Double virtual - impedance loop for inverters with repetitive and droop control in UPS applications

Abstract

A strategy for the implementation of the output impedance in multi - loop inverters with repetitive control for uninterruptible power supply applications is proposed. The implementation consists of a double virtual-impedance loop with a second-order generalized integrator to independently establish the output impedance in direct current, at the fundamental frequency of the output voltage and its harmonics, with a reduced computational cost. The proposal allows achieving an adequate distribution of the current of a non-linear reference load between inverters connected in parallel with droop control. Design and implementation is experimentally validated considering the requirement of dynamic response, voltage regulation, individual harmonic content and total harmonic distortion of the IEC62040 - 3 and IEC61000 - 2 - 2 standards for two 500 VA inverters prototype connected in parallel and in islanded mode.