An Enhanced $dq$-Based Vector Control System for Modular Multilevel Converters Feeding Variable-Speed Drives

Modular multilevel converters (M<inline-formula><tex-math notation="LaTeX">$^{\text{2}}$</tex-math></inline-formula> C) are considered an attractive solution for high power drive applications. However, energy balancing within the converter is complex to achieve, particularly when the machine is operating at low rotational speeds. In this paper, a new control system, based on cascaded control loops and a vector-power-voltage (<inline-formula> <tex-math notation="LaTeX">$vPV$</tex-math></inline-formula>) model of the M<inline-formula><tex-math notation="LaTeX"> $^{\text{2}}$</tex-math></inline-formula>C, is proposed. The control system is implemented in a <inline-formula> <tex-math notation="LaTeX">$dq$</tex-math></inline-formula>-synchronous frame rotating at <inline-formula> <tex-math notation="LaTeX">$\omega _e$</tex-math></inline-formula> rad/s with the external loop regulating the capacitor voltages using proportional-integral (PI) controllers. The internal loop controls the converter currents using PI and resonant controllers. In addition, the control systems required to operate the machine at other points, i.e., at medium and high rotational speeds, are also discussed in this paper. Experimental results obtained with an M <inline-formula><tex-math notation="LaTeX">$^{\text{2}}$</tex-math></inline-formula>C-based drive laboratory prototype with 18 power cells are presented in this paper.