Linear quadratic regulator based bumpless transfer in microgrids

This paper presents a Linear Quadratic Regulator (LQR) based bumpless transfer controller for achieving seamless transition between the grid connected and the islanded modes of operation in microgrids. During the grid connected mode, the inverter based Distributed Generators (DGs) operate in current controlled mode where they supply a constant active and reactive power. During the islanded mode, the inverters operate in voltage controlled mode. During transition between the two control modes severe transients can occur in the system which affect the voltage and frequency responses. The proposed bumpless compensator reduces the error between the outputs of two controllers at the instant of transition by optimally reducing an error measure based on LQR theory. The efficacy of the proposed controller is tested on a microgrid test system. Several operating scenarios on the microgrid have been tested using the proposed approach. It has been found that the proposed controller is effective in reducing the voltage and the frequency transients during transition.