Arcing Horns
Background
High voltage equipment, particularly that which is installed outside, such as overhead power lines, is commonly subject to transient overvoltages, which may be caused by phenomena such as lightning strikes, faults on other equipment, or switching surges during circuit re-energisation. Overvoltage events such as these are unpredictable, and in general cannot be completely prevented. Line terminations, at which a transmission line connects to a busbar or transformer bushing, are at greatest risk to overvoltage due to the change in characteristic impedance at this point.
An electrical insulator serves to provide physical separation of conducting parts, and under normal operating conditions is continuously subject to a high electric field which occupies the air surrounding the equipment. Overvoltage events may cause the electric field to exceed the dielectric strength of air and result in the formation of an arc between the conducting parts and over the surface of the insulator. This is called flashover. Contamination of the surface of the insulator reduces the breakdown strength and increases the tendency to flash over. On an electrical transmission system, protective relays are expected to detect the formation of the arc and automatically open circuit breakers to discharge the circuit and extinguish the arc. Under a worst case, this process may take as long as several seconds, during which time the insulator surface would be in close contact with the highly energetic plasma of the arc. This is very damaging to an insulator, and may shatter brittle glass or ceramic disks, resulting in its complete failure.
Operation
Arcing horns protecting bushings on a distribution transformer
Arcing horns form a spark gap across the insulator with a lower breakdown voltage than the air path along the insulator surface, so an overvoltage will cause the air to break down and the arc to form between the arcing horns, diverting it away from the surface of the insulator. An arc between the horns is more tolerable for the equipment, providing more time for the fault to be detected and the arc to be safely cleared by remote circuit breakers. The geometry of some designs encourages the arc to migrate away from the insulator, driven by rising currents as it heats the surrounding air. As it does so, the path length increases, cooling the arc, reducing the electric field and causing the arc to extinguish itself when it can no longer span the gap. Other designs can utilise the magnetic field produced by the high current to drive the arc away from the insulator. This type of arrangement can be known as a magnetic blowout.
Design criteria and maintenance regimes may treat arcing horns as sacrificial equipment, cheaper and more easily replaced than the insulator, failure of which can result in complete destruction of the equipment it insulates. Failure of insulator strings on overhead lines could result in the parting of the line, with significant safety and cost implications.
Arcing horns thus play a role in the process of correlating system protection with protective device characteristics, known as insulation coordination. The horns should provide, amongst other characteristics, near-infinite impedance during normal operating conditions to minimise conductive current losses, low impedance during the flashover, and physical resilience to the high temperature of the arc.
As operating voltages increase, greater consideration must be given to such design principles. At medium voltages, one of the two horns may be omitted as the horn-to-horn gap can otherwise be small enough to be bridged by an alighting bird. Alternatively, duplex gaps consisting of two sections on opposite sides of the insulator can be fitted. Low voltage distribution systems, in which the risk of arcing is much lower, may not use arcing horns at all.
The presence of the arcing horns necessarily disturbs the normal electric field distribution across the insulator due to their small but significant capacitance. More importantly, a flashover across arcing horns produces an earth fault resulting in a circuit outage until the fault is cleared by circuit breaker operation. For this reason, non-linear resistors known as surge divertors can replace arcing horns at critical locations.
Switch protection
Arcing horns are sometimes installed on air-insulated switchgear to protect the switch arm from arc damage. When a high voltage switch breaks a circuit, an arc can establish itself between the switch contacts before the current can be interrupted. The horns are designed to endure the arc rather than the contact surfaces of the switch itself.
Grading rings
Arcing horns shouldn’t be confused with grading rings, which are ring shaped conductors surrounding the high potential end of an insulator string on some high voltage transmission lines, attached to the line. The purpose of grading rings is not to provide a spark gap but to even out the potential gradient across the string. The electric field across a string of insulators is not distributed evenly along the string but is concentrated at the ends, so with an overvoltage the end insulator units will break down first. By distributing the electric field more evenly, grading rings increase the breakdown voltage of the string. However, sometimes grading rings have a secondary usage as arcing terminals, combined with arcing horns on the ground side of the insulator.
References
^ a b Short, Tom A.. Electric Power Distribution Handbook. CRC Press. p. 348. ISBN 9780849317910. http://books.google.co.uk/books?id=CTmcEVsLy_cC&pg=PA348&.
^ Guile, A. E.; Paterson, W. (1977). Electrical Power Systems. Pergamon. pp. 131132. ISBN 008021729X.
^ a b c d Electricity Training Association. Power System Protection. 2. IET. pp. 296297. ISBN 9780852968369.
^ Looms, J.S.T. (1988). Insulators for High Voltages. IET. p. 107. ISBN 9780863411168. http://books.google.co.uk/books?id=wn2LZLYt2DIC&pg=PA107.
^ Glover, J.D.; Sarma, M. (1987). Power System Analysis and Design. PWS-KENT. p. 416. ISBN 0534078605.
^ McCombe, John; Haigh, F.R. (1966). Overhead Line Practice (3rd ed.). Macdonald. p. 182.
^ Hordeski, Michael F.; Pansini, Anthony J. (2007). Electrical Distribution Engineering. Fairmont Press. p. 365. ISBN 9780881735468. http://books.google.co.uk/books?id=Z3DRIiujJCUC&pg=PA395.
^ Pansini, Anthony J. (1998). Electrical Transformers and Power Equipment. Fairmont Press. p. 185. ISBN 9780881733112. http://books.google.co.uk/books?id=f77zWwA3oS4C&pg=PA185&.
Categories: Electric power systems components | Electric arcs
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