Arm Overcurrent Analysis and Calculation of MMC-HVDC System with DC-link Pole-to-Pole Fault

Modular multilevel converter-based high voltage direct current (MMC-HVDC) technology has aroused extensive research and discussion. DC-link pole-to-pole short-circuit, the most severe fault type on DC side, has also been studied. This paper analyzes the DC-link pole-to-pole short-circuit characteristics and models the equivalent circuit of the fault current of each arm during the initial stage of the fault before the control system blocks the sub modules. Then, an improved equivalent analytic formula of calculating the arm overcurrent based on a regulation coefficient is proposed and the changing rule of the regulation coefficient is discussed under different operation conditions to provide reference for system design and relay protection setting. The calculated fault current based on the proposed methods are compared with the simulation results from PSCAD/EMTDC to verify that the results from analytic formula are accurate.