Voltage Ride through Control Strategy of Modular Multilevel Converter under Unbalanced Voltage Sag

This paper develops modeling and describes a control strategy for a modular multilevel converter (MMC) for grid-connected renewable energy systems. The proposed model can be used to simulate MMC activity during normal and faulty situations. Firstly, a dynamic model of a grid-connected MMC (GC-MMC), based upon the symmetrical component of voltages and currents, was designed. Then an adaptive robust control approach was established in order to follow the reference currents of the converter and stabilize the submodule (SM) capacitor voltage. The positive and negative sequences of reference currents that were given from the demanded active and reactive power during grid voltage disturbance and a normal situation were then utilized in control loops. Finally, the numerical results for the performance of the MMC throughout voltage sag conditions and the effect of uncertainties on the filter parameters during changing power demands were evaluated. The results specified that the current control strategy is more potent under voltage sag situations and able to fulfill the stability requirements of the MMC.