Date of Award

1-1-2013

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Herbert L. Ginn III

Abstract

Advances in power electronics and generation technologies have increased the viability of distributed generation systems. A microgrid is a special category of distributed generation systems that is distinguished by its size and the ability to operate independently as an islanded system. As long as a microgrid is connected to a large grid, quality of the voltage is supported by the main grid and each power source connected to the microgrid generates independently. In contrast, in the islanded operation of microgrids and in electrical islands such as shipboard distribution systems, dynamics are strongly dependent on the connected sources and on the power regulation control of the grid interfacing converters. In this mode, power sources in a microgrid should be controlled in coordination with each other so that a stable balanced three-phase sinusoidal voltage is provided.

In many cases, energy sources in a microgrid are interfaced through power electronic converters. A higher degree of controllability of converters as compared to electrical machines allows for the possibility of ancillary functions for power quality improvement when converters have unused capacity. The present work proposes a cooperative control approach for converters in a microgrid in which, by efficiently utilizing power converters in response to load demand and required ancillary functions, the operation of the microgrid is optimized. Efficient utilization of power converters is determined by a management system according to an optimization function.

A higher level control is also proposed in this work which exchanges set-point values with local controls through low bandwidth communication links in order to eliminate voltage magnitude deviation, frequency error, imbalance and harmonic distortion at a load bus.

Each of a converter's tasks can be expressed in terms of current components measured at a converter's point of connection to the system. Thus, current-based coordination of a microgrid is performed through a decomposition of current into orthogonal components. Different components of a converter's output current are controlled independently in order to enable optimization of various parameters of a microgrid. All converters in the system are considered including converters that are not actively interfacing an electrical energy source to the grid. Some power units in microgrids are controlled to generate active current according to a reference made by an internal control system such as MPPT system or SOC controller. The presented cooperative control approach is expanded to allow these units to supply active current in accordance with the local reference while they also contribute to generation of non-active currents in coordination with other units. Simulation results verify the benefits of the control approach developed here in both coordination and voltage quality improvement. Thus, the method allows operation of the microgrid to be improved by utilizing the available converters to the fullest extent possible. This reduces the need to connect additional resources to the microgrid.

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