Study On PD Characteristics Of Inverter-Fed Motor Insulation Based On UHF Method

Inverter-fed traction motor is one of the nine core components for electric-multiple-units (EMUs), whose secure operation are the key to guarantee the stability and reliability running of high-speed EMUs. In recent years, because the rapid develop of high-speed railway, the PWM technology is widely used. The voltage applied in the inverter-fed traction motor is no longer the traditional power frequency sine voltage, but the square wave pulse with a high pulse repetition frequency and steep rising/falling edge. If the insulation system of inverter-fed motor stator worked long term under the high frequency steep square wave pulse voltage, the insulation would accelerate ageing, and the service life would shorten, which become the hidden trouble for the safety operation of high speed railway. The research from domestic and foreign show that the partial discharge (PD) is one of the important reasons, which lead to the premature failure of inverter-fed traction motor under the condition of PWM pulse. Therefore, the detection and assessment of partial discharge has become a significant means of detection the insulation status for inverter-fed traction motor. And it could provide the theoretical basis for the design optimization and life evaluation on inverter-fed traction motor, meanwhile could guarantee secure and reliable operation of high-speed EMUs. Compared to the testing of PD under power frequency and DC, the steep rise edge of square wave pulse power is easily produced strong electromagnetic interference, which cause the PD signal would drown in the electromagnetic interference generated by the rising edge. Then the extraction of partial discharge under high frequency square wave pulse has become a problem..According to the principle of high frequency electromagnetic wave radiation and antenna receiving principle, a sleeve monopole antenna was developed to detect the ultrahigh frequency (UHF) signal of partial discharge. The structure parameters of the antenna were determined by optimizing the input impedance. And the performance of the antenna was analyzed, such as bandwidth, standing wave ratio and gain characteristic, which make it meet the requirements of detecting partial discharge signals. The experiment shows feasibility that the antenna could be used to test the partial discharge of inverter-fed traction motor insulation.The characteristics of partial discharge UHF signal were analyzed, and the UHF partial discharge detection system was built based on the sleeve monopole antenna. The distribution characteristics of time domain and frequency domain for the pulse power disturbance were studied. A high-pass filter was adopted to filter the coupling high frequency signal from antenna. The disturbance signal of the square wave pulse generator was eliminated effectively; the partial discharge signals were extracted accurately.The twisted pairs and nano-polyimide film which are used in the insulation for inverter-fed traction motor stator were selected as the test object. The partial discharge inception voltage (PDIV), average discharge, discharge times and its influence factors of the samples were studied under the condition of different pulse frequency and rise time. The partial discharge mechanisms of the samples were discussed in this paper. The accumulation of space charge in the air gap surface has a great influence on partial discharge. These will accelerate the aging and short service life of the insulation.