An Improved Multiple-loop Controller for Parallel Operation of Single-phase Inverters with No Control Interconnections

In this paper, we present a multiple-loop control scheme which is able to improve the transient response and power sharing accuracy of parallel-connected single-phase inverters with no control interconnections, and we also analysis the active and reactive circulating current of the parallel system model. A multiple-loop controller is developed, by adding four compensation loops to the traditional droop method. The output voltage and load current compensation loops are added in the scheme to improve the performance and robust of a single inverter. The instantaneous reference adjustment loop and variable output impedance adjustment loop are added to enhance dynamic performance of parallel inverters, as well as reduce the circulating current at the parallel inverter´s startup and running. The proposed controller provides both steady-state objectives and a good transient performance. Two 1 kVA DSP-based single-phase UPS inverters are designed and implemented. Simulation and experimental results are all reported, confirming the validity of the proposed control technique.