Frequency Domain Control Algorithm for Modular Multilevel Converter Under Grid Fault Conditions

This paper presents a novel control method that enables correct operation of Modular Multilevel Converters under grid unbalance or fault conditions such as single phase-to-ground, two phase-to-ground and phase-to-phase faults. The control method is based on frequency domain decomposition of supply voltages and overcurrent protection algorithm. The proposed control method produces positive sequence sinusoidal grid currents even during the presence of various types of grid faults. The paper also presents a modified feedback approach for active and reactive power controller loops in order to limit the maximum current during fault conditions. A new cost-\function based power reference calculation algorithm is also proposed in order to operate the converter at higher efficiency level during grid fault conditions. Simulation results show the effectiveness of the proposed control structure under various types of grid fault conditions