Emergency power system
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Emergency power systems are a type of system, which may include lighting, generators and other apparatus, to provide backup resources in a crisis or when regular systems fail. They find uses in a wide variety of settings from residential homes to hospitals, scientific laboratories and modern naval ships. Emergency power systems can rely on generators, deep cycle batteries, Flywheel energy storage or hydrogen fuel cells[1]. Finally, some homebrew emergency power systems use regular lead-acid car batteries, but these do not make a very efficient or reliable system.[2]
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[edit] History
Emergency power systems were used as early as World War II on naval ships. In combat, a ship may lose the function of its steam engines, which power the steam driven turbines for the generator. In such a case, one or more diesel engine(s) are used to drive back-up generators. Early transfer switches relied on manual operation; two switches would be placed horizontally, in line and the "on" position facing each other. a rod is placed in between. In order to operate the switch one source must be turned off, the rod moved to the other side and the other source turned on.
[edit] Operation in buildings
Utility power (known as normal power) can be lost due to downed lines, malfunctions at a sub-station, planned blackouts or in extreme cases a grid-wide failure. In modern buildings, most emergency power systems have been and are still based around on generators. Usually, these generators are diesel engine driven, although smaller buildings may use a gasoline engine driven generator and larger ones a gas turbine. However, lately, more use is being made of deep cycle batteries and other technology as Flywheel energy storage or hydrogen fuel cells. This as these latter systems do not produce polluting gases, hereby allowing the placement to be done within the building. Also, as a second advantage, they do not require a storage shed to be set up with additional fuels [3].
With regular generators, an automatic transfer switch is used to connect emergency power. One side is connected to both the normal power feed and the emergency power feed; and the other side is connected to the load designated as emergency. If no electricity comes in on the normal side, the transfer switch uses a solenoid to throw a triple pole, single throw switch. This switches the feed from normal to emergency power. The loss of normal power also triggers a battery operated starter system to start the generator, similar to using a car battery to start an engine. Once the transfer switch is switched and the generator starts, the building's emergency power comes back on (after going off when normal power was lost.)
Emergency lighting (also known as egress lighting) is almost always on a building's emergency power system. Unlike emergency lights, emergency lighting is not a type of light fixture; it is a pattern of the building's normal lights that provides a path of lights to allow for safe exit in case of an emergency, or lights up service areas such as mechanical rooms and electric rooms. Emergency lighting also includes exit signs. Fire alarm systems and the electric motor pumps for the fire sprinklers is almost always on emergency power. Other equipment on emergency power may include smoke isolation dampers, smoke evacuation fans, handicap doors and outlets in service areas. Hospitals use emergency power outlets to power life support systems and monitoring equipment. Some buildings may even use emergency power as part of normal operations, such as a theater using it to power show equipment because "the show must go on."
[edit] Electronic device protection
Computers, communication networks and other modern electronic devices need not only power, but also a steady flow of it to continue to operate. If the source voltage drops significantly or drops out completely these devices will fail, even if it is for a fraction of a second. Because of this, even a generator back-up does not provide protection because of the start-up time involved.
To achieve this, extra equipment as surge protectors, invertors, or a sometimes a complete uninterruptible power supply (UPS) is used. UPS systems can be local or building wide. A local UPS is a small box that fits under a desk or a telecom rack and powers a small number of devices. A building wide UPS can take on several different forms, depending on the application. It directly feeds a system of outlets designated as UPS feed and can power a large number of devices.
Since telephone exchanges use DC, the building's battery room is generally wired directly to the consuming equipment and floats continuously on the output of the rectifiers that normally supply DC rectified from utility power. When utility power fails, the battery carries the load without needing to switch. With this simple though somewhat expensive system, some exchanges have never lost power for a moment since the 1920s.
[edit] Structure and operation in utility stations
In recent years, large units of a utility power station are usually designed on a unit system basis in which the required devices, including the boiler, the turbine generator unit, and its power (step up) and unit (auxiliary) transformer are solidly connected as one unit. A less common set-up consists of two units grouped together with one common station auxiliary. As each turbine generator unit has its own attached unit auxiliary transformer, it is connected to the circuit automatically. For starting the unit, the auxiliaries are supplied with power by another unit (auxiliary) transformer or station auxiliary transformer. The period of switching from the first unit transformer to the next unit is designed for automatic, instantaneous operation in times when the emergency power system needs to kick in. It is imperative that the power to unit auxiliaries not fail during a station shutdown (an occurrence known as black-out when all regular units temporarily fail). Instead, during shutdowns the grid is expected to remain operational. When problems occur, it is usually due to reverse power relays and frequency-operated relays on grid lines due to severe grid disturbances. Under these circumstances, the emergency station supply must kick in to avoid damage to any equipment and to prevent hazardous situations such as the release of hydrogen gas from generators to the local environment.
[edit] Controling the emergency power system
For a 208 VAC emergency supply system, a central battery system with automatic controls, located in the power station building itself, is used to avoid long electric supply wires. This central battery system consists of lead-acid battery cell units to make up a 12 or 24 VDC system as well as stand-by cells, each with its own battery charging unit. Also needed are a voltage sensing unit capable of receiving 208 VAC and an automatic system that is able to signal to and activate the emergency supply circuit in case of failure of 208 VAC station supply.
[edit] See also
[edit] External links
- How Emergency Power Systems Work
- How to Install a backup generator
- Difference between car battery and deep-cycle batteries
