230 / 2018-09-23 16:30:55

Excess Current Characteristics of Partial Discharge of Polymer Film in Tri-electrode system under Power Frequency Voltage

Polymer film,Excess current,Pulse current,Tri-electrode,Surface current

Detection and diagnosis of partial discharge (PD) are the technical means to discover the sudden and developing insulation defects. PD is accompanied by a variety of transient physical phenomena, which are essentially the proliferation, disappearance and transfer of charge in the process of ionization. The current pulse coupled in the circuit is an important symbol to characterize the existence and strength of the discharge. Thus the narrow band and the broad band pulse current method developed by this principle has become the standard measurement method for insulation parts, such as factory, handover and off-line test of devices, and has become the only criterion for calibrating the discharge capacity. But the pulse current method has the problems of low signal-to-noise ratio and low sensitivity to low frequency signal. In order to overcome the limitations of traditional detection methods and determine the amount of discharge, we focus on the excess current in PD activity.

Excess current is defined as the associated low frequency current accompanying the transient physical phenomena of PD. The excess current generated during discharge can be quantified by measuring the dielectric response current flowing through the defect before and during PD. In this paper, an experiment platform based on tri-electrode structure is built to separate the internal current and surface current in the PD process of a polymer film by measuring the excess current and the pulse current in the PD process of the polymer film. By measuring and analyzing the excess current of two kinds of materials of polymer films, the basic characteristics of the low frequency excess current accompanied by the transient physical process of PD are verified and defined, and the composition of the surface current in the process of the film partial discharge is analyzed and explored. The surface current can be divided into a conductive portion and an inductive portion. A quadra-electrode system is introduced to separate these two parts of the surface current which the tri-electrode system cannot distinguish.

The experimental results show that the low frequency current which cannot be detected by the pulse current method does exist in the PD process. The amplitude of the excess current waveform is higher than the average discharge current, and the phase distribution is a little wider than the latter. The discharge quantity of the positive and negative current of the excess current is higher than that of the PD pulse current. The analysis of the net excess current reflects the current characteristics of the slow moving charge in the PD process. Through waveform analysis and geometric capacitance measurement, it can be seen that the surface current component of the net excess current contains the induced current generated by the geometric capacity coupling between the electrodes, and its composition accounts for a large proportion of the surface current. The presence of geometric capacity will have a large impact on the measurement of surface current.