Faulty VT contacts caused partial discharge to initiate on circuit Grid 3. The PD activity was detected by the ASM PD monitor and an alert was raised. Subsequent maintenance showed that this circuit would have failed if the PD activity had not been noticed.

The substation is fitted with Hawker Siddeley VMV 11kV switchgear


An ASM permanent PD monitor was installed in September 2007. TEV PD sensors were fitted to each of the 17 panels and HFCT sensors fitted to each of the 12 feeder cables in the substation.


On 20th Sept 2012, PD initiated in the switchgear of a transformer panel at the substation. The activity increased in both magnitude and intensity (Count) slowly over the next two weeks; an alert was sent to the substation owner recommending an investigation. The graph below shows the PD Peak and Count detected by the HFCT, as well as the TEV sensors monitoring the discharging panel.



PD testing carried out by an engineer on site showed that the source was in the VT on the panel. The circuit was switched out for maintenance on 8th October 2012. When re-engergised, the PD activity re-started, albeit at a lower level than it had previously been. Approximately one week after this, the PD rapidly increased in magnitude and intensity to levels higher than those seen prior to the maintenance.

On the 31st October 2012, the circuit was switched out and the VT was replaced. It was found that the contacts were misaligned and therefore not properly seated. This had caused significant discharge activity and internal burning.


The ASM PD monitor detected the initiation of high levels of PD activity on a Grid transformer switchgear panel. Alarms were raised and an alert was sent to the substation owner.

Subsequent maintenance work showed that there was significant discharge damage and burning to the VT contacts. It was determined that the circuit would have failed if no action had been taken.

Case Study – Rural Substation – Faulty VT