On-line partial discharge testing of in-service rotating machines in ex hazardous environments by employment of central monitoring solutions

Carrying out the on-line partial discharge (OLPD) testing of in-service high voltage (HV) rotating machines can be an effective technique to understand any insulation degradation mechanisms that may be present within the stator windings. Interpretation of OLPD measurements performed on rotating machines can be difficult due to the presence of noise and interference from variable speed drives (VSD´s) and/or motor exciter systems. The new PD measurement and monitoring techniques outlined in this paper are carried out at the switchgear-end of the machine´s feeder cable, at distances of up to 2km from the machine under test. It has been shown that this remote monitoring technique eliminates much of this high-frequency interference which can be seen at the machines´ terminal box. This is due to the effect of the high voltage power cable between the switchgear and the machine, with the cable acting as a low-pass filter, stripping out higher frequency signals whilst retaining lower frequencies. Also, most importantly for operators with motors located within `Ex´ hazardous gas zones such as in the oil & gas industry, the remote monitoring technique allows these motors to be monitored without the need to enter the `Ex´ zone to install sensors or monitors. The authors present case studies from two oil & gas companies showing how the technique has been used for monitoring PD of in-service motors located in Ex zones.