Control strategy for distributed integration of photovoltaic and energy storage systems in DC micro-grids

The interest on DC micro-grid has increased extensively for the more efficient connection with DC output type sources such as photovoltaic (PV) systems, fuel cells (FC) and battery energy storage systems (BESS). Furthermore, if loads in the system are supplied with DC power, the conversion losses from sources to loads are reduced compared with AC micro-grid. This paper proposes operation and control strategies for the integration of PV and BESS in a DC micro-grid. The proposed control enables the maximum renewable energy utilization during different operating modes of the micro-grid i.e., grid connected, islanded or transition between these two modes, whilst making an allowance for the DC voltage control and DC-loads supply. When the system is grid connected and during normal operation, active power is balanced by the AC grid converter to ensure a constant DC voltage. In order to achieve the system operation under islanding conditions, a coordinated strategy for the BESS, PV and load management including load shedding and considering battery state of charge (SoC), are proposed. Seamless transition of the PV converter control between maximum power point tracking (MPPT) and voltage control modes, of the battery converter between charging and discharging and that of grid side converter between rectification and inversion are ensured for different grid operation modes by the proposed control methods. The DC bus voltage level is employed as an information carrier to distinguish different modes and determine mode switching. MATLAB/SIMULINK simulations are presented to demonstrate the robust operation performance and to validate the proposed control system during various operating conditions. (C) 2012 Elsevier Ltd. All rights reserved.