Energy optimized control of MMC arm under unbalanced grid condition

In order to ensure the safe and stable operation of modular multi-level converter and reduce its demand for the capacitance of sub-module, aiming at the energy imbalance between arms and the increase of capacitance energy fluctuation of each arm under unbalanced grid condition, an optimal energy control strategy for MMC arms under unbalanced grid condition was proposed. Firstly, the energy change inside MMC was analyzed under the unbalanced grid voltage condition. Then, using the AC side current three-phase balance as the control target, the current components of the MMC arm were optimally controlled. Based on the phase-to-phase energy balance of each arm, the energy balance of the upper and lower arms was obtained. The circulation current control strategy was used to reduce the capacitance energy fluctuation of the sub-module. The energy of the internal arm under unbalanced grid voltage condition from the two aspects of energy balance and reduction of energy fluctuation, and the corresponding loop control strategy was used to reduce the energy fluctuation of the capacitor in the sub-module. The results show that the proposed control strategy can effectively control MMC internal energy characteristics and is helpful for stable operatation of power system. © 2020, Central South University Press. All right reserved.