Application of a Rogowski Coil Sensor for Separating Internal and External Partial Discharge Pulses in Power Transformers

Partial discharge (PD) measurement is an essential tool for evaluating the health condition of a transformer insulation system. The conventional PD measurement systems have a major drawback since they cannot discriminate the internal PD pulses and external pulseshaped interferences, especially in online and on-site conditions. Various denoising techniques (e.g., wavelets) have been also used in the literature, which are effective for extracting the PD signal from the noisy signals or classifying the PD types. However, the PD pulses and some of the randomly external pulse-shaped interferences have similar time-frequency characteristics, which makes them difficult to separate. In this article, a new cost-effective and simple logic method is proposed for the separation of these interferences. This uses the polarity comparison of the output voltages of the Rogowski coil sensor, mounted on the bushing, and measurement impedance, connected to the test tap. The same polarity represents the internal pulse, whereas the opposite polarity indicates the external pulses. A distribution transformer (20/0.4 kV, 500 kVA) and a condenser bushing (72.5 kV) were considered to evaluate the performance of the proposed system. The results confirm the effectiveness of the proposed PD measurement system for separating the pulses.