An approximated analytical model for pole-to-ground faults in symmetrical monopole MMC-HVDC systems

Developing pole-to-ground (PG) fault models for Modular Multilevel Converters (MMC) is not straightforward due to the fault asymmetry and converter switching concerning blocking characteristics. Various studies have been carried out regarding transient simulation of PG faults. However, there is a lack of analytical models for the first stage of the fault. Therefore, this work proposes an approximated analytical model for PG faults in half-bridge MMCs. Closed-form expressions for the MMC contribution to the fault and the fault current are derived. We show that separating the solutions in different resonant frequencies represents the system dynamics and facilitates the interpretation of the phenomena. When compared to system calculated by Ordinary Differential Equations (ODEs), the proposed model provided a good approximation for a wide range of parameters. When compared to the full PSCAD solution, the analytical model was able to precisely calculate the peak fault current value, which confirmed its validity.