Research On Improvement Of AC Line Protection In AC-DC Compound Power Grid

In recent years,with the gradual formation of the AC-DC compound power grid in China,the complex fault that AC failure causes commutation failure has become a crucial research direction in China's power grid.In AC-DC compound power grid,when the AC system fails and then the commutation fails,the fault characteristics felt by AC protection are greatly changed compared with the pure AC system.In severe cases,the AC protection will incorrectly operate or even cause DC blocking which will further impact the receiving AC system.A single AC line fault may develop into a systemic chain incident.Therefore,for ensuring the safe and stable operation of AC-DC compound power grid,it is of great significance to study the impact on AC protection of commutation failure caused by AC line faults in AC-DC compound system,and propose corresponding solutions.This paper mainly focuses on the AC protection of AC/DC interconnect system which is greatly affected by DC feed.The simulation models of AC system and AC-DC compound system are established by PSCAD/EMTDC.In the different situations when commutation failure occurs and no commutation failure occurs after AC line fault,the influence of DC feed on the protection action is compared and analyzed,and the corresponding optimization scheme is proposed to improve action performance of AC protection.For the traditional distance protection based on the differential full-cycle Fourier algorithm,an improved scheme based on the differential equation ranging algorithm is proposed.The differential equation algorithm can guarantee the fastness of the I segment of distance protection,and the distributed line parameter model is used to improve the accuracy of the differential equation algorithm.For the traditional differential current protection based on the current amplitude and phase at both ends of the protected line,the paper analyzes the problem of protection rejection in the case of high-resistance fault in AC-DC compound system.Analogous to the case of high-resistance fault under heavy load in AC system,the paper proposes an improved criterion by removing the influence of traverse current.For the misjudgement of the power-frequency variation direction component when the traditional pilot directional protection applied to the AC-DC compound system,combined with the specific case,the protection misoperation process is analyzed.In order to avoid the influence of power reverse,the misoperation of directional protection is avoided by setting a certain delay lock.