Voltage sag

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Tree limbs create a short circuit in electrical lines during a storm that spawned two tornadoes.

A power outage (also known as a power cut, power failure, power loss, or blackout) is the loss of the electric power to an area.

The reasons for a power failure can for instance be a defect in a power station, damage to a power line or other part of the distribution system, a short circuit, or the overloading of electricity mains. While the developed countries enjoy a highly uninterrupted supply of electric power all the time, many developing countries have an acute power shortage as compared to the demand.

Some developing countries and newly-industrialized countries have several hours of daily power-cuts in almost all cities and villages because the increase in demand for electricity exceeds the increase in electric power generation. Wealthier people in these countries may use a power-inverter (rechargeable batteries) or a diesel/petrol-run electric generator at their homes during the power-cut. The use of standby generators is common in industrial and IT hubs.

Blackout.

A power outage may take one of three forms:

Blackout

where power is lost completely. While the word "blackout" is one of the most common colloquial terms, "Load shedding" or a rolling blackout refers specifically to a controlled way of rotating available generation capacity between various districts or customers, thus avoiding wide area total blackouts.

Brownout

where the voltage level is below the normal minimum level specified for the system. 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. Some brownouts, called voltage reductions, are made intentionally to prevent a full power outage.

Dropout.

Dropout

where the loss of power is only momentary (milliseconds to seconds).

Power failures are particularly critical for hospitals, since many life-critical medical devices and tasks require power. For this reason hospitals, just like many enterprises (notably colocation facilities and other datacenters), have emergency power generators which are typically powered by diesel fuel and configured to start automatically, as soon as a power failure occurs. In most third world countries, power cuts go unnoticed by most citizens of upscale means, as maintaining an uninterruptible power supply is often considered an essential facility of a home.

Power outage may also be the cause of sanitary sewer overflow, a condition of discharging raw sewage into the environment. 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 diesel generator during extended periods of outage.

Power outages may also be caused by terrorism (attacking power plants or electricity pylons) in developing countries. The Shining Path movement was the first to copy this tactic from Mao Zedong.

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.

See also: Uninterruptible power supply

Blackout inevitability and electric sustainability

Self organized criticality

It has recently been argued on the basis of historical data^[1] and computer modeling^[2] that power grids are self-organized critical systems. These systems exhibit unavoidable^[3] 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.^[4] 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^[4].

Cascading failure becomes much more common close to this critical point. The power law relationship is seen in both historical data and model systems^[4]. 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^[4]. 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^[5]:

• 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^[5].

A complex network-based model to control large cascading failures (blackouts) using local information only was proposed in A. E. Motter^[6].

External links: power outage maps

See also

Energy portal

• List of power outages
• Brittle Power
• Energy efficiency
• Renewable energy
• Rolling blackout
• Uninterruptible power supply
• V2G
• Smart Power Grid

References

External links

Wikinews has related news:

Category:Disasters and accidents

• The Blackout History Project documents two New York City blackouts
• 3 Major Problems in Restoring Power After a Black Out Space Weather
• A. E. Motter and Y.-C. Lai, Cascade-based attacks on complex networks, Physical Review E (Rapid Communications) 66, 065102 (2002)
• Ontario Electricity articles
• Electricity Power Blackout and Outage tips
• First-Hand Account of NYC's 2003 Blackout
• Siemens AG - Blackout Prevention
• How Stuff Works - Blackouts