Research on Ultrasonic Detection Method for Partial Discharge of Dry Type Transformer

Partial discharge is an important cause of insulation degradation for dry type transformer. The longstanding discharge will accelerate the insulation aging of epoxy resin insulation, what would even lead to defects such as short circuit or insulation breakdown. The generation of partial discharge can produce ultrasonic signal. Ultrasonic detection method has strong resistance to electromagnetic interference, what makes it suitable for partial discharge defect detection of dry type transformer. In this paper, the propagation laws of ultrasonic signal in dry type transformer are studied by simulation, and the propagation laws of ultrasonic signal are compared under two different discharge defects: bubble discharge in epoxy resin insulation and air gap discharge at the junction of insulation and copper winding. The reasonable installation positions of ultrasonic sensors on dry type transformer are explored according to the results. The results show that the ultrasonic signal will decay rapidly during the transmission in dry type transformer. Under the same discharge fault, ultrasonic signal measured in the non-fault phase is much smaller than that in the fault phase. The refraction and reflection of ultrasonic signal between different materials can be ignored because the sizes of windings and epoxy resin structures are much smaller than ultrasonic wavelength, so the ultrasonic pressure at different measurement points mainly depend on the relative positions of the discharge point and measurement points. A sensor on the upper surface of the core in one phase can measure relatively stronger ultrasonic signal from threephase partial discharge, and a sufficient gain should be set for the sensor on the upper surface of the core to realize the monitor of partial discharge from three phases while using less sensors.