Winding Resistance Measurement transformers
Do not attempt to run an excitation current test immediately after any direct current (dc) test. Energizing with dc will leave a residual magnetism in the core and will ruin the results of the excitation current test.
If generation of ethylene, ethane, and perhaps methane in the DGAs indicates a poor connection, winding resistances should be checked. Turns ratio, sweep frequency response analysis (SFRA), Doble tests, or relay operations may give indications that dc testing is warranted. Winding resistances are tested in the field to check for loose connections on bushings or tap changers, broken strands, and high contact resistance in tap changers. Results are compared to other phases in wye-connected transformers or between pairs of terminals on a delta-connected winding to determine if a resistance is too high. Resistances can also be compared to the original factory measurements or to sister transformers. Agreement within 5% for any of the above comparisons is considered satisfactory. If winding resistances are to be compared to factory values, resistance measurements will have to be converted to the reference temperature used at the factory (usually 75 °C). To do this, use the following formula:
Rs = Resistance at the factory reference temperature (found in the transformer manual)
Rm = Resistance you actually measured
Ts = Factory reference temperature (usually 75 ºC)
Tm = Temperature at which you took the measurements
Tk = Constant for the particular metal the winding is made from: 234.5 ºC for copper, 225 ºC for aluminum
It is very difficult to determine actual winding temperature in the field; and, normally, this is not needed. The above temperature corrections are necessary only if resistance is to be compared to factory values. Normally, phase resistances are compared to each other or to sister transformers at the same temperature, and actual winding temperatures and corrections are not needed.
Compare winding resistances to factory values; change in these values can reveal serious problems. A suggested method to obtain an accurate temperature is outlined below. If a transformer has just been de-energized for testing, the winding will be cooler on the bottom than the top, and the winding hot spot will be hotter than the top oil temperature. The average winding temperature is needed, and it is important to get the temperature as accurate as possible for
The most accurate method is to allow the transformer to sit deenergized until temperatures are equalized. This test can reveal serious problems, so it’s worth the effort. Winding resistances are measured using a Wheatstone Bridge for values of 1 ohm or above and using a micro-ohmmeter or Kelvin Bridge for values under 1 ohm. A multi-amp (now AVO) makes a good instrument for these measurements, which is quick and easy to use. Take readings from the top of each bushing to neutral for wyeconnected windings and across each pair of bushings for deltaconnected windings. If the neutral bushing is not available on wyeconnected windings, take each one to ground (if the neutral is grounded) or take readings between pairs of bushings as if it were a delta winding. Be consistent each time so that a proper comparison can be made. The tap changer can also be changed from contact to contact, and the contact resistance can be checked. Make sure to take the first test with the tap changer “as found.” Keep accurate records and connection diagrams so that later measurements can be compared.