What is Partial Discharge & Why Test For It
Partial Discharge (PD) is an electrical discharge or spark across a void or gap in electrical insulation between two electrodes (e.g. in a motor the copper conductor and earth) without completely bridging across the two electrodes - hence the term "Partial".
Voids and gaps occur because of the manufacturing process, as a result of thermal deterioration over time, poor fitting coils and coil movement due to looseness and vibration etc. Also surface partial discharges can occur due to conductive contamination. End winding phase to phase partial discharge can occur between adjacent coils of different phases.
All high voltage motor windings will have some Partial Discharge PD even when new due to the nature of windings and the manufacturing methods required. As the insulation degrades the number and magnitude of the Partial Discharge PD increases. The increase in Partial Discharge PD causes further erosion of the insulation.
Partial Discharge PD Testing is a very useful tool for evaluating the condition of high voltage machine windings from around 4KV and above. With repeat testing over appropriate time intervals the rate of deterioration can be observed. Although the actual time of failure cannot be predicted the information can be used to assess the risk of failure and appropriate contingencies, maintenance and repair actives planned accordingly.
Partial Discharge PD Testing can be carried out off line using our potable test trailer and also on line following installation into the machine of appropriate sensors such as Capacitive Couplers, Rogowski coils, RF CT's etc.
In addition to rotating machine windings Partial Discharge PD testing can also be used on Switch Gear, Transformers and Cables.