Load sharing between parallel inverters using effective design of output impedance

Using conventional droop method, output impedance of inverters are considered to be mainly inductive; however, this is not always true, since the output impedance of inverter depends on the control strategy. This paper deals with the effective design of the output impedance of uninterruptible power system(UPS) inverters with parallel connection capability. In this paper a control technique based on conventional droop method is proposed which, by proper design of output impedance, allows the output impedance to have inductive behavior around fundamental frequency and resistive behavior for higher-order harmonics. By using the proposed method, parallel inverters can share linear and nonlinear loads properly while circulating current between them remains very small in both steady state and transient states. Simulation results are provided to prove the concept.