Control, analysis, and design of distributed inverter systems

As renewable energy sources and storage technologies become increasingly commonplace on the power grid, the number of power electronic inverters can be expected to grow. Since power delivery can be easily controlled with power electronic circuits, inverters over a wealth of opportunities in the coordination of highly distributed ac power systems. This dissertation is focused on the development of inverter coordination methods such that system distortion is reduced and stability is maintained. In particular, a method of switch interleaving is developed such that the distortion generated by a system of inverters is canceled. This method is applied to series connected H-bridge inverters which interface a distributed system of dc sources to an ac load. The resulting system can employ a very low switching frequency and small output filter while maintaining a low distortion ac output. A control system is developed which manages the cascaded H-bridge inverters during uniform and nonuniform conditions at the dc inputs. In addition, switch interleaving is applied to a system of parallel connected inverters. A closed-form expression is derived which describes the total current generated by a system of parallel interleaved converters. This result is presented with a level of generality such that it can be applied to both dc-dc and inverter systems. A technique for coordinating parallel inverters in a microgrid is formulated such that they are able to synchronize their ac outputs. The resulting microgrid is modular and does not require communication between inverters. A theoretical framework is developed which leads to a sufficient condition for inverter synchronization. It is shown that the synchronization condition is independent of the number of parallel inverters and the load parameters. Throughout the thesis, experimental and simulation results are used to substantiate the analytical results and illustrate the merit of the proposed techniques.