Comparative analysis of control strategies for modular multilevel converters

Performances analysis of a modular multilevel converter using two different control strategies is presented. The first approach uses a voltage control with a feedback loop on each individual cell to assure cell's voltage balancing. Nonetheless, this latter can be affected by the total cell's voltage ripple if not compensated. As a consequence, low frequencies are injected in the phase output voltage. To overcome this drawback, the voltage references are modified accordingly to compensate for the voltage ripple and reject low frequencies. A circulating current control is necessary in this approach to reduce harmonics contents in the arm's current. The second approach uses the energy as an image of the state of the system to assure the voltage balancing between the cells. In this method, even though there is no need to use a circulating current controller, the arm's currents are exempt of the 2^nd harmonic. Simulation results are presented to ascertain the operation of the system under steady and unsteady state conditions.