Breakdown Characteristics of Phase Change Coolant for Flexible HVDC Converter Valves
Wen Yingke1, Ruan Lin1,2
1. Institute of Electrical Engineering Chinese Academy of Sciences Beijing 100190 China; 2. University of Chinese Academy of Sciences Beijing 100049 China
Abstract：High voltage and large capacity is the development trend of flexible HVDC transmission technology. The reliability problem caused by overheating of IGBT converter valve limits the improvement of system capacity. The cooling capacity of the water cooling system is limited, and there are also hidden dangers such as blockage and leakage. It is of great significance to develop a safe and efficient new cooling technology for the converter valve. Phase change cooling technology has high cooling efficiency and reliability. It is expected to overcome the heat dissipation bottleneck of flexible HVDC converter valve and has a good application prospect in the field of converter valve cooling. In order to design the insulation system of the phase change cooling converter valve, it is necessary to study the insulation breakdown characteristics of phase change coolant. The existing researches on the breakdown characteristics of phase change coolant are almost carried out at power frequency, but the operation frequencies of converter valves are usually higher than power frequency, so the existing researches are not enough to guide the practical engineering application. In order to solve this problem, this paper firstly developed a frequency-adjustable insulation characteristics test platform for phase change coolant. The platform could realize the adjustment of the output voltage frequency, at the same time, the test chamber has sealing and visualization characteristics, which can meet the triphase state test requirements of phase change coolant. Considering the typical operating conditions of the converter valve, the insulation breakdown characteristics of the phase change coolant at different phase states, within 50~300 Hz were tested by using the platform. The change rules of the breakdown characteristics of phase change coolant with different phase states and voltage frequencies were obtained, the breakdown mechanism of phase change coolant at different phase states was analyzed, and the influence mechanism of phase state and frequency on the breakdown characteristics of phase change coolant was revealed. The following conclusions can be drawn from the study: (1) Under different frequencies, the change rule of the breakdown characteristics of the coolant with phase state is consistent: The average breakdown voltage of the liquid coolant is basically stable. The average breakdown voltage of gas-liquid coolant gradually decreases with the increase of heat flux, and finally approaches the value of gas coolant. The average value and data dispersion of gas phase breakdown voltage are the minimum. The breakdown voltage of liquid coolant and gas coolant obeys the Weibull distribution, while the breakdown voltage of gas-liquid coolant doesn’t. (2) With the increase of frequency, the average value and dispersion of breakdown voltages of liquid coolant and gas coolant decrease, and the gas phase breakdown voltage is more affected by frequency. The breakdown voltage of gas-liquid coolant shows different frequency dependence under different heat flux. When the heat flux is small or large, the average breakdown voltage of gas-liquid coolant decreases with the increase of frequency, however when the heat flux is during the middle transition zone, the average breakdown voltage of gas-liquid coolant increases first and then decreases with the increase of frequency. (3) The breakdown of coolant is caused by the collision ionization between charged particles and neutral molecules. The breakdown probability can be improved by increasing the number of charged particles, the free path of charged particles and the electric field strength. (4) The gas phase insulation performance of the phase change coolant studied in this paper is superior to SF6 and equivalent to transformer oil. In order to ensure the reliability of the phase change cooling converter valve, the main basis for the selection of coolant and the insulation design shall be the dielectric strength of gas coolant at the high operation frequency point of the converter valve. The research not only solves the problem of triphase state insulation test method of phase change coolant at high frequency, but also lays a foundation for the insulation design of phase change cooling flexible HVDC converter valve.
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2024, Vol. 39 
