A New Directional Coupler Type Partial Discharge Sensor Installed on the Power Lead of Rotating Machine

For on-line partial discharge (PD) monitoring of rotating machines, a novel PD sensor is proposed, which can be installed on the power lead inside the terminal box of the machine. The sensor has been designed to have high capacitance, and minimal reflection of measured pulses. As a sensitivity of the PD sensor, transfer impedance Zt has been measured and compared to conventional coupler-type PD sensors. A simple method is presented for measuring Zt of coupler sensors, using a vector network analyzer and a practical lead-cable of rotating machine. Through this method, it became possible to measure the Zt of coupler sensors including the installation environment of them. The Zt of the proposed sensor is higher than that of same sized other conventional couplers at frequencies between 30 and 92 MHz. Another sensitivity test has been performed using a PD calibrator as a test pulse source. The proposed sensor has higher measured peak voltage than the conventional coupler type sensors when the same charges were input.