126 / 2018-08-24 23:16:06

A Novel Series Resonance Test System for Partial Discharge Detection of XLPE Cables

XLPE Cable,series resonance test system,sinusoidal pulse width modulation,Equivalent Pulse Width Modulation,narrowband impulse interference,partial discharge deteciton,Time Domain Reflection

XLPE cables are a vital part of urban transmission and distribution networks because of their stability and safety. Partial discharge detection is becoming a common procedure in onsite diagnostic testing of XLPE cables in recent years. However, the traditional series resonant test system has a large amount of pulse interference, which makes it impossible to use for partial discharge detection. In order to achieve partial discharge detection and withstand voltage test simultaneously, this paper introduces a novel series resonant test system which can measure partial discharge.
The novel series resonant test system consists of rectification, inverter, exciting transformer, resonant reactor and measuring part. The basic principle of the novel system is the same as traditional system, exciting high voltage on cables by means of resonance. Whereas traditional series resonant test system widely uses Sinusoidal Pulse Width Modulation Technology to modulate variable-frequency sine waves. This technology cannot be used to achieve partial discharge detection because of its massive impulse noise caused by switching action. This novel system uses Equivalent Pulse Width Modulation to modulate sine wave, which can reduce the times of switching action in the inverter obviously. The paper introduces the control method and proves that it can produce a stable high voltage sinusoidal wave on cables. The relationship between resonant voltage and pulse width is obtained，using Fourier Transform and unified circuit model.
There still exist four impulse noises in each cycle of the sine wave, although using Equivalent Pulse Width Modulation. In order to obtain a pure partial discharge waveform, these four pulses must be eliminated. Using existing digital noise reduction techniques, it is difficult to eliminate narrowband impulse interference which is similar to partial discharge pulses. Fortunately, the phase of four pulses is known. It is very simple to eliminate four pulses in each cycle. Considering the width of pulse is shorter than 10μs, excluding 40μs per cycle of partial discharge data during analysis would not lose too much valid data, but can completely eliminate the impact of pulse interference on partial discharge.
The cable core is connected to system at one end, and the other end is open while testing. Partial discharge produces two pulses that are transmitted to both ends of the cable, one is transmitted directly to the measuring end and the other is transmitted to the measuring end after total reflection. The location of the partial discharge source can be calculated from the recorded time interval between the two pulse.
Finally a laboratory prototype is assembled and tested. The designed system is used for the PD detection of a 200m YJLV22-1×35mm2 XLPE cable circuit with a built-in fault. The detailed parameter values of the system are given in the paper. Partial discharge data before and after noise reduction processing is shown. Partial discharge positioning results are consistent with actual defect locations.
This paper introduces an improved series resonant test system which can conduct not only withstand voltage test but also partial discharge detection test of XLPE cables, using Equivalent Pulse Width Modulation, Time Domain Elimination and Time Domain Reflection Technology. Partial discharge detection and positioning analysis of the system are confirmed under laboratory conditions.