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How To Measure Transformer Winding Resistance

Transformer winding resistance is very important factor for the life of any transformer, this test is part of type tests, routine tests and field tests. The purpose of winding resistance test is to determine the following:

  1. Calculation of the I2R losses in transformer.
  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.

Typically transformer winding resistance test is performed at the installation site in order to check for abnormalities due to loose connections, broken strands of conductor, high contact resistance in tap changers, high voltage leads and bushings.

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On the other hand prior to dispatch the transformer out of the factory, each transformer is routine tested which is witnessed by a representative from the potential customers as required. The routine tests of the transformers include:

  • Measurement of each winding resistance and temperature at full load.
  • The voltage ratio shall be measured on each tapping.
  • The impedance voltage shall be measured in accordance to IEC76 – Power Transformers.  The impedance voltage should be corrected to the reference temperature of 75 deg C.
  • The no load and full load losses of the transformers are also need checking.
  • The transformer is checked for source voltage withstand test.
  • Partial Discharge test to IEC 270.
  • Transformer noise measurement.
  • Functional test for all setting point for alarm and trip value.

Transformer Winding Resistance Measurement Method

In the scenario of star connected winding, the resistance shall be measured between the line and neutral terminal and for  star connected auto transformers the resistance of the HV side is measured between HV terminal and IV terminal, then between IV terminal and the neutral. On the other hand delta connected windings, measurement of winding resistance shall be done between pairs of line terminals. As in delta connection the resistance of individual winding can not be measured separately, the resistance per winding shall be calculated as per the following formula:

Resistance per winding = 1.5 × Measured value
The resistance is measured at ambient temperature and then converted to resistance at 75oC for all practical purposes of comparison with specified design values, previous results and diagnostics.
Winding Resistance at standard temperature of 75oC Rt = Winding resistance at temperature t.
t = Winding temperature.
Generally transformer windings are immersed in insulation liquid and covered with paper insulation, hence it is impossible to measure the actual winding temperature in a de-energizing transformer at time of transformer winding resistance measurement. An approximation is developed to calculate temperature of winding at that condition, as follow.
Temperature of winding = Average temperature of insulating oil
(Average temperature of insulating oil should be taken 3 to 8 hours after de-energizing transformer and when the difference between top and bottom oil temperatures becomes less than 5oC.) The resistance can be measured by simple voltmeter ammeter method, Kelvin Bridge meter or automatic winding resistance measurement kit. (ohm meter, preferably 25 Amps kit)
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. Please note that Measurement of winding resistance of transformer shall be carried out at each tap.

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.

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. This is required for cooling purpose and winding temperature will become equal to its oil temperature.
  2. Measurement is done with D.C.Current voltage method of transformer winding resistance measurement
  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.
  5. The readings shall be taken after the 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.
  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.Current voltage method of transformer delta winding resistance measurement
  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.

Bridge Method of Measurement of Winding Resistance

The main principle of bridge method is based on comparing an unknown resistance with a known resistance. When currents flowing through the arms of bridge circuit become balanced, the reading of galvanometer shows zero deflection that means at balanced condition no current will flow through the galvanometer. Very small value of resistance ( in milli-ohms range) can be accurately measured by Kelvin bridge method whereas for higher value Wheatstone bridge method of resistance measurement is applied. In bridge method of measurement of winding resistance, the error is minimum.

kelvin bridge method of transformer winding resistance test
The resistance measured by Kelvin bridge,
All other steps to be taken during transformer winding resistance measurement in these methods are similar to that of current voltage method of measurement of winding resistance of transformer, except the measuring technique of resistance.
wheatstone method of transformer winding resistance measurement
The resistance measured by Wheatstone bridge, Rx = R A/B.

Transformer Site Tests

The transformers shall be fully tested on site prior into putting into service to ensure that all items are in proper working condition, correctly installed and free from damage. An authorized testing professional engineer shall be engaged to perform the site testing and commissioning. The following tests shall be made on the plant item together with any other tests which the client deems necessary.
Separate Source Power Frequency Withstand Voltage Test
The windings of each power transformer shall be power frequency voltage tested in accordance with BS 171 – Specification for Power Transformer.
Operation Tests
Transformer tap-changers, cooling equipment, emergency stop, door interlocks, temperature relays, their controls and all other auxiliary equipment shall be operated to prove that they are functioning satisfactorily before the transformers are put into service.
Secondary Wiring Test
The insulation of all secondary wiring shall be tested with injection test and megger (insulation) test after installation. Measurement of voltage ratio at every tap position and check of voltage sector.

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