A Clark Modulus Transform-based Protection Method for Metallic Return Flexible DC Grids

Developing protection strategies for disconnection faults begins with a thorough understanding of fault characteristics. This paper focuses on a bipolar DC system with a metal return line. It begins by examining the state variations of the modular multilevel converter (MMC) in a DC system during fault conditions. By applying the superposition principle, an equivalent circuit model for the MMC is derived, specifically for when faults occur in the DC system. This leads to the construction of an equivalent circuit for disconnection faults within the system. Subsequently, the study enhances the polar mode transformation matrix to deduce the composite modulus equivalent network for various fault scenarios. It further analyses the modulus properties of different types of disconnection faults. Utilizing the fluctuations in 2-mode and 0-mode currents, phase planes are developed to identify disconnection fault types, aiding in their classification. Additionally, the process of identifying disconnection faults, which hinges on the variations of mode 1 currents entering and leaving the discriminative zone, is introduced. The final part of the paper presents a simulation model, crafted in PSCAD/EMTDC. This model, set up for a four-terminal MMC metal return line bipolar DC network, serves to confirm the speed and dependability of the proposed protection approach through various simulation scenarios.