Study on Electrical Treeing of Polypropylene under Combined DC - Pulse Voltages in LN2

Polypropylene laminated paper (PPLP) is used as a primary insulation in low temperature insulated superconducting cables because of its good mechanical properties and high electrical strength in low temperatures. The operating impulse overvoltage is generated because of switching surges and commutation failure. The insulation property of polypropylene will change under this environment. In this work, electrical tree growth characteristics in polypropylene with application of combine DC-pulse voltage in liquid nitrogen (LN2) are studied. With the change of DC amplitude, different combinations of DC and pulse voltage are used as the applied voltage. Results show that under the DC voltage with the same polarity of the pulse voltage, the growth rage and accumulated damage is the largest. Under the DC voltage with the opposite polarity of the pulse voltage, the growth rage and accumulated damage is the smallest. Such phenomenon is related to higher effective value of the composite voltages resulting in tenser partial discharges activities under positive DC voltages.