A novel MMC bridge arm protection based on sum current of upper and lower bridge arms

Abstract

In flexible direct current transmission system, due to the nonlinear operation of the converter, the bridge arm current contains other components besides the power frequency component when a fault occurs at the outlet of DC line, which may lead to the calculated power frequency current amplitude greater than the setting value, and the existing bridge arm overcurrent protection will misoperate. In order to solve the problem, a novel converter bridge arm protection based on the sum current of upper and lower bridge arms is proposed. Firstly, the equivalent circuit of high voltage direct current system based on modular multilevel converter (MMC-HVDC) before blocking the insulated gate bipolar transistors (IGBTs) is established in the case of a bipolar short-circuit occurs at the outlet of DC line, the bridge arm current is deduced, and the influence factors and variation characteristics of each component in bridge arm current are studied. Secondly, the misoperation reasons of the existing overcurrent protection of converter bridge arms are analyzed. Finally, a novel MMC bridge arm protection based on the sum current of upper and lower bridge arms is proposed, and its action characteristics of the internal and external faults of the bridge arm are analyzed. To overcome the problem that the proposed protection cannot reflect the upper and lower bridge arms short-circuit, a supplementary criterion is proposed to identify this fault by measuring whether the upper bridge arm current and the lower bridge arm current are mainly power frequency components. The sum current protection and the supplementary criterion can reflect various faults in bridge arms, including the short-circuit between two bridge arms which can not be reflected by the existing bridge arm overcurrent protection, and they will not misoperate in the case of a fault occurs in DC line. The simulation results verifies the effectiveness of the proposed protection principle.