Yongpeng Meng

Dynamic behavior of surface charge distribution during partial discharge sequences

A surface charge measurement system based on Pockels effect was constructed to detect the variation of residual surface charge distribution in partial discharge (PD) sequences. The current form (ns level) of the PD sequence was measured simultaneously. By using this system, it is found that surface charge distribution caused by PDs appeared as several isolated charges spots similar to that in dielectric barrier discharges (DBD). The result also agrees with the previous models, suggesting that charge distribution is not uniform and that it plays an important role in the stochastic behavior of PDs. As the applied voltage increased, the number of charge spots increased and the hetero-discharge occurred. Besides, a charge ring along the void edge was observed after aging due to PD. An approximate linear relationship existed between PD magnitude and the corresponding accumulated surface charges. In the PD sequence, the discharges tended to take place at the sites having heterocharges at dielectric surface after voltage polarity reversed. Moreover, there were no homodischarges observed at the decreasing part of applied voltage even at the area without surface charges existing. This could not be explained by the accumulation of surface charges. It was also observed that more than one charge spot in one PD could occur. This was named simultaneous discharge in our paper, and may be attributed to the effects of photoionization and time lag.

Simulation of partial discharge sequences using fluid equations

Simulation methods employing continuity equations to describe particle transportation are used to examine the process of single discharge. However, it is difficult to simulate partial discharge (PD) behaviour accurately by these methods, because it will take too much computation time to simulate consecutive PD sequences. In this report, in order to save computation time, a simple three-dimensional network is used to simulate PD sequences in a small void under ac voltage. The model considers charge generation, charge transport and charge disappearance, and electrical field redistribution in three dimensions. The continuity equations for electrons, positive ions and negative ions, including the effects of ionization, attachment, recombination, electron diffusion and the transport of charge under electrical field are solved simultaneously with Poissons equation. With this model, the phase-resolved PD pattern and the current forms of each PD are obtained. Moreover, considering the propagation process of each PD and its influence on surface charge distribution, the fall part of PD current form and the effect of previous discharge on the subsequent one are also studied.

Study on the characteristics of partial discharges in voids under square voltage by detecting light emission intensity

In order to quantitatively study partial discharge (PD) behavior in voids under square voltage, the feasibility of PD measurement by detection of light emission intensity was investigated at first. Experiments were carried out to study the influences of void size on the correlation between light emission intensity and PD magnitude and its transition with PD degradation. It is confirmed that a good correlation existed between light emission intensity and PD magnitude in a void with diameter larger than 3mm and the correlation did not change in the PD degradation process. Then, the effect of square voltage rise rate on PD characteristics under square voltage was investigated by detecting light emission intensity. In the PD degradation process, an abnormal phenomenon that the maximum PD magnitude decreased with the increase of voltage rise rate was observed. This was interpreted in terms of the change of surface condition due to PD degradation.

The detection of the closing moments of a vacuum circuit breaker by vibration analysis

The vibration signals during the closing operation of three-phase vacuum circuit breakers (VCBs) were recorded for detecting the closing moment of each pole. The short-time energy (STE) analysis of the vibration signals with a moving Hamming window is used to derive the closing moments both under no-load conditions and load conditions. By determining a proper length for the moving window, the improved signal-to-noise ratio by the STE analysis greatly facilitates the identification of the closing moments. Under no-load condition, the results obtained from the vibration signals are in good agreement with those measured by the contact timing signals. Under load conditions, it is found that the effect of the making current on the detection results is too tenuous to be neglected. This makes it possible to apply this method to online detection of the closing moments of VCBs.

Study on law of negative corona discharge in microparticle-air two-phase flow media

To study the basic law of negative corona discharge in solid particle-air two-phase flow, corona discharge experiments in a needle-plate electrode system at different voltage levels and different wind speed were carried out in the wind tunnel. In this paper, the change law of average current and current waveform were analyzed, and the observed phenomena were systematically explained from the perspectives of airflow, particle charging, and particle motion with the help of PIV (particle image velocity) measurements and ultraviolet observations.

Condition monitoring of vacuum circuit breakers using vibration analysis

Vibration analysis is a potential effective method for condition monitoring of circuit breakers (CBs). In this paper, a time-frequency method using an algorithm of wavelet packet is applied in the subsequent analysis for vibration signals. The vibration signal collected on the framework of vacuum circuit breaker by an acceleration sensor is decomposed into individual frequency bands and those bands that contain the main signal energy are extracted to form individual condition eigenvector. All eigenvectors derived from various conditions are used for Condition classification of CBs. Based on this monitoring method, asynchronous feature between three poles have been simulated in a 35kV vacuum circuit breaker and difference between vibration signals during close operation with no-load and that with short-circuit current is investigated, which verify the feasibility and accuracy of the vibration analysis in condition monitoring of vacuum circuit breakers.

Effects of photoionization and overvoltage on simultaneous discharges

Simultaneous discharge, characterized by single current pulse corresponding with more than one discharge spot in one PD (partial discharge), was often observed in the partial discharge sequences, especially for the first discharge after voltage application. As it tends to be of larger magnitude than other PDs, to clarify its mechanism is important for understanding PD. In this paper, a combination measurement system of surface charge distribution and PD current was developed to identify simultaneous discharge in the samples with double-spheres plate electrode. Besides, a single square voltage pulse was applied so that only one discharge took place at a sphere electrode and the effect of residual charges can be excluded. Based on these, we investigated the effects of photoionization and overvoltage on the probability of simultaneous discharges. Photoionization effect was considered by adjusting the distance and adding a barrier between two sphere electrodes. When two spheres were separated by a glass slide, the simultaneous discharge probability was smaller than that without one. Besides, as the gap distance between two spheres increased, the simultaneous discharge probability diminished. These indicated that photoionization contributed to simultaneous discharges. Furthermore, simultaneous discharge probability became higher as the overvoltage increased, in both cases with and without glass slide.

Variation of surface charge distribution in PD sequences

Partial discharge (PD) occurring in gas void introduced during the manufacturing process of insulating constructions forms a serious threat to the life of insulation, hence its necessary to clarify its mechanism. In our paper, the surface charge measurement system based on Pockels effect was constructed to detect the residual surface charge distribution after PD occurred in a cylindrical void enveloped by PE. The real discharge current (ns level) of the PD sequences was obtained simultaneously. Depending on the combined measurement system, the current wave of a PD and corresponding discharge area were clearly observed. During continuous discharge sequences, PDs tended to take place at the sites having heterocharges at dielectric surface after voltage polarity reversed. Sometimes, the pulse current waveforms of more than two discharges showed overlapped. This phenomenon could be easily observed at initial discharges after voltage was applied. This may be due to the discharge time lag and photoionization effect. Besides, there were no discharges observed at the decreasing part of applied voltage though some area without surface charges existing.

The reliability of multi-parameter insulation diagnosis

On the basis of statistical theory, a method to determine quantitatively the reliability of multi-parameter diagnosis and to optimize the algorithm of multi-parameter diagnosis is put forward. Moreover, as an example, this method is applied to the estimation of residual breakdown voltage of generator bars and the optimized multi-parameter diagnosis algorithm is determined on the basis of actual data. Comparing with experimental data, it shows that the result of multi-parameter prediction model is more accurate than that of single-parameter prediction model when choosing the appropriate parameters group, however the quantity of parameters is not the more the better. To choose appropriate parameters for assessing insulation condition is important.

On-line condition monitoring system of medium-voltage switchgear

Switchgear is one of the key equipments of an electrical power system. Its operation condition is very important for system reliability. Mechanical failure (open rejecting, close rejecting, and mechanical malfunction), insulation failure, and contacts overheating are in the majority of all operation failures. In this paper, an online condition monitoring system for 12 kV switchgear is developed. In this system, the insulation condition of bushings, the temperature rise of the detachable contacts and the mechanical performance are detected online. For mechanical performance, the contact travel and the current of close and open release coils are detected. Thus the average speed and instant speed can be obtained from contact travel. A prototype of this online condition monitoring system has been developed. Experiments indicate that the monitoring system can perform accurate online detection of the operation condition of switchgear, and it can offer exact forecasting in time.