Application of Modular Multilevel Converter for Interfacing Grid-Connected Photovoltaic Conversion Plants

Abstract

This thesis investigates the applicability of the Modular Multilevel Converter (MMC) for interfacing grid connected photovoltaic conversion plants. A detailed three-phase 9-level simulation model is implemented in Simulink. Two control objectives are identified as distinctive for the MMC: Capacitor voltage balancing and suppression of circulating currents, both of which are included in the model. The MMC is controlled by a modified Level-Shifted Pulse Width Modulator. The model is verified by comparing its behaviour to that of the mathematical model of the MMC. The nature of photovoltaic power generation makes Maximum Power Point Tracking (MPPT) important to maximize the power yield from a pv module. All the pv modules connected to the same MPP tracker should have the same operating conditions. For largescale pv farms this is only feasible with multiple MPP trackers. Two pv inverter configurations are identified as suitable for grid connection of large-scale pv farms using the MMC: Cascaded dc-dc converters and multi-string inverter. With the former, the three phase legs share the same dc link voltage. With a multi-string topology, each submodule is fed by a separate pv string. Thus, power imbalance between the submodules are inevitable. This can be remedied by power imbalance compensation. For grid side control Synchronous Reference Frame Control (SRFC) and Model Predictive Control (MPC) is considered. MPC has the advantage of handling non-linear constraints on both states and variables. In addition it is reported to perform better than SRFC during dynamic conditions, which are likely to occur with power generation from pv modules. SRFC is implemented in the MMC simulation model. It synchronizes with the grid and delivers power at unity power factor.