Droop based control of parallel-connected single-phase inverters in D-Q rotating frame

In this paper, control of parallel single-phase inverters in direct-quadrature(DQ) rotating frame based on droop method is proposed. For each inverter, a secondary orthogonal imaginary circuit is created to provide the second phase required for the transformation; thus a DQ model of the inverter is obtained. The DQ transformation provides a time-invariant model of the inverter which makes the control design similar to that of dc-dc converters. The recognition of this analogy is important in that all of advanced dc-dc converters control techniques that have been previously developed can be applied to the control parallel inverters. The controller then is designed in DQ frame. Theoretically, infinite loop gain can be achieved in DQ rotating frame resulting in the elimination of the steady state error at the fundamental frequency of the inverter. The output impedance of the inverters is also considered, to ensure the decoupling between active and reactive power control using droop method. Simulation results are provided to prove the concept.