| Since the charging and discharging of electric vehicles is random and uncertain in time and space,it can be regarded as random load,which may bring negative effects to the safe and stable operation of power grid,and bring difficulties to the relay protection configuration and setting of distribution network.In this paper,the actual topology of electric vehicle charger is studied,and the calculation method of electric vehicle short circuit current considering converter control is put forward.On this basis,the random load characteristics of electric vehicle are analyzed,the influence of random load access on the distribution network protection is studied,and a protection scheme suitable for electric vehicle random load access is put forward.The main research contents and innovation point of this paper are as follows.A method of calculating the short circuit current of electric vehicle considering the control of the converter is proposed to adapt to the situation that the transient process of the electric vehicle is difficult to quantify.First,based on the actual topology of the electric vehicle charger,the basic structure and mathematical model of the voltage source PWM rectifier are analyzed and studied.Secondly,qualitative analysis and theoretical derivation of the transient process of electric vehicle fault are carried out,and the approximate analytical expression of short-circuit current of electric vehicle is obtained under short-circuit of distribution line.Finally,the fault characteristics of the electric vehicle in the fault situation are analyzed by simulation,and the correctness of the analytical expression of the short circuit current is verified.The short-circuit current calculation method will focus on the research of fault transient characteristics of electric vehicle charging and discharging motor,and focus on the control strategy of PWM Rectifier’s transient process.By approximating the linear differential equations with constant coefficients,the approximate analytical expression of the short circuit current is obtained.The random load characteristics of the electric vehicle and its influence on the current protection of the distribution network are studied,which makes the mechanism of the electric vehicle to affecting the protection more clearly.First,the influence of the large-scale electric vehicle charging on the regional load characteristics is analyzed.Then,combined with the theoretical analysis and simulation verification,the influence of the random load characteristics of electric vehicle on the distribution network protection is analyzed.The research results show that electric vehicle charging will generate impulse current instantly,which may lead to the incorrect operation of instantaneous current interruption protection.Moreover,when the large-scale electric vehicle is connected to the distribution network,the increased overcurrent protection setting value will cause the sensitivity of the protection to decrease or even refuse to move.A sensitivity-based backup setting method for current protection and a corresponding regional backup current protection scheme are put forward to solve the problem of electric vehicle random load access and the protection of long and short circuit coordination.First,in order to ensure the sensitivity of the backup section of the current protection,a setting method is proposed to calculate the fixed value only according to the sensitivity requirements.Then,in order to solve the problem of loss selectivity,a regional backup current protection scheme is proposed.The scheme takes feeder as the basic unit,and uses the coordination of multiple current protection between the upper and lower levels to form a new regional protection criterion,which ensures selectivity.Finally,the feasibility and correctness of the regional backup current protection scheme are verified by the example of IEEE34 node distribution system.The area protection method effectively considers the sensitivity and selectivity of relay protection.At the same time,this method does not protect the backup section according to the maximum load of the line,avoiding the influence of electric vehicle load access. |
PDF Full Text Request