Remote monitoring of Partial Discharge activity within high-voltage rotating machines in hazardous locations

On-line Partial Discharge (PD) testing of rotating machines has traditionally been carried out using pre-installed High Voltage Coupling Capacitor (HVCC) sensors connected at the terminal box of the machine. These provide high impedance to low frequency signals (<; 10 MHz) and low impedance to the high frequency PD pulses (>; 10 MHz). The use of these sensors allows PD measurements to assess the integrity of the stator winding insulation of the machine to be made without the need to interrupt the running of the machine. Although the HVCC sensor is useful for machine-end capture of PD signals, there is often a need to perform PD testing of a rotating machine some distance away from the machine, such as at the switchgear cable end. Carrying out PD testing of rotating machines at the remote, switchgear end of the feeder cable is advantageous for motors located within Ex, hazardous gas zones, such as within the petrochemical industry and off-shore oil and gas environments. The authors report on preliminary results from PD testing carried out on in-service 10 kV motors which show how different areas of PD activity occurring within the stator windings of the motors can be reliably measured by placing wideband High Frequency Current Transformer (HFCT) sensors at the switchgear end of the supply cable.