Research On Influence Of Distributed Capacitance Of AC UHV System On Line Protection

The UHV transmission project has obvious competitive advantages in terms of economy,technology and application,so it has been widely used and developed at high speed in domestic and foreign power systems.The development of 1000kV UHV transmission technology can effectively solve the 500kV ultra-high voltage power grid's inadequate about power transmission technology,which is widespread in the areas of power transmission,safety and stability,and social and economic benefits.However,because the voltage level of UHV transmission lines is doubled compared to 500kV UHV,and the transmission distance is particularly long,and the capacitive current caused by the distributed capacitance problem is correspondingly large.The transient component generated by the fault is obviously different from the transient component of the 500kV transmission system.The large distributed capacitance of the UHV transmission system also brings more problems to the academic research of UHV relay protection.It is significant to study how to effectively reduce the influence of distributed capacitance on UHV transmission line protection.This article compares commonly used differential compensation methods capacitor current steady-state compensation algorithm,time domain compensation algorithm and an improved adaptive capacitor current compensation method.All three algorithms improve the existing line protection criteria to achieve the purpose of capacitive current compensation.The protection algorithm based on the Bergeron model uses line distribution parameters to calculate line electrical quantities.The results are closer to reality and theoretically no longer affected by capacitive currents,which is of more research significance.Based on the in-depth study of the Bergeron model,this article further studies the single-ended transient current protection based on the Bergeron model.The single-ended protection theory can avoid the influence of communication channel and is more beneficial to the protection operation.Aiming at the limitation of the line length and the sampling frequency in the Bergeron model,the interpolation method is used to improve the Bergeron model formula to perform interpolation calculations on data that cannot be directly sampled.Based on the study of traveling wave theory,the wavelet transform modulus maximum is combined with the Bergeron model to complete the fault current calculation.Use zero-mode current to distinguish ground faults and non-ground faults.The discriminant formulas of the fault phase current abrupt changes before and after of the ground fault and the non-ground fault are calculated respectively.The direction of the fault is judged by the time difference between the first arrival of the traveling wave at two points.Using the traveling wave velocity and the time interval to the measurement point,the fault distance is calculated.And use the calculated short-circuit resistance and short-circuit current to verify the fault location value and select the correct fault location value.Finally,the Matlab simulation program was used to verify the interpolation method and the single-ended transient current protection theory based on the Bergeron model using the parameter model of the Jindongnan UHV project line.It is proved by simulation that the interpolation method meets the data requirements,and the simulation data and graphics conform to the theory of fault direction,fault location and fault phase selection,which verifies the correctness of the theory.