Grid-connected or utility-interactive photovoltaic systems are designed to operate in parallel with and interconnected with the electric utility grid. The primary component in grid-connected photovoltaic systems is the inverter or power conditioning unit (PCU). The PCU converts the DC power produced by the photovoltaic array into AC power consistent with the voltage and power quality requirements of the utility grid, and automatically stops supplying power to the grid when the utility grid is not energised (Fig. 5.4). A bi-directional interface is made between the photovoltaic system AC output circuits and the electric utility network, typically at an on-site distribution panel or service entrance. This allows the AC power produced by the photovoltaic system to either supply on-site electrical loads or to back feed the grid when the photovoltaic system output is greater than the on-site load demand. At night and during other periods when the electrical loads are greater than the photovoltaic system output, the balance of power required by the loads is received from the electric utility This safety feature is required in all grid-connected photovoltaic systems, and ensures that the photovoltaic system will not continue to operate and feed back onto the utility grid when the grid is down for service or repair.
Figure 5.4 Diagram of grid-connected photovoltaic system