Yifan Qin

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.

Wormholes effect in flashover process of alumina filled epoxy resin surfaces in SF 6

Alumina filled epoxy resin is important insulating material, which discharge characteristics are greatly determined by its material properties. In this paper, the surface flashover characteristics of the epoxy resin with various alumina contents (250- 400 phr) in 0.4 MPa SF6 are studied. The short-term and long-term withstand voltages and the application times are measured, and then the V-T characteristics are compared accordingly. The withstand voltage of the epoxy resin with certain alumina content decreases exponentially with time and the long-term withstand voltage is about 20-30% lower than the short-term withstand voltage. The discharge damages to alumina filled epoxy resin material are also studied through the carbonization traces and the microcosmic damage patterns. It is found that the epoxy resin with 350 phr alumina has the lowest short-term and long-term withstand voltages and most serious carbonizations according to the experimental results. The “wormholes” formation in the discharge and its effect in the flashover process are analyzed and discussed based on the carbonation traces and the microcosmic patterns. It is found that the flashover paths shift from creeping-discharge-based to bulk-breakdown-based due to wormholes effect with the increase of alumina content and the critical alumina content is about 300-350 phr.

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].

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 metallic particle on flashover characteristics of post insulator in SF 6 under negative DC voltage

The flashover of insulators is a bottleneck for the development of gas insulated transmission line (GIL). The operation experience of GIL shows that metallic particle plays an important role in flashover of the insulator. An experiment platform was established to study the influence of gas pressure on the flashover characteristics of the post insulator in SF6 under negative DC voltage. The impacts that the position and size of a metallic particle had on flashover characteristics were also studied. The distribution of electric field was processed by the finite element analysis software. The results indicate that the flashover voltage reaches the maximum value with the increase of gas pressure. This phenomenon is connected with the distribution of space charge. The flashover voltage drops rapidly when the metallic particle is attached to its high voltage electrode. The reduction of flashover voltage will become slow when the metallic particles length extends a certain value. The metallic particle will cause the greatest threat to insulation level when it is attached to the electrode of post insulator. The flashover voltage increases at first and then decreases when the distance between the metallic particle and high voltage electrode increases. The process of flashover was analyzed in the aspect of distribution of electric field.

Discussion on the control standards of moisture content in SF 6 electrical equipment

SF6 is widely used in high-voltage equipment due to its excellent insulation ability. However, when the moisture content in SF6 exceeds a certain value, its insulation could be damaged, threatening the normal operation of high-voltage equipment. Therefore its of great importance to establish proper control standards on moisture content in SF6. In this paper, the influence of moisture content on flashover voltages of cylindrical insulator models in SF6 is studied by controlling the moisture content in a specially designed organic glass chamber, and then the feasibility of three common moisture controlling methods are discussed. The results show that controlling moisture content in the form of volume fraction can directly limit the absolute content of water vapor and prevent the formation of harmful decomposition products. However, the measured values of volume fraction at different temperatures should be rectified to a common temperature, making it inconvenient to use; the form of relative humidity can reflect the condensation margin of water vapor and could be used without consideration of temperature, but it cannot directly show the absolute content of water vapor, making it difficult to control the production of discharge products; the dew point temperature cannot reflect the real condensation temperature of water vapor, so the method of controlling dew point temperature below zero degree is not scientific. In conclusion, 200 uL/L and 15% are suggested as the limiting values in the form of volume fraction and relative humidity according to the experimental results, which can serve as reference for the revision of the existing moisture control standards.

Firefly and Standing Still phenomena of free conducting wire-type particles in SF 6 under DC voltage

Compared to spherical particles, which are the most prevalent subjects studied by other researchers, free wire-type metallic particles have unique motion phenomena in electric fields under DC voltage, playing an important role in insulation flaw detection of power equipment. However, this is rarely researched sufficiently. Thus, the enclosed experimental chamber and the observation systems are specially designed and motion characteristics of the wire-type particles, with diameters 0.25 mm, 0.4 mm, 0.65 mm and lengths 1 mm, 2 mm, 3 mm, 4 mm, are studied. The results show that probabilities of Firefly and Standing Phenomena increase with lengths of wire-type particles increasing, and the phenomena are more likely to appear on the negative electrode. When the pressure decreases and radiuses of particles fall, probabilities of Firefly and Standing Phenomena increase. In the case of 1mm diameter, there are not any special phenomena under various conditions and the motion of particles is disorder. Lastly, the explanation of experimental phenomena is done in the view of space charge.

Turn-to-turn insulation breakdown characteristics under non-standard lightning impulse voltages

The non-standard lightning impulse voltages have variable front time, duration time and may have high-frequency components superimposed on the waveforms due to the reflection in the substation and other factors. These surges propagating into the transformer winding may cause resonance and excite high potential gradient, consequently leading to the breakdown of the winding insulation. In this paper, two electrode models are established to simulate the typical insulation configuration of windings in oil-immersed transformers. The influence of front time and oscillation frequency of impulse voltages on the breakdown characteristics are studied. The experimental results indicate that the breakdown strength increases with the decrease of front time. For the turn-to-turn insulation, its time lag is greatly influenced by the sickness of insulation layer and front time. Moreover, the sensitivity of time lag to the front time enhances with the increase of number of insulation layer. The turn to turn insulation can withstand a much more severe oscillation lightning impulse in comparison to standard lightning impulse. In addition, its 50% breakdown voltage increases with the increase of the oscillation frequency. Furthermore, while the total insulation paper thickness of turn-to-turn remains the same, the breakdown strength is reinforced as the number of insulation layers increasing.

The reversal of SF 6 /N 2 gas mixtures polarity effect under lightning impulse

In recent years, SF₆/N₂ gas mixture gains much popularity as an alternative insulating medium of pure SF₆ gas among gas-insulated equipment because of the greenhouse effect and poor economy effectiveness of the latter. This paper investigated the breakdown characteristics of SF₆/N₂ gas mixture with low SF₆ mixing ratio in slightly non-uniform electric field which is similar to the gas-insulated metal-enclosed transmission line (GIL) under positive and negative lightning impulse, and the reversal of the polarity effect was found. The research indicates that the breakdown voltage under negative lightning impulse is higher than that under positive lightning impulse in low gas pressure, but when pressure exceeds a critical value, the result converses. The rise of SF₆ mixing ratio will lead to the decrease of the critical value of gas pressure. The result is significant for guiding the application of the electric power equipment with SF₆/N₂ gas mixtures such as GIL, and it also plays a vital role in the future study of gas discharge mechanism of SF₆/N₂ gas mixtures.