Wenrong Si

Partial discharge characteristics over differently aged oil/pressboard interfaces

The choice of the materials used as the dielectric media in large transformers continues to be a combination of pressboard and mineral oil. However, a number of failures in the field have been attributed to the surface discharge or creeping discharge over the oil/pressboard interface. This paper reports on an experiment to study surface discharge over oil/pressboard. The TJ (triple-junction) model was used to study the characteristic of partial discharge (PD) from inception to flashover. The max PD charge, pattern, frequency and current pulse were measured during the flashover process. Four oil/pressboard specimens with different aging degrees were used and their influence on the flashover process over oil/paper interface was studied in this experiment. The results indicate that the max PD charge and PD frequency increase with the increase of applied voltage. The oil aging has great influence while the pressboard aging has little influence on PD characteristic over oil/pressboard. The PD pattern and current pulse shape show great difference for various applied voltages. The results therefore suggest that the PD detection on the oil/pressboard can be used effectively to implement the early monitor of surface discharge thereby to avoid the formation of surface flashover.

Measurement and simulation of partial discharge in oil impregnated pressboard with an electrical aging process

The continuous test on oil impregnated pressboard insulation with internal void defect was developed and the phase resolved partial discharge (PRPD) pattern of partial discharge (PD) signals during the electrical aging process was measured. Two different void structures which have different void volume were used in this experiment. It shows that the PD pattern could be classified into five stages and a great diversity in the first four stages is observed. The larger void volume leads to larger PD magnitude. The computer numerical simulation model which is based on a physical discharge process was used and the causes of PD pattern change were interpreted by comparison with computer numerical simulation results. The initial values and change tendency of gas pressure and surface conductivity were determined through experiment. The model parameters in different stages have been studied as well as the insight into the physical changes in the void during electrical aging. The results provide rules for the identification of the electrical aging stage through the partial discharge measurements.

Investigation of a comprehensive identification method used in acoustic detection system for GIS

Nowadays, the acoustic detection is widely used for defect diagnosis of gas insulated substations (GIS) in normal operation and factory tests. In this paper in order to develop a data analyzer for acoustic detection system to make an assistant diagnosis, the characteristic of acoustic signals generated by different artificial defects such as protrusions, floating shield, void in spacer and bouncing particles are investigated. Some meaningful parameters behind the detected acoustic signals are extracted and discussed, which are used to distinguish background noise, partial discharge (PD) phenomena or bouncing particles. Based on those works, a comprehensive identification method realized by processing the acoustic pulse sequences qi, ( Δ ti, qi) and (ti, qi) is introduced, which gives a recognition result with noise, PD type or bouncing particles. For the sequence (ti, qi), the backpropagation artificial neural network optimized by genetic algorithm (GA-BPANN) is used as a classifier based on the fingerprint consisting of 24 operators, which are derivate from typical 2D histograms of phase-resolved partial discharge (PRPD). And with considering the trigger source may having a phase difference from the working voltage, identification with phase compensation (IPC) is used as a try to deal with the challenge. Experimental results show that the comprehensive identification method is practical and effective.

Characteristics and Development Mechanisms of Partial Discharge in SF 6 Gas Under Impulse Voltages

It is well known that impulse voltages strongly affect the reliability of gas-insulated switchgear (GIS) under actual operational conditions. The alternating-current (ac) withstand voltage test and its corresponding partial discharge (PD) measurement are inadequate for the detection of all the hidden flaws in GIS. Such inadequacy can be compensated by additional impulse withstand voltage test and its PD detection measurement. However, the bulkiness and inefficiency of aperiodic impulse voltage generator make it unsuitable for on-site application. Therefore, in practical tests, oscillating impulse voltage (OIV) is generally adopted instead of aperiodic impulse voltage. In this paper, we measure the PD inception voltage (PDIV) and the breakdown characteristics of a needle-plate gap in SF6 gas under four types of impulse voltages by electrical and optical methods. The 50% probability PDIV is analyzed as a function of gas pressure. The physical mechanisms of the PD under the four impulse voltages are discussed. The PD characteristics under ac voltage are investigated for comparison. Experimental results reveal that the PD development process under the two aperiodic impulse voltages was mainly dominated by the precursor mechanism which consists of streamer and leader stages, while the PD propagation under the two OIVs was affected by the displacement current produced by oscillation. The difference of the PD characteristics under the aperiodic impulse voltage and OIV, and ac voltage are also presented.

Coupled Electromagnetic-Thermal Modeling the Temperature Distribution of XLPE Cable

Current carrying capacity of power cable should be determined by the maximum operating temperature which the insulation can withstand. The temperature rise in cable is mainly due to Joules loss in conductor and induced eddy loss in sheath. In this paper, based on the coupled electromagnetic-thermal theory, the power losses calculated in electromagnetic field analysis are used as the input data of thermal analysis to predict the temperature rise in different layers of cable. Using the simplified cable model, the temperature distributions in single phase and three phases cable with different arrangements are calculated and compared. The results show that three phases cable get the higher temperature distribution according to the proximity effects between the different phases compared to the single phase cable. As a result, in application, the optimized arrangement of cable can be applied to make the temperature rise of cable as low as possible.

Feature extraction methods for time frequency energy distribution of PD pulse

Feature extraction methods for time-frequency energy distribution (TFED) using moment, marginal and Renyi information are studied in this paper. They are developed by combination of TFEDpsilas own characteristics with mathematic statistic and information entropy. The Wigner-Ville distribution (WVD) of a simulated PD pulse is chosen as the subject investigated to illustrate those methods, which gives the result of time and frequency moments, time and frequency marginals and Renyi information for the TFED. The results show that those methods can exactly and easily make feature extraction for the TFED of PD pulse, which can be used to PD pattern recognition and in-depth study of PD mechanism in the electrical insulation system.

Simulating Propagation Characteristic of UHF Signal for PD Monitoring in Transformers by FDTD Technique

Partial discharge (PD) in transformer can be detected on-line by UHF technique. However, the relationship between the UHF signal received by UHF sensors and the PD sources is not very clear. It is very important studying the propagation properties of the PD signal. In this paper, the electromagnetic waves emitted by PD sources are investigated. The finite difference time domain (FDTD) technique is used to model wave transients. The model consists of a tank and a PD source. The wave transients at different time and position are simulated. The results show the propagation transients of the PD signal in the tank and wave propagation attenuates with distance between the source and the sensors.

UHF characteristic of PD in oil-paper insulation under DC Voltage

A test system with UHF and UWB detection technologies developed to measure PD under DC voltage is introduced in this paper. According to the typical defects in oil-paper insulation of converter transformer, six corresponding PD models are designed. The test results of PD UWB detection under DC voltage show that the PD current pulses have a very short rise-time, which are capable of radiating UHF signals at frequencies up to at least 300 MHz, and these electrical magnetic waves can be picked up by the UHF sensor. The DBM spectrum analysis is made for all UHF signals of the models, which shows that they differ with various defects, and the maximum amplitudes of the energy distribution against the UHF are also different. These indicate the on-line monitoring for the UHF PD signal under DC voltage should be realized utilizing the narrowband filter with adaptive center frequency.

Study of anti-interference technology for UHF PD-detection in power transformers

In this paper, the anti-interference technology for UHF PD-detection in power transformers is described. When we detect PD on power transformers using an UHF PD online monitoring system made by us, several kinds of interference signals are found in substations in China. The interference signals may cause that the sensitive of the measuring system get lower and the system give out a wrong indication. The interference signals include GSM mobile signal, TV and communication and some other signals with certain frequencies, partial discharges of other power equipments that are near the equipment to be measured. According to the characteristic of the interference signals, the solving methods are found to avoid to be influenced during the measurement. The new UHF PD online monitoring system with the methods of anti-interference can get rid of every pulse that is not created by the power transformer to be measured. The results show the methods are available and effective.

Propagation characteristic study of partial discharge UHF signal outside transformer

In the UHF detection of power transformers, the main method is detecting the UHF signals through UHF sensor in the transformer. The sensibility of this method is high but mounting way is trouble. Field detection and lab testing show that the UHF signals can diffract from the transformerpsilas joint. Some PD detector using external detection has used in field. But, the difference of signals in transformer and external transformer is not clear. In this paper, the feasibility of using external UHF sensor for detecting partial discharge activities is discussed, and then external propagation characteristic of UHF E-M wave is studied by lab experiments. Wavelet packet analysis of signals show the high frequency of external signals is bigger than inside signals. The relationship of distance of joint and UHF sensor is studied in this paper. The results show that UHF signals could leak outside the transformer through the joints after multiple refraction and reflection in the transformer tank. An antenna outside the tank could receive the signals. Study result will supply some signal acquisition direction for the UHF detection transformer PD.