A frequency-dependent droop scheme for parallel control of UPS inverters

This paper presents a frequency-dependent droop scheme for paralleling UPS inverters with no control interconnection, so the inverters can be tightly connected with the load. At the fundamental frequency, the reference voltag e of the inverter is generated as a reactor connected between the inverter and the load. The intention is to control the real and reactive power shari ng of inverters with the Q-V and P-omega droop scheme employed in the power system without needing a true transfer reactor. At the harmonic frequency, the reference voltage is generated as a resistor connected between the inv erter and the load. The intention is that the load harmonic current can be shared equally and the voltage waveform distortion will be low. Two 1KVA si ngle-phase inverters are designed and implemented, some simulation and expe rimental results are provided for demonstrating the effectiveness of the pr oposed approach.