Backfeeding

Backfeeding is flow of electrical energy in the reverse direction from its normal flow. For example, backfeeding may occur when electrical power is injected into the local power grid from a source other than a utility company generator.

Description

By definition, backfeeding causes electrical power to flow in the opposite direction from its usual flow. When studying backfeeding, engineers must focus on the transfer of electrical power, and not confuse this with momentary AC voltage or current flows viewed in isolation from the overall situation.

Power grid generators normally pump energy into the grid, making it available for others to use. A power station will typically backfeed (and thus consume power) when it is shut down, due to its own local loads (e.g. lights or repair equipment).

Power grid loads may backfeed if they also have distributed generation installed, such as a grid-connected photovoltaic solar power system or a microturbine-based power generator. It is also possible for an electric motor to temporarily backfeed if it is mechanically driven (see regenerative braking).

Design considerations

For cost reasons, many of the circuit (overcurrent) protection and power quality control (voltage regulation) devices used by electric utility companies are designed with the assumption that power always flows in one direction. An interconnection agreement can be arranged for equipment designed to backfeed between an electric utility customer with distributed generation and their power company. This type of interconnection can involve nontrivial engineering, and possibly equipment upgrade costs to keep the distribution circuit properly protected. Such costs may be minimized by limiting distributed generation capacity to less than is locally consumed, and guaranteeing this condition by installing a reverse-power cutoff relay that opens if backfeeding occurs.

Hazards

Because it involves transfer of significant amounts of energy, backfeeding must be carefully controlled and monitored. Personnel working on equipment subject to backfeeding must be aware of all possible power sources, and follow systematic protocols to ensure that equipment is fully de-energized before commencing work, or use special equipment and techniques suitable for working on live equipment.