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The main branch of 500kV high-voltage DC circuit-breaker (HVDC CB) adopts a series structure of multiple fast mechanical switches, which requires an effective distance within 2ms and can withstand over-voltage. Therefore, higher requirements on the dynamic voltage balancing and synchronization characteristics are put forward. In this paper, the action characteristics of 500kV HVDC CB and the stress of fast mechanical switches in main branch are analyzed. A model of fast mechanical switches in 500kV HVDC CB is built, and the distributed capacitance parameters are extracted. The voltage equalization measures of multi-switch series structure are studied. It is suggested that the voltage balancing capacitance can be 65nF, the damping resistance can be 200Ω, and the voltage balancing resistance can be 200MΩ. The voltage distribution of fast mechanical switches in different periods of operation is explored under operation lightning impulse (OLI), and the requirement of synchronization in the case of loss of redundancy is explored. The interval should not exceed 0.17ms.

With the wide application of high-voltage direct current (HVDC), cross-linked polyethylene (XLPE) cable may have some defects in production and operation. In the construction site, the method of using AC test voltage to detect the local defects of DC cable is lack of effective standards. At the same time, the existing research results do not provide the relationship between the co-simulation of electric heating and the initial discharge voltage of cable defects under AC and DC voltage. This makes the partial discharge (PD) phenomenon under AC test voltage difficult to correspond to the PD phenomenon under DC working voltage. Therefore, this paper uses the finite element software to simulate the joint of 320 kV DC cable under AC and DC voltage, and obtains the AC and DC voltage relationship when the defect occurs. The simulation results provide the threshold value of the power frequency initial discharge voltage and the DC initial discharge voltage when the internal defects of the DC cable occur partial discharge.

The basin-type insulator is one of the most important parts in SF 6 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 SF 6 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 particle's 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.

SF 6 is widely used in high-voltage equipment due to its excellent insulation ability. However, when the moisture content in SF 6 exceeds a certain value, its insulation could be damaged, threatening the normal operation of high-voltage equipment. Therefore it's of great importance to establish proper control standards on moisture content in SF 6 . In this paper, the influence of moisture content on flashover voltages of cylindrical insulator models in SF 6 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.

In recent years, SF 6 /N 2 was getting used as insulation medium in gas-insulated equipment for replacing pure SF 6 which causes greenhouse effect. For the application of SF 6 /N 2 in power equipment such as GIS and GIL, the breakdown characteristics of SF 6 /N 2 in varying electric field non-uniformity under standard lightning impulse were studied in this paper based on an impulse experimental platform. The experimental results demonstrates that the 50% breakdown voltage of SF 6 /N 2 rises linearly with rise of gas pressure when the electric field is slightly non-uniform, and the saturated trend appears when the electric field turns to be more non-uniform. The reversal phenomenon of polarity effect was found in SF 6 /N 2 with different mixing ratio. That is, when the pressure is relatively low, the 50% breakdown voltage under positive polarity is lower than which under negative polarity. When gas pressure reaches the critical pressure, the situation would be opposite. The augment of SF6 mixing ratio or decrease of electric field non-uniformity would both result in the decline of the critical pressure. Meanwhile the change of polarity effect with SF 6 /N 2 is later than that with pure single gas when slightly non-uniform electric field turns to be strongly non-uniform. In addition, the synergistic effect of SF 6 /N 2 weakens obviously with strengthening of electric field non-uniformity, and the negative synergistic effect even appears with strongly non-uniform electric field.

Gas insulation switchgear (GIS) has been widely used in electrical power systems for decades due to its good compactness and tightness, slight impact on the environment, and long maintenance cycle. However, it has been noticed that the failures of GIS insulation due to very fast transient overvoltages (VFTOs) increases with the increase of voltage levels. In this paper, a generating system of VFTO and standard lightning impulse (SLI) was established. Using the system, the breakdown characteristics of SF 6 gas gaps under VFTO and SLI were studied. The results show that SF 6 gap has polarity effect in both quasi-uniform field and highly inhomogenous electric field. Under quasi-uniform field, the 50% breakdown voltages under VFTO and SLI with positive polarity are higher than those with negative polarity. But for the highly inhomogenous electric field, the polarity effect is remarkable and polarity reversion phenomenon is observed. When the pressure of SF 6 is lower, the 50% breakdown voltages under SLI with negative polarity are higher than those with positive polarity. Nevertheless, the 50% breakdown voltages under SLI with positive polarity are higher than those with negative polarity when the pressure of SF 6 exceeds a critical pressure. The critical pressure decreases with the increase of the electric field non-uniform factor. Based on the analysis, it is concluded that the reasons for polarity effect can be explained by the migration and proliferation of space charge.

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.

In recent years, SF 6 /N 2 gas mixture gains much popularity as an alternative insulating medium of pure SF 6 gas among gas-insulated equipment because of the greenhouse effect and poor economy effectiveness of the latter. This paper investigated the breakdown characteristics of SF 6 /N 2 gas mixture with low SF 6 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 SF 6 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 SF 6 /N 2 gas mixtures such as GIL, and it also plays a vital role in the future study of gas discharge mechanism of SF 6 /N 2 gas mixtures.

In recent years, SF 6 gas insulated switchgear (GIS) is increasingly widely used among the power system of China and the flashover characteristics on GIS insulator have attracted widely attention of many researchers. According to the previous research, it's found that surface conditions of the insulators have a great impact on flashover voltage. In this paper, the influence of the surface roughness of the insulator on its flashover characteristics in SF 6 had been studied. A 1000kV standard lightning impulse (SLI) generator was constructed to study the influence of surface roughness of insulators on flashover voltage under SLI. 72.5kV supporting insulators with different roughness were obtained by polishing the surface with different mesh numbers of abrasive paper. A laser scanning confocal microscope (LSCM) was used to observe the roughness of the insulator. The results showed that the flashover voltage of insulator increases within the SF 6 pressure ranges from 0.1MPa to 0.4MPa under SLI and decreases with the increasing surface roughness. This phenomenon was analyzed in consideration of the surface area and adsorption. It's also found that the roughness has a bigger impact on the flashover voltage under the negative SLI. The flashover voltage of rough insulator under negative SLI is higher than that under positive SLI, which is different from the flashover characteristics of the new smooth insulator.