Jingfeng Wu

Dynamic and flexible effectivity evaluation system of live test approaches for power equipment

Ever since Chinese electric power reform in 2002, different live test approaches were introduced and applied in relative industries. There were at least 26 live test approaches applied in the field of power transformation, power transmission and power distribution. The essay introduced a comprehensive dynamic and flexible effectivity evaluation system, which helped to evaluate the strengths and weaknesses of different live test approaches.

Novel and necessary approaches ensuring statistically effective partial discharge testing for equipment in service

Serving for condition-based maintenance, researchers and engineers, who conducting, designing and researching partial discharge testing for equipment in service, used to focus on PD (partial discharge) locating. The novel idea of test sensor (instrument) locating was introduced. Locating the sensor instead of PD source would ensure PD testing results statistically effective. The concept of PD mapping and PD transmission zone were also introduced. The former takes effective sensor locating as prerequisite and the latter functions during PD locating. Available approaches and applications of sensor locating and PD mapping were demonstrated. The succession relationship of sensor locating and PD source locating were analyzed and the vistas of both were discussed.

Research on some related mechanisms of corrosive sulfur problems in mineral oils

Since a number of failures have happened on transformers and reactors due to the corrosive sulfur problem, especially for Dibenzyle-disulfide (DBDS) in mineral insulation oils, many researchers have carried out a series of studies on such issue. In this paper, some related mechanisms of corrosive sulfur problems in mineral insulation oils are introduced in detail. Such as the mechanism of DBDS how to change into copper sulfide (Cu2S), and which results are proved and checked by gas chromatography equipped with mass spectrometry (GC-MS), when there is copper sulfide deposited, what the surface color and microstructure of copper and insulation paper would be change are shown via scanning electron microscope (SEM), and the mechanism of Cu2S how to influence the dielectric properties of oil immersed insulation pressboard, together with the changing tendency of dissipation factor, permittivity and breakdown strength etc are introduced. Besides, the mechanism that how oxygen in oil to influence the Cu2S migrating from the copper surface into oil or insulation paper is given, and the activation energy of DBDS reacting with copper is obtained by Arrhenius equation, which is about 43.73 kJ/mol in air condition and 98.61 kJ/mol in nitrogen condition, at last, the mechanisms that how to inhibit such corrosive phenomena in mineral insulation oil via adding some passivators are introduced as well.

Research on anti-interferences methods in the PD detection on power transformer

The paper concerns improvement of the acoustic emission (AE) methods applied in diagnosing of insulations systems of electric power transformer. During detection of AE signals emitted by partial discharges (PD) occurring in insulation systems of power transformer, there exist different forms of interference sources, which may have a direct impact on the correct evaluation of the measured signals obtained using the AE method. Thus, analysis of the impact of interfering signals on the PD detection and classification is performed. In this study, a correlation-based wavelet base selection method is introduced. The proposed wavelet analysis approach capable of conserving a significant portion of the original signal, while the threshold value determined based on the relative difference between the original and noisy signal. The proposed method is applied to PD signals that obtained in the field. With the comparison of other de-noising methods, the de-noised PD signals indicate that the proposed method yields higher reduction in noise levels.

Mechanical fault diagnosis system based on acoustic feature analysis in gas insulated switchgear

Gas Insulated Switchgear has been widely used because of its high reliability. But in the long run discharge faults and mechanical faults often happen to GIS, early GIS fault detection are mainly for GIS partial discharge fault detection, but this paper focuses on the study of mechanical vibration fault. Based on acoustic signal, the paper introduces the common mechanical fault types and fault mechanism; and use the whole sound acquisition method to extract different mechanical fault data in the field; Then analysis the signal in the time domain, frequency domain and inverse frequency domain and summarize the rules of acoustic characteristics of different faults; Finally, the general and software of the system are designed and the system structure diagram and software flow chart and human computer interaction interface are given.

Discharge characteristics of SF 6 gas with and without insulators under VFTO and lightning impulse voltage

A generating system of very fast transient overvoltage (VFTO) and lightning impulse (LI) was established. The effects of voltage polarity, gas pressure and voltage waveform on discharge characteristics of SF6 gas with and without insulators under VFTO and LI were investigated. Experimental results show that the 50% breakdown voltage of rod-plane gap under VFTO is higher than that under LI in positive polarity. However, for negative polarity, the breakdown voltage and voltage-time curve under VFTO fall below those under LI at the pressure of 0.5MPa. Moreover, the polarity effect of the rod-plane gap under LI is reversed at a certain pressure. The flashover voltage of defective insulator under VFTO can be lower than that under LI by near 10%. The flashover voltage is the lowest when the particle is attached on some position nearby high voltage electrode of insulator. The decrease of pulse rise time will increase the discharge channel number. It is concluded that the polarity effect is related with the distortion effect of ion clusters formed by SF6 on electric field. The steepness and rise time of impulse play an important role in the initiation and further development of leader discharge in SF6 and on insulator surface.

Influence of Axial Magnetic Field on Cathode Plasma Jets in High-Current Vaccum Arc

The cathode plasma jets in vacuum arc and the effect of magnetic field on them have been studied for several years and these studies are helpful to understand the arc characteristics and improve the controlling effect of magnetic field on arc plasma. In this paper, the experiments of the cup-shaped axial magnetic field (AMF) contacts were conducted in which a Helmholtz coil was introduced to generate different external imposed AMFs. It was found that the appearance of arc plasma was changed by variation of the external imposed AMF. If the composite AMF in arc column increased gradually, the obvious individual cathode plasma jets were detected, the arc constriction decreased, and the arc stability increased. Meanwhile, the inclination of cathode plasma jets was observed and the inclination direction was found to be consistent with the magnetic field vector direction which is closely related to the composite AMF. By variation of the external imposed AMF and its direction, the inclination direction of cathode plasma jets also changed. The criterion to detect obvious cathode jets in arc column is obtained that at the contact peripheral where R> 80.5% R0 (R0 is the contact radius), the AMF strength is no less than 4.5 mT/kA. It was also found that if the composite AMF in interelectrode gap points to different directions at a certain moment, it may lead to the transfer of arc column from the center to the edge of the interelectrode region and the arc instability until when the composite AMF in the interelectrode gap points to the same direction. It can be concluded that the appearance of individual cathode plasma jets can be manipulated by the variation of AMF, which is helpful to study the interaction between arc plasma and electrodes, especially the anode, and improve the interruption performance at higher current.

Field application of partial discharge detection and location on 750kV reactor

In this paper, a series of problems of detecting partial discharge (PD) and locating PD sources using acoustic signal on high voltage reactor are systematically discussed. The complex circumstance of acoustic propagation in reactor and the diffraction influence on the acoustic velocity are fully considered. Based on abnormal signals detected on 750kV reactor in one substation of Shaanxi power grid and time-based distance calculation method, the PD source location was presented according to the acoustic emission (AE) associate with high frequency (HF) earthing current detection. Finally, the effectiveness of previous analysis was proved by the internal inspection.

The Influence of insulator shapes to the UHF signal propagation in GIS

The UHF detection method presently is one of the domestic and foreign research methods about partial discharge of GIS. But because the complexity of the internal structure of the GIS, in the dissemination process will be inevitably resonated, distorted and attenuated which makes the complexity of detection and evaluation increase. This paper studied the influence of insulator shape to UHF signal propagation in GIS. And the finite difference time domain method (FDTD) is used to simulate on the construction of a 220kV single phase bus model with the different shape of insulators. This paper gets the results: Firstly, the influence of the basin-type and the disc-type insulator to the voltage peak value and the energy of the UHF signal inside the GIS are basically the same, but the high-frequency element of the signal in back of the basin-type insulator is a little bit increased. Secondly, comparing with the disc-type insulator, the outside leakage UHF signal between 1.8GHz and 2.4GHz slightly attenuates.

Anti-interference measures for GIS partial discharge detection on field

It is well known that there is much electromagnetic interference on the gases insulated switchgear (GIS) operation environment, and which would bring some difficulties to distinguish the actual partial discharge signals with those interference. In this paper, three kinds of measures are introduced in detail, which have been proved to be effective to realize anti-interference for GIS partial discharge detection on field. The first one is aluminum foil coated shielding, the second one is acoustic-electric joint measuring, for which could verify each other, i.e. once there is a partial discharge defect in GIS, the ultrasonic and UHF signals should be appeared simultaneously, the third one is time difference method, which could judge the measuring signals are from GIS internal or outside. The three above mentioned measures could do some favor for the electric engineers who will carry out any similar partial discharge detection on field.