An Additional DC Voltage Control Strategy for Modular Multilevel Converter

This paper proposed an additional DC voltage controller to adjust the DC voltage to keep the modulation index of a converter within a proper range. Firstly, through the modulation theory of modular multilevel converter, it illustrates the minimum value of modulation index which is critical to guarantee the converter to generate staircase waveform with full voltage levels. Secondly modulation index should be lower than 1 to keep the converter stable operation. Therefore, the additional control strategy and its controller are introduced to realize a dynamic control of DC voltage which has a non-linear relationship with the modulation index. In addition, the calculation method of upper and lower limits for the output of the proposed controller is essential to maintain a proper modulation index for each converter, especially in a multi-terminal system. Simulation results show the proposed control strategy is effective and the controller can achieve the goals.