Control Strategy of Modular Multilevel Matrix Converter Under Two-Phase Operation

Modular multilevel matrix converter (M3C) has the capability of two-phase operation due to its high controlability, but the specific control strategy for the operation still needs to be investigated. Beginning with an analysis of fundamental control principles for low-frequency transmission systems (LFTS), this paper establishes a comprehensive mathematical model of the M3C under steady-state conditions. To achieve stable system control during two-phase operation, this paper systematically investigates the control objectives. Moreover the corresponding mathematical model is derived, demonstrating through theoretical analysis that two-phase operation at the low-frequency side does not compromise the stability of power-frequency side. A dedicated coordinate transformation based on two-phase system is used to ensure accuracy of inner-loop during two-phase operation. The effects of phase-to-phase insulation requirements and active power fluctuations are also taken into account to ensure stability of outer-loop. Based on this, voltage amplitude and fluctuation processing method are determined. The magnitude of the low-frequency output current is analyzed due to the power characteristics. The output current amplitude is determined through the effect of the maximum current. The effectiveness of the control strategy is validated on the PSCAD/EMTDC.