Overshoot Level Studies of PD Exposure in High-Voltage Motor Isolation

The ultrashort rise times of voltage pulses utilized in modern power electronic-based control in combination with voltage overshoots may accelerate aging of electric motor insulation systems. The latter being aggravated by both connecting cable length and impedance mismatches within motor windings. To understand these phenomena better, the aim of the presented work is to demonstrate the influence of both these factors on the resulting partial discharge (PD) activity. A test technique is presented that allows creating controllable overshoots in bipolar pulsewidth modulated (PWM) voltage waveforms of different rise times and experimentally exposing to them pigtail test objects that resemble the insulation of high-voltage motor windings. It is demonstrated that the mutual interrelation between the dc peak voltage level and the oscillation magnitude influences the resulting stress considerably. The PD extinction voltage level (PDEV) is particularly affected, and a notably increased stress is introduced on the insulation due to the presence of overshoots. It is thus implied that, when designing and performing accelerated aging tests, the possibility of appearing in them overshot stresses should be avoided, as their presence can yield erroneous conclusions in the evaluation of insulation material performance.