334 / 2018-10-13 01:34:01

Transient Space Charge Behavior in Low-density Polyethylene Films at Prebreakdown with Different Voltage Ramping Rates

transient space charge,LDPE,electric field distortion,charge packet,prebreakdown

DC breakdown tests are widely used to evaluate the electrical strength of insulating materials. However, only the applied electric field during breakdown is recorded in DC breakdown tests, and the internal electric field distortion caused by space charge is not considered. Meanwhile, pre-breakdown is a transient process. Injection, migration, accumulation, and dissipation processes of space charge in the dielectrics can be completed in a very short duration, which requires fast and continuous technology for space charge measurement. Low-density polyethylene film (additive free) with a thickness of 100 μm was purchased from GoodFellow Corp. An improved space charge measurement system was developed with a high-speed measurement interval (1 ms), a high voltage pulse of 120 V, and a pulse width of 5 ns. The test temperature was set as 70 °C. The voltage ramp rates were 0.5 kV/s, 1 kV/s, and 2 kV/s, respectively. The applied voltage was cut off until breakdown. The results show that in the linear ramping process of 0.5 kV/s, negative charge is rapidly injected and accumulated near the anode at a relatively low electric field. As the applied electric field gradually increases, the anode appears significant injection process of a positive charge packet, and the injection depth is less than 30 μm. For a 1 kV/s voltage ramp rate, there is still a negative charge injection and accumulation before positive charge packet injection, but the amount of negative charge is significantly less compared to that of 0.5 kV/s situation, and positive charge package injection depth is less than 25 μm. For a 2 kV/s voltage rate, the internal positive charge packet has not yet formed before prebreakdown. For the internal electric field distribution, the maximum electric field value within the samples during the linearly ramping process shifts from the vicinity of the anode to the middle of the sample, which should be related to the formation and migration of positive charge (packets) injection at the anode. When the prebreakdown happens, the maximum internal electric field distortion ratio could reach 56% (0.5kV/s), 49% (1kV/s) and 44% (2kV/s), respectively. It is analyzed that the cathode side with the semiconducting layer is much easier to inject charges than the anode side with the aluminum electrode. On the other hand, the conduction band is bent at the interface between the electrode and the sample due to the applied voltage, and the energy band in the sample will be tilted, which leads to the results that the potential barrier to be overcome for hole injection at the anode is higher than that for electron injection at the cathode. Therefore, the anode begins to inject holes only with the continuous enhancement of the external field, forming positive space charges. In addition, the charge migration distance becomes shorter and shorter with the increase of voltage ramp rate partially because the charge does not have enough time to inject and migrate due to the shorter time before the pre-breakdown. On the other hand, the negative differential mobility effect also contributes to the shorter migration distance. This study provides new methods and new ideas for evaluating the breakdown properties of insulating materials - considering space charge effects.