Control strategies for a grid-connected hybrid energy system

This paper presents the control strategies to operate a grid connected hybrid system. The hybrid system consists of a Photovoltaic (PV) array and a Proton Exchange Membrane Fuel cell (PEMFC). Due to the intermittent nature of the solar energy the PV array becomes uncontrollable in delivering maximum power to the load. In coordination with the PEMFC the hybrid system output power becomes controllable. The coordination of the two control modes Unit Power Control (UPC) mode and feeder-flow control (FFC) mode can be applied to the system. In UPC mode the DGs regulate the voltage magnitude at the connection point where source power is injected. In this mode if a load increases anywhere in the micro-grid the extra power comes from the grid, since the hybrid source regulates power output to a constant. In FFC mode the DGs regulate the voltage magnitude at the connection point and the power flowing in the feeder. In FFC mode extra load demands are picked up by the DGs which maintain a constant load from utility point of view. The proposed control strategies always operate the PV array at the maximum output power and the PEMFC at its high efficiency band improving the performance of the system operation with reduced number of mode changes.