Shimin Li

RTV silicone rubber pre-coated ceramic insulators for transmission lines

The paper gives the state-of-the-art on pre-coated ceramic insulators for transmission lines. The usage of pre-coated ceramic insulators has mainly been in coastal areas where sea salt causes flashovers of ceramic insulators. The experience to date has been very good everywhere with very few problems reported. Interest in the pre-coated insulator concept continues to be strong everywhere. The paper also provides guidelines to assist the user in the specification of pre-coated ceramic insulators.

Effect of gamma-ray irradiation on the TSC in polyethersulfone

When polyethersulfone was irradiated by /spl gamma/ rays and then poled at 37/spl deg/C, a new TSC (thermally stimulated current) peak appeared at /spl sim/100/spl deg/C. This new peak and other peaks near 40 and 150/spl deg/C increase with an increase in dose of /spl sim/0.6 kGy, and this is considered to be due to trapped charges and other ionic carriers becoming mobile when rearrangement of scissioned chains occurs. However, if the dose exceeds 0.6 kGy, the peak magnitude decreases. Furthermore, if the TSC was poled at 120/spl deg/C, only the peak near 150/spl deg/C appeared and this peak monotonically decreased with an increase in the irradiation dose. A similar decreasing tendency was also observed in the conduction current and in the residual voltage. These decreasing features are considered to be caused by the radiation-induced change that rearrangement of scissioned chains decreases free volume for ionic transport.

DC inclined-plane tracking and erosion test of insulating materials

The paper reviews previous work on the DC inclined plane test and suggests equivalent DC voltage levels in parallel to AC voltage in the ASTM inclined plane tracking and erosion test. The aim of this work is to provide a basis for standardizing the inclined plane test for DC voltage. Round robin tests done in five laboratories on five specimens of a silicone rubber material were done with the purpose of establishing appropriate ratios by which the equivalent DC voltages can be determined with respect to the corresponding AC voltages. These levels were determined as 67% and 87%, for +DC and -DC respectively, of the AC initial tracking voltage, and for practical purposes, these levels are rounded to 70 and 90%.

A mechanism transition between two consecutive breakdowns at short rod-plane vacuum gap

The objective of this paper is to propose a mechanism transition between two consecutive breakdowns at short rod-plane vacuum gap. A rod-plane vacuum gap is designed in a vacuum interrupter. A 50Hz power frequency voltage is applied across the vacuum interrupter to measure the pre-breakdown field emission current. A compensation algorithm removes the displacement current from the measured current and then only field emission current left. The voltage and field emission current fit the Fowler-Nordheim theory and two parameters β value and effective emission area can be obtained. Three vacuum interrupters are applied the power frequency voltage twice and two consecutive breakdowns happen. The experimental results show that: The first breakdown (BD) voltages of the three vacuum interrupters are 49.5 kV, 44.7 kV, 41.1kV (peak value), and the β value are 260,219, 257 respectively. The second breakdown (BD) voltages of the three vacuum interrupters are 51.5 kV, 48.3 kV, 44.1kV (peak value) but pre-breakdown currents β values and effective emission area are too low to be determined. Thus a mechanism transition between the two consecutive breakdowns in a short rod-plane vacuum gap are suggested as follows: field emission current induced breakdown and particle induced breakdown both exist at 1mm vacuum gap. The first breakdown is induced by field emission current and the second breakdown is induced by particles.

The influence of the cathode melted layer on vacuum insulation

The objective of this paper is to determine the influence of the depth of cathode melted layer on vacuum gaps insulation. A pair of rod-plane electrode was used. A DC arcing circuit produced different depth melted layer on the rod anode through different arcing time, which were 10ms, 46ms and 73ms respectively. Then the vacuum gap was adjusted at 1mm to measure the breakdown voltage. Positive polarity standard 1.2/50μs lighting impulse voltage was applied by a basic up-down method and the rod was the cathode. Experimental results revealed that the breakdown probability distribution followed a Weibull distribution when the breakdown voltage saturated. The 50% breakdown voltage U50 of different arcing time 0ms, 10ms, 46ms, 73ms corresponded to 55.6kV, 73.3kV, 75.5kV, 77.9kV, respectively. The U50 with the melted layers increased 37% at least. However, the U50 with various depth of melted layers were similar. Therefore, the melted layer does improve the breakdown voltage but the depth of melted layer has no influence on the breakdown voltage.