Winding resistance test of transformers are very important test to perform at factory and at site. While transformers are transported, loded and unloaded it is important to check the following calculations during the testing of transformer prior commissioning, which can be carried out through winding resistance test.
- Calculations of the I²R component of conductor losses.
- Calculation of winding temperature at the end of a temperature test cycle.
- As a diagnostic tool for assessing possible damage in the field.
Moreover the transformer are subject to vibration which may occur fault or problems due to poor design, assembly, handling, poor environments, overloading or poor maintenance.
By measuring the resistance of the windings, it assures that the connections are correct and the resistance measurements indicate that there are no severe variations or opens connections. Usually transformers have taps built into them. These taps allow the ratio to be increased or decreased by fractions of a percent. If any of the ratio changes involve a mechanical movement of a contact from one position to another, these tap changes should also be checked during a winding resistance test.
Methods of Winding Resistance Test
- Current voltage meathod
- Bridge method
- Kelvin Bridge Method
- Wheatstone Bridge Method
1. How To Perform Current Voltage Method
- 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.
- Measurement is done with D.C.
- To minimize observation errors, polarity of the core magnetization shall be kept constant during all resistance readings.
- 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
- 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.
- 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.
- 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.
- For star connected three-phase winding, the resistance per phase would be half of the measured resistance between two line terminals of the transformer
- 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.
- 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.
2. How To Perform Bridge Method
The principle of the bridge method is based on comparing an unknown resistance with a known resistance. When currents flowing through the arms of the bridge circuit become balanced, the reading of galvanometer shows zero deflection that means at balanced condition no current will flow through the galvanometer.
A very low value of resistance can be accurately measured by the kelvin bridge method whereas for higher value wheatstone bridge method of resistance measurement is applied. In bridge method of measurement of winding resistance, the errors are minimized.
Kelvin Bridge Method
The resistance measured by Kelvin bridge,
Wheatstone Bridge Method
The resistance measured by Wheatstone 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.