07 March, 2007

Power Failures,Protection from outages and Restoration

Power Failures :
A power outage (Also power cut, power failure or power loss) is the loss of the electricity supply 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 acute power shortage as compared to the demand. Countries such as Pakistan have several hours of daily power-cuts in almost all cities and villages except the metropolitan cities and the state capitals. Wealthier people in these countries may use a power-inverter or a diesel-run electric generator at their homes during the power-cut.


A power outage may be referred to as a blackout if power is lost completely, or as a brownout if the voltage level is below the normal minimum level specified for the system, or sometimes referred to as a short circuit when the loss of power occurs over a short time (usually seconds). 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. 'Load shedding' is a common term for a controlled way of rotating available generation capacity between various districts or customers, thus avoiding total wide area blackouts.

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.

Live Examples of breakdown in interconnected grid system
In most parts of the world, local or national electric utilities have joined in grid systems. The linking grids allow electricity generated in one area to be shared with others. Each utility that agrees to share gains an increased reserve capacity, use of larger, more efficient generators, and the ability to respond to local power failures by obtaining energy from a linking grid.

These interconnected grids are large, complex systems that contain elements operated by different groups. These systems offer the opportunity for economic savings and improve overall reliability but can create a risk of widespread failure. For example, a major grid-system breakdown occurred on November 9, 1965, in eastern North America, when an automatic control device that regulates and directs current flow failed in Queenston, Ontario, causing a circuit breaker to remain open. A surge of excess current was transmitted through the northeastern United States. Generator safety switches from Rochester, New York, to Boston, Massachusetts, were automatically tripped, cutting generators out of the system to protect them from damage. Power generated by more southerly plants rushed to fill the vacuum and overloaded these plants, which automatically shut themselves off. The power failure enveloped an area of more than 200,000 sq km (80,000 sq mi), including the cities of Boston; Buffalo, New York; Rochester, New York; and New York City.

Similar grid failures, usually on a smaller scale, have troubled systems in North America and elsewhere. On July 13, 1977, about 9 million people in the New York City area were once again without power when major transmission lines failed. In some areas the outage lasted 25 hours as restored high voltage burned out equipment. These major failures are termed blackouts.

The worst blackout in the history of the United States and Canada occurred August 14, 2003, when 61,800 megawatts of electrical power was lost in an area covering 50 million people. (One megawatt of electricity is roughly the amount needed to power 750 residential homes.) The blackout affected such major cities as Cleveland, Detroit, New York, Ottawa, and Toronto. Parts of eight states—Connecticut, Massachusetts, Michigan, New Jersey, New York, Ohio, Pennsylvania, and Vermont—and the Canadian provinces of Ontario and Qu├ębec were affected. The blackout prompted calls to replace aging equipment and raised questions about the reliability of the national power grid.

The term brownout is often used for partial shutdowns of power, usually deliberate, either to save electricity or as a wartime security measure. From November 2000 through May 2001 California experienced a series of planned brownouts to groups of customers, for a limited duration, in order to reduce total system load and avoid a blackout due to alleged electrical shortages. However, an investigation by the California Public Utilities Commission into the alleged shortages later revealed that five energy companies withheld electricity they could have produced. In 2002 the commission concluded that the withholding of electricity contributed to an “unconscionable, unjust, and unreasonable electricity price spike.” California state utilities paid $20 billion more for energy in 2000 than in 1999 as a result, the head of the commission found.

The commission also cited the role of the Enron Corporation in the California brownouts. In June 2003 the Federal Energy Regulatory Commission (FERC) barred Enron from selling electricity and natural gas in the United States after conducting a probe into charges that Enron manipulated electricity prices during California’s energy crisis. In the same month the Federal Bureau of Investigation arrested an Enron executive on charges of manipulating the price of electricity in California. Two other Enron employees, known as traders because they sold electricity, had pleaded guilty to similar charges. See also Enron Scandal.

Despite the potential for rare widespread problems, the interconnected grid system provides necessary backup and alternate paths for power flow, resulting in much higher overall reliability than is possible with isolated systems. National or regional grids can also cope with unexpected outages such as those caused by storms, earthquakes, landslides, and forest fires, or due to human error or deliberate acts of sabotage.


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 the 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.

Power Analytics
Power Analytics is the term used to describe the management of electrical power distribution, consumption, and preventative maintenance throughout a large organization’s facilities, particularly organizations with high electrical power requirements. For such facilities, electrical power problems – including the worst-case scenario, a full power outage – could have a devastating serious impact. Additionally, it could jeopardize the health and safety of individuals within the facility or in the surrounding community.

Power Analytics use complex mathematical algorithms to detect variations within an organization’s power infrastructure (measurements such as voltage, current, power factor, etc.). Such variations could be early indications of longer-term power problems; when a Power Analytics system detects such variations, it will begin to diagnose the source of the variation, surrounding components, and then the complete electrical power infrastructure. Such systems will – after fully assessing the location and potential magnitude of the problem – predict when and where the potential problem will occur, as well as recommend the preventative maintenance required preempting the problem from occurring.

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 absence of grid power, a so-called black start needs to be performed to bootstrap the power grid into operation.

For more information on this visit the below links :
http://www.windows.ucar.edu/spaceweather/cold_start.html
http://blackout.gmu.edu/home.html

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