Optimal Decentralized Supplementary Inverter Control Loop to Mitigate Instability in an Islanded Microgrid with Active and Passive Loads

The growing penetration levels of passive and rectifier interfaced active loads can cause instability in an islanded inverter based microgrid. This may be due to the dynamics associated with passive loads and interaction between the control loops of active loads and inverter-interfaced DGs. This paper proposes a decentralized supplementary control loop around each inverter-interfaced distributed generation (DG) unit to mitigate the instability caused by both the active as well as passive loads. The proposed control loop is based on a robust linear quadratic regulator with prescribed degree-of-stability (LQRPDS). In order to obtain the optimal values of the state and control weighting matrices of LQRPDS, a bi-objective optimization problem has been formulated and solved using a fast and elitist multi-objective non-dominated sorting genetic algorithm (NSGA-II). The proposed decentralized supplementary control loop does not interfere with the steady state performance and provides robust control performance under various load configurations as well as step load disturbances. Eigenvalue analysis and time-domain simulations have been used to validate the effectiveness of the proposed decentralized supplementary control loop.