Transmission and Distribution
10EE53
Arcing horns
From Wikipedia, the free encyclopedia
Arcing horns on each side of a tension-type insulator string
Arcing horns (sometimes arc-horns) are projecting conductors used to protect insulators on
high voltage electric power transmission systems from damage during flashover. Overvoltages
on transmission lines, due toatmospheric electricity, lightning strikes, or electrical faults, can
cause arcsacross insulators (flashovers) that can damage them. The horns encourage the
flashover to occur between themselves rather than across of the surface of the insulator they
protect.[1] Horns are normally paired on either side of the insulator, one connected to the high
voltage part and the other to ground. They are frequently to be seen on insulator strings on
overhead lines, or protectingtransformer bushings.
The horns can take various forms, such as simple cylindrical rods, circular guard rings, or
contoured curves, sometimes known as 'stirrups'.
Contents
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1 Background
2 Operation
3 Switch protection
4 Grading rings
5 References
Background[edit]
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, orswitching surges during circuit reenergisation.[2] 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.[3]
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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.[1] 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 relaysare 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[edit]
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. [3] 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
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designs can utilise the magnetic field produced by the high current to drive the arc away from the
insulator.[4] This type of arrangement can be known as amagnetic 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 asinsulation 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.[5]
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.[6]Alternatively, duplex gaps consisting of two
sections on opposite sides of the insulator can be fitted.[3] 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 acircuit outage until the fault is cleared by
circuit breaker operation. For this reason, non-linear resistors known as varistorscan replace
arcing horns at critical locations.[3]
Switch protection
Arcing horns are sometimes installed on air-insulated switchgear and transformers 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 thecurrent can be interrupted. The horns are designed to
endure the arc rather than the contact surfaces of the switch itself.[7][8]
Grading ring
Arcing horns are not to 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 over voltage 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
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secondary usage as arcing terminals, combined with arcing horns on the ground side of the
insulator.