A surge arrester is a protective device used to limit voltage in equipment by discharging or bypassing surge current. Surge arresters prevent continued flow to follow current to ground and are capable of repairing these functions as specified by ANSI (American National Standards Institute - a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States) standards. Arresters do not absorb or stop lightning, but rather divert it, limiting the voltage and protecting the equipment it is installed next to. Surge arresters have many applications, ranging from protecting a home to an electrical substation. They can be installed on circuit breakers, in pad mounted transformers, on pole mounted transformers, and more.
Power distribution lines will see various voltage surges from time to time, the main source of which is lightning. Lightning is random and unpredictable, and approximately 100 lightning strikes occur on the surface of the Earth every second. Other sources of voltage surges include switching surges and temporary overvoltages. A switching surge is an overvoltage resulting from changes in the operation conditions within the system. These are the most common voltage surge for station-class arresters. Switching surges involve the trapping of energy and subsequent release of it. Temporary overvoltages occur due to ground faults on one phase or voltage rises on unfaulted phases until the fault can be cleared.
To work, a surge arrester contains a series of metal oxide varistor (MOV) blocks. These MOV blocks function similarly to a voltage-controlled switch, which acts as an insulation with line voltage. When the voltage arrester experiences a rise of voltage above the reference voltage, the MOV blocks go into conduction. Because the MOV blocks are non-linear, once the voltage drops back down below the reference voltage, the conduction ends.
Surge arresters are required to be capable of withstanding the continuous power-frequency voltage in which it is intended to operate. The arrester must discharge transient energy in the form of current, while simultaneously preventing the voltage across the equipment from becoming excessive. Arresters must operate in the same environment as the equipment they are protecting. Temporary overvoltage (TOV) capability, the allowable overvoltage and duration that an arrester can withstand without damage, should be displayed on the device.
Arresters can fail for a variety of reasons, including: TOV condition that lasted longer than the device could withstand, the arrester was undersized, lightning surges were greater than the device’s rating, gap degradation in silicon carbide arresters, deterioration of the housing, wildlife interference, or disk aging. After experiencing an end-of-life event, porcelain arresters may break apart, while polymer arresters may experience a blow out the side or the disconnector will separate the arrester from the ground.
There are four main types of surge arresters: secondary arresters, distribution arresters, intermediate arresters, and station class arresters. Secondary arresters are used to protect against secondary surges and are rated under 1000 V. When using a secondary arrester, transformer failure rates are significantly reduced. Distribution arresters are rated 1-36 kV. Among distribution arresters, there are three subclasses: light duty, normal duty, and heavy duty, each used in different levels of lightning. Intermediate arresters offer better discharge voltages, can withstand high fault currents, and are available in ratings ranging from 3 to 120 kV. Finally, station class arresters provide the best discharge voltages & highest energy handling capabilities, can withstand the highest current fault, and are available in ratings from 3 to 684 kV. For demanding applications, station class arresters have varying cantilever strengths.
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