Effect of Repetitive Rates of Impulse Voltage on Electrical Tree in Silicone Resin for Inverter-fed Traction Motor

Silicone resin, as the main insulation material for VPI (vacuum pressure impregnation) of traction motor coil, withstands the high repetitive impulse voltage due to the application of inverter, which may accelerate the growth of electrical tree and eventually give rise to the deterioration of stator insulation. Therefore, it’s necessary to investigate the effect of repetitive rates of impulse voltage on electrical tree in silicone resin. The samples constituted of cured silicone resin impregnating varnish and needle-plate electrode were tested under the bipolar square waveform impulse voltages of 7 kV with the repetitive rates of 0.5, 1 and 2 kHz, respectively. A real-time microscope digital imaging system was established to observe the morphological characteristics, initiate and growth of electrical trees in samples. The test results indicate that the morphological characteristic of electrical tree in silicone resin for inverter-fed traction motor is greatly affected by the repetitive rate of applied impulse voltage. The increased repetitive rate would cause the decrease of fractal dimension of electrical trees and the significantly promotion of average radial growth rate, which may be accounted by the enhanced partial discharges in the tree channels, while the repetitive rate only has trivial influence on the initiation time.