DC high-voltage test research requires a DC voltage generator higher than (+1000 kV). This device can be used for dielectric strength and polarity reversal research of DC transmission equipment, converter station equipment, and insulating materials under DC high voltage. It can also be used to detect leakage currents of AC/DC power supply equipment and other high-voltage test equipment, such as surge voltage generators, surge current generators, etc. With the large-scale deployment of HVDC transmission projects, the number and frequency of high-voltage DC generators have greatly increased. To improve the efficiency of high-voltage DC generators, it is necessary to increase the speed of polarity reversal.
The full polarity reversal of a DC high-voltage generator involves a series of processes such as generator voltage reduction, grounding, silicon stack polarity switching, grounding, and voltage boosting. Each step in the polarity reversal process requires a lot of time. To shorten the polarity switching time and achieve fast polarity switching, it is necessary to shorten the time of each process.
(1) Accelerating the voltage reduction speed is one of the methods to shorten the polarity switching during the voltage reduction process. This is mainly achieved by increasing the discharge resistance to accelerate the voltage reduction speed. Increasing the discharge resistance does not increase the total resistance of the discharge resistor, but rather increases the number of resistors in parallel with the voltage divider having the same resistance value. The more columns, the greater the total resistance. The lower the discharge resistance, the faster the voltage reduction speed. At the same time, the lower the discharge resistance, the more likely it is to cause an increase in leakage current during normal operation of the device due to the discharge resistance being too small.
(2) During the grounding process of the grounding switch, it takes a long time for the test voltage to be grounded to zero or near zero. When the voltage of the DC high-voltage generator drops from the test value to a certain level, directly grounding the fast grounding switch can greatly shorten the grounding process of the grounding switch. However, a fast grounding switch that meets high-voltage requirements must be able to withstand all high voltages and large currents, which is extremely demanding for the fast grounding switch.
(3) Simply put, the silicon stack needs to achieve fast polarity reversal to realize the internal single-tube polarity reversal. To achieve this goal, we must first consider the driving mechanism. Currently, there are three simple driving methods: electric, pneumatic, and hydraulic. According to the structural characteristics of the DC high-voltage generator, it is impossible to achieve high-voltage power supply. Pneumatic devices have the compression ratio of the cylinder's air source. If all silicon reactors are connected in series through air pipes, when the humidity in the gas is high, the entire device will be discharged from the air pipes, causing equipment damage and other issues. Using hydraulic method with vacuum-treated alkylbenzene insulating oil will not cause discharge. Secondly, the compression ratio of oil is much smaller than that of gas, so the silicon reactor can be fixed. Fixed polarity reversal avoids the problems of the other two methods. In comparison, it can be seen that the hydraulic selection method is reasonable.
After hydraulic transmission, the internal single tube and the end hydraulic cylinder form a set of transmission mechanisms. When the hydraulic cylinder starts, the internal single tube begins to smoothly change its polarity until the hydraulic cylinder completes, and the polarity switching is in place. At this time, the oil in the pipeline returns, activates the pressure relay, and sends a control signal indicating that all silicon stacks are in the correct position. Hydraulic pipes are used to connect the silicon stacks, and each silicon accumulator pipe and cylinder are connected in parallel to ensure short polarity switching operation time for each silicon stack.
