Travelling wave propagation of partial discharges along generator stator windings

Generator windings have complicated structures which can significantly attenuate and distort high frequency (above 10-50 MHz) partial discharge (PD) pulses transmitted along the winding. In the lower frequency range under 100-300 kHz, however, the PD pulses suffer only small attenuation and distortion. The dependence of PD pulse transmission on frequency is therefore important for PD measurement on generators. Investigations have been carried out on a number of generators by the authors during the last decade. It has been found from theoretical analysis and test results that within a certain frequency range, a generator stator winding can always be approximately represented by a transmission line. A low voltage impulse test method has been developed to determine the frequency range using spectral analysis and digital filtering techniques. It has also been found that in this frequency range, the travelling wave speed increases with frequency and reaches a constant value which can be used to determine the transit time of the winding. Using the travelling wave of PDs, more accurate apparent charge measurements can been achieved and a new dual CT technique has been developed for the detection and location of partial discharges on generators.