Straight forward vector control of the Modular Multilevel Converter for feeding three-phase machines over their complete frequency range

Abstract

The paper presents a control strategy for the Modular Multilevel Converter (MMC), which allows feeding a three-phase machine over its complete frequency range. The machine is controlled by a standard field oriented control in the outer closed loop. The inner control has to met the challenge of balancing the energy stored in the capacitance of the converter arms. In this approach two operation modes are used: a low frequency mode for start-up and low speed operation plus a high frequency mode for higher speed. A special control scheme for the low frequency mode has to be applied to achieve low energy pulsation in the arm capacitances. It uses the common mode voltage of the three-phase machine together with inner circulating currents to ensure a symmetrical energy distribution in the arms of the MMC and to avoid any AC-currents in the DC-source.