Lingli Zhang

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.

Influence of spike defect on the impulse breakdown characteristics of SF6 gas gap in GIS

By using 1 MV Marx generator and a 220 kV GIS bus, an experiment platform has been established, and a stainless steel needle is applied to simulate spike defect, so as to study the influence of pressure, wave front time and electric field on 50 % breakdown voltage (U50) and voltage-time (V-t) characteristics of SF6 gas gap with existence of a spike defect on the bus of GIS under double exponential impulse voltage. When wave front time is short, 50% breakdown voltage of the SF6 gap hardly changes with various pressures, and U50%-P curve turns to be a hump shape when wave front time increases. However, pressure has no significant effect on V-t characteristics. The slope of V-t characteristics curve is negative when wave front time of applied voltage is short (0.08~0.9μs), and it will turn to be positive when wave front time becomes longer (several ~15μs). A new method to fit these V-t characteristics curves under impulse voltage with longer wave front times is proposed. It is found that 50 % breakdown voltage decreases almost linearly with the increase of electric field non-uniformity factors (f), and V-t characteristics cluster with valley-like profile have been drawn at different f. Dispersion of V-t characteristics cluster reduces and the flat region expands with the increase of f, and minimum value of breakdown voltage of SF6 gap with spike defect under impulse voltage, which is of guiding significance for engineering, can be obtained.

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 impulse wave front time on flashover characteristics of coaxial bus line of 220kV GIS

In order to find out the detection effectiveness of GIS insulation defects when suffering different overvoltages, a coaxial bus line of 220kV with a needle attached to it was established as an experimental sample to simulate the practical condition that there existed a metallic particle on the bus line of real GIS. Research was carried on with the help of 1MV steep-fronted impulse generator. By changing the wave front resistance of Marx generator, different kinds of double exponential impulse voltages could be produced. Influence of wave front time (tf) on 50% discharge voltage (U50) and voltage-time (v-t) characteristic of GIS was studied in this article. Results indicate that with tf increasing, U50 presents an increasing tendency overall; with pressure (P) increasing, when tf is short, U50 hardly changes with P, but when tf becomes long enough, U50 changing with P appears as a hump curve. As for v-t characteristic, it turns out that, with the length (l) of needle decreasing, both breakdown voltage and its dispersibility increase; its also found that the shorter the needle is, the shorter delay that turning point occurs becomes. When tf changes, shape of v-t characteristic changes too, the v-t characteristic integration of different wave front times looks like a concave basin. The above phenomena are explained in the view of the ionization, diffusion and recombination of space charges.

Research on impulse withstand test effectiveness from insulation characteristics of SF 6 gas gap with highly inhomogeneous electric field

Gas-insulated metal-enclosed switchgear (GIS) is an important switching equipment in modern power system. Since GIS equipment has a fully enclosed structure, once failure occurs, it will always be followed by a huge economic loss and an adverse social impact. On-site lightning impulse (LI) test is very important to improve the quality of the GIS equipment since it is particularly sensitive to some fixed insulation defects formed in the transportation and installation process. The on-site LI waveform, restricted by the large load capacitance and impulse generator inherent inductance, has a long wavefront time Tf compared with standard LI, called non-standard LI. The non-standard LI test has been carried out in the field. But its insulation defect detecting effectiveness is not clear. In this paper, a generating system of impulses with different waveform parameters, including double-exponential impulses with different wavefront times in the range of 0.08 ∼ 23.5 μs and an oscillating LI with wavefront time around 10 μs, was established. The insulation characteristics of SF6 gas gap with highly inhomogeneous electric field under impulses with different waveform parameters were investigated. Experimental results showed that the insulation defects detecting effectiveness of oscillating LI with long Tf around 10 μs is lower than that of standard LI. According to the voltage-time characteristic, it can be inferred that the Tf plays an important role in the insulation defect detection. Prospective voltage is introduced to evaluate the insulation defects detecting effectiveness first. The insulation defect detecting effectiveness using the impulse test is reflected in the value of the prospective voltage. The prospective voltage-time characteristics confirm the conclusion that with the increase of impulse voltage Tf, the detection effectiveness for GIS insulation is reduced. Consequently the standard or impulse LI test with Tf < 1.56 μs is to be preferred for installation detection, because it offers a greater probability of detecting insulation defects.

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.

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.

Discharge performance of conductor spikes in GIS under AC voltage

Gas insulated switchgear (GIS) is widely applied in power systems due to its excellent insulation performance. The reliability of GIS will be influenced by various insulation defects generated during the manufacture, installation, adjustment, operation and maintenance of the apparatus. Some defects, including conductor spikes, free metal particles, etc., make local electric field higher. Due to the HUMP effect of SF6 gas, the partial discharge inception voltage and the breakdown voltage of these defects are approximate to each other in non-uniform electric field under operating conditions. These defects cannot be effectively detected through partial discharge detection method as a result of this phenomenon. Different geometry of conductor spikes are used in this article, the partial discharge inception voltage and the breakdown voltage of these defects are measured by using ERA method, the results indicate that the breakdown performance is singular because of existence of space charge, and the partial discharge performance is closely related to the uniformity of gas gap.

Discussion on lightning impulse test waveform according to breakdown characteristics of SF6 gas gaps

The standard lightning impulse (LI), defined as a smooth full lightning impulse voltage having a front time of 1.2 μs and a time to half-value of 50 μs, obtained from field observation, has remained unchanged from IEC Ed. 1 in 1962 through to Ed. 3 in 2010. LI test plays an important role in the equipment design and insulation defects detection. In order to evaluate the reasonability of the standard LI waveform from the point of insulation characteristics, breakdown characteristics of GIS under LI with different wavefront times Tt and wavetail times Tt were studied. Those experimental results show that the LI waveform parameters have little influence on the breakdown characteristics of a sound GIS equipment, but have great influence on the GIS equipment with defects. For a highly inhomogeneous electric field, with the wavefront time increasing, the 50% breakdown voltages U50% have remarkable change and the U50%−Tt curve tends to be U-shaped. The 50% breakdown voltages decrease significantly with the increase of impulse wavetail time, especially when wavetail time is in the range of 1.5 to 15 μs, but the difference of the values decreases after 15 μs. Ignoring the generating possibility in the field for the large series inductance and load capacitance, a LI with wavefront time in the range of 0.2 to 0.7 μs is proposed in the field for it may increase the probability of detecting insulation defects. Finally, the polarity reversion phenomenon and critical radius phenomenon, observed from the breakdown characteristics, can be explained by the migration and diffusion of space charges.

Influence of impulse waveform parameters on the breakdown voltage in SF 6 highly inhomogeneous electric field

Very fast transient overvoltages (VFTOs) generated during the routine operations of disconnector switch in GIS make great threat to the insulation of power equipment such as GIS. So far, the research results of the insulation characteristics of GIS under VFTO have big dispersion and poor comparability for the waveform parameters of VFTO have not been standardized. A generating system of double-exponential impulses with front times in the range of 0.08∼1.2 μs and wave tail times in the range of 1.5∼50 μs was established to simulate VFTO. Using this impulse generator, the influence of impulse wave front time and wave tail time on the breakdown voltage in SF6 highly inhomogeneous electric field was studied. The results show that with the rise of gas pressure, the hump phenomenon occurs in the U50%-P curves. With the increase of impulse wave front time, the 50% breakdown voltages change significantly and the U50%-rf curves tend to be U-shaped. The bigger the electric field factor f is, the more obvious the U-shaped trend is. Meanwhile, the 50% breakdown voltages decrease significantly with the increase of impulse wave tail time. Analysis reveals that the reasons for the phenomenon above may be explained by the differences of the migration and diffusion of space charges and discharge time delay under impulses with different waveform parameters.