V-t Characteristics of Twisted-pair with Polyvinyle Formal Wires Undergoing Thermal Stress in Mineral Oil

This paper deals with lifetime estimation (or V-t characteristics) of twisted-pair with PVF wires undergoing thermal stress at 180 °C for two months in mineral oil prior to the V-t test. An attempt is made to detect partial discharges (PD) in V-t characteristics measurement to discuss the degradation mechanism till the sample breakdown. Twisted-pair PVF wire specimen with copper conductors of 550 μm in diameter and insulating layer of 17 μm in thickness were used. Thermal aging was conducted for the twisted-pair specimen kept in mineral oil with less than 25 ppm water content in mineral oil in a sealed container at 180 °C for two months, which provides equivalent 60 years aging period to the sample by assuming the ten-degrees Celsius half-life law. V-t characteristics of the thermally aged PVF twisted -pair sample were measured for applied voltage V
_a
=1.5 kV ( at 1080 Hz) and 2.0, 2.5, 4.0 kV (at 540 Hz) to accelerate the test while PD were detected. Experimental results revealed that V-t characteristics of the thermal aged sample are classified into two regions: (i) above 4.6 kV, PDs continuously occur till breakdown, or PD begins to occur not from the beginning but a certain time after the voltage application, and ii) below 4.6 kV; breakdown suddenly occurred without PD at 2kV to 4 kV. It was found that the thermally aged PVF wire at 180 °C for two months still maintains sufficient insulation performance, i.e. providing life more than 30 years at 1.5 kV.