Elimination of MMC differential currents via a feedback LTI control system

The interesting features of the modular multilevel converter makes it the VSC solution of choice by the manufactures in the HVDC field. Far more complex than the older VSC solutions, the MMC generates less harmonics and losses. One of its drawbacks is that, internally, this topology creates a double line frequency current component which flows between phases. This work describes the origin of the double line frequency current and presents, step-by-step stationary reference frame circulating current suppress controller (S-CCSC) as a feedback linear time invariant (LTI) control to cancel out this source of additional losses. The proposed S-CCSC solution was compared to the previously proposed 2ω rotating reference frame CCSC (R-CCSC). Both solutions show very fast results, removing almost instantaneously the second harmonic current without leaving any evidence of them in steady state. Moreover, the LTI solution is demonstrated to demand 33% less computational time, which is a clear advantage from a practical application perspective.