An Improved Current Differential Protection in the MMC-based DC Distribution Network

Abstract

When a fault occurs on an AC line connected to a Modular Multilevel Converter (MMC) station in a flexible DC distribution network, the short-circuit current on the MMC side is influenced by its control strategy—such as phase angle control and current limiting—resulting in characteristics significantly different from those in traditional distribution networks. Consequently, conventional current differential protection (CDP) may exhibit insufficient sensitivity or even fail to operate. First, based on the symmetrical component method, this paper analyzes the substantial difference in fault current magnitude between the MMC side and the grid side. Furthermore, the impact of the MMC station on the performance of CDP is specifically discussed under high transition resistance conditions. Given the current-limiting behavior and the theoretically infinite negative-sequence impedance on the MMC side, a novel current amplitude differential protection scheme is proposed. This method utilizes the current amplitudes from both ends, reducing the requirement for synchronized sampling and making it particularly suitable for distribution networks with limited communication capabilities. Finally, a simulation model is developed in MATLAB, and the results validate the effectiveness of the proposed approach.