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.

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Investment for renewables has been growing rapidly since the beginning of the new century, and the momentum is expected to sustain in order to mitigate the impact of anthropogenic climate change. Transition towards higher renewable penetration in the power industry will not only confront technical challenges, but also face socioeconomic obstacles. The connected between environment and energy systems are also tightened under elevated penetration of renewables. This paper will provide an overview of some important challenges related to technical, environmental and socioeconomic aspects at elevated renewable penetration. An integrated analytical framework for interlinked technical, environmental and socioeconomic systems will be presented at the end.

The low-carbon transformation of power sector is significant for achieving the goal of carbon peak and carbon neutrality in China. Based on the evaluation of power carbon budget, three power transformation scenarios of deep low-carbon, zero carbon, and negative carbon were built, the key boundary conditions such as power consumption demand were studied, and a path planning optimization model was established in the paper. Using the GESP-V software package for optimized analysis, the low-carbon transformation paths were determined for power structure, power carbon emissions, and power supply costs under different scenarios. The major issues that are critical for the low-carbon transformation of the power system were discussed, including coal power development, renewable energy development and utilization, diversified supply of clean energy, and electric power balance. Several suggestions were further proposed. Specifically, the top-level design should be strengthened to steadily plan the transformation pace, major low-carbon technologies should be developed to coordinate the overall technology and industrial layout, and the market mechanism with balanced interests should be improved while establishing a green finance policy system. The high-quality low-