Winding Resistance Test of Transformer

A winding resistance test of a transformer measures the DC resistance of the windings and their connections. It is a type test, a routine test and a field test.

IEC 60076-1 lists winding resistance as a routine test. Factories also take it during a temperature-rise type test. In the factory the measured values are used for 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.

On site the test checks for loose connections, broken conductor strands, high contact resistance in tap changers and faults in high voltage leads and bushings.

Procedure of Transformer Winding Resistance Measurement

For a star-connected winding, measure between the line and the neutral terminal. That reading is the phase resistance. A line-to-line reading on the same star winding is twice the phase resistance.

For star-connected autotransformers the HV-side resistance is measured between the HV terminal and the IV terminal, then between the IV terminal and the neutral.
For delta-connected windings, measurement of winding resistance is taken between pairs of line terminals. An individual delta winding cannot be isolated, so winding resistance is found from the line-to-line reading with the following formula:

Resistance per winding = 1.5 × Measured value

The resistance is measured at the winding temperature of the test and converted to 75°C so it can be compared with design values, earlier results and later diagnostics. 75°C is a common oil-immersed reference; IEC 60076-1 also uses other reference temperatures for some ratings.

Winding resistance at a standard temperature of 75^oC

R_t = winding resistance at temperature t
t = winding temperature

Transformer windings sit in insulating liquid and paper, so the metal temperature cannot be read directly on a de-energised unit at the time of transformer winding resistance measurement. A common approximation is:

Temperature of winding = Average temperature of insulating oil

Take the average oil temperature 3 to 8 hours after the transformer is de-energised, once the difference between top and bottom oil temperatures is less than 5^oC. That rest rule is workshop practice, not an IEC 60076 pass/fail limit.

Resistance can be measured with a voltmeter and ammeter, a Kelvin bridge meter or an automatic winding-resistance kit (an ohmmeter, often a 25 A kit).

Caution for the voltmeter-ammeter method: keep the test current at or below 15% of the winding rated current. IEEE C57.12.90 uses that cap so the winding does not heat and change resistance during the reading. IEC 60076-1 requires the current not to heat the winding enough to change the reading, but it does not set 15%.

Note: The measurement of winding resistance of a transformer shall be carried out at each tap.

Current Voltage Method of Measurement of Winding Resistance

Transformer winding resistance can be measured by the current-voltage method. A test current is injected into the winding and the voltage drop across the winding is measured. Ohm’s law then gives R_x = V ⁄ I.

Procedure of Current Voltage Method of Measurement of Winding Resistance

1. Before measurement, the transformer should be off and without excitation for 3 to 4 hours. This allows the winding temperature to match the oil temperature.
2. Measurement is done with 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
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 the 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.

Bridge Method of Measurement of Winding Resistance

A bridge compares an unknown resistance with a known resistance. When the arm currents are balanced, the galvanometer shows zero deflection and no current flows through it.

Milliohm-range values are measured with a Kelvin bridge. Higher values use a Wheatstone bridge. The bridge arrangement keeps lead errors small.

The resistance measured by a Kelvin bridge is:

The rest of the transformer winding resistance measurement steps are the same as the current-voltage method of measurement of winding resistance of transformer. Only the resistance measuring technique changes.

The resistance measured by a Wheatstone bridge is: