Circuit breaker
Circuit breakers are circuit breakers that are designed for switching high currents. In contrast to load switches , circuit breakers can not only switch regular operating currents and low overload currents, but also switch on high overload currents and short-circuit currents (generator circuit breakers up to 800 kA ) in the event of faults , hold these fault currents for a specified time and switch them off safely without damage. Circuit breakers are single-pole or three-pole.
Basics
The overload current relates to the weakest link in the system downstream of the circuit breaker, viewed in the direction of energy flow. In order to protect such downstream systems from damage caused by overload or short circuit, the circuit breaker should be able to switch these currents in connection with the network protection devices .
The interruption of the current flow leads at least briefly to a voltage flashover between the two contacts for each switch , since the distance during the separation process is not yet sufficient for insulation. If there is gas between the two poles, if there is a correspondingly high voltage difference between the poles, it is ionized by the flashover and a self-sustaining gas discharge is formed , which is called an arc .
This plasma not only continues to conduct electricity , but also reduces the service life of the component; with strong currents it can even destroy the switch. In contrast to disconnectors , circuit breakers are designed in such a way that the arc that arises when the switching contacts are opened is extinguished quickly and without damaging the switch, thus interrupting the flow of current.
operation area
Apart from the trivial application as a simple switching element, circuit breakers are used as network protection in the event of faults such as short circuits or earth faults . Circuit breakers can be controlled using different mechanisms and primarily depends on the power to be switched. The drives are manual drives and so-called power drives used. Power drives are drives that are not actuated by human force. There are the following power drives:
- Magnetic drives
- Motor drives
- Spring drives
- Pneumatic drives.
A particular advantage of power drives is the possibility of remote control of the switch.
Low voltage applications
There are four groups:
- air circuit breaker (ACB)
- Molded Case Circuit Breakers (MCCB)
- Load Break Switches (LBS), versions with / without fuses
- Miniature Circuit Breaker (MCB)
Low-voltage circuit breakers are electromagnetic circuit breakers. Their mode of operation corresponds in principle to the mode of operation of miniature circuit breakers . They are usually equipped with a thermal and a magnetic release and thus have the same structural elements as circuit breakers. However, they are designed for higher rated currents, and the releases of circuit breakers, unlike miniature circuit breakers, can be set separately.
In the low-voltage range, the switches are also used as motor protection switches .
Non-adjustable circuit breakers with tripping characteristics K and Z according to EN 60947-2 (VDE 0660-101) are used in practice like miniature circuit breakers and are also referred to as such.
Structure of a low-voltage circuit breaker
Functional areas of a tripping characteristic
Example of setting options on a circuit breaker
Medium voltage
Circuit breakers serve as protective devices in the medium voltage level, which can also separate short circuits from the network in the event of a fault. Triggering in the event of an error (off command) takes place via pull magnets , which release locking mechanisms which release mechanical energy present in a memory (pneumatic pressure accumulator, spring accumulator ). This means that the contact pieces (contacts) are moved away from one another or towards one another at high speed. After the switching process z. B. wound up again with an electric motor of the spring accumulator.
Circuit breakers, like switch disconnectors, have the task of switching objects such as generators , transformers , coupling of busbars as well as cables and overhead lines.
Compared to switch disconnectors, circuit breakers are able to switch safely even in the event of a fault (e.g. short circuit).
Arc extinction
A distinction is made between the type of arc extinction :
- Circuit breaker with zero point cancellation
- Circuit breaker with short-circuit current limitation.
Delete the so-called zero-quencher at zero crossing of the AC -Schaltlichtbogen. In the case of circuit breakers with short-circuit current limitation, the surge short-circuit current is limited to a smaller forward current. The quick opening of the contact pieces is achieved in the low voltage range via the switching mechanism . The arc is extinguished in a deion chamber . There are also other shutdown mechanisms such as B. a drop armature or the triggering by the force of two parallel current-carrying contact pieces.
Extinguishing agent
In the case of circuit breakers in the high-voltage range - these switches belong to the group of high-voltage switches - various extinguishing media can be used to extinguish the arcs that occur during switching. The following extinguishing media are used in different types of circuit breakers at high voltage.
air
Compressed air is compressed to a pressure of 30 bar by a compressed air generation, storage and distribution system and, during the switching process, ensures that arcs are blown out via appropriate nozzles. The air was cleaned, compressed and dehumidified by these treatment systems.
In outdoor technology, ambient air is still used today to isolate it and as an extinguishing medium. Compared to circuit breakers insulated with inert gas, this leads to a larger space requirement, since the extinguishing capacity of air is less.
Insulating gas SF 6
The sulfur hexafluoride gas ( SF 6 ) is used in circuit breaker technology as an insulating gas to extinguish arcs that occur when the circuit breaker is switched. It is suitable for this due to its high electrical strength and high thermal conductivity. Furthermore, after dissociation in the burning arc, it has good recombination ability. Free charge carriers in the switching path are quickly eliminated and the stress hardening occurs more quickly. SF 6 is used for high breaking capacities in blow piston switches, while self-blowing switches are used for medium breaking capacities . SF 6 is the most powerful known greenhouse gas .
oil
Oil was used until around 1940 in one type of oil switch, the boiler oil switch. Since 1975 oil switches have been and are being replaced by the more powerful and efficient SF6 circuit breakers. As of the 2010s, low-oil circuit breakers can still be found in high-voltage networks.
In the case of oil switches, the high temperature of the arc breaks down some of the oil in the arcing chambers into a gas. The gas pressure creates a flow of oil that extinguishes the arc. With small switching capacities, the lower oil flow was supported by a pumping effect. This type of switch can e.g. B. switch a current of 4 kA at a voltage of 240 kV. With lower switching voltages, the switching currents are significantly higher.
vacuum
With vacuum switches, the contacts are under vacuum to avoid an electric arc. These switches are preferred for medium-voltage switchgear with voltages up to approx. 40 kV.
Switching principles
The switching principles of the individual circuit breakers differ essentially in their design. Encapsulated and non-encapsulated circuit breakers, which are often available as panels in combination with other high-voltage devices such as work earths and isolators, voltage and current transformers, drives, short-circuit making-proof high-speed earth electrodes and busbar feeders, are now available. In the following, however, only the insulating and extinguishing medium of the actual circuit breaker will be explained, since this medium can vary within a field, which is often referred to colloquially as a circuit breaker.
Non-enclosed switches
Unencapsulated switches are circuit breakers that are not shielded from the surrounding air. These can be located both outdoors and in control cabinets. The lack of shielding means that only air can be used as an extinguishing medium.
Air-insulated switches
Air- insulated switches (English: Air Insulated Switchgear , short: AIS ) are circuit breakers that extinguish an arc due to the switching movement length of the contacts. These switching movements depend on the voltage level in which the switch is used. AIS up to 20 kV nominal voltage is usually used indoors. Higher voltage levels are switched in encapsulated circuit breakers or in outdoor switches.
Outdoor switch
Outdoor switches separate a live line by moving the two contacts apart. The distance is designed in such a way that the plasma channel of the arc breaks down even at high humidity. This can mean that, depending on the voltage level, this isolating distance can be several meters. Therefore, these systems are mostly in the open air ("open air").
Encapsulated switches
Encapsulated switches are circuit breakers that are shielded from the environment by a system of pipes, housings and enclosures. This shielding also makes it possible to use media other than ambient air to extinguish the arc.
Compressed gas switch / 2-chamber switch
Here air is used as an extinguishing agent for the arc. Air is usually also used to operate the switch. It is turned on or off with high pressure. The switching time of the contacts is very short due to the high pressure. When switching off, there is also the fact that the resulting arc is elongated by the air flow until it breaks off (since the voltage over this length is not sufficient to allow the arc to exist). These switches were preferred when they had to be switched on or off frequently. Compressed gas switches have hardly been used for several decades due to their high level of noise generation. They have largely been replaced by self-blowing switches or flow switches - mostly with SF 6 gas.
Blow piston switch
In switches that have been developed according to the blow piston principle, the extinguishing unit consists of a fixed contact and the movable blow cylinder contact. During the switch-off movement, the volume of the blowing cylinder is steadily reduced and thus the pressure of the enclosed gas is increased until the fixed and moving contact separate. The separation of the contacts creates an electric arc, which increases the pressure of the gas further. The main compression of the gas is done by the switch drive. If the pressure is high enough, the compressed gas can flow out and draw energy from the arc through the flow movement and finally blow it out. The design of the two contacts as nozzle contacts results in optimal flow and extinguishing properties.
Auto-blow switch
The required extinguishing pressure for the self-inflating switch is generated during the switch-off movement, as is the case with the blower piston switch. However, the energy of the arc is used intensively here in the so-called heating volume to increase the pressure of the insulating gas. As a result, the drive of the self-blowing switch only has to use the energy for the switching movement of the switch and only insignificantly for compressing the insulating and extinguishing gas. This results in an energy saving of around 80 percent, which means that the drive can also be constructed in small dimensions. Examples of this type of circuit breaker are the gas-insulated switchgear , which is available in modular form as panels.
Flow switch
Flow switches are offered for all alternating voltages (currently up to 765 kV, soon up to 1100 kV) in the extra-high voltage network . Typical maximum permissible breaking currents are in the range of 25–63 kA, in special cases also 80 kA and more. For some time now, sulfur hexafluoride (SF 6 ) gas has been used increasingly instead of oil .
literature
- Fighter, Stefan; Kopatsch, Gerald (Ed.): Switchgear Manual. 12. Edition, Cornelsen, Berlin 2011, ISBN 978-3-06-450726-5 ( online )
Individual evidence
- ^ A b Adolf J. Schwab: Electrical energy systems generation, transport, transmission and distribution of electrical energy. 3rd revised edition, Springer Verlag, Berlin-Heidelberg-Dartredt-London-New York 2012, ISBN 978-3-642-21957-3 .
- ↑ a b c Wilfried Knies, Klaus Schierack: Electrical systems technology; Power plants, networks, switchgear, protective devices. 5th edition, Hanser Fachbuchverlag. 2006 ISBN 978-3-446-40574-5 .
- ↑ Ernst Hörnemann, Heinrich Hübscher: Electrical engineering specialist training in industrial electronics. 1st edition. Westermann Schulbuchverlag GmbH, Braunschweig, 1998, ISBN 3-14-221730-4 .
- ↑ a b c d Theodor Schmelcher: Low-voltage manual, project planning information for switchgear, switchgear and distribution boards. 1st edition, Siemens Aktiengesellschaft (Abt. Verlag), Berlin and Munich, 1982, ISBN 3-8009-1358-5 .
- ↑ Basics of low-voltage switching technology. Siemens AG, 2008, p. 336 , accessed on March 1, 2014 (chapter Selection criteria for low-voltage switchgear , table distribution with fuses and circuit breakers . Various Siemens circuit breakers and symbols of their characteristic curves. For 3WN, for example, the current-dependent delayed (a ) and instantaneous electromagnetic short-circuit release characteristic (s) adjustable (see also with overload relay on page 333).
- ↑ a b c d Hans-Günter Boy, Uwe Dunkhase: The master's examination in electrical installation technology. 12th edition, Vogel Buchverlag, Oldenburg and Würzburg, 2007, ISBN 978-3-8343-3079-6 .
- ↑ a b Réne Flosdorff, Günther Hilgarth: Electrical energy distribution. 4th edition, Verlag BG Teubner, 1982, ISBN 3-519-36411-5 .
- ^ Klaus Tkotz, Peter Bastian, Horst Bumiller: Electrical engineering. 27th revised and expanded edition, Verlag Europa-Lehrmittel Nourney Vollmer GmbH & Co. KG, Haan Gruiten 2009, ISBN 978-3-8085-3188-4 .
- ^ A. Senner: Electrical engineering. 4th edition. Verlag Europa-Lehrmittel, 1965.
- ^ Günter Springer: Electrical engineering. 18th edition, Verlag Europa-Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9 .
- ↑ compressed gas switch, ABB Management AG, Baden, Aargau, CH, document identification DE4427163A1 8 February 1996 Online (accessed on 4 June 2012).
- ↑ Electrical pressure gas switch, Mitsubishi Denki KK, Tokio / Tokyo, JP, document identification DE2811509C3 June 24, 1993 online (accessed June 4, 2012).
- ↑ Air piston switch European patent application EP 0 039 096 A2 Online (accessed on June 4, 2012).
- ↑ Self - inflating switch European patent application No. EP 2 299 464 A1 Online (accessed on June 4, 2012).