Harmonic Balance-Based Space Vector Approach for MMC Inter-Leg Equalization

This paper proposes a space-vector modelling formalism and a harmonic balance control strategy for the voltage equalization of a Double-Star Modular Multilevel Converter employed as an active interface between a three-phase AC grid and a HVDC grid. The balancing procedure is based on the control of the power flow through the converter legs, with the aim to stabilize the energy stored in each switching module to the same value. By properly exploiting both the space-vector/zero-component and the common/differential mode decompositions, the proposed approach is able to highlight all the viable interactions between the system voltages and currents. The harmonic balance control is then formulated in its most general form in order to effectively increase the degrees of freedom available to the voltage equalization while guarantying the hardware constraints and functional requirements of the converter. The resulting control algorithm, based on a subset of the harmonic control variables, has been implemented on a controller board and experimentally validated on a 1.5 kW test bench.