Partial discharges detection in medium voltage systems using directional antenna sensors

Partial discharge (PD) measurement is a fundamental tool to evaluate the state of apparatuses of medium voltage (MV) power systems. An alternative to traditional PD sensors (e.g. current transformers, capacitive couplers) for online measurements can be constituted partly by electromagnetic field sensors, capable to pick up the signals generated by PD occurring in the insulation system and traveling in the surrounding media. In this paper, the results of on-line PD measurements, performed on two PILC MV power lines by means of a UWB PD detector and a narrow band log periodic antenna sensor, are presented and commented. The sensor was chosen in order obtain the maximum sensitivity on the basis of the frequency content of the input signal (i.e. electromagnetic pulse generated by PD) and to give an optimized output signal according to the PD detector characteristics (such as input impedance and bandwidth). The results of the on-field tests show that the system composed by the PD detector and the antenna sensor is able to detect PD occurring in cable accessories. Rejection of noise and of external disturbances is also possible by means of time-frequency mapping of recorded pulses. The correct identification of the PD phenomenon and the consequent risk assessment can thus be performed. The used antenna sensor is directional and allows localized investigation on each component of the system to be performed and information about the PD source location to be gathered.