A Nonlinear Multimode Controller for Seamless off-Grid of Energy Storage Inverter Under Unintentional Islanding

Abstract

A nonlinear multimode controller (NMC) is proposed to achieve the whole process seamless off-grid of energy storage inverter (ESI) from the grid-connected state of current control mode (CCM) to the islanded state of voltage control mode (VCM) under unintentional islanding. Based on the presented design idea of intelligent equivalent substitution, NMC is composed of multimode voltage controller (MVC), multimode power compensator (MPC) and multimode current controller (MCC). MVC can control load voltage in islanding and maintain manipulated variables of load current in other modes. MPC can compensate the power flowing into grid and help inner loop reference value achieve the smooth drop, and also compensate for load current variation during grid-connection. MCC can track the given current during grid-connection and make MPC quickly obtain the compensation of load power variation. The reference values of inner loop are logically split into the output of MVC, MPC and MCC, so that the manipulated variables can be combined nonlinearly to achieve seamless off-grid of ESI under unintentional islanding. This paper also analyzes the ESI stability with NMC during the off-grid process, and analyzes the relationship between manipulated variables by simulations. Finally, the correctness and effectiveness of proposed NMC are verified by experiments.