Stability improvement of microgrids using a novel reduced UPFC structure via nonlinear optimal control

A reduced Unified Power Flow Controller (UPFC) structure is proposed to enhance transient stability of small-scale micro grids. The micro grid utilizes Photovoltaic (PV) unit as the Distributed Generation (DG) unit that is connected to the grid through dc-dc buck converter and inverter. The dc-dc converter provides a constant dc voltage at the output that is connected to the inverter. The inverter generates the proper ac voltage according to the system requirements to feed the gird. The reduced UPFC model exploits dc link of the DG unit to generate appropriate series voltage and inject it to the power line to enhance transient stability. It employs the nonlinear discrete-time Hamilton-Jacobi-Bellman (HJB) optimal control to ensure that the stability of the system is realized through minimum cost for the system. A Neural Network (NN) is used to approximate the cost function based on the weighted residual method. In order to verify efficiency of the proposed model and control scheme, the DG unit and proposed UPFC structure are tested in simulations and experiments. The results shows effective performance of the proposed approach in damping oscillations in the system.