engleski

Ultrasonic Method for Testing of Power Transformers

Partial discharges (PD) in power transformers with oil-paper insulation result in electrical, chemical and acoustic effects. Their detection can be carried out with several techniques, according to the effects that they produce. Standard method in HV testing technique is an electrical method, which is a part of the quality control in a factory. PD in a transformer produces gases, which are the result of a chemical degradation of oil and paper. This is a basis for Dissolved Gas Analysis (DGA) method that determines the amount of characteristic gasses dissolved in oil. Even thou this methods are useful, they only give information of the presence and/or magnitude of PD but not their location. This is the reason why the ultrasonic method imposes as a solution for location of the PD source. 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. These waves are detected with piezzo-electric sensors, which transform this mechanical wave in to an electrical signal. From time difference of wave detection on different sensors, a possible location of the source can be estimated. Since the method detects ultrasonic acoustic waves spreading trough the tank it can detect other transformer deficiencies such as loose contacts and local overheating of oil (T>200°C) which gives this method an unique diagnostic status. Loose contacts can cause local overheating in oil. When oil analysis points to local overheating, the tap changer is one of the first parts that raise suspicion, since most of the contacts are on it. This is the case where DGA indicates the thermal problem in the oil, and the result of ultrasonic testing points at TC contacts. If PD occurs at low voltages, it is possible that an oversight in production was made. This was the case with two transformers, where electrical method detected high levels of PD at voltages much lower than the nominal. Ultrasonic method detected errors during assembly, in one case, a poor contact between the high-voltage terminal and the capacitor body of the bushing, and in the second case, a non-grounded electrode of the core outer legs screen. Long lasting PD can result in breakdown of isolation, and fallout of the transformer from the grid, which can have major consequences on the stability of the power system and significant financial consequences for distributors. The case was analyzed in which a failure of transformer, namely a breakdown from the HV bushing oil side screen, initiated a series of tests on similar transformers installed at the same substation. The assumption was that PD occurred in the screen epoxy insulation and eventually caused the breakdown.

diagnostics; power transformer; partial discharges; ultrasound; acoustic emission

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