An air circuit breaker is a special switching device that is used only in high-voltage circuits. Extinguishing the arc and moving the contact power group is carried out by a strong stream of compressed air pumped by a separate mechanism. Since this device must perform operations with high voltage, its reliability and insulating properties must always be at a high level. Its design is carried out in accordance with GOST R52565–2006. In world practice, they are used mainly in the post-Soviet space in circuits of 35 kV and above. After SF6 and vacuum high-voltage circuit breakers were invented, the presentation and implementation of which took place back in the 60s of the last century, this type of circuit breakers began to gradually disappear from switchgear in the most developed countries.
In modern electrical engineering, at the moment, only an air circuit breaker is used that is capable of performing switching actions, as well as reliably protecting electrical receivers from emergency short circuit conditions or overloads. In principle, this is the same ordinary machine, only very rarely it is produced for voltages above 1000 Volts.
Operating principle
The principle of operation of air circuit breakers is based on extinguishing the electric arc that appears when the load breaks. This process can occur in two types of air movement:
- Longitudinal;
- Transverse.
An air circuit breaker may have several contact breaks, and this depends on the rated voltage for which it is designed. To facilitate extinguishing particularly large types of arc, a shunt resistor is connected to the arc extinguishing contacts. Automatic air switches, operating on the principle of extinguishing the arc in conventional chambers, without the presence of compressed air, do not have such elements. Their arc extinguishing chamber consists of partitions that break the arc into small parts, and therefore it does not flare up and quickly goes out. In this article we will talk more about the operation of high-voltage (above 1000 Volts) switches that are not equipped with built-in switches, but have control in the circuit of which relay protection is included.
The operating principle of a high-voltage circuit breaker with compressed air differs from each other in design features, and in particular, with and without a separator.
In switches that are equipped with separators, the power contacts are connected to special pistons and form one contact-piston mechanism. The separator is connected in series to the arc extinguishing contacts. That is, the separator with arc-extinguishing contacts forms one pole of the switch. In the closed position, both the arc-extinguishing contacts and the separator are in the same closed state. When a shutdown signal is given, a mechanical pneumatic valve is activated, which in turn opens the pneumatic actuator, while air from the expander acts on the arc extinguishing contacts. An expander, by the way, is also called a receiver by experts. In this case, the power contacts open, and the resulting arc is extinguished by a stream of compressed air. After which the separator itself turns off, breaking the current that remains. The air supply must be clearly adjusted so that it is sufficient to reliably extinguish the arc. After the air supply stops, the arcing contacts take on the on position, and the circuit is broken only by an open switch. Therefore, when working on electrical installations that are powered by such switches, it is imperative to open the disconnectors for safe work. Just turning off the pneumatic switch is not enough! Most often, in circuits up to 35 kV, a design with open separators is used, and if the voltage at which the switch operates is higher, then the separators are already made in the form of special air-filled chambers. Switches with a separator, for example, were produced in the Soviet Union under the brand name VVG-20.
If the high-voltage air circuit breaker does not have a separator, then its arcing contacts also serve as breaking the circuit and extinguishing the resulting arc. The drive in them is separated from the environment in which the quenching occurs, and the contacts can have one or even two stages of operation.
Description
Devices of this type are subject to certain requirements, including ensuring safe long-term use and reliable protection against overloads and short circuits in the network.
The quality of the device has a special role, since the operation of air switches can occur in different temperature and humidity conditions, in the presence of vibration loads and frequent switching. Automatic air circuit breakers simultaneously control and protect the network. They are classified into several types according to the response time, which is allotted for opening the contacts from the moment of the signal:
- selective;
- standard;
- fast-acting (have a current-limiting function).
Device classification
Features of application and design of limit switches
All air high-voltage circuit breakers, except for their design (with and without separator), also differ in purpose:
- Network. They are designed for voltages of 6000 volts and higher and are used in alternating current circuits to turn consumers on and off in normal non-emergency operating modes, as well as shutdown in the event of short circuits;
- Generating. They are used in networks with operating voltages from 6 to 24 thousand Volts, for connecting generators to these circuits. Withstands starting currents, as well as short-circuit modes;
- For electrothermal installations. The operating voltage at which normal switching is possible is 6–220 kV. Can also work in emergency modes.
- Special purpose. They are not mass-produced, but made to order and manufactured taking into account local operating conditions.
And also switches that have a pneumatic installation for operation are divided according to the type and location of this mechanism that pumps air into the device:
- Support;
- Hanging. They have a structure suspended from the portal installed on the outdoor switchgear;
- Roll-out. Equipped with a mechanism for rolling out of the switchgear;
- Built into switchgear (complete switchgear).
Fastening
The design of the device consists of a dielectric housing. Automatic air circuit breakers used for low voltage are fixed at the installation location using a DIN rail. Wiring is connected to the screw elements, and the device is turned off and on using a lever.
The case is held on the rail by a special latch - this way the device can be quickly removed by first moving it away. Fixed and moving contacts are necessary for the circuit switching process. The moving element uses a spring to allow the contacts to separate. This action can be performed by a magnetic or thermal splitter.
Advantages and disadvantages
Features of the use of remote control switches
The advantages of such outdated devices are few, here are the main ones:
- Due to long-standing use, there is extensive experience in both operation and repair;
- Unlike other more modern counterparts (especially SF6), these switches can be repaired.
Among the shortcomings I would like to highlight the following:
- Availability of additional pneumatic equipment or compressors for operation;
- Increased noise during shutdown, especially during emergency short circuit conditions;
- Large, out-of-date dimensions, which causes an increase in the territory allocated for outdoor switchgear;
- They are afraid of humid air and dust. Therefore, additional measures are taken for air systems; equipment aimed at reducing these harmful factors is installed.
Additional elements for air circuit breakers
Features of operation and application of the Tesla resonant transformer
The switch itself cannot generate the compressed air flow on which its operation is based, so the following main components are required for its operation:
- Compressor for creating compressed air;
- Sealed pneumatic drive system;
- Receiver for storing ready-made compressed air.
In connection with the use of these components, also according to GOST, the following is required:
- Pressure gauges. They show the actual pressure in the switch reservoir;
- A minimum pressure switch whose contacts will provide a signal in the event of a decrease in a certain pressure that is normalized. The same role can be played by a pressure gauge containing an electrical contact part;
- General shut-off valve, which is installed on the air duct;
- A check valve that ensures reliable shutoff of the compressed air outlet from the tank when the pressure in the supply air line decreases;
- A filter that cleans the air from various, conductive and other dust;
- A device for draining air or water from the lowest point of the tank.
Excerpt
The shutdown process can be characterized by the presence of a delay or its absence. The type of switch, in particular the speed of its response, depends on the time interval during which the existing value is exceeded and the contacts diverge. Thus, high-speed, selective and standard switches have become widespread.
The last two options do not have the ability to limit current. In selective devices, network protection is carried out using installed switches that have different response speeds: the consumer has the minimum value, and gradually this parameter increases towards the power source.
Air preparation
If the distribution substation is equipped with this type of circuit breakers, then a number of requirements are also imposed on the air supplied to them, aimed at preparing the air, cleaning it and removing moisture. Dust in the air, even very fine dust, reduces the discharge voltage and also clogs the valves. Of particular concern is humidity, which can condense in the air duct due to changes in the environment. Because of this, in winter it is possible that valves and pipes may ice up and the air passage under pressure may be impaired. Steel elements quickly rust and wear out. The appearance of condensation on the inner surface of insulators leads to a deterioration in electrical strength and even breakdowns. To clean the air, oil filters are used, which are installed on the suction pipes. They should be cleaned regularly, and the higher the dust content, the shorter the period between them. Reducing moisture in the air is done by subjecting it to compression at twice the rated pressure. The moisture trapped in the coil is released, and the compressed air passes through the reducer, which reduces its pressure. Additional drying can be accomplished using absorbents that capture water from the air. These two unprecedented measures make it possible to achieve a significant reduction in moisture to almost zero.
Magnetic splitter
The mechanism of action of the magnetic device is driven by a movable core. A splitter of this type is a solenoid, through the winding of which the current flowing through the switch passes; when the rated value is exceeded, the core begins to retract. The magnetic type has the property of instantaneous operation, which the thermal type cannot boast of, but the reaction occurs only if the set threshold is significantly exceeded. Several varieties are used that have varying degrees of sensitivity.
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During the splitting process, there is a possibility of an electric arc. To prevent this, an arc extinguishing grid is placed next to the contacts, and the elements themselves are made in a special shape.
Types of manufactured high-voltage air circuit breakers
VVB series switches
They are produced by the Elektroapparat software and are designed to work with U from 110 to 750 kV. Their key elements are installed on columns made of reliable porcelain insulators. The operating pressure that the compressor must create is from 2 to 2.6 MPa; this factor depends on the voltage at which the device will be operated.
VVBK series switches
They are designed to operate in networks with voltages of 110–500 kV. In their systems, the compressed air pressure should not be less than 4 MPa. To improve arc extinction at such voltages, a two-way supply of purified air is used. A simple pneumatic system was replaced by a more advanced pneumomechanical one, which is what made it possible to significantly reduce the response time during shutdowns, which is important in such circuits.
Switches series VVG-20
They are exclusively used for generators. They are designed to operate with a rated voltage of 20 kV and a rated current of 20 kA, with a breaking current of 160 kA. The air pressure is around 2 MPa. When the switch is turned on, the separator is activated first, and then the arc extinguishing mechanism itself. They are intended for indoor installation only.
You should be especially careful when working with compressed air and dangerous high voltages, as these two types of energy can not only cause personal injury, but also death.
Peculiarities
Any VVB air circuit breaker has a set short-circuit voltage limit; if there is a current higher than the existing parameter, there is a possibility of welding or burning of the contacts, and as a result, breakdown of the device. It can be made in a retractable or stationary version, and have a motor or manual drive. The drive can have pneumatic, remote, electromagnetic and other actions and is intended to turn the device off and on.
A relay with a direct mechanism of action acts as a splitter. In this case, thermobimetallic or electromagnetic parts ensure shutdown if the primary network is characterized by a lack of current, as well as in the event of overload and short circuit. The splitting design includes tripping springs, rocker arms, latches and levers. In addition to tripping the switch, it is used to prevent the possibility of closing.
Video about the structure and purpose of switch parts
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