Load Current Distribution between Parallel Inverters based on Capacitor Voltage Control for UPS Applications

Many challenges are encountered when uninterruptible power supply (UPS) modules are operated in parallel to meet the power demand of the load. The circulating current is one prominent example of these challenges that may cause serious damage to equipment. Hence, control algorithms must ensure that the current is shared equally or in proportion to the power ratings of the parallel inverters, while mitigating the circulating current. Another challenge is that all parallel inverters should have equal output voltage, phase, and frequency. This paper proposes a new method to control parallel UPS modules. The method is based on regulating the capacitor voltage of the LCL output filter of each inverter to indirectly control its output current. The method can achieve high accuracy in terms of equal current distribution between the power sources. Simulation and experimental results are presented to verify the theoretical analysis.