Backfeeding | Wikipedia audio article

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
By definition, backfeeding causes electrical power to flow in the opposite direction from
its usual flow. When studying backfeeding, engineers must
understand the transfer of electrical power, and not confuse this with momentary AC voltages
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

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