Arcing horns
Encyclopedia
Arcing horns are projecting conductors used to protect insulator
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...

s on high voltage electric power transmission
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...

 systems from damage during flashover
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

. Overvoltages on transmission lines, due to atmospheric electricity
Atmospheric electricity
Atmospheric electricity is the regular diurnal variations of the Earth's atmospheric electromagnetic network . The Earth's surface, the ionosphere, and the atmosphere is known as the global atmospheric electrical circuit...

, lightning
Lightning
Lightning is an atmospheric electrostatic discharge accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms...

 strikes, or electrical faults, can cause arcs
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

 across 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. Horns are normally paired on either side of the insulator, one connected to the high voltage part and the other to ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....

. They are frequently to be seen on insulator strings on overhead lines, or protecting transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

 bushings
Bushing (electrical)
A bushing is a hollow insulating liner through which a conductor may pass. Bushings appear on switchgear, transformers, circuit breakers and other high voltage equipment.- Description :...

.

The horns can take various forms, such as simple cylindrical rods, circular guard rings, or contoured curves, sometimes known as 'stirrups'.

Background

High voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...

 equipment, particularly that which is installed outside, such as overhead power line
Overhead power line
An overhead power line is an electric power transmission line suspended by towers or utility poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of transmission for large quantities of electric energy...

s, is commonly subject to transient overvoltage
Overvoltage
When the voltage in a circuit or part of it is raised above its upper design limit, this is known as overvoltage. The conditions may be hazardous...

s, which may be caused by phenomena such as lightning strike
Lightning strike
Lightning strikes are electrical discharges caused by lightning, typically during thunderstorms.Humans can be hit by lightning directly when outdoors. Contrary to popular notion, there is no 'safe' location outdoors. People have been struck in sheds and makeshift shelters...

s, 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
Busbar
In electrical power distribution, a bus bar is a strip of copper or aluminium that conducts electricity within a switchboard, distribution board, substation or other electrical apparatus....

 or transformer bushing, are at greatest risk to overvoltage due to the change in characteristic impedance
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...

 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
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 which occupies the air surrounding the equipment. Overvoltage events may cause the electric field to exceed the dielectric strength
Dielectric strength
In physics, the term dielectric strength has the following meanings:*Of an insulating material, the maximum electric field strength that it can withstand intrinsically without breaking down, i.e., without experiencing failure of its insulating properties....

 of air and result in the formation of an arc
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...

 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 relay
Protective relay
In electrical engineering, a protective relay is a complex electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault is detected...

s are expected to detect the formation of the arc and automatically open circuit breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...

s 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
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...

 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 form a spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...

 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
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

 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
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

 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
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...

. More importantly, a flashover across arcing horns produces an earth fault resulting in a circuit outage
Downtime
The term downtime is used to refer to periods when a system is unavailable.Downtime or outage duration refers to a period of time that a system fails to provide or perform its primary function...

 until the fault is cleared by circuit breaker operation. For this reason, non-linear resistors
Varistor
A varistor is an electronic component with a "diode-like" nonlinear current–voltage characteristic. The name is a portmanteau of variable resistor...

 known as surge divertors can replace arcing horns at critical locations.

Switch protection

Arcing horns are sometimes installed on air-insulated switchgear
Switchgear
The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults...

 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 the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

 can be interrupted. The horns are designed to endure the arc rather than the contact surfaces of the switch itself.

Grading rings

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
Potential gradient
A potential gradient is the local space rate of change of the potential with respect to displacement.In electrostatics then, it is the local space rate of change of the electric potential:Units are volts per meter...

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.
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