Let learn about vacuum circuit breaker

The structure of a vacuum circuit breaker

The ZN63A-12 Indoor High Voltage Vacuum Circuit Breaker consists of a vacuum arc extinguishing chamber, operating mechanism, transmission mechanism, chassis, etc. Vacuum arc extinguishing consists of a static contact, a moving contact, a shield, a shell, a bellows, a protective rod, an end cover, etc., as shown in the figure. The disconnection distance between the dynamic and static contacts of a 10kV vacuum circuit breaker is generally 10~15mm.

Vacuum arc extinguishing chamber contacts are generally docked, and it is easy to cause the head to bounce. In order to prevent overvoltage caused by bouncing, there are special requirements for the material of the contacts, and sufficient initial pressure and final pressure are required during the closing process. The bounce time should not exceed 2ms.

The material requirements of the ZN63A-12 Indoor High Voltage Vacuum Circuit Breaker contacts are resistant to fusion welding, arc resistance, low moisture content, and low interception levels. In order to be able to reliably fit the short circuit current. Generally, multi-alloys are used to make contacts. Commonly used are copper-tungsten-bismuth-zirconium quaternary alloys, copper-tungsten-nickel-antimony quaternary alloys, and alloys such as copper-tellury-selenium, copper-bismuth-cerium, and copper-bismuth-aluminum.

The components of the vacuum arc extinguishing chamber are sealed in a glass shell, and the glass shell itself also acts as an insulator. In order to seal the dynamic conductive rod, a corrugated tube is used, which is an elastic element. Through it, the vacuum arc extinguishing chamber can complete dividing and closing operations under the action of the operating mechanism without damaging the vacuum level.

The ZN63A-12 Indoor High Voltage Vacuum Circuit Breaker does not reignite after the arc is extinguished at zero due to its high insulation strength under high vacuum and the rapid diffusion of charged particles generated by the arc between contacts in extremely thin gases during splitting operation. Metal vapors and charged particles during arc combustion are absorbed and condensed by a shield in intense diffusion. There are three Archimedes screw grooves on the running surface of the contact, causing the arc current to generate a transverse magnetic field between the contacts on the flowing route, causing the arc current to move rapidly in the tangential direction on the main contact, thereby reducing the temperature of the contact and reducing the burnout of the contact.

Insulation characteristics of vacuum

Vacuum has strong insulation characteristics. In the ZN63A-12 Indoor High Voltage Vacuum Circuit Breaker, the gas is very thin, the free travel of gas molecules is relatively large, and the chance of colliding with each other is very small. Therefore, free collision is not the main cause of real space gap breakdown, but the electrode is used by the electrode under the action of a high-strength electric field The precipitated metal points are the main factor causing insulation damage.

The insulation strength in a true space gap is not only related to the size of the gap and the uniformity of the electric field, but is also greatly affected by the properties and surface conditions of the electrode material. The true space gap has higher insulation characteristics than high-pressure air and SF6 gas in the case of a small distance gap (2—3 mm), which is why the contact opening of vacuum circuit breakers is generally small.

The influence of electrode materials on breakdown voltage is mainly manifested in the mechanical strength (tensile strength) of the material and the melting point of the metal material. The higher the tensile strength and melting point, the higher the insulation strength of the electrode under vacuum.

Experiments have shown that the higher the vacuum level, the higher the breakdown voltage of the gas gap, but it remains basically the same at 10-4 trays or more. Therefore, to maintain the insulation strength of the vacuum arc extinguishing chamber, the vacuum level should not be lower than 10-4 blocks.

The formation and extinction of an electric arc in a vacuum

The vacuum arc is very different from the gas arc discharge phenomenon we have learned before. The free gas phenomenon is not the main factor that creates the arc. Vacuum arc discharge is formed in the metal vapor evaporated from the contact electrode. At the same time, the characteristics of the arc performance are also different depending on the magnitude of the switching current. We generally divide it into low-current vacuum arcs and high-current vacuum arcs.

1. Low current vacuum arc

When the contact is turned off in a vacuum, cathode spots where current and energy are concentrated are generated. A large amount of metal vapor evaporates from the cathode spot. The density of the metal atoms and charged mass points in it is very high, and the arc burns in it. At the same time, the metal vapor and charged mass points in the arc column continue to diffuse outward, and the electrodes are constantly evaporated with new mass points to supplement them. When the current is over zero, the energy of the arc decreases, the temperature of the electrode drops, the evaporation effect decreases, the density of the mass in the arc column decreases, and finally, at zero, the cathode spot disappears and the arc is extinguished.

Sometimes, the evaporation effect cannot maintain the diffusion rate of the arc column, and the arc suddenly goes out, causing interception.

2. High current vacuum arc

When the contact disconnects the large current, the energy of the arc increases, and the anode also heats up seriously, forming a strong cluster arc. At the same time, the effect of electric power is also obvious. Therefore, for high-current vacuum arcs, the magnetic field distribution between contacts has a decisive influence on the stability and arc extinguishing performance of the arc. If the current is too high and exceeds the limit of the switching current, it will cause the disconnection to fail. At this time, the touch hair is very hot, and it still evaporates after the current has passed zero. The medium is difficult to recover, and the current cannot be disconnected.

The structure

There are many manufacturers of vacuum circuit breakers, and there is a wide variety of models. According to the conditions of use, it is divided into two types: indoor (ZnX—**) and outdoor (zWx—**). It is mainly composed of the frame part, the arc extinguishing chamber part (vacuum bubble), and the operating mechanism part.

The main body of the ZN63A-12 Indoor High Voltage Vacuum Circuit Breaker consists of a conductive circuit, an insulation system, a seal, and a housing. The overall structure is a three-phase common box type. Among them, the conductive loop is connected to the inlet and outlet wire conductive poles, inlet and outlet insulation bearings, conductive clips, soft connections, and vacuum arc extinguishing chambers.

The mechanism is electric energy storage, electric split gate, and has manual function. The whole structure is composed of closing springs, energy storage systems, overcurrent buckles, split gate coils, manual splitting and closing systems, auxiliary switches, energy storage indicators and other components.

How it works

Vacuum circuit breakers use high vacuum when current flows through zero, plasma diffuses rapidly and extinguishes the arc to complete the purpose of cutting off the current.

Principle of action

Energy storage process: When the energy storage motor is connected to the power supply, the motor drives the eccentric wheel to rotate, and drives the arm and plate to swing through a roller close to the eccentric wheel to push the energy storage pawl to make the ratchet rotate. When the pin on the ratchet and the plate of the energy storage axle sleeve are attached, the two move together to make the closing spring hanging on the energy storage shaft sleeve lengthen. The energy storage shaft sleeve is fixed with a positioning pin to maintain the energy storage state. At the same time, the crank arm on the energy storage shaft sleeve pushes the stroke to switch off the power of the energy storage motor, and the energy storage claw is lifted to reliably separate from the ratchet.

Closing operation process: When the mechanism receives the closing signal (the switch is disconnected, energy storage state), the core of the closing electromagnet is sucked down to move, pull the locator to rotate counterclockwise to release the energy storage maintenance, the closing spring drives the energy storage axle sleeve to rotate counterclockwise, and its cam presses the drive shaft sleeve to drive the plate and rocker arm to hold the rocker arm in half shaft, so that the mechanism is closed. At this time, the chain device locks the locator so that the positioning cow cannot rotate in a counterclockwise direction, achieving the purpose of institutional joint sales, and ensuring that the mechanism cannot close the door at the closing position.

Splitting operation process: After the circuit breaker closes, the split electromagnet receives a signal. The iron core sucks, and the ejector in the tripper moves upward to make the release shaft rotate, drive the ejector rod to move upward, move the jacking plate and drive the semi-axis to rotate in a counterclockwise direction.

The half shaft and rocker arm are unlocked. Under the action of a split spring, the ZN63A-12 Indoor High Voltage Vacuum Circuit Breaker completes the split operation.