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In this paper, a calculation method is proposed to predict the dielectric strength (DS) of gases and the DS of some promising SF 6 alternative gases and their mixtures is further evaluated by DC breakdown tests. The calculation method is based on the density functional theory (DFT), so that the DS of the gases can be obtained by only calculating their molecular parameters, including the average static electronic polarizability and adiabatic ionization energy. The paper also presents the DC breakdown strength of the C 4 F 7 N, C 5 F 10 O, and HFO-type gases, and it is found that the experimental DS values are in good agreement with the calculated values. In addition, DC breakdown tests for gas mixtures are also conducted and their synergistic effect is discussed in detail. The results show that the DS of C 4 F 7 N mixed gases is equivalent to that of pure SF 6 gas even under high pressures, indicating their potential applications in HV electrical equipment. The DS of C 5 F 10 O mixed gas is slightly lower than that of C 4 F 7 N mixed gas. Compared with CO 2 , mixing with air brings stronger synergistic effect in C 5 F 10 O and C 4 F 7 N in our experiment. The DS of HFO mixture is substantially less than that of SF 6 because of the weak synergistic effect when mixed with CO 2 or N 2. This paper presents an effective method to evaluate DS of SF 6 alternative gases and the results are useful for their future engineering applications. [ABSTRACT FROM AUTHOR]

This paper addresses the prediction of microparticle-initiated breakdown in mm-scaled air gaps which has a transition Pd value of about 200 Pa.m. Predictions by Townsend theory and streamer inception are compared with experimental measurements and the results indicate that the Townsend theory is in better agreement. Based on this method, the breakdown voltage of an air gap with a single microparticle is calculated and drops quasi-linearly with its radius at a decay factor about 35 V/μm. The relative reduction rates of U b caused by single r m =10 and 100 μm microparticle reaches 3.1 and 37.3%, respectively. However once r m =10 μm microparticles cluster, the reduction rate rises to 7.2 and 35.5%. This provides a prediction method to assess the effects of microparticles on gas breakdown, especially in mm-scaled gap with transition Pd value and contributes to development of insulation performance. [ABSTRACT FROM AUTHOR]

The C 4 F 7 N/CO 2 gas mixture has attracted much attention due to its excellent insulation property and eco-friendly character. In order to evaluate the feasibility of its application in DC GIL, this paper focuses on the surface charging characteristics of spacers under DC stress in the C 4 F 7 N/CO 2 mixture. First, the DC insulation strength was tested under uniform field. The results show that the critical dielectric strength of pure C 4 F 7 N is more than twice that of SF 6 and the dielectric strength of 15%∼20% C 4 F 7 N/CO 2 mixture is equivalent to that of SF 6. Next, the surface charge measurement of spacers was performed in C 4 F 7 N/CO 2 mixtures, which reveals that the surface charge density decreases slightly with the increase of C 4 F 7 N content. However, the surface charging level in 25% C 4 F 7 N/CO 2 mixture is still much higher than that in SF 6. It is found that most of the surface charge stems from partial discharges (PDs) near the flange. The PD measurement shows that in SF 6 , there are more PDs with low magnitude, but much less PDs with high magnitude compared to that in the 15%∼25% C 4 F 7 N/CO 2 mixtures. It is the different PD behaviors between C 4 F 7 N/CO 2 mixtures and SF 6 that lead to their different surface charging characteristics. [ABSTRACT FROM AUTHOR]

C5-PFK has attracted much attention as one of the most promising SF 6 substitutes for medium voltage (MV) power equipment because of its high insulation strength and low global warming potential (GWP). In this paper, the dielectric properties of C5-PFK-Air and C5-PFK-CO 2 mixtures are experimentally investigated, to explore the most effective solution for C5-PFK mixtures for application in MV power equipment. The basic physical properties of C5-PFK and its mixtures are calculated and compared with those of C4-PFN based gases and SF 6 -N 2 mixtures. The GWP and saturated vapor pressure for gas mixtures are calculated, and a relative value R-GWP is defined to evaluate the greenhouse effect at specific pressure. The 50% breakdown voltages of C5-PFK-Air and C5-PFK-CO 2 mixtures under lightning impulse voltage are determined experimentally. The mixture ratio and filling pressure are determined taking into account of the ambient temperature and pressure for MV apparatus. The possibility of practical applications for these mixtures are analyzed by combining the saturated vapor pressure characteristics and the breakdown electric field strength. The synergy effects of the C5-PFK-CO 2 and SF 6 -N 2 mixtures are greater than those of the C5-PFK-Air and C4-PFN-CO 2 mixtures. The C5-PFK-Air and C5-PFK-CO 2 mixtures have higher dielectric strength than SF 6 at 0.1 MPa when the C5-PFK is about 28% under the temperature limitation of −5°C. The pressure increments of 0.014 and 0.066 MPa are required for C5-PFK-CO 2 and C5-PFK-Air mixtures, respectively to reach the dielectric strength of pure SF 6 at 0.1 MPa under the temperature limitation of − 25°C. [ABSTRACT FROM AUTHOR]

Perfluoronitriles (PFN) and their mixtures have garnered much attention as a great potential SF6-substitute gas, owing to their excellent dielectric strengths and minimal impacts on the environment. The dielectric properties of C4-PFN/CO2 and SF6/N2 mixtures are investigated and compared by experiments and calculations, with taking into account the limitations of saturated vapor pressure on the mixed ratio at certain ambient temperatures. The saturated vapor pressures of these two mixtures are determined by combining the Antoine Equation and the basic law of vapor liquid equilibrium. Then, the 50% breakdown voltages V50% of the C4-PFN/CO2 and SF6/N2 mixtures are determined by tests with up-and-down method, and the values of E/p are deduced. Finally, the dielectric properties of these two mixtures are analyzed by combining the saturated vapor pressure characteristics and the breakdown electric field strengths. The results show that a proper choice of the mixed ratio and gas pressure of the C4-PFN/CO2 mixture can provide a stronger DS than the SF6/N2 mixture and pure SF6. This indicates a great potential of the C4-PFN/CO2 mixture as a SF6-substitute gas for high voltage apparatus. [ABSTRACT FROM AUTHOR]

CO2 attracts a lot of attention of researchers as a possible substitute for SF6. The arc interruption capabilities of SF6-CO2 mixtures for different mixed ratios are investigated in this paper by carrying out tests with a 126 kV puffer gas circuit breaker prototype and calculations. Based on the measured arc voltage and current around current zero, the post-arc current characteristics and the thermal interruption properties are investigated for SF6-CO2 mixtures under different rise rate of the recovery voltage RRRV and di/dt before current zero. It is found that the interruption performance of SF6-CO2 increases greatly with increasing SF6 concentration, and the critical RRRV and di/dt before current zero of SF6-CO2 mixtures for SF6 content of 0, 20 and 50% are about 39, 45 and 70% of that of SF6 respectively, at the short-circuit current of 10 kA. The pressure increment at current zero is found to have a great effect on the interruption of the short-circuit current. In addition, the pressure buildup in CO2 is much lower than that in SF6 for both no-load and loaded operations. [ABSTRACT FROM AUTHOR]

Studies of circuit breakers are of interest to high voltage direct current (HVDC) power system. The method for investigation of the switching arc characteristics of a 500 kV metallic return transfer breaker (MRTB) is presented in this paper, by coupling magneto-hydro-dynamics (MHD) arc model with the resonant circuit equations. 2-D MHD arc model taking account of important physical processes of mass flow, Ohmic heating, radiation energy transfer, momentum exchange, Lorentz force and current flow is developed, and then the influence of stray resistor is analyzed under the interrupting current of DC 4 kA. Calculation results of the distribution of temperature and gas pressure in the nozzle region are presented, where the shock wave and negative voltage-current characteristic of arc are discussed. The calculated arc current and voltage are in agreement with the experimental results to some extent, which could verify the validity of the calculation method partially. [ABSTRACT FROM AUTHOR]