Energy independence is using on-site power generation to meet all energy needs (usually to ensure power reliability and/or power quality) in one of two configurations:
Grid-connected — using grid power as a backup electricity source during failure or maintenance of the on-site generator: also referred to as grid-parallel.
Grid-independent — stand-alone power generation, usually including energy-saving approaches and an energy storage device for backup power.
When using DG as a backup power supply, IEEE 1547 requires that the generator be disconnected from the grid in order to prevent an unintentional Island. During a power outage, the transfer switch ensures that there is no backfeed of electricity from the DER device into the utility's electric distribution system. Backfeeding creates a potentially dangerous situation for utility line workers and may also damage equipment. As shown in the diagram below, a transfer switch opens the connection to the utility in the event of a power outage to eliminate the possibility of backfeeding and unintentional Islanding the DER device.
The following animations illustrate the proper use of DG as a backup, where the site disconnects from the grid. The second animation illustrates the creation of an unintentional Island, where the generator backfeeds power to the electric grid.