Ultrasonic method for testing of power transformers

Abstract

Ultrasonic method of detecting PD is based on the fact that the electrical energy of the PD transforms in a mechanical energy, an ultrasonic acoustic wave that spreads through the transformer to the tank wall. From time difference of wave detection on different sensors, a possible location of the source can be estimated. Ultrasonic method can detect other transformer deficiencies such as loose contacts and local overheating of oil (T>200°C). Three case studies are given in this paper. The first case was where DGA indicates the thermal problem in the oil and the result of ultrasonic testing points at OLTC contacts. In the second case an ultrasonic method was performed after electrical method detected high levels of PD at voltages much lower than the nominal. Ultrasonic method detected non-grounded parts of the returning limb electrostatic screen. In the third case, a failure of transformer, namely a breakdown from the HV bushing end shield, initiated a series of tests on similar transformers installed at the same substation. The assumption was that PD occurred in the shied epoxy insulation and eventually caused the breakdown.