Application of Modular Multilevel Converter for Interfacing Grid-Connected Photovoltaic Conversion Plants
MetadataShow full item record
- Institutt for elkraftteknikk 
This thesis investigates the applicability of the Modular Multilevel Converter (MMC) forinterfacing grid connected photovoltaic conversion plants.A detailed three-phase 9-level simulation model is implemented in Simulink. Two controlobjectives are identified as distinctive for the MMC: Capacitor voltage balancing andsuppression of circulating currents, both of which are included in the model. The MMC iscontrolled by a modified Level-Shifted Pulse Width Modulator. The model is verified bycomparing 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 modulesconnected to the same MPP tracker should have the same operating conditions. For largescalepv farms this is only feasible with multiple MPP trackers.Two pv inverter configurations are identified as suitable for grid connection of large-scalepv farms using the MMC: Cascaded dc-dc converters and multi-string inverter. With theformer, 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 submodulesare inevitable. This can be remedied by power imbalance compensation.For grid side control Synchronous Reference Frame Control (SRFC) and Model PredictiveControl (MPC) is considered. MPC has the advantage of handling non-linear constraintson both states and variables. In addition it is reported to perform better than SRFC duringdynamic 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 anddelivers power at unity power factor.