Three Degree of Freedom Robust Voltage Controller for Instantaneous Current Sharing Among Voltage Source Inverters in Parallel

Voltage controlled voltage source inverters (VCVSI) are predominantly used as an interface between source and grid in distributed generation. Modularity of system is achieved by parallel operation of several VCVSI of reduced rating. In this paper, a 3-DOF control scheme is proposed for parallel operation of three phase inverters to enable equal load sharing even during transients while tracking a common sinusoidal voltage reference. The voltage reference is either free running or derived from grid voltage and can be used to synchronize a parallel inverter module with any utility grid. The control algorithm for each inverter is identical, and it is independent of terminal parameters of other inverters, granting N+1 modularity to the system. The proposed fast inner voltage loop with second-order controller and lead compensators enable stable operation at low switching frequencies. A voltage correction is added to the reference to ensure sharing of higher order load current harmonics among inverters. A method to estimate the system tolerance to parametric uncertainties and delays is developed using μ-analysis and a method is presented to improve it. The analysis is validated with simulation and experimental results on two 110 V_ac/2.5 kVA three-phase inverters, paralleled to form a stand-alone grid and feeding a nonlinear load.