Partial Discharge Detection And Recognition Technology Of Large Generator

The large generator is one of the important devices in power system. Partial discharge (PD) accured in its insulation can lead to the deterioration of insulation, resulting in insulation breakdown and short circuit. Traditional off-line test can not present the real status of generators in operation station, so it can not ensure safe-operation of generators. Insulation faults were still reported now and then. As a result, it's necessary to develop a complete and practicable on-line partial discharge monitoring system for the large generator stator winding to find out the real insulation status effectively and to improve the operation reliability of generator units.On the basis of knowledge from many documents, the generator partial discharge generation reasons and characteristics of three different types of PD in the large generator stator winding are mainly elaborated in this paper. And the propagation law of PD pulse in the large generator stator winding is also analyzed. According to these theories and the application of the electrical research association in the generator partial discharge detection, a new type of on-line PD monitoring system for the large generator stator winding is developed,including hardware and software development. The hardware system is composed of the wide band sensor, the signal prepoccessing circuit and the PD signal collecting unit. The software system is developed based on the VC++ software development platform,realizing function of controlling the data acquisition card and communication between the upper computer and the lower computer.Considering different types of interferences which always submerge PD signals possibly encounter in the weak partial discharge signals, the first important problem of on-lining monitoring is how to pick up the nonstationary random signals from the interferences. Due to the variety of interference exsiting in the field, a measurement aimed to the hierarchically processing of signals is given in this paper, a method using different processing means to suppress different interferences. Firstly,considering the strong periodic narrow-band interference of PD signals has distinct difference from PD pulse singals in the frequency domain, a method is put forward to eliminate the interferences of partial discharge based on FFT and wavelet theory. Secondly, because of the similarity of white noises and PD pulse signals, wavelet packet transform is apt to eliminate white noises. Simulation proves that the wavelet transform combine with FFT method can be more effectively to eliminate the strong narrow-band interference of partial discharge signals and preserve the original signals very well. Meanwhile, the wavelet packet transform can pick up the partial discharge signals from the white noises, and it is much better than the wavelet transform.Further more, PD tests have been made on the high voltage generator stator winding using the system designed in this paper and the signals were processed using the interference suppression methods proposed in this paper, the result shows that the measurement aimed to the hierarchically processing of signals can pick up the PD signals effectively from the interferences. With the development of the signal processing technology, methods to process signals will be more perfect,so there is a practical future with this measurement.