A high-voltage circuit breaker is designed to make, carry, and interrupt electric currents under its rated voltage. The rated voltage is "the maximum system voltage for which the equipment is designed," according to the definition given by the International Electrotechnical Commission (IEC). According to the IEC., a high-voltage circuit breaker operates both: under normal operating conditions, for example, to connect or disconnect a power line in an electrical network; under specified abnormal conditions, particularly to clear a short circuit on the network caused by lightning (see also the specific section) or other causes. Because of its characteristics, a circuit breaker is the essential switching device for protecting a high-voltage network, as it is the only device capable of interrupting a short-circuit current and thus preventing equipment connected to the network from being damaged by such a fault.
Principle of operation The interruption of an electric current by a high-voltage circuit breaker is achieved by separating contacts in a gas (air, SF6, or natural-origin gases) or in another insulating medium (oil or vacuum). Immediately after contact separation, the current continues to flow in the circuit through an electric arc established between the breaker contacts.
Today, high-voltage circuit breakers 72.5 kV to 1200 kV mainly use gas or oil for insulation and interruption. Vacuum interruption technology is limited mainly to medium-voltage applications, although recent developments exist for rated voltages of 84 kV or 145 kV. For example, a vacuum circuit breaker model has been designed for a rated voltage of 145 kV and uses a naturally occurring gas for insulation outside of the vacuum interrupters. In gas circuit breakers, the current is cut off when sufficient gas flow is applied to the electric arc to cool and interrupt it. Under normal conditions, the gas contained in the circuit breaker is insulating and withstands the network voltage connected to its terminals. When the breaker contacts separate, the gap between the contacts is subjected to a strong electric field. The current then flows through an arc, which is a plasma (or ionized gas) composed of decomposed gas molecules, electrons, and ions. The temperature of the arc becomes extremely high and may reach 20000K or more at its core, depending, among other factors, on the magnitude of the short-circuit current, the type of interrupting medium, and the cooling rate of the arc. Under the effect of the blowing applied to the arc during breaker operation, the arc temperature decreases, electrons and ions recombine, and the fluid regains its insulating properties. Current interruption is then achieved. For high-voltage circuit breakers, the selected interruption principle is current interruption at the moment when the current passes through zero (which occurs every ten milliseconds in the case of alternating current at convert 50 Hz. This is because it is at this moment that the power supplied to the arc by the network is at its minimum (this apparent power supplied is even zero at the moment when the instantaneous current value is zero). With sufficient gas flow, this time interval—when the current is small—can therefore be used to cool the arc sufficiently so that its temperature decreases and the space between the contacts becomes insulating again.
Interruption techniques The first high-voltage circuit breakers, introduced at the end of the 1890s and the early 20th century, used oil or compressed air for interruption. The first circuit-breaker patent for high voltage, based on the separation of two contacts in oil (and in air), was filed by Sebastian Ziani de Ferranti in July 1895. This principle was later improved and led to the development of oil circuit breakers. Interruption in atmospheric air (a naturally occurring gas) was developed for high voltage A, the idea being to lengthen the arc sufficiently in air to cool it, extinguish it, and then withstand the network voltage. The oil-based switching technique was later replaced by interruption in compressed air, SF6 gas, naturally occurring gases, and vacuum.
Oil circuit breakers
Interruption in oil became widely used in high voltage after first being developed in medium voltage(or high voltage A). Under the action of the electric arc, the oil decomposes and several type of gases are produced (mainly hydrogen and acetylene). The energy from the arc is used to decompose and evaporate the oil, which cools the medium between the contacts and consequently interrupts the current as it passes through zero. The first oil circuit breakers had interrupting contacts immersed in oil contained in a metal tank at ground potential, hence the name dead tank. They are called "large-volume oil circuit breakers". Some are still in service today, for example, in the United States. Subsequently, in the 1950s, "low-volume oil circuit breakers" were designed to reduce the amount of oil required and, above all, to limit the fire risk inherent in bulk oil circuit breakers. The arc develops inside an insulating cylinder to limit its length and control as much as possible the energy contained in the arc. This energy is used to generate the blow-out by vaporizing the oil, as explained above. This technique, know as "self-blow-out," was later used for SF6 gas circuit breakers. It has been applied for rated voltage up to 765 kV and very high fault currents of up to 50 kA. The main disadvantages of these circuit breakers were the need for many interrupting units in series (to maintain voltage) and the need for extensive and delicate maintenance (replacement of used oil). They have been replaced by SF6 gas circuit breakers, which require little maintenance and have a long service life.
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