Partial Discharge Diagnosis of Stator Insulation

Using Damped AC Voltages

¹Edward Gulski Senior Member IEEE, ²Paul P. Seitz, ²Ben Quak

¹ Delft University of Technology, The Netherlands

²  Seitz Instruments AG, Switzerland

Since many years the condition of stator insulation in service is investigated using partial discharge (PD) measurements and dielectric losses (tan ∂). In particular, off-line and on-line methods of applying PD detection have become common practice.

 

The off-line diagnosis is based on applying to single phases of the stator insulation an external AC 50(60) Hz power supply, detecting PD levels in accordance to IEC60270 and the estimation of dielectric losses. In this way, the PD behavior: PD inception voltage, PD level, PD intensity PD pattern are measured, and the presence of discharging defects e.g. slot discharges, end-winding discharges can be identified and evaluated for insulation condition assessment purposes. Moreover using tan ∂ values the insulation degradation can alos be evaluated.

 

For a modern on-site diagnostic technique several aspects are important:

 

a)       non-destructive for the stator insulation and uses for service conditions representative AC voltage stress;

b)       to support the interpretation uses number of generally accepted quantities and parameters e.g. pC, kV, Δtan ∂;

c)       provides distinction between different types of insulation problems;

d)       provides indication of the potential fault location;

e)       compact, user friendly solution which can be easily operated under different circumstances;

 

In this contribution results of systematic testing generator stator insulation using PD detection and dielectric losses measurement at damped AC voltages 50Hz-300Hz are presented. Based on laboratory investigation on a full scale stator insulation system, as well as during field inspections on large motors a comparison of above mentioned diagnostic parameters will be discussed for the point of view of the usefulness of damped AC voltages.