A novel virtual impedance method for droop controlled parallel UPS inverters with wireless control

This paper proposes a novel virtual impedance method for uninterruptible power system (UPS) inverters with parallel connected capability. To achieve wireless control, the droop control method is adopted. However, the power sharing accuracy is highly sensitive to the output impedance of inverter. So, a virtual output impedance is usually added to alleviate the unbalance of output impedance between each inverter and then, improve power sharing accuracy. The virtual impedance is implemented as the time derivative of the inverter output current, the differential progress will amplify the noise presented in output current, result in high sensitivity to the noise for the virtual impedance. To solve this problem, a novel virtual impedance by combining with a low-pass filter is proposed, which not only balances the output impedance of inverter, but also achieves better output voltage THD when sharing nonlinear loads. The double loop control method for inverter provids a good tracking of output voltage reference and high quality output waveform. Simulation results are presented from a parallel connected UPS system to validate this method.