Reduced Modular Multilevel Converter Model to Evaluate Fault Transients in DC Grids

This paper proposes a reduced model for a modular multilevel converter (MMC) that can be used to evaluate the first transient after a short circuit fault in a DC grid. Detailed modelling of an MMC involves a large number of electrical nodes, hence requiring high computational effort. Reduced converter models have been proposed in the literature. However, calculation times can still be high for large grids. The reduced model proposed in this paper is based on an RLC-circuit that models the capacitive discharge phase of the MMC during DC faults. Therefore, it can be used to efficiently evaluate fault detection criteria that must act within the first transient. By performing transient simulations for a pole-to-pole fault at the converter terminals, the suitability of the model to represent the MMC during faults is demonstrated. Furthermore, it is shown by transient simulations that the model can adequately represent the reflection of travelling waves due to faults in a multiterminal system.