Study On Improved Algorithm For Differential Protection Of UHVDC Line

UHVDC(Ultra High Voltage Direct Current)plays an important role in the construction of power grid in our country with unique technical advantage and good economic benefit.The UHVDC transmission lines are long,the geographical environment and the climatic environment along the line are complex and changeable,and the probability of failure of the line is high.In view of the fact that the conventional main protections in DC line have the problems of low reliability and weak anti-interference ability,it is necessary to develop a new protection algorithm to improve and optimize the DC line protection.This paper systematically analyzes the characteristics of traditional differential protection algorithm about DC line.The traditional current differential protection for HVDC is significantly influenced by high frequency harmonic disturbance,triggering factors including power electronics and distributed capacitance,etc.Due to the lack of effective restriction method,it is difficult to coordinate the threshold and the action time,and the performance of the protection is not high.Aiming at these problems,the additional fault model and the fault model about HVDC transmission lines are established.An improved differential protection algorithm based on fault mutation and an improved differential protection algorithm based on fault totality are proposed.The core idea of this improved algorithm comes from the longitudinal impedance.It utilizes the relationship of equivalent electrical triangle balance among the specific voltage data,current data and line resistance,combined with the low-pass filtering effect of the integral algorithm,to construct the proposed criterion.The sum of the currents at both ends of the line is taken as the action amount,and the ratio of the difference of the voltage at both ends of the line to the line resistance is used as the braking amount.Theoretical analysis shows that: for external faults,the movement is significantly less than the brake;for internal faults,the movement is greater than the brake,the criterion can be set in a simple and clear way.Meanwhile it is insensitive to the sampling frequency and independent of distributed capacitive current.Based on the parameters of ±800kV Yun-Guang transmission line,PSCAD/EMTDC software is used to build a bipolar UHVDC transmission system simulation model.Theperformance of improved algorithms are validated by simulation data in the PSCAD/EMTDC and MATLAB software.Comprehensive simulation experiments show that: the improved algorithm based on mutation operates with high reliability and selectivity under different fault conditions with high fault resistance,it has the characteristics of high reliability and sensitivity,and can recognize the external or internal fault precisely and rapidly;The improved algorithm based on fault totality has high reliability and long validity period,and it can be used as backup protection for mutation improvement algorithm.The above improved algorithm of differential protection have good practical value in engineering.