Mode) + CL-G (GST L Mode) ≈ CL-H + CL-G CH-L (UST Mode) + CH-G (GST H Mode) ≈ CH-L + CH-G 11. e. Top oil temperature wattmeter reading are taken after an interval of one hour. e) Thermal equilibrium (steady state) is reached when the temp rise does not very more then 1°C per hour during 4 conservative hourly reading. f) After the temperature rise steady state is reached the input is reduced to a value which result in the circulation of rated current at rated frequency in the winding and this current is maintained for 1 hour. . The top oil temperature is calculated at an interval.ambient surrounding temperature. g) After 1 hour the supply is switched off and the hot resistance of any one phase of the HV&LV winding is measured about 5 to 6 reading are taken to make the plotting of graph of resistance Vs time easier. To avoid errors due to time lag between variations in the temperature of the thermometers are inserted in cups filled with oil. exactly at the time and switching off supply. Radiator top and bottom and temperature of radiator bank inlet and outlet temperature. h) Graph of resistance Vs time is plotted for the winding to get hot resistance at zero time i. i) The temperature of the winding is then determined by this resistance method. rated frequency and rated voltage. Once the voltage wave shape is . j) Calculation sheet is attached herewith which explains the calculation of temperature rise of windings from the test. The drop in average oil temperature during last 1 hour is taken into account when calculating the temp rise of winding above the average oil temperature. The other line terminals are direct connected to the earth in case of star winding & through shunts in case of delta winding. WAVE SHAPE DETORMINATION AND CALIBRATION The very first activity involved in the testing sequence is application of low voltage impulse to the winding and circuit parameters are varied till the desired wave shape is obtained. Neutral terminal is directly connected to earth through a neutral shunt value shall not be higher than 500 ohms for supplementary current recording. k) The temperature rise of winding and oil so found from the test are compared with the guaranteed values. Impulse Test The line terminal of transformer under test is connected to the output of impulse Generator and voltage divider. 12. The temperature rise of winding above the average oil temperature determined in the first part of test will give the temperature rise of the winding above the cooling medium temperature for total losses at rated current. Other winding are shorted & connected to earth. From the oscillographic record the actual level of the voltage.2026/81. Significant variations in the wave shape of voltage or current between successive records at test voltage could from on evidence of the insulation failure. c) One reduced chopped wave. a) One reduce full wave impulse (between 50% to 75% level) b) One 100% full wave. VOLTAGE APPLICATION Voltage application is made as per test sequence given below.established a calibration shot is applied at 50% to 75% of impulse level as per I.5 of I. d) Two 100% chopped wave.1 of I. that particular shot is repeated as per I.S.4 of I. Interpretation of oscillograms is based on comparison of voltage and current waves between successive records at rated test voltage. At least two independent recording channels are necessary or Digital impulse Measuring system (DIMS) to be used. The charging voltage required for the calibration shot is recorded and the charging voltage for the 100% impulse is calculated. Polarity generally negative as per clause 12. e) Two 100% full wave.However winding with low impedance it is difficult to obtain impulse shape.2 ± 30% and 50 ± 20% microseconds as per clause 12. . In this case wider tolerances may be applied by agreement between the parties as per clause a 12.2 The test impulse shall be a full standard lightning impulse 1. Applied voltage is one of the channels whereas neutral current measurement of the winding under test is adopted as second channel.12.2026/81.2026 If during application any flashover in the external circuit or any oscillographic recording fails. In case of the Delta connected windings the neutral is formed by joining the other two terminals & earthing them through current measuring shunt. divider ratio and standard deflection of a DC line on the oscilloscope.1.S.2026/81 (part-111) CI.S.S.1.2026/1981.S. the gap between two electrodes in the oil cup is adjusted to 2.Electric Test of Oil for BDV Test a) Oil Sampling is taken from the bottom sampling valve of transformer. The Oil is allowed to drop from the valve in the other Vessel First and then oil cup is washed with the oil thoroughly. e) Such Six reading are taken and average of six reading is being treated as final break down value of oil. c) The oil is allowed to settle down for 15 minutes.Comparison of chopped wave records is normally not done.mtr whichever is lower. f) BDV of oil is not found satisfactory.5mm with the help of 2. The pressure shall be maintained for 12 Hours during which time no leakage shall occur.I.5mm marked gauge. . 13. Oil Leakage Test Ref: C.P. as it is impossible to get identical instants of chopping by use of trigration triggered chopping gaps.B. d) After oil is settled the voltage across electrode is increased. 14. Oil pressure test to be conducted on tank with turret and all other accessories as assembled for routine test by filling completely with oil of at a pressure corresponding to twice the normal head of oil or to normal pressure plus 35KN/Sq. Di . the oil will be filtered further till it gives the satisfactory results. b) After taking sample of oil.