Seamless Transition between Grid-Connected and Islanded Operation Modes for Hybrid PV-BESS Combination used in Single-Phase, Critical Load Applications

Abstract

A single-phase, grid-connected, hybrid combination of Solar Photo Voltaic (PV) and Battery Energy Storage System (BESS) incorporating seamless transfer between grid-connected and islanded modes is presented in this paper. The purpose of the proposed combination is to supply reliable and uninterrupted power for critical load applications. Combination of BESS and Solar PV when connected to single-phase, AC utility mains enhances system reliability during transient conditions such as, variation in solar irradiation, sudden grid outage and resynchronisation with grid at the point of common coupling (PCC). Perturb and Observe (P&O) Maximum Power Point Tracking (MPPT) algorithm is used for the control of DC/DC boost converter between solar PV array and DC bus to extract maximum power at a given irradiation level. The control strategy for bi-directional, DC/DC converter regulates DC bus voltage and charging/discharging of BESS under different operating conditions. Controlled VSC operation ensures that the required critical load demand is supplied and the balance power gets distributed between the utility grid and the BESS. The system operates in Grid Control Mode (GCM) when connected to utility and in Islanded Mode (IM) on occurrence of grid outage. Synchronisation controller ensures smooth and transient free transition between GCM and IM respectively. Percentage (%) Total Harmonic Distortion (THD) of grid current (when tied to the grid) and load voltage (during grid outage) is observed within 5% respectively. The control strategy proposed for such a PV-BESS hybrid system is robust in nature and simple to realise. The proposed system and corresponding control strategy is designed for a 5 kW Solar PV array rating. The system is simulated in MATLAB/Simulink environment in Discrete-Time Frame (DTF) for a converter switching frequency (fs) of 10 kHz.