Improved Power Control of Inverter Sources in Mixed-Source Microgrids

The Consortium for Electric Reliability Technology Solutions (CERTS) microgrid concept is an established approach for controlling many distributed sources on either an isolated or grid-connected power system that does not need a fast communications network for control. This research investigates the impact of load transients on the operating characteristics of CERTS microgrids. For voltage source-based distributed generators, load changes can overload microgrid sources that have low operating margins, causing significant overcurrents, instability and voltage collapse. This research investigates the dynamic characteristics of inverter microsources in microgrids and determines the impediments to fast load redistribution and rebalancing. Based on these results, control techniques are developed that maintain microgrid stability and rapidly redistribute load changes among the sources to minimize the stresses on sources with limited overload power and energy capabilities. Linear operating point analysis and simulation techniques are used to explore the dynamic characteristics of microgrids under various network parameter sets. Attention is focused on a particularly challenging microgrid configuration consisting of photovoltaic (PV) sources having low amounts of stored energy. The power regulation bandwidth of a photovoltaic inverter must be fast enough to conserve its limited available energy and avoid collapse of its DC bus voltage during load transient conditions. Both grid-forming PV inverter controllers using frequency droop regulation and grid-following inverter controllers using current regulation are developed and compared to evaluate their dynamic power regulation capabilities and their impact on microgrid performance. This comparative analysis leads to the development of techniques for improving the power regulation bandwidth of grid-forming controllers that reduce the risk of instability and faults instigated by load transients. One of the key findings of this research is a limit on the maximum frequency droop gain to insure microgrid stability that is directly proportional to the resistance of the electrical lines/cables interconnecting the microgrid sources. Two control techniques are demonstrated for increasing this limit on the maximum frequency droop gain and power regulation bandwidth: 1) by actively increasing the effective line resistance, or 2) by decreasing the effective frequency droop gain. Key results of this work are experimentally verified using a laboratory CERTS microgrid testbed.