a presentation on Measurement of Winding Resistance and Insulation Resistance of 3 Phase 11 kV/ 433 V, 16KVA Oil cooled type Transformer PRESENTED BY: Mr. Dardhit Patel & Mr. Abhishek patel Asst.prof Vadodara institute of Eng.
ABSTRCT This Paper consist the specification of transformer and its Procedure of measurement of winding resistance between two line terminals of HV winding and LV winding as well by current voltage method and its actual results before and after short circuit test. Actual results of Insulation resistance of HV to LV winding, HV winding to LV + Earth, LV winding to HV + Earth before and after short circuit test. All results is taken from laboratory of Industry.
Specification of Transformer 1. Type : Out door, Oil cooled 2. KVA rating : 16 3. Rated voltage H.V. (Volts) : 11000 L.V. (Vol : 433 4. Rated current H.V. (Amp.) : 0.84 L.V. (Amp.) : 21.33 5. Number of phases : 3 6. Connection H.V./L.V. : Delta/Star 7. Frequency (Hz.) : 50 8. Type of cooling : ONAN 9. Temperature rise of oil : 45oC 10. Percentage Impedance : 4.5 % + IS Tolerance
Specification of Transformer 11. Primary winding conductor : Aluminium bare wire dia 0.89 mm 12. Secondary winding conductor : Aluminium strip (8.9mm x 2.7mm) bare 13. Quantity of oil (Litre) : 53 14. Weight of oil (Kg.) : 44 15. Weight of core and winding : 75 (kg.) 16. Total weight (Kg.) : 207 17. Vector group : Dyn-11 18. Year of manufacture : 2009 19. Insulation level H.V. : 28 kVrms 20. Insulation level L.V. : 3 kVrms 21. Total losses at 75 0C (Watts) : 60 (at 50 % load) 22. Total losses at 75C (Watts) : 360 (at 100 % load)
Measurement of Winding Resistance The resistance of each winding, the terminals between which it measured and the temperature of the windings shell be recorded. Direct current shell be used for the measurement. In all resistance measurements care shall be taken that self-inductive effects are minimized. During that cold resistance measurements, the time for measuring current to become steady should be noted and use for guidance when making hot resistance measurement following a temperature rise type test
Dry type transformer The temperature recorded shall be the average reading of several (at least three) thermometers place on winding surface. Winding temperature and resistance shall be measure simultaneously, and the temperature of winding, as measured by thermometer, should approximately equal to the surrounding medium.
Oil-Immersed Type Transformers After the transformer has been under oil without excitation for at least 3 hours, 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 purposes of the temperature rise test, special efforts shall be made to determine the average winding temperature accurately. Thus the difference in temperature between the top and bottom oil shall be small. To obtain this result more rapidly, the oil may be circulated by pump.
Transformer winding resistance measurement is carried out as a type test, routie test and also as a field test. it helps in determining the following: (1) Calculation of the I2R losses. (2) Calculation of winding temperature at the end of temperature rise test of transformer. (3) As a benchmark for assessing possible damages in the field.
Current Voltage Method of measurement of winding resistance The transformer winding resistances can be measured by current voltage method. In this method of measurement of winding resistance, the test current is injected to the winding and corresponding voltage drop across the winding is measured . By applying simple Ohm’s law. i.e. Rx = V ⁄ I, one can easily determine the value of resistance.
2) Measurement is done with 1000V D.C. Procedure of current voltage method of measurement of winding resistance 1) Before measurement the transformer should be kept in OFF condition without excitation at least for 3 to 4 hours. During this time the winding temperature will become equal to its oil temperature. 2) Measurement is done with 1000V D.C. 3) To minimize observation errors, polarity of the core magnetization shall be kept constant during all resistance readings. 4) Voltmeter leads shall be independent of the current leads to protect it from high voltages which may occur during switching on and off the current circuit.
Procedure of current voltage method of measurement of winding resistance 5) The readings shall be taken after the electric current and voltage have reached steady state values. In some cases this may take several minutes depending upon the winding impedance. 6) The test current shall not exceed 15% of the rated current of the winding. Large values may cause inaccuracy by heating the winding and thereby changing its resistance. 7) For expressing resistance, the corresponding temperature of the winding at the time of measurement must be mentioned along with resistance value. As we said earlier that after remaining in switch off condition for 3 to 4 hours, the winding temperature would become equal to oil temperature. The oil temperature at the time of testing is taken as the average of top oil and bottom oil temperatures of transformer.
Procedure of current voltage method of measurement of winding resistance 8) For star connected three phase winding, the resistance per phase would be half of measured resistance between two line terminals of the transformer. 9) For delta connected three phase winding, the resistance per phase would be 0.67 times of measured resistance between two line terminals of the transformer. 10) This current voltage method of measurement of winding resistance of transformer should be repeated for each pair of line terminals of winding at every tap position.
Procedure of current voltage method of measurement of winding resistance For star connected winding, the resistance shall be measured between the line terminals shows in Fig 2 and for delta connected windings, measurement of winding resistance shall be done between pairs of line terminals which is shown in Fig 1. FIG.1 FIG.2
Caution for voltmeter ammeter method Current shall not exceed 15% of the rated current of the winding. Large values may cause inaccuracy by heating the winding and thereby changing its temperature and resistance.
Results of Winding Resistance Measurement at top oil temperature: 40.1C LV Winding resistance (m) HV Winding resistance () u – v v – w w –u U – V V – W W –U 151.77 152.08 152.2 126.16 125.42 125.73 Measurement at top oil temperature: 28.3 C LV Winding resistance (m) HV Winding resistance () u – v v –w w-u U – V V – W W –U 195 195.37 175.47 195.558 178.9
Measurement of Insulation Resistance The oil/air temperatures shall be measured and recorded immediately prior to the’test. The insulation resistance of each winding, in turn, to all the other windings, core and frame or tank connected together, and to earth shall- be measured and recorded.
Results of Insulation Resistance Top oil temp.: 28.3C, Measured at Voltage: 1000 V DC IR value (M) HV to LV winding >2000 HV winding to LV +EARTH LV winding to HV + EARTH Top oil Temp.: 40.1C, Measured at Voltage: 1000 V DC IR value (M) HV to LV winding 1400 HV winding to LV +EARTH >2000 LV winding to HV + EARTH 800
CONCLUSION It can be conclude that, the number of free electron increase with increase in temperature that means if temperature increase, more number of electrons comes to the conduction bands from valance band by crossing the forbidden energy gap. As the number of free electron increase, resistance would decrease with increase in temperature which can show from the results of winding and insulation resistance.
References: [1] http://electrical-engineering-portal.com/measurement-of-insulation-resistance-2#2. [2] http://www.electrical4u.com/transformer-winding-resistance-measurement/. [3] IS:2026 (Part -1, Reaffirmed 2001) 1977, Bureau of Indian Standards. [4] IS:2026 (Part -2, Reaffirmed 2001) Edition 2.2, 1977, Bureau of Indian Standards.
THANKS