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. September 24, 1912 (filed in 1905). 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.
Vacuum tube circuit breakers The first research and patents on vacuum interrupters (switches) were conducted by the California Institute of Technology around 1926. The first industrial applications were realized in the late 1950s when the technological difficulties of implementation were resolved, particularly the guarantee of a high vacuum for at least twenty years, which requires perfect sealing of the interrupter.
In a vacuum circuit breaker, the arc is sustained by particles emitted from the contacts. The high dielectric strength achieved in a high vacuum allows the device to withstand the transient recovery voltage between contacts after current interruption. The passage of current through specially shaped contacts generates a magnetic field that causes the arc to rotate and prevents it from remaining attached to the same contact surface. This avoids melting of the arc contacts and excessive production of metallic particles that would limit the dielectric strength after current interruption.
Circuit breakers incorporating vacuum interrupters are currently in service at 84 kV in Japan. The breaking capacity of a vacuum circuit breaker can reach 63 kA or even more. A model of circuit breaker model with a single interrupting unit per pole manufactured by Siemens allows vacuum interruption up to 145 kV. Research is ongoing to design of vacuum circuit breakers with higher rated voltages, up to a rated voltage of 363 kV and a breaking capacity of 80 kA.
Compressed-air or natural-gas circuit breakers
The gas contained in compressed air circuit breakers is maintained at high pressure ( to ) using a mechanical compressor. This high pressure ensures dielectric strength and causes the blowing needed to extinguish the arc.
The intense gas blast used in these breakers made it possible to obtain very high performance (currents interrupted up to 100 kA at high voltage) and very short fault-clearing times, allowing good stability of power networks during faults.
For a long time they had a monopoly on very high performance and were the preferred choice in very high voltage networks during the 1960s and 1970s, particularly in North America.
One drawback of compressed-air circuit breakers is the very high noise level during opening. In addition, they require periodic maintenance, particularly of their compressors. This explains why they have gradually been replaced by a new generation of circuit breakers: SF6 gas circuit breakers.
The compressed-air technique is still the only one capable of achieving the highest breaking capacities (275 kA at 36 kV), which are required for generator circuit breakers.
Research and development activities are currently being conducted by industry regarding the use of nitrogen (a natural-origin gas) as an interrupting medium without compressors, particularly in gas-insulated substations.
High-voltage SF6 (sulfur hexafluoride) circuit breakers History The use of SF6 gas for insulation was patented in the United States by Franklin Cooper of General Electric in 1938. November 12, 1940 (filed in 1938) Its use for current interruption was also claimed in 1938 in a German patent by Vitaly Grosse of AEG (Allgemeine Elektrizitäts-Gesellschaft).
The first industrial application of SF6 gas for switching dates from 1953, when Westinghouse used it for high-voltage load-break switches rated from 15 kV to 161 kV with a breaking capacity of 600 A.
The first high-voltage SF6 gas circuit breaker was developed in 1956 by Westinghouse, but the breaking capacity was limited to 5 kA at a voltage of 115 kV (1,000 MVA), and the device required several interrupting units in series per pole to achieve the required performance (six interrupting chambers per pole).
At the same time, in 1957, the Ateliers de Constructions Électriques de Delle built a 23 kV, 250 MVA circuit breaker for distribution switchgear, followed by a grounded-tank (dead tank) circuit breaker for a 25 kV locomotive with an apparent power of 200 MVA.
It was not until 1959 that Westinghouse produced the first high-breaking-capacity SF6 circuit breaker: 41.8 kA at 138 kV (10,000 MVA) and 37.6 kA at 230 kV (15,000 MVA). This three-pole operated circuit breaker included three interrupting chambers in series per pole. It operated with an SF6 pressure of 13.5 bar relative (above atmospheric pressure) for blowing and 3 bar relative to ensure dielectric strength. Good electrical performance was achieved thanks to the high pressures used; however, these devices presented a risk of SF6 gas liquefaction at temperatures below 5 °C. For certain climates, it was therefore necessary to plan for the maintenance of the temperature of the high-pressure tank.
The excellent electrical properties of SF6 led to large-scale production of the gas during the 1960s and its use for the development of high-voltage circuit breakers at increasingly high rated voltages. Since 2009 in China, high-voltage SF6 circuit breakers have been designed for rated voltages up to 800 kV, and later 1,100 kV.
The development of high-voltage networks and the need to bring these networks into urban and industrial areas led to the design of new compact types of high-voltage substations known as "gas-insulated" or "metal-enclosed". To ensure insulation, the naturally occurring air was replaced by SF6 gas, which has very good dielectric properties, which has greatly reducing the bulk of high-voltage equipment.
The application of this technology in high-voltage systems made it possible, as early as 1966, to build the first metal-enclosed substation as an experimental 220kV prototype installed at Le Plessis-Gassot near Paris, and the first metal-enclosed circuit breaker in 1967 in Levallois-Perret, followed in 1969 by the installation at the Vaise substation in Lyon.