An impulse test is only of use if the detection method is sensitive enough to detect any breakdown inside the voltage transformer. With voltage transformers, there is a problem of detecting the breakdown due to lack of sensitivity.
Why?
A Voltage Transformer is wound with very high number of turns of fine magnetic wire and has very high inductance and low series capacitance.
Courtesy: figure 1 and figure 2 – Fault analysis on voltage transformers under impulse test conditions by J. C. Bradbury (B.Sc from UNSW)
Hence, the capacitance current is small and the inductance current takes time to build up to a larger value (many micro seconds) during impulse testing. Hence, looking at the voltage trace only, makes it difficult, if not impossible to detect a breakdown due to turns fault in the high voltage winding of a VT.
Now, to increase the sensitivity of breakdown measurement, you can observe the current wave shape on the oscilloscope. In case, if there is a minor fault, it is indicated by changes in amplitude of the oscillations on the oscilloscope.
Normally the current shunt is in the neutral circuit and is a resistive shunt. This is not sensitive enough. Hence, one can use a capacitance shunt instead of resistor shunt for the grounding connection. This should produce a more sensitive failure detection on the current trace.
There is a “bleeder” resistor connected in parallel with the capacitor.
When you have a turn to turn fault, the current waveform is “integrated” by the capacitor for the later part of the current waveform. If the current waveform deviates more from the current trace on the reduced wave impulse shot then it has a insulation failure. The selection of the capacitor is important to maximize the current trace sensitivity.
The other alternate method is to measure the second (low voltage) winding wave shape for increased measurement sensitivity detection. The transferred secondary voltage is used as a detection method for High voltage winding failure.
Read about our visit to UNSW High Voltage Impulse lab test here. https://www.tortech.com.au/visit-to-unsw-impulse-testing-lab-by-tortech/