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.
Development and economization of consumer power generation equipment such as wind turbines and photovoltaic systems has led 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.
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.
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.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.
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.
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.
Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines which facilitate this movement are known as a transmission network. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution. The combined transmission and distribution network is part of electricity delivery, known as the "power grid" in North America, or just "the grid". In the United Kingdom, India, Tanzania, Myanmar, Malaysia and New Zealand, the network is known as the National Grid.
In electrical engineering, the power factor of an AC electrical power system is defined as the ratio of the real power absorbed by the load to the apparent power flowing in the circuit, and is a dimensionless number in the closed interval of −1 to 1. A power factor of less than one indicates the voltage and current are not in phase, reducing the average product of the two. Real power is the instantaneous product of voltage and current and represents the capacity of the electricity for performing work. Apparent power is the product of RMS current and voltage. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power may be greater than the real power. A negative power factor occurs when the device generates power, which then flows back towards the source.
Electric power distribution is the final stage in the delivery of electric power; it carries electricity from the transmission system to individual consumers. Distribution substations connect to the transmission system and lower the transmission voltage to medium voltage ranging between 2 kV and 35 kV with the use of transformers. Primary distribution lines carry this medium voltage power to distribution transformers located near the customer's premises. Distribution transformers again lower the voltage to the utilization voltage used by lighting, industrial equipment or household appliances. Often several customers are supplied from one transformer through secondary distribution lines. Commercial and residential customers are connected to the secondary distribution lines through service drops. Customers demanding a much larger amount of power may be connected directly to the primary distribution level or the subtransmission level.
The electric power industry covers the generation, transmission, distribution and sale of electric power to the general public and industry. The commercial distribution of electric power started in 1882 when electricity was produced for electric lighting. In the 1880s and 1890s, growing economic and safety concerns lead to the regulation of the industry. What was once an expensive novelty limited to the most densely populated areas, reliable and economical electric power has become an essential aspect for normal operation of all elements of developed economies.
A substation is a part of an electrical generation, transmission, and distribution system. Substations transform voltage from high to low, or the reverse, or perform any of several other important functions. Between the generating station and consumer, electric power may flow through several substations at different voltage levels. A substation may include transformers to change voltage levels between high transmission voltages and lower distribution voltages, or at the interconnection of two different transmission voltages.
The utility frequency, (power) line frequency or mains frequency is the nominal frequency of the oscillations of alternating current (AC) in a wide area synchronous grid transmitted from a power station to the end-user. In large parts of the world this is 50 Hz, although in the Americas and parts of Asia it is typically 60 Hz. Current usage by country or region is given in the list of mains electricity by country.
An electricity meter, electric meter, electrical meter, or energy meter is a device that measures the amount of electric energy consumed by a residence, a business, or an electrically powered device.
Grid energy storage is a collection of methods used for energy storage on a large scale within an electrical power grid. Electrical energy is stored during times when electricity is plentiful and inexpensive or when demand is low, and later returned to the grid when demand is high, and electricity prices tend to be higher.
An emergency power system is an independent source of electrical power that supports important electrical systems on loss of normal power supply. A standby power system may include a standby generator, batteries and other apparatus. Emergency power systems are installed to protect life and property from the consequences of loss of primary electric power supply. It is a type of continual power system.
A transfer switch is an electrical switch that switches a load between two sources. Some transfer switches are manual, in that an operator effects the transfer by throwing a switch, while others are automatic and trigger when they sense one of the sources has lost or gained power.
The Texas Interconnection is an alternating current (AC) power grid – a wide area synchronous grid – that covers most of the state of Texas. The grid is managed by the Electric Reliability Council of Texas (ERCOT).
An engine-generator is the combination of an electrical generator and an engine mounted together to form a single piece of equipment. This combination is also called an engine-generator set or a gen-set. In many contexts, the engine is taken for granted and the combined unit is simply called a generator. An engine-generator may be a fixed installation, part of a vehicle, or made small enough to be portable.
A network protector is a type of electric protective device used in electricity distribution systems. The network protector automatically disconnect its associated distribution transformer from the secondary network when the power starts flowing in reverse direction. Network protectors are used on both spot networks and grid networks. The secondary grid system improves continuity of service for customers, since multiple sources are available to supply the load; a fault with any one supply is automatically isolated by the network protector and does not interrupt service from the other sources. Secondary grids are often used in downtown areas of cities where there are many customers in a small area.
Load management, also known as demand-side management (DSM), is the process of balancing the supply of electricity on the network with the electrical load by adjusting or controlling the load rather than the power station output. This can be achieved by direct intervention of the utility in real time, by the use of frequency sensitive relays triggering the circuit breakers, by time clocks, or by using special tariffs to influence consumer behavior. Load management allows utilities to reduce demand for electricity during peak usage times, which can, in turn, reduce costs by eliminating the need for peaking power plants. In addition, some peaking power plants can take more than an hour to bring on-line which makes load management even more critical should a plant go off-line unexpectedly for example. Load management can also help reduce harmful emissions, since peaking plants or backup generators are often dirtier and less efficient than base load power plants. New load-management technologies are constantly under development — both by private industry and public entities.
An electric power system is a network of electrical components deployed to supply, transfer, and use electric power. An example of an electric power system is the grid that provides power to an extended area. An electrical grid power system can be broadly divided into the generators that supply the power, the transmission system that carries the power from the generating centres to the load centres, and the distribution system that feeds the power to nearby homes and industries. Smaller power systems are also found in industry, hospitals, commercial buildings and homes. The majority of these systems rely upon three-phase AC power—the standard for large-scale power transmission and distribution across the modern world. Specialised power systems that do not always rely upon three-phase AC power are found in aircraft, electric rail systems, ocean liners and automobiles.
An electrical grid, electric grid or power grid, is an interconnected network for delivering electricity from producers to consumers. It consists of:
A grid-connected photovoltaic system, or grid-connected PV system is an electricity generating solar PV power system that is connected to the utility grid. A grid-connected PV system consists of solar panels, one or several inverters, a power conditioning unit and grid connection equipment. They range from small residential and commercial rooftop systems to large utility-scale solar power stations. Unlike stand-alone power systems, a grid-connected system rarely includes an integrated battery solution, as they are still very expensive. When conditions are right, the grid-connected PV system supplies the excess power, beyond consumption by the connected load, to the utility grid.
A generator interlock kit is a device designed to allow safe powering of a home by a portable generator during a power outage. It is a less-expensive alternative to purchasing and installing a dedicated transfer switch. The kit achieves the same function by adding an external interlock onto an existing breaker panel that allows the main breaker to be turned on or one designated load breaker to be turned on, but not both at the same time. The interlocked load breaker is repurposed as the "backfeed" breaker, and a generator is connected to it.
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The electrical power grid that powers North America is divided into multiple wide area synchronous grids. The Eastern Interconnection and the Western Interconnection are the largest. Three other regions include the Texas Interconnection, the Quebec Interconnection, and the Alaska Interconnection. Each region delivers power at a nominal 60 Hz frequency. The regions are not usually directly connected or synchronized to each other, but there are some HVDC interconnections.