A conflict in control strategy of voltage and current controllers in Multi-Modular single-phase UPS inverters system

This paper proposes an optimised design approach for Multi-Modular (M-M), single-phase UPS inverters system. The Voltage Source Inverters (VSIs) have low output impedance for good voltage regulation, which conflict with the requirement of high impedance in the parallel configuration to restrain Inter-Modular (I-M) circulating currents. Conventional dual-loop approach of Proportional-Resonant (PR) compensators in inner and outer loops has a drawback of poor current sharing. This paper suggests the use of Proportional (P) and PR compensators in respective voltage and current control loops along with the Instantaneous Average Current Feed-Forward Sharing (IACFFS) control to achieve the required voltage regulation and proper current sharing. This current sharing control scheme is employed to introduce high impedance to the I-M circulating currents without disturbing voltage waveform quality. The paper thereafter demonstrates a better current sharing capability with the presented control algorithm through mathematical modelling and simulations in MATLAB environment.