Energy Storage Sizing and Optimal Operation Analysis of a Grid-Connected Microgrid

Due to the multi-task functionality of Energy Storage Systems (ESSs), they are an indispensable component of Microgrids (MGs). ESSs ease the intermittency of Renewable Energy Sources (RESs) and power fluctuations of the power grid. Moreover, employing ESSs reduces the operating cost of MGs and enhances the reliability of the system. However, the challenging aspects of employing these devices are their high cost and limited lifetime duration. Therefore, optimising the size of ESS is an utmost of importance. This paper proposes a nested optimisation strategy to obtain the optimal capacity of ESS for a grid-connected MG with critical loads. The inner layer aims to reduce the operating cost of the MG by solving the Unit Commitment (UC) problem. In this layer, the variable operating cost of ESS, peak demand cost, and cost of power drop have been taken into account. On the other hand, the outer layer minimises the investment cost of the ESS. The Particle Swarm Optimisation (PSO) and Mixed Integer Quadratic Programming (MIQP) methods are employed to solve the outer layer and inner layer problems, respectively. The application of the proposed method to a real case study with critical loads verified its effectiveness and functionality. Moreover, the optimal operation of MG devices is achieved by solving the UC problem.