170 / 2018-08-25 21:25:46

Partial Discharge Behavior of Epoxy/paper Insulated Tubular Busbars with Artificial Defects

partial discharge; simulation; insulation material

As known that epoxy resin insulated tubular busbars used in low power voltage grid have been playing an important role in safe and steady operation of national grid, so that the normal operation of tubular busbars should be guaranteed as long as possible regardless of the inevitable aging of epoxy resin. Epoxy/paper insulated materials, with the merits of great insulation performance, excellent mechanical strength, low cost, widely served in many applications. However, the electric field strength and temperature in the central conductor are high, and the interfacial effects，such as charges accumulating, voids generating, between the epoxy layer and insulating paper are severe. The partial discharge (PD) will arise in the interfacial layer because of the voids generated during the curing process. The PD in a cavity inside solid insulating materials is known to be severely affect the lifetime of power equipment. Numerous researches concentrated on the PD in oil-impregnated insulating paper, but few works study the PD in epoxy/paper insulated materials. Consequently, the understanding of PD in epoxy/paper insulated materials is beneficial to improve the long-term performance of equipment. Here, two kinds of epoxy/paper insulated tubular busbars that have voids defect only and voids with metallic impurity defect, were prepared. The simulation and PD measurement are present in our works.
In this work, the PD in epoxy/paper insulated tubular busbars with two types artificial defects mentioned above and different aging time were measured. The experimental voltage imposed on the samples was 18 kV, and the temperature was 428 K. The results showed that partial discharge inception voltage was lower but the apparent discharge was higher as the aging time increased. In addition, the defect of void with metallic impurity showed larger apparent discharge magnitude, compared with the samples with voids only. For example, the measured largest apparent discharges of tubular busbars including the defect of void with metallic impurity was 8337 pC, larger than the value of 2266 pC measured in tubular busbars with voids defect only.
Furthermore, we simulate the partial discharge activity in epoxy/paper insulated material with two types of defects. The MATLAB Simulink tools are used to simulate the PD activity according to simplified model. The simplified model incorporated three capacitors indicating the capacitor of void, the capacitor of the material cascading to the void, and the capacitor of the left material parallel to the void, respectively. Besides, the epoxy resin composite specimen with defects were simplified into a series-parallel circuit consist of capacitors and resistors. The capacitor and resistor values were changed according to the different types of defects. Considering the fact that resistor of void will decrease once PD activity appeared, the resistor of void was divided into two parts including the volume resistor and surface resistor. To simulate the actual PD activity, an ideal switch and changing surface resistor were adopted. In different stages of PD, the surface resistor of void surrounded by epoxy resin changed variously, thus some subsystems were used to decrease the surface resistor in different ways. In addition, AC voltage sources, inductors, current measurement devices, voltage sources devices and scopes were adopted in simulated circuit. The results showed that the specimen including defect of void with metallic impurity influenced the voltage waveform more obvious than the one with defect of voids only, although two both of them changed the normal waveforms imposed on the specimen. Moreover, different defects will cause various electrical field distribution leading to different PD activities. In consequence, finite element analysis (FEA) method was applied to simulate the electrical field distribution. The result revealed that the metallic impurity in epoxy resin composites specimen caused local electrical field distorting and accelerated the aging of material around the defect. The simulated results were consistent with the former experimental results.