Modelling and control of inverter sources within a low voltage distributed generation system

The Microgrid has been proposed as a way of combining distributed generation which can have a positive impact on power quality. To solve many of the challenges a microgrid presents, transient modelling is important. This work develops a library for creating a transient model of a microgrid based on transfer function representations of microgrid components, and drawing from techniques to implement simple models of inverters. The developed library allows for the fast, accurate simulation of multiple parallel sources connected to a single load through coupling inductances. Typically, in a microgrid, a controller with a power-frequency droop is used in the sources. This allows for power sharing when the microgrid is in island mode. In this work, the model library was used to develop simulations which illustrated the known problems with the basic droop control implementation and some of the solutions proposed in the literature. One area which had not been addressed in detail in relation to droop control, was the high likelihood of unbalanced loads in a stand-alone system. In this work, a modification to the droop controller was developed which allowed for the reduction of voltage imbalance in the presence of unbalanced loads. The controller calculates the amount of negative sequence voltage at the load, and compensates for this at the source. This unbalance controller was then integrated into a new controller developed to address many of the weaknesses of the standard droop control configuration. Due to the lack of inertia in the power sources, the typical droop controller setup requires storage on the DC side of the inverter (typically a battery) in order to allow for step changes in load demand. In this work, a controller was developed for an AC grid connected storage device (such as a flywheel) which was effective in reducing the need for individual storage for each source. The advantage to this approach is that the grid connected storage device can be chosen to be more environmentally friendly than the batteries which are typically used for the DC storage.