柔直换流阀用相变冷却工质击穿特性研究

Breakdown Characteristics of Phase Change Coolant for Flexible HVDC Converter Valves

相变冷却技术冷却效率高、安全可靠,有望攻克柔直换流阀散热瓶颈,实现其高压、大容量发展,在换流阀冷却领域具有良好的应用前景。为实现相变冷却换流阀绝缘系统可靠设计,该文设计研制了三相态相变冷却工质可调频绝缘特性测试专用平台,并以换流阀用相变冷却工质为研究对象,依据换流阀典型运行工况,利用专用测试平台开展了相变冷却工质三相态、50~300 Hz频率范围的绝缘击穿特性研究。得到了相变冷却工质击穿特性随工质相态及电压频率的变化规律,分析了相变冷却工质不同相态的击穿机理,揭示了工质相态和电压频率变化对冷却工质击穿特性的影响机制。研究工作不仅解决了高频下相变冷却工质的三相态耐压测试方法问题,且所得研究成果为相变冷却柔直换流阀的绝缘设计奠定了基础,同时可指导相变冷却技术在其他电力电子器件及装备上的应用。

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.