Small-signal modeling and analysis of parallel-connected voltage source inverters

Numerous benefits promote the proliferation of renewable and distributed energy resources in power grids. Some recent research has focused on the concept of microgrids, in which a number of renewable energy sources can be connected together to supply power to customers as a complement to traditional power sources or as main power sources. In a microgrid, multiple distributed energy sources are operated in parallel through power converters with several control loops of different control objectives. Thus the dynamic stability problem is increasingly important in microgrids due to the existence of a large number of power converters, and has not yet been widely studied. This paper presents a small-signal model of parallel operated voltage source inverters, in which the variations in d-q axis currents and output d-q axis voltages are taken as state variables, the variations in the power inverter duty cycles are used as the control input, and the variations in the DC power source voltages are considered as external disturbances. Frequency-domain characteristics of the open-loop and closed loop systems are analyzed. Control strategies are designed for two parallel connected inverters. Time-domain simulation studies are carried out to verify the small-signal model and controller design.