No Load Transformer Inrush Current Restraining Scheme Based On Closing Voltage Amplitude Controlling Method

Inrush current causes series of problems,such as protection malfunction,decrease of transformer service life,plenty of harmonics and abnormal operation of electrical equipment.Inrush current suppression has always been one of the key issues in transformer protection,although there are lots of mature theories and practical applications focusing on inrush current identification and suppression,these methods have shortcomings more or less.Therefore,suppressing the inrush current by analyzing the causes has theoretical and practical significance.Based on the study and summarization of the previous research work,this paper proposes a no load transformer inrush current restraining scheme based on closing voltage amplitude controlling method.At first,a scheme of restraining transformer inrush current based on control by closing voltage amplitude is proposed based on analyzing the influence factors of transformer inrush current and deriving the mathematical relationship between inrush current and influence factors.In this scheme,add a closing voltage control circuit between the transformer and the system power supply,in order to making sure that the magnetic flux produced by the transformer no-load closing does not exceed saturated magnetic flux,the voltage amplitude should be controlled by selecting the appropriate curve,thus attaining the goal restraining the transformer inrush current.When the transformer closing voltage close to the system power supply voltage cut off the closing voltage control circuit,the system power supply directly to the transformer,the transformer smoothly into the normal operation.Subsequently,the control function of voltage amplitude is derived from the magnetic flux inequality and the attenuation law of the transient flux component,the value range and particular value of initial value and slope in the voltage amplitude control function is deduced.The no-load closing process of single-phase and three-phase transformer in different conditions is simulated by using PSCAD/EMTDC,and the parameters such as magnetic flux,excitation inrush current and differential current are analyzed in this paper.The simulation results show that by means of controlling the voltage amplitude,the inrush currents can be restrained.Using this scheme,the value of inrush current can be kept in a small range which can overcome the influence of transformer inrush current better.Finally,remanence situation is classified into three cases by firstly analyzing the remanent flux level and the opening angle of three-phase transformer in both external fault and normal operation conditions,and then summarizing the characteristic of remanence in extreme and special values.For each extreme remanence and closing angle case,simulation circuits are built for no-load closing simulation under different conditions,the results show favorable applicability of the proposed scheme in any remanence and closing angle case.On the basis of the scheme,a new voltage amplitude control scheme is developed to overcome the inaccurate remanence estimate and complicated calculation.The new scheme illustrates two conditions.The first one is that the flux cannot exceed the saturation flux during the closing process;the second one is that the flux needs to reach the specified threshold value at the end of the voltage amplitude control scheme,The validity and superiority of the proposed scheme are verified by theoretical analysis and simulation results.