Discharge detection techniques for stator windings

Partial-discharge measurements are being used increasingly to detect localised or damaged areas in modern high-voltage stator windings. However, ambiguities may arise because of the complex LC network between coils. Discharge pulses propagate in a variety of modes, each having different attenuation factors. Tests with diamond-wound motor stators and a 500 MW generator stator show that pulse detection in a limited frequency range, for example 20¿300 kHz, is most suitable. Here, attenuation errors are restricted to within a factor of two to three, whereas a wideband detection with the bandwidth in excess of 1 MHz may produce up to a 50-fold attenuation of signals from sites remote from the terminals of the stator windings. Electromagnetic probes and ultrasonic scanning techniques are useful for locating the sites of high discharge. Their accuracy and limits are reported.