Research On Suppression Technology Of Transformer Inrush Current

Power transformer is one of the most important parts in the power system,which is connected with the power grid and the user.It is responsible for switching voltage,affecting the power quality of the system.Although after a long period of use and continuous improvement,the stability of closing and the safety of operating in the transformer has not been guaranteed.Because of the randomness of the input time and the nonlinearity of ferromagnetic materials,the working area of the transformer is in the saturation zone and the working point of the transformer is more than the knee point.The excitation current will increase rapidly.It is called inrush current.The existence of inrush current affects the related protection of transformer.The transformer will be abnormal and not close normally.The circuit breaker and the winding of transformer will be damaged due to the large inrush current.It will cause pollution to the power grid.So it is very important to suppress the inrush current when the transformer is unloaded.This paper is consulted the recent literatures,to understand the mechanism of inrush current,to discuss the necessity to suppression of inrush current and to summarize the identification and suppression technology of inrush current.The identification technology of inrush current is widely used and easily fit to protector,but the suppression technology of inrush current can eliminate the inrush current.There are a lot of inrush current suppression technology.By comparison,it can be concluded that the phase selection and closing technique is effective and easy to realize.Phase selection and closing technology becomes the focus of this paper.In order to observe and analysis of transformer inrush current during no-load transformer closing and to understand the transient process,the theory of transformer flux is deduced and the MATLAB software is used to simulate the inrush current of single-phase transformer in this paper.By checking the inrush current waveform,the characteristics and the related parameters of inrush current are analyzed,preparing for the next job.Phase selection and closing technique under different strategies is used for three phase transformer.By using MATLAB software for simulation,it is confirmed that the Synchronous switching strategy in phase selection and closing technique is suitable for three-phase transformer.By observing the magnetic flux,it can be concluded that the three-phase transformer can completely eliminate the inrush current when closed in the phase angle which is same to the angle of the residual magnetism.The voltage integral method is used to estimate residual magnetism.Using MATLAB to simulate the time of the three phase transformer,closure and transient recovery voltage will be measured.On the basis of considering the action time,this paper is designed the system of excitation inrush current suppression in transformer.The system is based on the synchronous switching strategy in phase selection and closing technique and used TMS320F2812 as the controlling core.The system is divided into two aspects:hardware circuit and software program.Hardware circuit is used for signal acquisition and implementation.Software program is used for data memory and processing.Through a number of experiments,the inrush current suppression system can achieve its suppression function,the maximum value of the inrush current will be reduced to about 3 times of the rated current.