Study on the Short-Circuit Current of Asymmetrical AC Fault Near MMC

Abstract

The mechanism of the contribution of modular multilevel converter (MMC) on short-circuit current under asymmetrical fault at the point of common coupling (PCC) was studied according to operation characteristics and control strategy applied in fault. A ± 320kV two-terminal MMC system was established in PSCAD/EMTDC to verify the conclusion. Simulation results show that after fault, MMC performs as a current source at AC side because of the characteristics of the inner loop current controller. After the controllers respond, the fault current does not contain DC component. The steady-state short-circuit current is symmetrical, which can be computed by the reference value of the inner loop controller. The limits of PI of the outer and inner loop controller affect steady-state short-circuit current. The flow directions of active power of MMC before and after fault are consistent, while the flow direction of reactive power is related to the control mode and operation state. The research provides valuable guide to parameters tunning of MMC controller and relay protection.