Research On Isolation Switching Power Supply For Medium And High Voltage Motor Soft Start Equipment

Comparing medium voltage and high voltage motor soft start system with low voltage motor soft start system, the essential difference is the thyristor trigger circuit. The key part of the transistor trigger circuit in the low voltage system is the trigger transformer, which not only plays an important role in triggering transistor but also isolates signals, and prevents signal interference from the main circuit. However, in order to solve the insulating problem of signal transmission channel in the medium voltage and high voltage motor soft start system, we usually use optical fiber to transmit signal. Both optical electrical converter and trigger circuit work in the high voltage level, so we need a high voltage accessory isolating source to provide the power. In the isolated power supply, transformer is a common means for energy isolation. In order to improve the isolation voltage level of transformer and reduce the size of transformer, we make high voltage wires to pass through the magnetic ring transformer, and transmit energy based on the principle of high frequency current transformer. When we put the high frequency inverter current into the primary side of the transformer, the secondary side will induce high frequency alternating current. Then we provide power to optical electrical converter and trigger circuit through the rectifier, the filtering and the steady voltage circuit. This structure makes the isolation level of the transformer mainly depend on the isolation level of high voltage wires, and therefore ensures the whole system's stability.The research objective in this thesis is the square wave current source of the primary side, and uses the method of external power supply in the low voltage side. We use the double Buck converter circuit as the main circuit of the constant current source, and design the current closed loop control strategy for the constant current source. The rationality of control strategy is tested with simulation.Based on the topology analysis of the full bridge inverter circuit and the particularity that the input power is current source, the whole control scheme of full bridge inversion circuit is designed. The scheme allows two switches of the same bridge arms to be on at the same time. It verifies the feasibility of the whole design and achieves the designed control performance through the simulation of full bridge inverter circuit.To improve the power source with excellent dynamic and static performance, the mathematic model of the power topology is established with the help of the circuit averaging method, and the parameters of current controller is designed. Through simulation of the whole circuit, the accuracy of the parameters is verified. Through experiments, alternating square wave current source whose average is 2A and the frequency is 70kHz can be realized. The testing results show that the power source can meet the design requirements and they further validate the rationality of the whole power scheme.