Multivariable current control of Modular Multilevel Converters with disturbance rejection and harmonics compensation

Voltage Source Converter (VSC) technology is very common in high-voltage direct current (HVDC) transmission systems. Compared to conventional VSC technology Modular Multilevel Converters (MMCs) offer advantages such as higher voltage levels, modular construction, longer maintenance intervals and improved reliability. For power transmission, stability, robustness of the control and especially the behavior during grid failures are important economic issues. Furthermore, compensation of harmonics can be an additional feature to improve the voltage quality of the connected grid. Regarding the innovative MMC topology, special control features are needed due to the higher number of control variables. This paper presents a multivariable control approach to realize an optimal current control of the positive, negative and zero phase-sequence converter currents without steady-state error and the compensation of higher harmonics using an extended estimator.