An Accurate Accelerated Steady-State Model for Modular Multilevel Converters with Very High Voltage Levels

Compared with the conventional two-level voltage source converters (VSCs), multilevel VSCs (MVSCs) have lower common mode voltage, lower voltage stress on power switches, lower dv/dt ratio, and better harmonic content. Among the available MVSCs, modular multilevel VSCs are particularly suitable for an high voltage direct current (HVDC) transmission system. Nevertheless, for an HVDC system, each phase of the MMC terminal may consist of several hundreds of submodules (SMs). This imposes a particularly challenging in terms of steady-state simulation and modeling. Steady-state modeling is usually needed for determining the operating point for the MMC controller design and for predicting harmonics propagation in a power network. In this paper, a highly efficient and accurate steady-state model for an MMC system with high voltage levels is proposed. The model is based on a model reduction technique recently proposed, which is named Adaptive Residual-Time Restarting Krylov Subspace (ARTRKS) method. As will be shown in the paper, the proposed method is able to model various MMC systems. The results obtained from the proposed model are highly agreeable with those from PSCAD/EMTDC. Moreover, it will also be shown that proposed method can drastically reduce the computation time for obtaining steady state solutions.