High Sensitivity Differential Protection for AC Line in MMC-HVDC System

Due to the influence of MMC control mode, the differential protection of AC transmission line in MMC-HVDC system has low sensitivity. To solve this problem, a high sensitivity differential protection scheme based on phase correction is proposed. The phase offset affected by the control link is calculated and the additional phase Angle is obtained. The additional phase Angle is introduced in the calculation of differential protection. In view of different fault phases, positive and negative sequence current is used to correct the phase. The influence of MMC control strategy on differential protection is reduced. The scheme can effectively identify faults and has high sensitivity. At the same time, it can adapt to different operation modes of MMC and has strong resistance to transition resistance.

I. Introduction

Modular multilevel converter (MMC) has the advantages of high modularity, no commutation failure and easy decoupling control. The flexible DC power grid based on MMC can realize the flexible control of power flow in transmission system. At present, it is widely used in wind farm grid connection, passive system, weak grid power supply and urban power supply.