Haoyang You

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.

A fully enclosed, compact standard lightning impulse generator for testing ultra-high-voltage-class gas-insulated switchgears with high capacitance

At present, conducting standard lightning impulse (LI) tests in the field for gas-insulated switchgear (GIS) equipment is difficult because of the high capacitance of the test equipment and large circuit inductance of traditional impulse devices, which leads to a wavefront time T(f) ≥ 2.5 μs. A novel fully enclosed, compact standard LI generator for testing ultra-high-voltage-class GIS equipment with high capacitance is presented to solve the problem of T(f) exceeding the standard during LI voltage tests for actual large-sized equipment. The impulse generator is installed in a metal vessel filled with SF6 or SF6/N2 gas mixture at a pressure of 0.3-0.5 MPa, providing a more compact structure and a lower series inductance. A newly developed conical voltage sensor is used to accurately measure the output voltage waveform. Two test modes (via bushing docking and direct docking) for the GIS test based on the impulse generator are introduced. Calculation results show that the impulse generator can generate an LI test waveform following the present IEC standard for the test of equipment with capacitance >10,000 pF.

Reverse recovery characteristics of high power thyristors in HVDC converter valve

The reverse recovery characteristics of high power thyristor are important device physical properties influencing the reliable operation of HVDC thyristor valve. In this paper, the reverse recovery characteristics of high power thyristor are investigated from the aspects of both external circuit and internal device physics. By analyzing the depletion of carrier during the reverse recovery of thyristors, a determination method of minority carrier lifetime is proposed to construct a connection between the macroscopic reverse recovery characteristics and the microscopic device physical parameter such as minority carrier lifetime. Meanwhile, the influence of the current with a power frequency on the characteristics of reverse recovery especially on the reverse recovery time is investigated experimentally, and the relationship between external circuit and internal physical property is discussed. The results show that thyristor reverse recovery storage time is independent of sinusoidal currents under power frequency, which is approximately equal to its minority carrier lifetime in the base region of high power thyristors; Finally, the influences of reverse recovery characteristics on the protection strategy of thyristor unit are analyzed, and the related methods of thyristor unit routine test are also discussed. This work will be beneficial to the operational maintenance of thyristor valve in power system.

Study on insulation characteristics of GIS under combined voltage of DC and lightning impulse

Operating experience shows that residual DC voltage remains in GIS bus bar for a long time after opening of disconnectors. This DC voltage will not only cause accumulation of charges and contaminants on the insulator surface, but will also easily lead to motion of free metallic particles in SF6 gas gap, weakening the insulation ability of GIS. At the time of disconnectors reclosing, impulse voltage could be generated in the bus bar and superimpose on the pre-existing DC voltage. As a result, GIS will be under combined voltage of DC and impulse. This paper experimentally studies the insulation characteristics of GIS under combined voltage of DC and lightning impulse. It is found that, for non-contaminated insulators, when DC and LI voltage are of the same polarity, flashover voltage under combined voltage is nearly the same as the flashover voltage under LI alone. However, when they are of the opposite polarity, flashover voltage decreases with increasing DC voltage. In cases there are metallic particle or powder around insulator, pre-stressed DC voltage will cause adhesion of contaminants on the insulator surface and lead to decrease of flashover voltage. Specially, for the particle-contaminated insulator, it is found that the surface charge accumulation caused by DC voltage will have big influence on the flashover voltage, and the most critical condition is when DC and LI voltage are of the opposite polarity. In addition, the breakdown characteristics of SF6 gas gap with free metallic particle are investigated. It is found that under pre-stressed DC voltage, particle will do standing motion or bouncing motion. And breakdown voltage becomes lowest when particle is doing standing motion. The results of the experiments show that residual DC voltage in GIS could deteriorate its insulation ability and the combined voltage of DC and impulse is found to be a critical condition for GIS insulation. Moreover, the combined voltage is shown to be more sensitive to detect some insulation defects in GIS than applying impulse voltage alone, especially for free metallic particles, which are commonly found in GIS but up to now no effective detection method is proposed. Therefore combined voltage of DC and impulse could be considered to serve as a supplement of field test for GIS.

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.

Reverse recovery time characteristics of high power thyristors

The reverse recovery characteristics of the high power thyristor becomes increasing important to the converter valve designers as commutation failure often occurs in its reverse recovery process. Therefore, the recovery protection of converter valve is provided by thyristor control unit. The design of thyristor control unit is associated with the relationship between reverse recovery characteristics and external operating conditions. In this paper, the thyristor reverse recovery characteristics has been investigated to determine the reverse recovery time based on analyzing and testing the reverse recovery process of thyristor. Various types of recovery as well as some important circuit influences on the recovery characteristics of high power thyristors are considered. The effects of peak current during conduction, conduction pulse width, and snubber circuit are presented. Thereafter, the influences of thyristor reverse recovery time on the design of thyristor control unit have been investigated.

The research of discharge dispersion of SF 6 breakdown voltages with the size of electrodes under lightning impulse

Generally, as using the up-and-down method to research the discharge character of SF6 under impulse voltage, the requirement for dispersion of breakdown voltage is less than 3%, which guarantees the reliability of experimental data. But it is found that the dispersion of SF6 breakdown voltage, comparing with other gases, is usually large and even larger than 5%. This research has been still lack of systematic studies. Therefore, the changing law of SF6 breakdown voltage dispersion with electrode sizes and gas pressures under positive and negative standard lightning impulse (LI) is discussed in this paper. The results show that when gap length d=33 mm and radius of electrode r=40, 15, 8 mm, the dispersion is lower as the pressure grows. The changing trend of dispersion, as the field non-uniformity factor grows, is alike “U” curve trend under positive impulse, which is declining at first and then growing. And the changing trend of dispersion is alike inverse “U” curve trend under negative pulse, which is declining at first and then growing.

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.

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.