Rotating Machine (Generator / Motor)

Online Partial Discharge Monitoring of Generator and Motor
For many years, the measurement of partial discharge (PD) has been employed as a sensitive means of assessing the quality of new insulation as well as a means of detecting localized sources of PD in used electrical winding insulation arising from operational stresses in service. Compared with other dielectric tests (i.e. the measurement of dissipation factor or insulation resistance) the differentiating character of partial discharge measurments allows localized weak points of the insulation system to be identified. Especially on-line PD measurements are not only sensitive to partial discharges but also to various arcing and sparking phenomena.
With regard to condition assessment of rotating machines, the measurement of partial discharges can provide information on:

• points of weakness in the insulation system;
• degradation processes;
• maintenance measures and intervals between overhauls.
  PD on-line measurements are recorded with the rotating machine experiencing all of the operating stresses; thermal, electrical, environment and mechanical. On-line PD testing has the following advantages:
• the voltage distribution across the winding is the same as during opration;
• the measurements are made at operating temperature;
• normal mechanical forces are present.
  Due to the realistic stress impact on the winding during measurement and due to the fact that the measurement is performed during normal operation, on-line PD testing has become very popular. Since no service interruption is required, once the PD sensors are installed during a scheduled unit outage, and no external power source is needed, on-line testing is usually cost effective compared to off-line PD measurement. Condtion changes of the stator winding insulation system can be identified and evaluated at an early stage based on a real-time condition assessment and thus condition-based and predictive maintenance strategies can be improved.