Characterization and Control of a System with Multiple Offshore Power Inverters Connected in Parallel with Long Cables

An emerging trend in the electricity industry is a paradigm shift from large-scale centralized power plant to small-scale distributed energy resources (DERs) located at the point of utilization. Regardless of the type of DER, inverters, which convert DC into AC power, are crucial devices for injecting generated energy into the macro-grid. In order to offer a high degree of modularity, scalability, adaptability, maintainability, and autonomic behavior, it is more advantageous to use multiple low-power parallel-connected inverters than a single high-power inverter unit. In many applications, such as small-scale wind farms and wave generator systems, those low-power inverters are connected together through long cables. Due to possible mismatch among the output impedances of the inverters, cable characteristic impedance, load characteristics, and grid impedance, the entire system could be dynamically unstable. Furthermore, high-order output filters exhibit multiple resonant frequencies that would cause output oscillation. An existing remedial measure to alleviate this problem is to apply a passive damper in the power stage or an active damping technique in the controller, but they would cause either extra power loss or limit the system dynamics.This project aims to enable a breakthrough in multi-parallel-connected inverter technology by investigating 1) interactions among the inverters, cables, loads, and power grid, 2) predictive control algorithms for controlling active and reactive power flow, 3) an active damping technology at the point of common coupling, and 4) a fault diagnosis technique for DERs. The findings will lay foundation and research directions for new-generation inverters to 1) regulate the power flow to the grid and the voltage at the interface independent of the value of the transmission line impedance, and 2) pickup up appropriate share of load in a rapid and seamless fashion when any inverter islands from or reconnects to the power grid, and the system to 3) keep the operation stable under normal condition, and 4) automatically detect and isolate the system from any fault or abnormal condition.