Fault current calculation in MTDC grids considering MMC blocking

In this paper, a generic fault current calculation method based on coupled differential equations is proposed which is valid for both blocked and unblocked states of the MMC. The first part of the paper concerns the modeling aspects of MMCs in blocked and unblocked states, while the second part focusses on the representation of the DC grid. Overhead lines are modeled as lumped RL-elements, whereas cables are represented by an adapted pi-model in order to consider the capacitor discharge. A simplified DC Circuit Breaker model is integrated into the set of coupled differential equations, which allows to estimate the energy absorption requirements during fault current suppression. The results of the method are compared to EMT simulation results and validated throughout different fault scenarios. The method enables the representation of temporary MMC blocking for a large timeframe of tens of milliseconds after fault occurrence and is convenient for the sizing of DC Circuit Breakers and DC reactors in MTDC grids.