Research On Controlled Switching Technology Of No-Load Transformers Based On FPGA

Transformers are static motors which work depend on the principle of electromagnetic induction, they are the frequent switching equipments in the power system. Transformers are inductive load, so inrush current exits when energized with no-load, there will be adverse affect to power system if inrush current is large enough. To reduce the damage caused by the inrush current, controlled switching technology is a good solution. This paper is based on the application of controlled switching technology of no-load transformer. By controlling the opening and closing time of transformer, the impact of switching transient process will be weakened, then inrush current can be restrained.This paper analyzes the mechanism of inrush current, and expounds controlled switching technology. According to the electromagnetic theory, formula of the optimal closing time is given. By building the simulation model of no-load transformer system, simulation shows the effects of random switching and controlled closing strategy with different remanence. The results were compared, and it verify the effect of reducing inrush current by controlled switching technology. According to theoretical analysis and simulation results, the paper proposes to build a controlled switching system of no-load transformer with the core of FPGA based on delay closing technology.In this paper, the application of FPGA is explained. Compared to other existing control chip, FPGA's advantage as controller is pointed out. According to the functional requirements, the hardware and software of the control system are designed. The design of hardware focus on control circuits of FPGA, associated peripheral circuit, and the connection relationship between different parts; the software is modular design, after completing the program, timing simulation is executed to verify the correctness of the program.After finishing the controller, the paper proposes to build the experimental platform for controlled switching of no-load transformer, building driving circuit, load circuit and others. First, we test the capability of the mechanical switch, and the trigger experiment with no load is done, proving that the control accuracy is within 0.2ms; electromagnetic compatibility test is conducted, parts of the experiment reach level 3; finally, closing inrush current experiment of single-phase and three-phase transformer is done, we gather the current waveform of controlled switching and random switching, and compare their effect. From the results of experiment, the system designed in the paper achieves the function of controlled switching, and trigger time is accurate under laboratory conditions; the control effect of single-phase system is stable, with repeatability; the control effect of three-phase system conforms to the expectation. Compared with random switching, suppression effect of inrush current by controlled switching is obvious.