| dc.description.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. | |
