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Power outage
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A power outage (also known as a power cut, power failure, power loss, or blackout) refers to the short- or long-term loss of the electric power to an area.
There are many causes of power failures in an electricity network.
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Encyclopedia
A power outage (also known as a power cut, power failure, power loss, or blackout) refers to the short- or long-term loss of the electric power to an area.
There are many causes of power failures in an electricity network. These causes include for example faults at power stations, damage to power lines, substations or other parts of the distribution system, a short circuit, or the overloading of electricity mains.
Power outages are categorised into three different phenomena, relating to the duration and effect of the outage:
- A dropout is a momentary (milliseconds to seconds) loss of power typically caused by a temporary fault on a power line. Power is quickly (and sometimes automatically) restored once the fault is cleared.
- A brownout is a drop in voltage in an electrical power supply, so named because it typically causes lights to dim. Systems supplied with three-phase electric power also suffer brownouts if one or more phases are absent, at reduced voltage, or incorrectly phased. Such malfunctions are particularly damaging to electric motors.
- A blackout refers to the total loss of power to an area and is the most severe form of power outage that can occur. Blackouts which result from or result in power stations tripping are particularly difficult to recover from quickly. Outages may last from a few hours to a few weeks depending on the nature of the blackout and the configuration of the electrical network.
Power failures are particularly critical at sites where the environment and public safety are at risk. Institutions such as hospitals, sewerage treatment plants, mines etc. would typically have backup power in the form of standby generators which automatically start up when electrical power is lost, which would allow them enough time to either complete their work or to initiate a controlled shutdown of their process and/or evacuation or personnel.
Other applications such as data storage augment their use of generators with uninterruptible power supplies, which allows computers to "ride through" the initial outage without switching off, while the standby generators start up. Other life-critical systems such as telecommunications are also required to have emergency power. Telephone exchange rooms usually have arrays of lead-acid batteries for backup and also a socket for connecting a generator during extended periods of outage.
Effects of a brownout
Different types of electric devices respond in different ways to an undervoltage condition. Some are severely impacted while other devices may not be affected at all.
- Resistive devices vary their heat output based on the supplied voltage. An incandescent lamp will dim due to the lower heat emission from the filament. No damage occurs but functionality is reduced. (Overvoltage results in a much brighter lamp and rapid failure due to increased heat emission.)
- Commutated electric motors (also called universal motors) vary their speed in response to voltage changes, so they will slow down during a brownout. This does not harm the motor but will reduce the speed of the device operated by the motor.
- AC induction motors and three-phase motors will draw more current to compensate for the decreased voltage, which may lead to overheating and damage of the insulation on the motor's field windings.
- A linear power supply (consisting of a transformer and diodes) will produce a lower voltage for electronic circuits, resulting in slower oscillation and frequency rates. In a CRT television, this can be seen as the screen image shrinking in size and becoming dim and fuzzy. The device will also attempt to draw more current, potentially resulting in overheating.
- A switching power supply may be minimally affected if it was designed to compensate for over/under-voltage. However this is highly design-dependent, and it can malfunction and destroy itself if operated outside its normal voltage range.
Protecting the power system from outages
In power supply networks, the power generation and the electrical load (demand) must be very close to equal every second to avoid overloading of network components, which can severely damage them. In order to prevent this, parts of the system will automatically disconnect themselves from the rest of the system, or shut themselves down to avoid damage. This is analogous to the role of relays and fuses in households.
Under certain conditions, a network component shutting down can cause current fluctuations in neighboring segments of the network, though this is unlikely, leading to a cascading failure of a larger section of the network. This may range from a building, to a block, to an entire city, to an entire electrical grid.
Modern power systems are designed to be resistant to this sort of cascading failure, but it may be unavoidable (see below). Moreover, since there is no short-term economic benefit to preventing rare large-scale failures, some observers have expressed concern that there is a tendency to erode the resilience of the network over time, which is only corrected after a major failure occurs. It has been claimed that reducing the likelihood of small outages only increases the likelihood of larger ones. In that case, the short-term economic benefit of keeping the individual customer happy increases the likelihood of large-scale blackouts.
Title XIII of the Energy Independence and Security Act of 2007, signed by President Bush on December 19, 2007, makes it the policy of the United States to upgrade the United State's existing electricity grids with advanced communications and embedded sensors to create a smart grid that can avoid power outages (in addition to lowering grid-related CO2 and reducing energy consumption). The Electric Power Research Institute (EPRI) has estimated that each year power outages and disruptions cost Americans more than $100 Billion.
Restoring power after a wide-area outage
Restoring power after a wide-area outage can be difficult, as power stations need to be brought back on-line. Normally, this is done with the help of power from the rest of the grid. In the total absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation. The means of doing so will depend greatly on local circumstances and operational policies, but typically transmission utilities will establish localised 'power islands' which are then progressively coupled together. To maintain supply frequencies within tolerable limits during this process, demand must be reconnected at the same pace that generation is restored, requiring close coordination between power stations, transmission and distribution organizations.
Blackout inevitability and electric sustainability
Self organized criticality
It has recently been argued on the basis of historical data and computer modeling that power grids are self-organized critical systems. These systems exhibit unavoidable disturbances of all sizes, up to the size of the entire system. This phenomenon has been attributed to steadily increasing demand/load, the economics of running a power company, and the limits of modern engineering. While blackout frequency has been shown to be reduced by operating it further from its critical point, it generally isn’t economically feasible, causing providers to increase the average load over time and/or upgrade less often resulting in the grid moving itself closer to its critical point. Conversely, a system past the critical point will experience too many blackouts leading to system-wide upgrades moving it back below the critical point. The term critical point of the system is used here in the sense of statistical physics and nonlinear dynamics, representing the point where a system undergoes a phase transition; in this case the transition from a steady reliable grid with few cascading failures to a very sporadic unreliable grid with common cascading failures. Near the critical point the relationship between blackout frequency and size follows a power law distribution. Other leaders are dismissive of system theories that conclude that blackouts are inevitable, but do agree that the basic operation of the grid must be changed. The Electric Power Research Institute champions the use of smart grid features such as power control devices employing advanced sensors to coordinate the grid. Others advocate greater use of electronically controlled High-voltage direct current (HVDC) firebreaks to prevent disturbances from cascading across AC lines in a wide area grid.
Cascading failure becomes much more common close to this critical point. The power law relationship is seen in both historical data and model systems. The practice of operating these systems much closer to their maximum capacity leads to magnified effects of random, unavoidable disturbances due to aging, weather, human interaction etc. While near the critical point, these failures have a greater effect on the surrounding components due to individual components carrying a larger load. This results in the larger load from the failing component having to be redistributed in larger quantities across the system, making it more likely for additional components not directly affected by the disturbance to fail, igniting costly and dangerous cascading failures. These initial disturbances causing blackouts are all the more unexpected and unavoidable due to actions of the power suppliers to prevent obvious disturbances (cutting back trees, separating lines in windy areas, replacing aging components etc). The complexity of most power grids often makes the initial cause of a blackout extremely hard to identify.
Mitigation of power outage frequency
The effects of trying to mitigate cascading failures near the critical point in an economically feasible fashion are often shown to not be beneficial and often even detrimental. Four mitigation methods have been tested using the OPA blackout model:
- Increase critical number of failures causing cascading blackouts - Shown to decrease the frequency of smaller blackouts but increase that of larger blackouts.
- Increase individual power line max load – Shown to increase the frequency of smaller blackouts and decrease that of larger blackouts.
- Combination of increasing critical number and max load of lines – Shown to have no significant effect on either size of blackout. The resulting minor reduction in the frequency of blackouts is projected to not be worth the cost of the implementation.
- Increase the excess power available to the grid – Shown to decrease the frequency of smaller blackouts but increase that of larger blackouts.
In addition to the finding of each mitigation strategy having a cost-benefit relationship with regards to frequency of small and large blackouts, the total number of blackout events was not significantly reduced by any of the above mentioned mitigation measures.
A complex network-based model to control large cascading failures (blackouts) using local information only was proposed in A. E. Motter.
See also
External links
- Space Weather
- A. E. Motter and Y.-C. Lai, Physical Review E (Rapid Communications) 66, 065102 (2002)
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