Qiaogen Zhang

The design of Rogowski coil with wide band using for partial discharge measurements

The elaborated Rogowski coil with improved frequency characteristics is applied to measure partial discharge (PD) in XLPE cable. By selecting magnetic cores and optimizing chief parameters, the Rogowski coil has both wide bandwidth and high sensitivity. The coil was placed at the grounded wire of a 10 kV XLPE cable for measuring PD current. The cable was designed with an artificial defect of needlepoint pricked from the sheath. The results show that PD signals in XLPE cable can be detected by this coil with no distortion, and the real PD characteristics of needlepoint defect can be obtained from the PD signal.

Insulator surface charge accumulation under DC voltage

The phenomena of surface charging are investigated using the capacitive probe method. In order to decrease the charge leakage a new type probe with high resolution is designed. The influence of voltage amplitude, duration, polarity and surrounding humidity on the surface charging are studied. The accumulation competition between positive charges and negative charges is observed. The experimental results show that at different places on the insulator surface the surface charging has different saturation time constants.

The effect of impulse rising steepness on streamer to leader transition in non-uniform field gap in SF6

This paper describes the mechanism of discharge development in non-uniform field gap in SF6 under positive impulses with different rising steepness. Based on theoretical consideration, an expression of the transition delay time from streamer to leader is presented, which can help to better understand the process of streamer to leader transition under impulses with different rising steepness in SF6. Also, the calculated results of the proposed expression for the transition delay time are quite in agreement with the experimental ones.

Streamer corona propagation of an inhomogeneous field gap in SF6 gas stressed by steep-fronted impulse

Based on experimental observations and theoretical explanation of the streamer spatial extension for an inhomogeneous field gap in SF6, a new model is developed, taking into account the probability of the appearance of an efficient electron. The model, which can help one to better understand the breakdown characteristics of the impulse-steepness dependence, relates the spatial extension of the streamer corona in an inhomogeneous field gap in SF6 gas to the steepness of applied positive impulses.

Metal inserted spacer influence on particle motion in nonuniform electrical field under DC voltage

In gas insulated systems (GIS), particles near spacer in GIS tend to cause apparatus faults by leading flashover breakdown along spacer surface. Metal inserted spacer as a method is designed in purpose to prevent particles from lifting and adhering to spacer. The effect of electrode inserted is investigated by calculation of electrostatic force acting on spherical particles in 2-dimension model through finite element method (FEM). Three type spacers with different shapes are studied and the results show that metal inserted spacer is effective in hurtless treatment of particles in DC GIS.

Initial conditions of surface charge accumulation under impulse voltage

The surface charge distributions under impulse voltage are measured. Two types of insulators are investigated. The influence of impulse voltage magnitude on the surface charge accumulation is studied. It is shown that the pre-discharge current has a neutralization effect on charge accumulation, which makes the surface charging very weak when the lightning voltage is close to the flashover voltage. The initial conditions of surface charge accumulation are discussed in the present paper, and the source of charging is also studied. It is concluded that partial discharge in gas near the surface of the insulator is the initial condition of charge accumulation under impulse voltage.

Electrical measurement of different discharge from spherical and filamentary particles

Particles will move freely at different motion modes in GIS, resulting in micro-discharges with different characteristics upon impacting with electrodes. A non-inductive resistor is adopted to detect the micro-discharge. The measurement results show that the discharge waveform from particles can be used to distinguish spherical and filamentary particles and to estimate the motion modes of different particles.

Influence of waveform on the accumulation of charges along a particle contaminated spacer surface in SF6 stressed by impulse voltage

The influence of the waveform on the surface charging of a spacer stressed by impulse voltages is investigated in this paper. The experimental study was carried out using a simplified spacer with an adhering conducting particle under homogeneous conditions in SF6 gas. A capacitive probe was specially designed to yield the surface charge data through potential measurement. The results show that the impulse waveform has a significant influence on charge accumulation on the spacer surface. Also, the measurements of the optical signal occurring in the predischarge process can give an explanation for the mechanism of charge accumulation on the spacer surface in the presence of impulse.

Effect of surface charges on impulse flashover voltages of spacer in SF/sub 6/

The positive and negative charges were injected onto the spacer surface by DC corona discharges, and the flashover characteristics were investigated by a sudden change from DC to lightning or steep-fronted impulse in the presence of a conducting-particle attached on the spacer in SF/sub 6/ gas. Experimental results showed that the polarity of corona charges has a great influence on impulse flashover voltages. For the opposite polarity impulse to corona charges, there is a significant shift of the time domain where the minimum flashover voltage appears in the U-shaped curves of relationships between flashover voltage and wavefront duration (U/sub f/-t/sub f/). Also, the influence of DC polarity reversal on the corona discharge current has been studied.

A method for calculating low probability impulse breakdown voltages in SF/sub 6/-filled gaps

From the analysis of the abnormal discharge in SF/sub 6/-filled gaps, a method for calculating low probability impulse breakdown voltage is put forward, which consists of three limiting lines: the streamer breakdown line, the low probability impulse breakdown level and the streamer onset line in different gas pressure ranges. The calculated value gives a safety margin compared with the measured value.