Optical Ultrasonic Detection for Partial Discharge Under Pulsed High Voltage

Medium voltage power electronic transformer (PET), composed of power electronics modules and a high-frequency transformer (HFT), plays a significant role in the hybrid ac/dc distribution power grid, with renewable energy sources and electrified loads. Stressed by the high frequency, high voltage, and high temperature, the insulation of HFT is the weak point. However, it is difficult to get access to the discharge signal under severe electromagnetic interference (EMI) due to the fast-switching of the power electronics modules. An optical interference-based partial discharge (PD) detection is proposed and verified to evaluate the insulation status of HFT and PET. In this article, the method of detecting and extracting PD signals under high-voltage and high-frequency pulse waveform is studied by using a skeleton optical fiber sensor (OFS), Sagnac interference structure, and noncorrelation coefficient algorithm. The algorithm distinguishes PD activities from interference signals by analyzing the correlation of signals. The ultrasonic interference characteristics of frequency below 20 kHz and pulse voltage rise time above 50 ns are analyzed. The results demonstrate that the main frequency range of the disturbing signal is from 20 to 100 kHz, which overlaps with the frequency distribution of the PD signal. The noncorrelation coefficient between the signals is proportional to the PD strength. The PD signal is detected successfully on the HFT prototype. It is expected to provide a convenient approach to insulation detection and evaluation of high power, high frequency, and high voltage power apparatus.