Jingtan Ma

Follow phenomenon of breakdown voltage in SF 6 under lightning impulse

Follow Phenomenon, defined as a series of continuous breakdowns or withstands without sufficient time lag of successive applied voltages, is observed during the research of SF6 discharge characteristics under lightning impulse. Follow Phenomenon of SF6 gas plays a decisive role for the insulation recovery time between continuous discharges, but little relevant research has been reported before. In this work, Follow Phenomenon as a function of gas pressure, time lag of applied voltages in quasiuniform and non-uniform field, under positive and negative lightning impulse is studied. It is found that, when the time lag t=1 min, Follow Phenomenon is more obvious under positive lightning than that under negative lightning, and it becomes more pronounced under larger pressure; as time lag t increases, the increasing speed of Follow Phenomenon with pressure becomes lower. When time lag t=5 min, the measure uncertainty influenced by Following Phenomenon could be avoided when P≤0.2 MPa in quasi-uniform field and P≤0.3 MPa in non-uniform field. As pressure rises, a longer time lag is needed to eliminate the influence of Follow Phenomenon. At the end, analysis is made which shows that Follow Phenomenon is caused by the detachment of negative ion formed at previous discharge processes and by “the Sweeping effect” of effective initial electrons.

Motion and discharge characteristics of metal particles existing in GIS under DC voltage

Free particles formed in the discharge process or equipment aging severely threaten the operation security of GIS, which draws great attention of researchers. However, seldom research focuses on disc-type particle, which is the majority of shaped particles in practical GIS. In this paper, an elliptic coordination system is built for obtaining the theoretical lift-off electric field of disc-type particles, and experimental system and observation system are established for studying motion behavior and discharge characteristics of metal particles. Results show that for spherical particles, while the distance between two particles d<9rs, the downward force caused by impurities on electrodes can be neglected; for wire-type particles, as lengths of particles increase, lift-off electric fields and probabilities of Firefly tend to be larger. Besides, lift-off electric fields grow exponentially with the increases of radius. When length of particle exceeds 3 mm, the gap between practical values and theoretical values becomes larger with the decrease of radiuses; for disc-type particles, with the increase of radiuses, dispersions of lift-off electric field becomes larger; the difference between practical and theoretical values broadens with the fall of radiuses. Besides, lift-off electric fields of disc-type particles are higher than that of wire-type particles with same qualities and with the increase of radius, probabilities of Firefly increase in an unapparent speed in comparison to wire-type particles; in the end, discharge processes in SF6 gap are illustrated. When particles are close to the bottom electrode, the lower end of particles will breakdown first. Then the top end of particles will form streamer discharge. With the continuous development of streamer, the gap will eventually breakdown.

On-site standard lightning impulse test for 1,100-kV gas-insulated switchgear with large capacitance

Gas-insulated metal-enclosed switchgear (GIS) was developed in the mid-1960s [1]-[5]. Compared with traditional air-insulated switchgear, the GIS concentrates the bus, isolating switch, circuit breaker, lightning arrester, transformer, and many other devices in an enclosed grounding metal case. It has many advantages such as reliability, compactness, long maintenance cycle, and low environmental impact, which have led to it being widely used in electrical power systems [6]-[16]. However, increased operating voltages and more extensive power grids have given rise to increased GIS failure rates [17], [18].

Note: A phase synchronization photography method for AC discharge

To research discharge physics under AC voltage, a phase synchronization photography method is presented. By using a permanent-magnet synchronous motor to drive a photography mask synchronized with a discharge power supply, discharge images in a specific phase window can be recorded. Some examples of discharges photographed by this method, including the corona discharge in SF6 and the corona discharge along the air/epoxy surface, demonstrate the feasibility of this method. Therefore, this method provides an effective tool for discharge physics researchers.

Simulation and motion analysis of spherical free conducting particle between coaxial electrodes

The trend of development of Gas Insulated System (GIS) recently includes: miniaturization, intelligentization and high reliability. This needs a higher insulation performance of GIS. Free conducting particle in GIS is a main reason that causes the degradation of GIS insulation. Free conducting particle will bring a high risk to GIS insulation when it approaches to the high voltage conductor or reaches the disc-type insulator surface. Particle in radial non-uniform electrical field is apt to move along the radial direction in GIS, and probably moves into the high insulation risk region. According to the distribution of the electrical field between coaxial electrode systems, the dynamic model of spherical particle in axial non-uniform electrical field under AC voltage is amended. The motion phenomenon of particle is explained by solving the dynamic equations. Gravitational force, Coulomb force, electrical gradient force and gas viscous force are considered. Improving the applied voltage can increase the motion distance in vertical direction. The criterion can explain the incremental quantity of the motion onset voltage of particle in uniform field under AC voltage than it under DC voltage. Observations indicate that voltage exerts an incremental effect on maximum height. By analyzing improved dynamic model of spherical particle, another novel criterion to judge the penetration of the motion of spherical particle in uniform field under AC voltage is brought forward, as well.

Defect diagnosis of HVDC bushings based on SF 6 decomposition and partial discharge characteristics

It is of great significance to detect different defect types of HVDC bushings effectively for the safe operation of power system and the condition maintenance of power transformers. In order to simulate the insulation defects of the main insulation of the HVDC bushings, five samples of the main insulation of the HVDC bushing with a maximum DC voltage of 30kV are fabricated by a vacuum impregnation process with an intact one and four defective ones. During the experiment, 30 kV DC voltage is applied to each sample. SF6 decomposition is recorded at every 12 hours, reaching a maximum of 96 hours. And the c(SOF2)/c(SO2F2) and c(SOF2+SO2F2)/c(CF4) are selected as statistical characteristic quantities of SF6 decomposition. After the partial discharge waveform is stabilized, the partial discharge is recorded for 30 seconds. And the skewness 5k, the steepness Ku and the local peak number Np are selected as the statistical characteristic quantities of the map. At the same time The results showed that from the SF6 decomposition, all defects except for capacitor screen breakdown can be diagnosed. However, there are significant differences in the shapes and statistical features of the partial discharge spectra when different types of defects existed in the main insulation of the HVDC bushings. Therefore, compared with SF6 decomposition, the spectrum of partial discharge under DC voltage can be used as the basis for the identification of defect type, and provides a reference for the condition maintenance of the HVDC bushings.

Influence of trace SF 6 on discharge characteristics of SF 6 /N 2 gas mixture under lightning impulse

As one kind of synergistic effect gas, the discharge characteristics of SF<inf>6</inf>/N<inf>2</inf> gas mixtures would be influenced significantly by trace amounts of SF<inf>6</inf>. That is, the discharge characteristics of pure N2 is quite different from that of SF<inf>6</inf>/N<inf>2</inf> gas mixtures with relatively low content of SF<inf>6</inf>. For the application of SF<inf>6</inf>/N<inf>2</inf> gas mixtures in power system equipment, this paper studied on the discharge characteristics of SF<inf>6</inf>/N<inf>2</inf> gas mixtures with low SF<inf>6</inf> mixing ratio in extremely non-uniform electric field under positive and negative lightning impulse. Meanwhile, the characteristic of SF<inf>6</inf>/N<inf>2</inf> gas mixtures were compared with that of pure N<inf>2</inf> and SF<inf>6</inf>. The experimental results indicate that the 50% breakdown voltage increases linearly with the rise of gas pressure under negative lightning impulse, and the rising rate of breakdown voltage with pure N2 is greater than SF<inf>6</inf>/N<inf>2</inf> gas mixtures even greater than pure SF<inf>6</inf> gas. At high gas pressure, the 50% breakdown voltage of pure N2 is higher than that of SF<inf>6</inf>/N<inf>2</inf> gas mixtures, that is to say, the negative synergistic effect appeared. Under positive lightning impulse, the influence of gas pressure on 50% breakdown voltage is weaker, but the N-curve characteristic appeared because of the effect space charge. Meanwhile, the negative synergistic effect also appeared under positive lightning impulse at high gas pressure. The experimental results indicate that the breakdown time delay of pure N2 is much longer than that of SF<inf>6</inf>/N<inf>2</inf> gas mixtures with 1% SF<inf>6</inf> mixing ratio because of the obvious difference between the formative time delay of N2 and SF<inf>6</inf>/N<inf>2</inf> gas mixtures. The phenomenon of breakdown time delay demonstrates that the discharge form of N<inf>2</inf> is different from that of SF<inf>6</inf>/N<inf>2</inf> gas mixtures, that is to say, the streamer propagation changes to streamer-leader propagation.

Evaluation of the Martin empirical formulae for transformer oil: Statistical meaning of the time parameter

To study the statistical meaning of the time parameter t63% in the Martin empirical formula, we took Kunlun 25# transformer oil as the liquid insulating dielectric and utilized an impulse voltage generator, the impulse width of which could range from 50 ns to 1000 µs, along with the corresponding measuring equipment. The relationships between the breakdown voltage, the breakdown time lag and the discharge probabilities were acquired through experiments under impulses with quasi-uniform field. The results show that the discharge probabilities of different voltage levels under the fixed impulse waveform and the breakdown time lag under the same voltage level both could be well fitted to the three-parameter Weibull distribution functions, whose shape parameter was a constant only correlated to the electrode geometry. Meanwhile, on the basis of the Weibull distribution functions and corresponding impulse waveforms, we defined the effective duration and show the statistical meaning of the time parameter t63%.

Flashover characteristics of basin-type insulator with metallic particles on its surface under standard lightning impulse

The basin-type insulator is one of the most important parts in SF6 gas insulated switchgear (GIS). Flashover of basin-type insulators will cause serious harm to the electrical system. The fact that metallic particle attached to the surface cannot be detected in the delivery test under the standard lightning impulse (SLI) will pose potential threat to GIS. This paper studied flashover characteristics of the basin-type insulator with metallic particle contamination attached to its high voltage electrode in SF6 under SLI. An 1800 kV lightning impulse generator and a specialized pulse-voltage divider were used in the study. The results show that the flashover voltage drops rapidly when the metallic particle is attached to its surface. At high pressure, the basin-type insulator will be more sensible to the metallic particle. The metallic particle may cause danger to the system even its size is small. The reduction of flashover voltage will become slow with the increase of the metallic particles length. The gas pressure has little effect on the flashover voltage rate when the metallic particle exceeds a certain length. The flashover voltage under negative polarity of SLI is higher than that under positive polarity. Polarity effect will be obvious when the gas pressure is low. The discharge development process of basin-type insulator has been studied on the aspect of space charge accumulation at last.

Influence of electric field non-uniformity on discharge characteristics in SF 6 /N 2 gas mixtures under power frequency voltage

In recent years, SF6/N2 gas mixtures with low SF6 mixing ratio was getting used as insulating medium in gas-insulated metal-enclosed transmission line in order to replace pure SF6. For the application of SF6/N2 gas mixtures in power equipment, this paper studied on the breakdown characteristics of SF6/N2 gas mixtures with low SF6 mixing ratio in different electric field non-uniformity under power frequency voltage, and the characteristic of partial discharge initial voltage was studied in non-uniform electric field. In order to study the breakdown characteristic of SF6/N2 gas mixtures under high voltage grade, a fully enclosed power frequency voltage test device was set up. The sphere-plane and rod-plane electrodes were used to simulate the slightly non-uniform field and local-field enhancement in GIS or GIL. The research indicates that the breakdown voltage of SF6/N2 gas mixtures increases linearly with increase of gas pressure in slightly non-uniform electric field. With the increase of the electric field non-uniformity, the linearity of breakdown voltage weakens and the stable corona discharge appears. Because of the space charge effect, the N-curve characteristic of breakdown voltage appears in SF6 and its mixtures. When electric field non-uniformity exceeds a certain value, the stable corona discharge appears before the electric breakdown of electrodes gap. But the breakdown occurs immediately with the appearance of corona when the gas pressure exceeds a certain value. Meanwhile, the synergistic effect was strengthened and then weakened. And the amplitude and range of N-curve extend when electric field non-uniformity increased.