New control Approach for Modular Multilevel Converter Under Unbalanced Load Conditions

This paper presents a new method of balancing the cell voltages of a DC to 3AC Modular Multilevel Converter (M2C) under unbalanced conditions on the AC terminals. When the load current is not a symmetrical sinusoidal three phase system, the cell capacitors of the M2C are stressed differently. This new control approach uses a sliding window discrete Fourier transform (DFT) to identify and compensate the negative sequence of the capacitors energy ripple. Additional internal currents compensate the negative sequence energy ripple to even out the stress of each capacitor in the M2C.