Analysis and Control of Grid Tied Modular Multilevel Converter under Unbalanced PV Power Generation and Unbalanced Grid Conditions

In this paper, Modular Multilevel Converter (MMC) is considered for integrating the giga-watt PV farms with the power grid. In the considered MMC configuration, the PV panels are directly connected across the sub modules of MMC. This MMC-configuration suffers from power imbalance between the arms, phases or both due to non-uniform illumination of the PV panels. In addition to the internal abnormalities in MMC, faults like Line to Ground (L-G) and 3-φ faults are very frequent in a power system. In this paper., the behavior of MMC under various grid conditions and for varying PV power injection is investigated and analyzed. The mathematical formulation shows that the arm current and circulating current may increase drastically which subsequently increases the ripple in the capacitor voltage. To handle this issue, a control algorithm is proposed to keep the circulating currents, arms currents and capacitor voltage ripple within limits under the various grid faults and internal abnormal conditions in MMC. The proposed algorithm controls the power from the PV Converter (PVC) and ensures minimal circulating current and ripple in capacitor voltage. To validate the claims, the system is simulated in MATLAB/Simulink environment. Various waveforms are showcased to demonstrate the efficacy of the proposed algorithm.