Influence And Mechanism Of Square Wave Voltage On Partial Discharge And Motor Insulation Life

Power electronic drives using asynchronous motors are extensively used in various fields because they offer a number of remarkable advantages such as improved controllability and energy efficiency. Using pulse width modulation technology, an inverter output voltage with high frequency and fast-changing features, which brings new problems to motor insulation systems. Partial discharge (PD), induced by the motor terminal overvoltage, is generally recognized as the main reason for the premature failure of insulation in inverter-fed motors. To reduce the effect of the aforementioned problem, the International Electrotechnical Commission (IEC) released related technical specifications (TS), which require PD inception voltage (PDIV) and corona resistance performance to be tested for low-voltage random-wound and high-voltage form-wound motors, respectively, to avoid premature insulation failure and thus, increase insulation reliability. However, when carrying out the TS, there are following problems should be solved.①The overlapping of disturbance and the PD pulses in the time and frequency domains makes PD signal extraction difficult and thus, low signal to noise ratio (SNR).②To choose suitable repetitive square wave voltage parameters when performing PDIV measurements, the effect of voltage parameters to PD features should be considered accordingly. However, this effect is unclear.③When performing life tests on stator or insulation samples under repetitive square wave voltage conditions in high-voltage motors, the influence of voltage parameters on test results could cause inaccurate test results.Therefore, to solve the aforementioned problems, this study focuses on the following aspects:First, by using an insulated-gate bipolar transistor (IGBT) and power amplifier technologies, an impulse generator, which can output repetitive square wave voltages at50ns (the shortest) rise time, and with adjustable frequency, duty cycle, and voltage magnitude, was designed. To suppress the disturbance appearing at the rise and fall flanks of the impulse voltage, both the high-frequency current transformer (HFCT) and ultra-high frequency (UHF) methods with suitable filtering and frequency shifting technologies were adopted. In this manner, the PD measurement system for off-line detection and online monitoring was designed. Second, resorting to the aforementioned experimental system, the statistical features of the PD were studied by focusing on the single PD specimen made from enameled wires insulated by polyimide. Based on the high number of data, the effects of repetitive square wave voltages on PD features were summarized. The mechanisms of these effects were also provided. Lastly, electrical aging was performed on a single PD specimen under repetitive square wave voltages with different parameters. Statistical analysis was performed based on life data, and explanations for the results were provided based on PD data obtained during electrical aging.Experimental results showed that the overvoltage caused by waiting for initial electrons could change the PD time and frequency domain characteristics. PD magnitude and high frequency electromagnetic energy increase as the increase of impulse voltage rise time. High-frequency square wave voltage can give rise to PD events with low magnitudes and phases. Changing the voltage duty cycle can cause the asymmetric shapes of phase resolved partial discharge (PRPD) maps. When stressed by sinusoidal and square wave voltages with the same peak-to-peak voltages and frequencies, the insulation life of enameled wires insulated by polyimide exhibited significant differences. Insulation lifetime does not decrease linearly with frequency because of the influence of square wave voltage frequency on surface charge accumulation.The conclusions of this study will provide theoretical and experimental references to revise the TS for PD measurement on inverter-fed motors. Consequently, insulation-testing technologies for low-voltage random-wound and high-voltage form-wound motors will also be improved; thus increasing their insulation reliability.