Search

In recent years, GIS compact design has gradually been recognized and adopted by the industry. In the trend of the compact design, the insulation performance of the cone-type spacer is getting more and more attention by GIS manufacturers and grid operators and maintenance departments. In particular, under the action of DC residual pressure, the insulation properties of cone-type spacer become the focus of attention of all parties. Based on the field equipment operating conditions, the experimental study of DC residual voltage for the true cone-type spacer insulation performance is significant. In order to obtain the proposed pre-stressed time of DC voltage and distributions of surface charge, for the insulation test of 500 kV cone-type spacer under lightning impulse with DC voltage superimposed. A full scale surface charge measurement platform is build up for 500 kV GIS spacer. Then, long-term DC voltage experiments were carried out to obtain the distributions of surface charge on spacer in stationary state under different voltages. Moreover, the saturation time of surface charge accumulation under different DC electric stress is calculated and discussed. The study results show that (1) the distribution of surface charge on spacer is not uniform. It can be observed that on most areas of 500 kV spacer concave surface is dominated by the electric conduction through the volume with positive charges. (2) The time constant of surface charge accumulation decreases with the residual DC voltage amplitude, it can be estimated that the saturation time of surface charge accumulation on 500 kV cone-type spacer is about 23 h for the residual DC voltage reach to 1.0 p.u. The study results are thought to be helpful to the insulation test of 500 kV conetype spacer under impulse voltage with a superimposed residual DC voltage. [ABSTRACT FROM AUTHOR]

The problem of surface charge accumulation on a gas-spacer interface is one of the critical factors that limits the application of DC gas-insulated transmission lines (DC GIL). In previous works, the models of surface charge accumulation under different temperatures on a spacer were investigated. However, these simulation models lacked experimental verification. In this paper, a coupled electro-thermal field simulation model for charge accumulation on a spacer under DC stress is introduced, and an experimental platform with the ability to load the thermal field on a spacer is built. Then, the effect of the thermal gradient on the surface charge of a spacer is investigated by measurements and simulations. The results indicate that for the spacer sample in the experiment, the average surface potential increased 277 %, and the variation range of that increased 309 % as the conductor temperature increased from 303.2 to 343.0 K. The coupled electro-thermal field simulation model of charge accumulation on a spacer under DC stress was verified by experimental results. The simulation model will hopefully be helpful for the insulation design of DC-GIL. [ABSTRACT FROM AUTHOR]

The modification of insulating materials is regarded as an important way to improve the insulation performance of DC spacers. Many simulations have been carried out to guide the development of insulating materials. However, the impacts of temperature on surface charge accumulation and electric field distributions are neglected in many simulations. Since the conductivities are strongly impacted by temperature, the thermal stresses should be considered in the simulations of charge accumulation. To solve these problems, a coupled electro-thermal field simulation model of charge accumulation under DC stress is proposed in this paper. Then, a DC spacer model is introduced as an example, and the parameters in the simulation are shown. Based on the simulation model, the effects of the spacer volume conductivity on the distribution of the surface charge, the saturation time of charge accumulation and the distribution of the electric field along the spacer are investigated. In addition, the influencing mechanism is explained. The results indicate that the volume conductivity of the insulating material should be decreased by two orders of magnitude from the existing AC materials to reduce the surface charge accumulation. With the reduced volume conductivity, 1) the peak value of the charge density on the upper surface decreases 52.9%, and that of lower surface decreases 51.0%; 2) the peak value of the tangential component of the electric field strength along the cone-type spacer in DC-GIL decreases by 12.2%; and 3) the duration of the capacitive-resistive field transition increases by 38.5-fold. The simulation method and advice on the material volume conductivity can be referenced in the DC space design. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the modifications to nanofluid-impregnated pressboards by different shapes of nanoparticles are different. To investigate the effect of nanoparticles shapes on creeping flashover characteristics at the interface of nanofluid-impregnated pressboard (NP), creeping flashover behavior of spherical nanoparticle fluid/pressboard (NSFP) and nanorod fluid /pressboard (NRFP) were compared to that of oil/pressboard (OP) which was used as a reference. Creeping flashover experiments under AC and positive and negative impulse voltages were carried out. Results show that the modifications by spherical and rod nanoparticles on creeping flashover strength are distinct and different: negative creeping flashover voltages of NSFP and NRFP are increased up to 32 and 15% respectively, and the positive ones are separately improved as high as 15% and 5%. Whereas, experiments under AC voltage reveal quite a different trend: creeping flashover voltage of NSFP and NRFP are almost increased to the same extent, nearly 18%, while partial discharge inception voltage improvement in the increment of the former is 5.6 times larger than that of the latter. Charge behaviors at oil/pressboard interfaces were displayed by thermally stimulated depolarization current method. Space charge accumulation at the surface of NSFP is much less than that of NRFP and the dissipation rate of the former is faster, due to a larger trap density and a lower trap energy level, which weakens electric field distortion and results in the improvement of creeping flashover strength. The underlying mechanism is that spherical nanoparticles have a specific surface area 1.3 times as large as that of nanorods at the same concentration, and it is a source of shallow traps. [ABSTRACT FROM AUTHOR]

With this expected future advance of HVDC, the use of gas insulated transmission lines (GIL) for dc application are getting increasingly interesting. For now, the problem of surface charge accumulation on gas-insulator interface is one of the critical factors for the development of DC-GIL. In many previous works, the model of surface charge accumulation on insulator was investigated. However, the quantitative relationship between temperature and surface charge accumulation on insulator was not exactly obtained since the lack of complicated heat transfer progress in the model. In this paper, the heat transfer surface charge accumulation model of operating DC-GIL was developed, including the nonlinear relationship between volume current in gas and electric field. Moreover, the space charge was also considered in the model. Based on the developed model, temperature distributions in DC-GIL insulator under different current are obtained. Afterwards, the temperature impact on space charge density in the insulator, the saturation time of surface charge accumulation, the surface charge on the insulator surface, and the electric field distribution on the insulator were investigated. It was proven that the tangential component of the electric field reaches to 5.3 kV/mm on lower interface and 5.0 kV/mm on upper interface for Ti=378 K. This value increase 17.8% on lower interface and 17.6% on upper interface along with the conductor temperature from 298 K to 378 K. The data can be referred in the insulation design of DC-GIL. [ABSTRACT FROM AUTHOR]

It was observed that partial discharges were not detected from GIS insulators with the crack defects operated at 1100 kV in the UHV substation in China. The question is that whether crack defects can induce partial discharge at the operated voltage. In order to obtain the relationships between PD occurrence and crack configuration on GIS insulator operated at 1100 kV, the electric field strengths in the crack, changed with crack length, crack depth, crack direction, crack position, were simulated on a 1100 kV GIS insulator at operation voltage. The simulated results indicated that the field strength in the cracks, which located on the surface of the insulator or paralleled to the electric field direction, were close to that in the insulator around the crack, and the crack cannot induce partial discharge. However, the field strength in the cracks, which located in the deep position of insulator and in perpendicular to the electric field direction, was much higher than that in the insulator around the crack. For example, in the case of the crack located close to high voltage conductor, field strength in deep crack perpendicular to electric field direction could be 5.6 times of the field strength in the insulator around the crack, and it close to the field criteria of gas discharge, leading partial discharge occurred at operation voltage. The simulated results were verified by the experiment conducted on the insulators with cracks. The experimental results indicated that the crack could produce partial discharge, only when it is located in deep insulator material and in perpendicular to the electric field direction, and close to high voltage conductor. [ABSTRACT FROM AUTHOR]

The problem of surface charge accumulation on gas-insulator interface is one of the critical factors for the development of DC Gas Insulated transmission Lines (GIL). In order to solve this problem, researchers have proposed a variety of mathematical models in which the volume conductivity of SF6 was assumed to be constant for calculating the distribution of surface charge. However, the conductivity of SF6 was inconstant, affected by electric field strength. In this paper, a gas model is developed, taking into account the generation, recombination and motion of charge carriers in SF6 of DC-GIL. The surface charge density and the electric field distribution on the insulator surface under DC voltage were simulated. Afterwards, the electric conduction through the volume of the insulator was proved to be the dominating accumulation mechanism. In order to improve the insulation performance, the influence of practical insulator shape on the electric field distribution in DC-GIL was studied. Three different shapes of insulator which were disc insulator, conical insulator and obtuse conical insulator were introduced and compared. The simulation results indicate the obtuse conical insulator was the most suitable configuration for DC-GIL. [ABSTRACT FROM AUTHOR]