Non-contact Method Of On-Line Monitoring Partial Discharges For The Stator Insulation Of Large Generators

The partial discharge detection technology for stator insulation of large generators has been developed for several tens of years, but still been far away from accurately deciding the type, severity and position of partial discharges. In this dissertation, a method named non-contact monitoring method is established, and a large amount of study work was done in respect of non-contact measurement of partial discharges, suppression of narrow-band frequency noises, elimination of outside pulse noises, elimination of inside pulse noises and partial discharge resource location. Some experience or conclusions drew from these studies will be helpful for realization of condition based maintenance of large generators.Using models of isolated phase bus and stator bar of generators, such studies was carried out as follows, the traveling process of pulses signal in isolated phase bus structure, the detection principle of electromagnetic sensor, influence of large power frequency current in bus and reflected wave from electrical equipments connected with bus on detection of partial discharge. A non-contact electromagnetic sensor is designed considering the structure of isolated phase bus, which has measuring frequency band from 0.1 MHz to 20MHz and be able to detect 15mV partial discharge from the bus. In laboratory 100% pulse noise propagating from outside to generator direction along the bus bar are recognized accurately.After analyzing experiment data from a small-scale generator module and measurement data from a large hydro-generator, the coupling mechanism and rules between stator windings and excitation windings is put forward, and a three-phase comparison method for eliminating pulse interferences from generator's excitation winding, sparkle discharge of slip-ring in rotor, ground wire of generator and environment is presented.A Fourier series anti-interference method is developed to restrain narrow-band frequency noises, and this method's anti-interference principle, efficiency and influencing factors are introduced. Besides this method was verified by field data and comparison analysis with other anti-interference methods. It has obvious superiority in respect of remaining original shape, amplitude and polarity information of partial discharge, increasing ratio of signal to noise to a large extent.By experiment carried on a turbo generator, the author has found the pulses can propagate by magnetic of stator windings. The coupling mechanism, frequency domain characteristics and propagation rule of three propagation paths of partial discharge signal...