Emergency power system
From Wikipedia, the free encyclopedia
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 laboraties and modern naval ships. Emergency power systems can rely on generators or uninterruptible power supplies.
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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 switchs 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 a modern building, most emergency power sytstems rely on a diesel engine driven generator, although smaller buildings may use a gasoline engine driven generator. An automatic transfer switch is used to tie the emergency power in. 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 light 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 that may be on emergency power can 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 theatre using it to power show equipment because "the show must go on."
[edit] Electrionic device protection
Computers, communication networks and other modern electronic devices depend on a steady stream of power 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 envolved. To achieve this protection an uninterruptible power supply (UPS) is used. UPS systems can be local or building wide. Local UPS's are 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 serveral 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.
[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] Fueling the emergency power system
For a 230 V AC emergency supply system, a central battery system with controls, located in the power station building itself, is used to avoid long electric supply leads. This central battery system consists of lead-acid battery cell units to make up 240 V DC as well as a few more stand-by cells, each with its own battery charging unit. Also needed are an inverter unit capable of receiving 230 V AC and an automatic switch-less system that is able to signal to and activate the emergency supply circuit in case of failure of AC 230 V station supply.
