On-line partial discharge monitor for high voltage power transformers

On-line partial discharge (PD) monitoring of high voltage power transformers is limited due to cost and, especially, electrical interference at the different power station and sub-station sites. It is known that a PD can produce a number of signals at different locations within a large transformer including a discharge current in the neutral, a displacement current through the capacitive tapping of a bushing, a radiated RF impulse and a radiated ultrasonic impulse. By the selection of appropriate signals it is possible to design a relatively interference free monitoring system with adequate selectivity and sensitivity for use on site as well as in the factory. This paper describes a monitoring system which utilises the radiated RF and ultrasonic impulses. By suitable signal conditioning and digital signal processing techniques, the monitor establishes the presence or otherwise of a PD. A system was laboratory tested and temporarily installed in a new 330 kV transformer. More recently, another prototype monitoring system was installed in a 132 kV power transformer for field trials at a Pacific Power sub-station