To investigate the breakdown mechanism in oil/PB composite insulation systems applied to power transformers, we have measured the electric field in partial discharge occurrence by the application of lightning impulse voltage using the electric field measurement system by electro-optical Kerr effect. It has confirmed that the system has sufficient time response for the measurement of the electric field during lightning impulse application. In the oil/PB composite insulation system, which has multi oil gaps, we could have confirmed the increase of the electric field of the other gaps when the partial discharge occurred in one of the oil gaps. Furthermore, we have confirmed that the obtained optical phase shift is similar to estimated value by electric field simulation.
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