Research on a New Grid-Connected Inverter Technology for Building-Integrated Microgrid

Description

In today’s environmentally-conscious world, there is an urgent need to find ways to improve the quality of electrical power system and explore new sustainable renewable power sources. An emerging trend in the electricity industry is a shift from large power plant to small distributed generation (DG) systems located at the point of consumption to form a microgrid. Such architecture has the advantages of optimizing asset utilization, power quality, and system reliability, flexibility and capacity.Regardless of the type of renewable power source, the grid-connected inverter is a crucial component that converts the energy generated by the source to the grid. The power conversion stage of the inverter mainly consists of a high-frequency switching circuit and an output filter. The switching circuit converts the incoming dc power into a high-frequency ac power. The output filter allows the line-frequency power to pass and attenuates the switching harmonics to the grid. There has been much research in reducing the physical size and weight of the output filter design and advancing the control technology for meeting the static and dynamic power flow requirements.In principle, the physical size of the output filter decreases with an increase in the system order. It is thus an attractive strategy to use a high-order filter to increase the power density, energy efficiency and dynamic response of the inverter. However, high-order 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 conversion stage or an active damping technique in the controller, but they would introduce extra power loss and limit the system dynamics. Therefore, dominant research in the field still focuses on the use of low-order filters.This project aims at enabling a breakthrough in grid-connected inverter technology by the research of the methods for controlling inverters with high-order output filters. A new concept of utilizing the power transfer characteristics and resonant properties of the output filter to perform independent active and reactive power control, and control of the output oscillation will be studied. It allows the inverter to regulate power flow to the grid and the voltage at the interface independent of the value of the transmission line impedance. It will also pick up appropriate share of load in a rapid and seamless fashion when the microgrid islands from or reconnects to the macrogrid.