Capacitor voltage balancing and circulating current suppression of modular multi-level converters under unbalanced load condition

Modular multilevel converters can provide higher reliability, improved performance during fault condition, increased efficiency and more flexibility than two level and conventional multilevel converters. By utilizing these structures, higher voltage amplitude can be achieved with lower rated power devices. In this paper, a novel modified capacitor voltage balancing algorithm for regulation of each arm's capacitors is proposed and each capacitor is balanced about its nominal value and in each period, the summation of upper and lower arm capacitor voltages are equalized. So, the decreased deviation between upper and lower half-bridge capacitors can decrease the circulating current. Also, the circulating current among phases (a, b, and c) can be removed in unbalanced load and fault condition. During unbalanced load conditions (such as single phase fault, unbalanced voltage and nonlinear loads), the generated circulating current in each phase can distort the converter performance. The frequency of circulating current is twice the fundamental network frequency that can increase the THD and decrease the converter efficiency. In this paper, the resonant controller with a novel balancing algorithm are employed to improve the converter performance. Finally, simulation in MATLAB/ SIMULINK environment is used to validate the theories.