Knowledge points:

The circuit breaker is an important control and protection equipment in power plants and substations. It can not only cut off and close the no-load current

and load current of the high-voltage circuit, but also cooperate with the protection device and automatic device to quickly cut off the fault current in case

of system failure, so as to reduce the scope of power failure, prevent the expansion of accidents, and ensure the safe operation of the system. Since the early

1990s, oil circuit breakers in power systems above 35kV in China have been gradually replaced by SF6 circuit breakers,.

1、 Basic principle of circuit breaker

The circuit breaker is a mechanical switch device in the substation that can open, close, bear and break the load current under normal circuit conditions,

and can also bear and break the fault current under abnormal circuit conditions within a specified time. The arc-extinguishing chamber is one of the most

important parts of the circuit breaker, which can extinguish the arc generated during the on-off process of power equipment and ensure the safe operation

of the power system. The arc-extinguishing principle of high-voltage AC circuit breaker is determined by the insulation medium used. Different insulation

media will adopt different arc-extinguishing principles. The same arc-extinguishing principle can have different arc-extinguishing structures. The arc-

extinguishing chamber of SF6 circuit breaker mainly includes two types: compressed-air type and self-energy type. The compressed air arc extinguishing

chamber is filled with 0 For SF6 gas of 45MPa (20 ℃ gauge pressure), during the opening process, the compressor chamber makes relative movement to

the static piston, and the gas in the compressor chamber is compressed, forming a pressure difference with the gas outside the cylinder. The high pressure

SF6 gas strongly blows the arc through the nozzle, forcing the arc to extinguish when the current passes zero. Once the opening is completed, the pressure

difference will soon disappear, and the pressure inside and outside the compressor will return to balance. Because the static piston is equipped with a check

valve, the pressure difference when closing is very small. The basic structure of self-energy arc extinguishing chamber is composed of main contact, static

arc contact, nozzle, compressor chamber, dynamic arc contact, cylinder, thermal expansion chamber, one-way valve, auxiliary compressor chamber, pressure

reducing valve and pressure reducing spring. During the opening operation, the operating mechanism drives the transmission shaft and its inner crank arm

in the support, thus pulling the insulating rod, piston rod, compressor chamber, moving arc contact, main contact and nozzle to move downward. When the

static contact finger and the main contact are separated, the current still flows along the static arc contact and moving arc contact that are not separated.

When the moving and static arc contacts are separated, the arc is generated between them. Before the static arc contact is separated from the nozzle throat,

the high temperature generated by arc combustion The high-pressure gas flows into the compressor chamber and mixes with the cold gas in it, thus increasing

the pressure in the compressor chamber. After the static arc contact is separated from the nozzle throat, the high-pressure gas in the compressor chamber is

ejected from the nozzle throat and the movable arc contact throat in both directions to extinguish the arc. During closing operation, the operating mechanism

moves in the direction of the static contact with the moving contact, nozzle and piston, and the static contact is inserted into the moving contact seat to make

the moving and static contacts have good electrical contact, so as to achieve the purpose of closing, as shown in the figure.

 
2、 Classification of circuit breakers

(1) It is divided into oil circuit breaker, compressed air circuit breaker, vacuum circuit breaker and SF6 circuit breaker according to arc extinguishing medium;

Although the arc-extinguishing medium of each circuit breaker is different, their work is essentially the same, which is to extinguish the arc generated by the

circuit breaker during the opening process, so as to ensure the safe operation of electrical equipment.

1) Oil circuit breaker: use oil as arc extinguishing medium. When the arc burns in the oil, the oil rapidly decomposes and evaporates under the high temperature

of the arc, and forms bubbles around the arc, which can effectively cool the arc, reduce the arc gap conductivity, and promote the arc to extinguish. An arc-

extinguishing device (chamber) is set in the oil circuit breaker to make the contact between oil and arc close, and the bubble pressure is increased. When the nozzle

of the arc-extinguishing chamber is opened, gas, oil and oil vapor form a stream of air and liquid flow. According to the specific arc-extinguishing device structure,

the arc can be blown perpendicular to the arc horizontally, parallel to the arc longitudinally, or combined vertically and horizontally, to implement strong and effective

arc blowing on the arc, thus accelerating the deionization process, shortening the arcing time, and improving the breaking capacity of the circuit breaker.

2) Compressed air circuit breaker: its arc extinguishing process is completed in a specific nozzle. The nozzle is used to generate high-speed air flow to blow the arc

so as to extinguish the arc. When the circuit breaker breaks the circuit, the high-speed air flow generated by compressed air not only takes away a large amount of

heat in the arc gap, thus reducing the temperature of the arc gap and inhibiting the development of thermal dissociation, but also directly takes away a large number

of positive and negative ions in the arc gap, and fills the contact gap with fresh high pressure air, so that the strength of the gap medium can be recovered quickly.

Therefore, compared with the oil circuit breaker, the compressed air circuit breaker has strong breaking ability and fast action The breaking time is short, and the

breaking capacity will not be reduced in automatic reclosing.

3) Vacuum circuit breaker: use vacuum as insulation and arc extinguishing medium. When the circuit breaker is disconnected, the arc burns in the metal vapor

generated by the contact material of the vacuum arc extinguishing chamber, which is called vacuum arc for short. When the vacuum arc is cut off, because the

pressure and density inside and outside the arc column are very different, the metal vapor and charged particles in the arc column will continue to diffuse outward.

The interior of the arc column is in the dynamic balance of the continuous outward diffusion of charged particles and the continuous evaporation of new particles

from the electrode. As the current decreases, the density of metal vapor and the density of charged particles decrease, and finally disappear when the current is close

to zero, and the arc goes out. At this time, the residual particles of the arc column continue to spread outward, and the dielectric insulation strength between the

fractures recovers rapidly. As long as the dielectric insulation strength recovers faster than the voltage recovery rising speed, the arc will be extinguished.

4) SF6 circuit breaker: SF6 gas is used as insulation and arc extinguishing medium. SF6 gas is an ideal arc extinguishing medium with good thermochemistry and

strong negative electricity.

A. The thermochemistry means that SF6 gas has good heat conduction characteristics. Due to the high thermal conductivity of SF6 gas and the high temperature

gradient on the surface of the arc core during the arc combustion, the cooling effect is significant, so the arc diameter is relatively small, which is conducive to arc

extinction. At the same time, SF6 has a strong thermal dissociation effect in the arc and sufficient thermal decomposition. There are a large number of monomer

S, F and their ions in the arc center. During the arc combustion process, the energy injected into the arc gap of the power grid is much lower than that of the circuit

breaker with air and oil as the arc extinguishing medium. Therefore, the contact material is less burnt and the arc is easier to extinguish.

B. The strong negativity of SF6 gas is the strong tendency of gas molecules or atoms to generate negative ions. The electrons generated by arc ionization are strongly

adsorbed by SF6 gas and halogenated molecules and atoms generated by its decomposition, thus the mobility of charged particles is significantly reduced, and

because negative ions and positive ions are easily reduced to neutral molecules and atoms. Therefore, the disappearance of the conductivity in the gap space is very

rapid. The conductivity of the arc gap decreases quickly, which causes the arc to extinguish.

(2) According to the structure type, it can be divided into porcelain pole circuit breaker and tank circuit breaker.

(3) According to the nature of the operating mechanism, it is divided into electromagnetic operating mechanism circuit breaker, hydraulic operating mechanism

circuit breaker, pneumatic operating mechanism circuit breaker, spring operating mechanism circuit breaker and permanent magnetic operating mechanism

circuit breaker.

(4) It is divided into single-break circuit breaker and multi-break circuit breaker according to the number of breaks; The multi-break circuit breaker is divided

into circuit breaker with equalizing capacitor and circuit breaker without equalizing capacitor.

3、 Basic structure of circuit breaker

The basic structure of the circuit breaker mainly includes the base, operating mechanism, transmission element, insulation support element, breaking element, etc.

The basic structure of the typical circuit breaker is shown in the figure.

Disconnecting element: It is the core part of the circuit breaker to connect and disconnect the circuit.

Transmission element: transfer operation command and operation kinetic energy to the moving contact.

Insulating support element: support the circuit breaker body, bear the operating force and various external forces of the breaking element, and ensure the ground

insulation of the breaking element.

Operating mechanism: used to provide opening and closing operation energy.

Base: used to support and fix the circuit breaker.