Backfeeding: Difference between revisions
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Backfeeding is the flow of [[electric power]] in the reverse direction of the generally understood or typical flow of power. Depending on the source of the power, this reverse flow may be intentional or unintentional. If not prevented (in the case of unintentional backfeeding) or properly installed (in cases of intentional backfeeding), backfeeding may present unanticipated hazards to electrical grid equipment and service personnel. |
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== Types of Backfeeding == |
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'''Backfeeding''' is the delivery or flow of [[electric power]] in the reverse direction of the "normal" flow of power, which is from [[power station]]s, through [[electric power transmission]] and [[electric power distribution]], to homes and buildings, to electrical appliances. It includes feeding power into the "load" side of a distribution panel, rather than the "line" side. |
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=== Intentional Backfeeding === |
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One example of backfeeding is a [[portable generator]] supplying [[electrical power]] ''into'' a house or building via an [[electrical outlet]] or a load-side [[circuit breaker]] (instead of "feeding" power through a correctly wired electrical inlet and a [[transfer switch]]). Backfeeding into a building is typically improvised during a blackout, in order to quickly restore electrical power to [[freezer]]s, [[heating, ventilation, and air conditioning]] equipment, lighting, communications, pumps, and business operations, to prevent damage (by thawing or freezing), inconvenience, loss of business, etc., until power is restored. It is vital to disconnect from the local power grid, typically by turning off all circuit breakers or fuses that connect to the external mains, or pulling the electric meter. Failure to disconnect from the external mains can be expensive, due to unintentionally powering neighboring buildings and homes. Failure to disconnect from the external mains is illegal. It could energize downed power lines, potentially killing or injuring people who assume that the lines are de-energized. It could cause unexpected fires. |
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Development and economization of consumer power generation equipment such as [[wind turbine]]s and [[photovoltaic systems]] has lead to an increase in the number of consumers that may produce more electrical power than they consume during peak generating conditions. If supported by the consumer's [[electric utility]] provider, the excess power generated may be fed back into the electrical grid. This process makes the typical consumer a temporary producer while the flow of electrical power remains reversed. When backfeeding is performed this way, electric utility providers will install a specially engineered [[electrical meter]] that is capable of [[net metering]]. |
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=== Unintentional Backfeeding === |
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Another example is a house or building supplying [[electrical power]] ''into'' the local [[power grid]].<ref>{{cite web|url=https://www.electricianslibrary.com/what-is-backfeeding/|title=What is Backfeeding? – Electricians Library|website=www.electricianslibrary.com|language=en-US|access-date=2018-07-04}}</ref> This is usually unintentional, and it is dangerous. It can be caused by feeding power into an electrical outlet of a building (as above). It can be caused by feeding power into the main power inlet of the building without disconnecting it from the external mains. Wires, fuses, and almost all circuit breakers carry power in either direction. |
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A common source of unintentional backfeeding is an electrical generator (typically a [[portable generator]]) that is improperly connected to a building electrical system. A properly installed electrical generator incorporates the use of a [[transfer switch]] to ensure the incoming electrical service line is disconnected when the generator is providing power to the building. In the absence (or improper usage) of a transfer switch, unintentional backfeeding may occur when the power provided by the electrical generator is able to flow over the electrical service line. Because an electrical [[transformer]] is capable of operating in both directions, electrical power generated from equipment on the consumer's premises can backfeed through the transformer and energize the distribution line to which the transformer is connected.<ref>{{cite web |last1=J.W. |title=What is Backfeeding |url=https://www.electricianslibrary.com/what-is-backfeeding/ |website=Electrician's Library}}</ref> |
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=== Intrinsic Backfeeding === |
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==Description== |
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Backfeeding also exists in other instances where a location that is typically a generator becomes a consumer. This is commonly seen when an electrical generation plant is shut down or operating at such a reduced capacity that its [[parasitic load]] becomes greater than its generated power.<ref>{{cite web |last1=J.W. |title=What is Backfeeding |url=https://www.electricianslibrary.com/what-is-backfeeding/ |website=Electrician's Library}}</ref> The parasitic power load is the result of the usage of: pumps, facility lighting, HVAC equipment, and other control equipment that must remain active regardless of actual electrical power production. Electrical utilities often take steps to decrease their overall parasitic load to minimize this type of backfeeding and improve efficiency.<ref>{{cite web |last1=Vesel |first1=Richard |title=Utilities Can Improve Power Plant Efficiency, Become Emission-Compliant in Short Term |url=https://www.power-grid.com/2012/11/01/utilities-can-improve-power-plant-efficiency-become-emission-compliant-in-short-term/#gref |website=Power Grid International}}</ref> |
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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. |
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== Grid Design Considerations == |
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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). |
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⚫ | For manufacturing cost and operational simplicity reasons, most 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 from the consumer's equipment to the electrical utility provider's distribution system. This type of interconnection can involve nontrivial engineering and usage of costly specialized equipment designed to keep distribution circuits and equipment properly protected. Such costs may be minimized by limiting distributed generation capacity to less than that which is consumed locally, and guaranteeing this condition by installing a reverse-power cutoff relay that opens if backfeeding occurs. <ref>{{cite book |last1=Ram |first1=Badri |last2=Vishwakarma |first2=D.N. |title=Power system protection and switchgear |publisher=Tata McGraw-Hill |isbn=0074623508 |pages=456}}</ref> |
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== Safety and Operational Hazards == |
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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]]). |
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==Design considerations== |
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⚫ | For cost |
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==Hazards== |
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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. |
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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When working on de-energized power conductors, [[lineworker]]s attach |
When working on de-energized power conductors, [[lineworker]]s attach temporary protective grounding assemblies or "protective ground sets", which short all conductors to each other and to an earth ground. This ensures that no wires can become energized, whether by accidental switching or by unintentional backfeeding. |
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Because of the hazards presented by unintentional backfeeding, the usage of equipment that defeats engineered or standardized safety mechanisms such as double-ended power cords (an electrical cord that has a male electrical plug on both ends) is illegal and against the United States [[National Electrical Code]]. |
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Feeding power ''into'' an electrical outlet energized the blades of a ''male'' connector. If the power cord is unplugged from the outlet while the generator is running, the exposed blades of the male connector present a risk of shock, fire, and damage to the generator. |
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When the power comes back on, the backfeeding connection must be undone in the correct order. The generator must be disconnected before the main breaker is turned on, or else the generator could be damaged. |
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==See also== |
==See also== |
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*[[Islanding]], a similar phenomenon involving local generators |
*[[Islanding]], a similar phenomenon involving local generators. |
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*[[Transfer switch]] |
*[[Transfer switch]] |
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==External links== |
==External links== |
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{{Electricity delivery}} |
{{Electricity delivery}} |
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[[Category:Electric power distribution]] |
[[Category:Electric power distribution]] |
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{{Electric-power-stub}} |
Revision as of 16:09, 14 April 2020
Backfeeding is the flow of electric power in the reverse direction of the generally understood or typical flow of power. Depending on the source of the power, this reverse flow may be intentional or unintentional. If not prevented (in the case of unintentional backfeeding) or properly installed (in cases of intentional backfeeding), backfeeding may present unanticipated hazards to electrical grid equipment and service personnel.
Types of Backfeeding
Intentional Backfeeding
Development and economization of consumer power generation equipment such as wind turbines and photovoltaic systems has lead to an increase in the number of consumers that may produce more electrical power than they consume during peak generating conditions. If supported by the consumer's electric utility provider, the excess power generated may be fed back into the electrical grid. This process makes the typical consumer a temporary producer while the flow of electrical power remains reversed. When backfeeding is performed this way, electric utility providers will install a specially engineered electrical meter that is capable of net metering.
Unintentional Backfeeding
A common source of unintentional backfeeding is an electrical generator (typically a portable generator) that is improperly connected to a building electrical system. A properly installed electrical generator incorporates the use of a transfer switch to ensure the incoming electrical service line is disconnected when the generator is providing power to the building. In the absence (or improper usage) of a transfer switch, unintentional backfeeding may occur when the power provided by the electrical generator is able to flow over the electrical service line. Because an electrical transformer is capable of operating in both directions, electrical power generated from equipment on the consumer's premises can backfeed through the transformer and energize the distribution line to which the transformer is connected.[1]
Intrinsic Backfeeding
Backfeeding also exists in other instances where a location that is typically a generator becomes a consumer. This is commonly seen when an electrical generation plant is shut down or operating at such a reduced capacity that its parasitic load becomes greater than its generated power.[2] The parasitic power load is the result of the usage of: pumps, facility lighting, HVAC equipment, and other control equipment that must remain active regardless of actual electrical power production. Electrical utilities often take steps to decrease their overall parasitic load to minimize this type of backfeeding and improve efficiency.[3]
Grid Design Considerations
For manufacturing cost and operational simplicity reasons, most 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 from the consumer's equipment to the electrical utility provider's distribution system. This type of interconnection can involve nontrivial engineering and usage of costly specialized equipment designed to keep distribution circuits and equipment properly protected. Such costs may be minimized by limiting distributed generation capacity to less than that which is consumed locally, and guaranteeing this condition by installing a reverse-power cutoff relay that opens if backfeeding occurs. [4]
Safety and Operational 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.
When working on de-energized power conductors, lineworkers attach temporary protective grounding assemblies or "protective ground sets", which short all conductors to each other and to an earth ground. This ensures that no wires can become energized, whether by accidental switching or by unintentional backfeeding.
Because of the hazards presented by unintentional backfeeding, the usage of equipment that defeats engineered or standardized safety mechanisms such as double-ended power cords (an electrical cord that has a male electrical plug on both ends) is illegal and against the United States National Electrical Code.
See also
- Islanding, a similar phenomenon involving local generators.
- Transfer switch
References
- ^ J.W. "What is Backfeeding". Electrician's Library.
- ^ J.W. "What is Backfeeding". Electrician's Library.
- ^ Vesel, Richard. "Utilities Can Improve Power Plant Efficiency, Become Emission-Compliant in Short Term". Power Grid International.
- ^ Ram, Badri; Vishwakarma, D.N. Power system protection and switchgear. Tata McGraw-Hill. p. 456. ISBN 0074623508.