The power demand increases rapidly in China; however, the areas of huge power demands are of long distance from most areas of abundant energy resource in the country. Therefore, China put in great effort to develop ultrahigh voltage (UHV) power transmission systems to optimise its energy allocation. This includes (i) systematically developing key technologies such as overvoltage suppression, external insulation design, and electromagnetic environment control, and (ii) developing key equipment such as UHV alternative current (UHVAC) transformers, circuit breakers, and series compensated equipment, and UHV direct current (UHVDC) converter transformers, smoothing reactors, converter valves, and DC transmission control and protection systems. Eight AC UHV projects and 11 DC UHV projects have been built in China, which play an important role in the optimal allocation of energy. Plus there are one more UHVAC and three more UHVDC transmission projects in construction, while UHVAC gas-insulated lines will be applied in the UHVAC line crossover the Yangtze River and the ±1100 kV UHVDC power transmission technology is in development. Here, the development and application of UHV power transmission technologies in China are described, some main challenges the UHV projects encountered are discussed, and experiences obtained from the operation of UHV systems are introduced. It is concluded that China obtained mature experience in developing, constructing, and operating UHV systems and successfully realised long-distance, large-capacity power transmission, and the UHV power transmission technology has become an important measure for energy allocation in large areas.

No abstract

SUMMARYA novel mathematical model for accurately calculating the currents flowing along the conductors of grounding system below high voltage a.c. substations and nearby floating metallic structure buried in horizontal multilayer earth model has been developed in this paper. Not only the mutual conductive and capacitive coupling influences of leakage currents, but also mutual inductive coupling influence of network currents flowing along the conductors of grounding system and nearby floating metallic structure in the horizontal multilayer earth model have been considered in this model, and only propagation effect of electromagnetic wave within limited area of the substation has been neglected. The quasi-static complex image method and closed form of Green's function are introduced into this model to accelerate the calculation. The model is then implemented in a computer program, which can be used to calculate currents distribution along the conductors of any configuration of grounding system, and with or without floating metallic structure under some hundreds of kHz frequency harmonic wave.

SUMMARYIn the simulation of quasi-static electromagnetic fields produced by a point source located in both horizontal and vertical multilayer media, the conventional image method requires an infinite number of images to get an accurate solution, while the method of quasi-static complex images needs only a few ones. Based on the method of quasi-static complex images, the closed form of Green's function of a point source in both horizontal and vertical multilayer earth model is derived through matrix pencil (MP) approach. The fast convergent Galerkin's type of boundary element method (BEM) is taken to simulate and analyse a grounding system including floating electrode with any complicated structure, which can be located anywhere in horizontal or vertical multilayer earth model.

Epoxy polymer-based dielectric materials play a crucial role in advanced electronic devices and power equipment. However, high voltage-stress applications impose stringent requirements, such as a high dielectric strength, on epoxy polymers. Previously reported studies have shown promising material architectures in the form of epoxy polymer–nanoparticle dielectrics, which can restrict the movement of high-energy electrons by the interface charge traps associated with the various interfacial regions. However, these high-energy electrons inevitably traverse the epoxy polymer matrix and destroy the molecular structure, thereby creating a weak link for dielectric breakdown. In this study, a general strategy is developed to improve the dielectric strength by constructing interface charge traps in the molecular structure of the epoxy polymer matrix, using the −CF3 group in partial replacement of the −CH3 group. The proposed strategy increases the dielectric strength (39.5 kV mm–1) and surface breakdown voltage (26.9 kV) of the epoxy polymer matrix by 22.08% and 13.3%, respectively, because the interface charge trap hinders the movement of high-energy electrons. At the same time, the strategy does not degrade the mechanical and thermal properties. The results hold potential for wide application in the manufacturing of advanced future electrical and electronic equipment requiring resilience to high-voltage stress.

Leader discharge plays a key role in the breakdown of long air gap and the evolution of natural lightning discharges. Prior to the formation of continuous leaders, streamer corona bursts with dark periods in between generally appear. To attain a better physical understanding of the dynamics of discharge during dark periods, experimental studies with a specially designed electrode based on an improved quantitative schlieren system were performed. The experiments measured gas density and temperature changes of the leader discharge channel within the dark period for the first time. Temperature changes in a single discharge channel and corresponding current were obtained simultaneously. Measurements show that at the axis of the streamer stem and the unstable leader within the dark period the gas temperature is below 2000 K and decreases monotonically with time. These measurements confirmed the prediction of a previous theoretical study. To analyse the properties of discharge during the dark period, the measurements were compared with simulation results based on a detailed 1D thermo-hydrodynamic model. The measured temperature drop was less significant than that computed from simulation. This discrepancy is believed to be caused by heating from the ionic current, which was neglected in the previous studies. The ionic current helps to maintain the gas temperature during the long dark period but cannot reverse its decline. From a comparison and analysis, several suggestions are proposed to improve both the measurement system and the numerical model for further quantitative investigations of the dynamics of leader initiation.

Very-fast transient overvoltage (VFTO) is a serious threat to the insulation of extra-high voltage and ultra-high voltage power-system equipment and should be suppressed. A simpler and more reliable VFTO suppressing method than a shunt resistor is presented in this paper, which uses magnetic rings installed on the center conductor of gas-insulated switchgear (GIS). Two kinds of magnetic materials that can work under high frequency were adopted, one of which is ferrite R2KB and the other is amorphous FJ37. Measurements in a scale model with an air-insulated busbar show that ferrite R2KB rings have a better suppressing effect than amorphous FJ37 when the ring-cluster length is larger than a certain value. The ferrite R2KB rings were applied to 252-kV GIS. The test results show that ferrite R2KB rings can significantly suppress the peak value and the time to peak of VFTO. When the ring-cluster length is 40 cm and the test voltage is 140 kV of power frequency, the overshoot factor and the front steepness of VFTO can be reduced more than 50% and 20%, respectively. The test results are exiting and indicate that using magnetic rings to suppress VFTO is feasible. Index Terms-Ferrite, gas-insulated switchgear (GIS), magnetic ring, suppression, very-fast transient overvoltage (VFTO).

No abstract