Continuous online monitoring of PD activity in the medium voltage distribution network

Partial discharges (PD) occur as a result of dielectric breakdown in electrical insulation under high voltage stress. They can be caused by the presence of gas pockets or voids in solid insulation / gas bubbles in liquid insulators (internal), insulator surface imperfections / irregularities (superficial) and around an electrode in gas (corona). PD activity, once initiated, causes chemical and physical degradation of insulating material and can lead to catastrophic failure. Early warning of PD activity and localisation is considered essential to ensure the long term reliable operation of high voltage equipment including cables, cable accessories and switchgear. PD detection in high voltage cable and cable accessories is usually carried out with test objects offline, requiring potentially significant interruption of service. An online detection system is preferable but difficult to calibrate apparent charge magnitudes due to changes in real distribution network loading and topologies. An online PD monitoring system with continuous calibration has been developed to determine the apparent charge magnitudes of PD activity and locations in a section of an in-service distribution network. System components including the online calibration pulse injector, capacitive sensors for high frequency PD pulse detection, and data acquisition, processing and communication components at each monitoring site will be presented. Long term parameter trending data from the ongoing trial in a typical urban distribution network in Spain will also be presented.