Reliability issues in high frequency/transient environments of adjustable speed drives-insulation coordination

As the power electronics develops more compact, higher operating frequency systems, reliability engineering issues exist that need to be addressed. High-voltage, high-frequency transients and related waveforms that are part of powering power supplies, power conditioning, and advanced loads such as EV traction motors, may damage the insulating materials in components, wiring harnesses, interfaces, and connectors. A new technique for testing the reliability performance of the thin dielectrics used in such insulation has recently been realized. The relatively new environment of modern high power electronics contains new stresses of high-voltage (i.e., > Paschen minimum of nominally 140 volts peak), short-duration, high-repetition-rate pulses that are applied to such thin dielectric films. Under voltages above the Paschen minimum, these fast transients result in creating as intense-corona environment, causing premature electrochemical aging of the insulation, followed by premature insulation failure. Understanding the aging rates within such systems will permit adequate engineering safety margins to be incorporated therein to achieve full life-cycle reliable operation. These test conditions were prompted by the similar environment foreseen for a number of applications and by the particular understanding about the films which is gained. Polypropylene films, 9 micrometers thick, representative of typical wire and connector bulk insulation, were placed between a rod-plane electrode set and subjected to high-voltage pulses until breakdown occurred, for voltages sufficiently elevated that significant corona activity was in evidence. The effect of voltage magnitude and pulse repetition rate (rep-rate) on the number of pulses the films were able to survive was recorded. Standard statistical techniques for life-testing were employed, along with several novel modifications developed for this work, in the data analysis. In this study it was found that the voltage dependence followed nearly that seen for intense corona dc and ac voltage applications, while the higher rep-rate permitted a greater pulse life than did lower rep-rates. Conclusions regarding aging rates of insulation systems under intense corona activity will be presented along with recommended engineering practices to obviate the observed very high aging rates of such systems.