question: How the power grid is able to work even when some circuits are not closed
comment: The circuits are connected in parallel. If you cut off one of the circuits, the others are still connected. Consider [this diagram]( URL_0 ). If you remove one of the bulbs it only cuts off that circuit, but electricity can still flow through the other bulbs.
passages: The structure, or "topology" of a grid can vary depending on the constraints of budget, requirements for system reliability, and the load and generation characteristics. The physical layout is often forced by what land is available and its geology. Distribution networks are divided into two types, radial or network.
  
  The simplest topology for a distribution or transmission grid is a "radial" structure. This is a "tree" shape where power from a large supply radiates out into progressively lower voltage lines until the destination homes and businesses are reached. However, single failures can take out entire branches of the tree.
   Section::::Size.
  
  Section::::Size.:Wide area synchronous grid.
   Electricity grids are the networks used to transmit and distribute power from production source to end user, when the two may be hundreds of kilometres away. Sources include electrical generation plants such as a nuclear reactor, coal burning power plant, etc. A combination of sub-stations and transmission lines are used to maintain a constant flow of electricity. Grids may suffer from transient blackouts and brownouts, often due to weather damage. During certain extreme space weather events solar wind can interfere with transmissions. Grids also have a predefined carrying capacity or load that cannot safely be exceeded. When power requirements exceed what's available, failures are inevitable. To prevent problems, power is then rationed.
   On arrival at a substation, the power will be stepped down from a transmission level voltage to a distribution level voltage. As it exits the substation, it enters the distribution wiring. Finally, upon arrival at the service location, the power is stepped down again from the distribution voltage to the required service voltage(s).
  
  Electrical grids vary in size from covering a single building through "national grids" which cover whole countries, to "transnational grids" which can cross continents.
  
  Although electrical grids are wide spread, 1.4 billion people are not connected to an electricity grid.
   Section::::Features.
  
  Section::::Features.:Voltage.
  
  Grids are designed to supply voltages at largely constant amplitudes. This has to be achieved with varying demand, variable reactive loads, and even nonlinear loads, with electricity provided by generators and distribution and transmission equipment that are not perfectly reliable. Often grids use tap changers on transformers near to the consumers to adjust the voltage and keep it within specification.
  
  Section::::Features.:Topologies.
  
  Transmission networks are complex with redundant pathways. For example, see the map of the United States' (right) high-voltage transmission network.
   However, in practice, they are never run flat out simultaneously. Typically, some generators are kept running at lower output powers (spinning reserve) to deal with failures as well as variation in demand. In addition generators can be off-line for maintenance or other reasons, such as availability of energy inputs (fuel, water, wind, sun etc.) or pollution constraints.
  
  "Firm capacity" is the maximum power output on a grid that is immediately available over a given time period, and is a far more useful figure.
  
  Section::::Features.:Interconnectors.
   Direct Current (DC) electricity from sources such as hydro, wind or solar is passed to an inverter which is grid tied. The inverter monitors the alternating current mains supply frequency and generates electricity that is phase matched to the mains. When the grid fails to accept power during a "black out", most inverters can continue to provide courtesy power.
  
  Section::::Battery-to-grid.
   A large failure in one part of the grid - unless quickly compensated for - can cause current to re-route itself to flow from the remaining generators to consumers over transmission lines of insufficient capacity, causing further failures. One downside to a widely connected grid is thus the possibility of cascading failure and widespread power outage. A central authority is usually designated to facilitate communication and develop protocols to maintain a stable grid. For example, the North American Electric Reliability Corporation gained binding powers in the United States in 2006, and has advisory powers in the applicable parts of Canada and Mexico. The U.S. government has also designated National Interest Electric Transmission Corridors, where it believes transmission bottlenecks have developed.
   Normally, the electric power used within the plant is provided from the station's own generators. If all of the plant's main generators are shut down, station service power is provided by drawing power from the grid through the plant's transmission line. However, during a wide-area outage, off-site power from the grid is not available. In the absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation.
   Grid-tied electrical system
  
  A grid-tied electrical system, also called "tied to grid" or "grid tie system", is a semi-autonomous electrical generation or grid energy storage system which links to the mains to feed excess capacity back to the local mains electrical grid. When insufficient electricity is available, electricity drawn from the mains grid can make up the shortfall. Conversely when excess electricity is available, it is sent to the mains grid.
  
  When batteries are used for storage, the system is called battery-to-grid (B2G), which includes vehicle-to-grid (V2G).
  
  Section::::How it works.
   A premises is generally said to have obtained "grid connection" when its service location becomes powered by a live connection to its service transformer.
  
  A power station is generally said to have achieved "grid connection" when it first supplies power outside of its own boundaries. However, a town is only said to have achieved "grid connection" when it is connected to several redundant sources, generally involving long-distance transmission.
  
  Section::::Balancing Authority, Tie Facility and Interconnection.
   Most transmission grids offer the reliability that more complex "mesh networks" provide. The expense of mesh topologies restrict their application to transmission and medium voltage distribution grids. Redundancy allows line failures to occur and power is simply rerouted while workmen repair the damaged and deactivated line.
  
  Other topologies used are "looped" systems found in Europe and "tied ring" networks.
   Power failures are particularly critical at sites where the environment and public safety are at risk. Institutions such as hospitals, sewage treatment plants, mines, shelters and the like will usually have backup power sources such as standby generators, which will automatically start up when electrical power is lost. Other critical systems, such as telecommunication, are also required to have emergency power. The battery room of a telephone exchange usually has arrays of lead–acid batteries for backup and also a socket for connecting a generator during extended periods of outage.
  
  Section::::Failure.:Black start.
   Normally, even when the load and production are exactly matched, the so-called "balanced condition", the failure of the grid will result in several additional transient signals being generated. For instance, there will almost always be a brief decrease in line voltage, which will signal a potential fault condition. However, such events can also be caused by normal operation, like the starting of a large electric motor.
   Grid connection
  
  In electrical grids, a power system network integrates transmission grids, distribution grids, distributed generators and loads that have connection points called busses. A bus in home circuit breaker panels is much smaller than those used on the grid, where busbars can be 50 mm (~5 in) in diameter in electrical substations. Traditionally, these grid connections are unidirectional point to multipoint links. In distributed generation grids, these connections are bidirectional, and the reverse flow can raise safety and reliability concerns. Features in smart grids are designed to manage these conditions.
   Section::::List of countries or regions that legally allow grid-tied electrical systems.
  
  BULLET::::- Armenia
  
  BULLET::::- Australia
  
  BULLET::::- Bangladesh
  
  BULLET::::- Bosnia and Herzegovina
  
  BULLET::::- Brazil
  
  BULLET::::- Canada
  
  BULLET::::- Dominican Republic
  
  BULLET::::- El Salvador
  
  BULLET::::- European Union
  
  BULLET::::- Guatemala
  
  BULLET::::- India
  
  BULLET::::- Iran
  
  BULLET::::- Israel
  
  BULLET::::- Japan
  
  BULLET::::- Jordan
  
  BULLET::::- Mexico
  
  BULLET::::- New Zealand
  
  BULLET::::- Pakistan
  
  BULLET::::- Panama
  
  BULLET::::- Philippines (via MERALCO)
  
  BULLET::::- Singapore
  
  BULLET::::- South Africa (Only by arrangement with municipality)
  
  BULLET::::- Sri Lanka
  
  BULLET::::- United States of America
  
  BULLET::::- Venezuela (no legal restrictions)
  
  Section::::See also.
  
  BULLET::::- Cost of electricity by source
  
  BULLET::::- Distributed network
  
  BULLET::::- Electric power transmission
  
  BULLET::::- Electranet
   A grid-friendly device can respond to changes in frequency by reducing or interrupting the demand for electric power (called "load") when the frequency drops below a certain threshold, and/or increasing load when the frequency rises. Although a single grid-friendly device may be a very small load, the fraction of the total load that can be controlled by frequency at any time is usually sufficient to provide under-frequency protection to the system before more drastic measures like black-outs are required.
   The frequency of the power being delivered to the grid is a function of the supply, one that the inverters carefully match. When the grid source is lost, the frequency of the power would fall to the natural resonant frequency of the circuits in the island. Looking for changes in this frequency, like voltage, is easy to implement using already required functionality, and for this reason almost all inverters also look for fault conditions using this method as well.
   Electrical grid
  
  An electrical grid, electric grid or power grid, is an interconnected network for delivering electricity from producers to consumers. It consists of
  
  BULLET::::- Generating stations that produce electrical power
  
  BULLET::::- electrical substations for stepping electrical voltage up for transmission, or down for distribution
  
  BULLET::::- High voltage transmission lines that carry power from distant sources to demand centers
  
  BULLET::::- Distribution lines that connect individual customers.
   A "black start" is the process of restoring an electric power station or a part of an electric grid to operation without relying on the external electric power transmission network to recover from a total or partial shutdown.
   Section::::Trends.
  
  Section::::Trends.:Demand response.
  
  Demand response is a grid management technique where retail or wholesale customers are requested either electronically or manually to reduce their load. Currently, transmission grid operators use demand response to request load reduction from major energy users such as industrial plants.
  
  Section::::Trends.:Aging infrastructure.
  
  Despite the novel institutional arrangements and network designs of the electrical grid, its power delivery infrastructures suffer aging across the developed world. Contributing factors to the current state of the electric grid and its consequences include:
   BULLET::::- High-precision resistance measurements using a Kelvin connection
  
  BULLET::::- The difference in voltage between ground and neutral, between the neutral wire and the ground in domestic AC power plugs and sockets, can be fatal. A properly installed electrical system connects them together at only one location, leading many people to the fatally incorrect conclusion that they are at "the same" voltage, or that the safety ground is "redundant and unnecessary"
  
  BULLET::::- The Seebeck effect and the Peltier effect
  
  BULLET::::- Joints involving aluminium wire
  
  Dots used to mark nodes on a circuit diagram are sometimes referred to as "meatballs".
   Power grids have been related to complex biological systems in many other contexts. In one study, power grids were compared to the dolphin social network. These creatures streamline or intensify communication in case of an unusual situation. The intercommunications that enable them to survive are highly complex.
  
  BULLET::::- Random fuse networks
   In addition, there's often central control, which can change the parameters of the AGC systems over timescales of a minute or longer to further adjust the regional network flows and the operating frequency of the grid. For timekeeping purposes, the nominal frequency will be allowed to vary in the short term, but are adjusted to prevent line-operated clocks from gaining or losing significant time over the course of a whole 24 hour period.
   Section::::Terminology.
  
  A highly fault-tolerant system might continue at the same level of performance even though one or more components have failed. For example, a building with a backup electrical generator will provide the same voltage to wall outlets even if the grid power fails.
  
presuppositions: If one circuit is cut off the power grid is unable to work.
corrections: Circuits are connected in parallel, so if once circuit is cut off, the others are still connected;  this enables the power grid to continue to function.
raw_presuppositions: If one circuit is cut off the power grid is unable to work. A power grid needs all circuits closed in order to work.
raw_corrections: Circuits are connected in parallel, so if once circuit is cut off, the others are still connected;  this enables the power grid to continue to function. Circuits are connected in parallel, so if once circuit is cut off, the others are still connected;  this enables the power grid to continue to function.