Research On Controlled Switching Of No-Load Transformers With Cable

Power transformers are the applied widespreadly as the pressure equipment in Power System. The number of transformers and the frequency of its operating also increase year by year. The opening and closing operation of no-load transformer is associated with a lot of transient processes. The transient process has great influence on the safe of Power System. Overvoltage of Current Chopping will occur during the opening operation. The inrush current would abrupt change when no-load transformer close, as a result of iron core flux saturation. To avoid these adverse effects, using the synchronous switching technology effectively can inhibit inrush current.This paper analyzes the Overvoltage of Current Chopping’s generating principle during transformer opening. The no-load transformer switching model built by ATP/EMTP, the simulation results show the amplitude of overvoltage can be effectively reduced when opening no-load transformer at the peak voltage. The overvoltage would change when the no-load transformer connect with cable, due to the capacitance effect of cable, the longer cable the smaller intercepting overvoltage.In this paper, we will study the principle of inrush current and synchronous switching technology. The no-load transformer switching model built by ATP is used for analyzing the results based on switching technology with different residual magnetism. The comparison of inrush current and harmonic content will be shown in the following chapters to the different closing angle. This paper talks about the basic principle of using the different closing strategy under the different condition of residual magnetism. Through the simulation of three kinds of switching strategy, results show the synchronous switching strategy can effectively suppress inrush current. Also the influence on the closing phase selection produced by different length of cable connected to the transformer is taken into consideration in this study. These simulations prove that the synchronous switching technology can effectively inhibit the inrush current. This technology has effectively maintained the power transformer and has ensured their stable operation in power system.Finally, the paper introduces the design of Synchronous Switching control system based on DSP. The system is applied to choose different closing strategy while different residual magnetism appeared in transformer’s iron core. The system could collect voltage and current, implement FIR filter function, extract the zero point of signal and control discharging capacitance’s voltage, and other functions. This control system can realize the accuracy of Synchronous Switching technology. And use this controller to the no-load transformer test, the test results proved the practicability of the proposed controller.