The installation and interconnection of DER devices require a transfer switch. 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. Backfeed creates a 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 backfeed.
There are a wide variety of transfer switches presently available, based on the type and size of the DER device to be installed and the type of application. The traditional interconnection approach (shown below), frequently used for back-up residential generator sets, requires an electrical permit and a licensed electrician to install a transfer switch and a breaker sub-panel. In this configuration, the manual or automatic transfer switch is wired between the main breaker panel and the new sub-panel. The electrician hard-wires the circuits of the sub-panel to supply electricity to specific transferable loads (within the power limit of the DER device) during a utility power outage. When the outage occurs, the DER device is connected by the transfer switch and only the circuits fed from the sub-panel can be powered by the DER device. The sub-panel generally contains 4-10 hard-wired circuits and requires a licensed electrician for future wiring changes.
Other types of switches allow for the entire electric load (as opposed to specified circuits) of a building or home to be transferred to the back-up DER device during a power outage. In this type of installation and interconnection, which also requires a licensed electrician, the transfer switch is wired between the meter and the breaker panel (shown in the diagram below) and a sub-panel is not required. When an outage occurs, the DER device either automatically or manually provides electricity to all of the circuits.
PEPCO Technologies, LLC recently received Underwriters Laboratories certification for its manual GenerLink device, which is installed outside the home at the meter by utility personnel. The GenerLink does not require the installation of a hard-wired sub-panel, but rather allows the user to manually choose the circuits to be powered during an outage at the main breaker panel.
In addition to traditional transfer switch devices, which allow only for grid-independent interconnection of back-up DER devices, a number of companies (including PEPCO Technologies, LLC) are developing or have already developed grid-interconnection equipment that permits two-way power flow when the electric grid is operational. When an outage occurs, the interconnection device operates in the same manner as a traditional transfer switch. These devices allow DER equipment to operate in a grid-parallel mode (e.g., operation of the DER device and the grid at the same time), giving users the opportunity to sell electricity back to the grid through net-metering programs.
Many DER equipment manufacturers are working closely with power electronics companies to integrate the inverter/s and grid interconnection apparatus into a single package within the DER device. In the future, it is possible that the control scheme of the DER device will incorporate "smart electronics". During a grid power outage, the "smart electronics" of the DER device will choose when it can or cannot send electricity to a load, eliminating the need for a sub-panel or manual selection of circuits. It is also anticipated that future grid interconnection equipment will provide "bumpless" transfer between grid-parallel operation and grid-independent operation. If this becomes possible, then the user will ideally have electricity 100% of the time without any indications of utility power outages.