Tree initiation in polyethylene by application of DC and impulse voltage

The investigations of the tree initiation in polyethylene were carried out using the de and the impulse voltages in the rise times ranging from 1µs to 700µs. The results of the impulse voltage supply show that the time required for the build-up of the space charges at the tip of needle electrode at 25°C ∼ 30°C is about 30µs ∼ 80µs. The tree initiation voltages are decreased when the impulse voltage is superposed on the de voltage of the opposite direction and they are increased when the impulse voltage is superposed on the de voltage of the same direction. The specimens are prestressed with the de voltage for 10 minutes, then subsequently are tested using an impulse voltage with the duration of wave front of 1µs after the waiting period of 2, 30 and 60 minutes, respectively. The tree initiation voltages are reduced the same as the result of the de superposed tests when the polarity of the impulse voltage is opposite to the de prestressing voltage.