Research On The Influence Of High Voltage Internal Transformer Excitation Inrush Current On Zero Sequence Current Protection And Its Countermeasure

With the current increase of power grid capacity and the decrease of system impedance,increasing the short-circuit impedance can reduce the short-circuit current and reduce the cost of circuit breaker.Therefore,the use of high impedance transformer is of great significance to the growing power system.The high-voltage winding builtin high impedance transformer has the characteristics of no additional primary equipment,low design cost and small load loss.Therefore,it has obvious advantages in the application of high impedance transformer.However,because the high-voltage winding of the transformer is closer to the iron core,there is a new case of zero sequence current protection maloperation of adjacent upstream lines caused by air drop of highvoltage built-in transformer.It is urgent to conduct a comprehensive study and put forward countermeasures.This paper studies the particularity of inrush current of high-voltage built-in transformer,analyzes and simulates its impact on three kinds of zero sequence overcurrent protection configured in the system,puts forward a variety of suppression and identification measures for protection in case of misoperation,and establishes a protection risk assessment model.The main research contents and achievements of this paper are as follows:(1)The inrush characteristics of high voltage built-in transformer and its influence on zero sequence overcurrent protection are studied.Firstly,the generation mechanism and characteristics of zero sequence inrush current of three-phase transformer are analyzed,and the expression of zero sequence inrush current amplitude is deduced;Secondly,the special winding structure of high-voltage built-in transformer is analyzed combined with the expression,and verified by Matlab / Simulink simulation modeling.It is concluded that it has the characteristics of larger amplitude of zero sequence inrush current,faster attenuation speed and longer attenuation time than ordinary transformer;The three influencing factors of inrush current,remanence,operation mode and closing angle are simulated and analyzed,and their influence on inrush current is obtained.Finally,according to the actual power grid model,combined with the specific setting principle of protection,the effective value of zero sequence inrush fundamental wave is compared with the protection setting value by changing the iron core remanence and line length,the critical line length and iron core remanence are deduced,and the conclusion that the zero sequence overcurrent protection in Section II of upstream line is more prone to misoperation is obtained.(2)Three countermeasures against zero sequence inrush current of high voltage built-in transformer are studied.Firstly,on the basis of theoretical derivation,five inrush suppression strategies are simulated and analyzed,and their advantages and disadvantages are obtained;Then,the external operating conditions of the transformer are improved,and four optimization measures are proposed;Finally,based on the three characteristics of the second harmonic content of zero sequence inrush current,a new second harmonic identification method based on zero sequence current sudden variable is proposed,and a simulation model is built to analyze three examples to verify the effectiveness of the new criterion.(3)The risk assessment model of protection based on MATLAB / Simulink is studied.Firstly,the risk assessment parameters and the characteristics of the traditional protection risk assessment model are analyzed;Then,according to the inrush current influencing factors,a protection risk assessment system based on closing angle is proposed.Considering the difficulties in obtaining residual magnetism and operation mode,a weighted protection risk assessment model with two influencing factors missing is designed;Finally,the risk assessment of line zero sequence overcurrent protection is carried out,and the weight of missing factors and protection misoperation probability are solved.It is verified that this model can provide effective early warning for the operation of high-voltage built-in transformer.