A Universal Controller Under Different Operating States for Parallel Inverters with Seamless Transfer Capability

A universal controller is proposed in this article to operate parallel inverters in both grid-connected (GC) state and standalone (SA) state and ensure seamless transfer between them without reconfiguring the control structure. The universal controller is mainly composed of frequency-locked-loop and three cascaded control loops: a grid current loop, capacitor voltage loop, and inductor current loop. A proportional-integral regulator is adopted in the grid current loop, and a limiter is inserted after the integrator. In the GC state, the proposed controller accurately regulates the grid current of an individual inverter. When islanding occurs, the proposed controller can automatically convert from grid current control to $v_{C}\hbox{--} i_{g}$ -based droop control; critical islanding detection is not needed. This universal controller has several advantages. First, it can realize seamless transfer of parallel inverters; microgrid reliability can be guaranteed in the SA state, and power sharing can be achieved among parallel inverters without communication lines. Second, compared with droop control, it can regulate the grid current accurately and output constant power when the grid voltage fluctuates in the GC state. Third, the grid current harmonics in the GC state and capacitor voltage harmonics in the SA state can be mitigated. Simulation and experimental results verify the effectiveness of the controller.