The most important component of vacuum circuit breaker: vacuum interrupter

1. Structure of vacuum interrupter:

1.1 Gas-tight insulation system

It consists of a hermetically sealed insulating shell made of glass or ceramic, a movable end cover plate, a fixed end cover plate, and a stainless steel bellows.In order to ensure good air tightness between glass, ceramic and metal, it is required that the air permeability of the material itself be as small as possible and the internal air discharge volume be limited to a minimum value.

The function of the stainless steel bellows can not only isolate the vacuum state inside the vacuum interrupter from the external atmospheric state, but also make the movable contact and conductive rod move within the specified range, completing the breaking operation of the vacuum switch.

1.2 Conductive system

The fixed conductive rod, the fixed running arc surface, the fixed contact, the moving contact, the moving running arc surface and the moving conductive rod constitute the conductive system of the vacuum interrupter.

• Fixed conductive rod, fixed running arc surface and fixed contact are collectively called fixed electrode.
• Movable contacts, moving arc surfaces and moving conductive rods are collectively called moving electrodes.

The operating mechanism closes the two contacts through the movement of the moving conductive rod to complete the connection of the circuit.

In order to reduce the contact resistance between the two contacts as much as possible and keep it stable, and the vacuum interrupter has good mechanical strength when bearing dynamic stable current, the vacuum switch is equipped with a guide sleeve at one end of the moving conductive rod, and a set of compression springs is used to keep the two contacts at a rated pressure.

When the vacuum circuit breaks the current, the two contacts of the vacuum interrupter are separated and an arc is generated between them, until the arc is extinguished when the current naturally crosses zero, thus completing the circuit breaking.

1.3 Shielding system

The shielding system is mainly composed of shielding cylinder, shielding cover and other parts.
The main functions are:

• Prevent the contact from generating a large amount of metal vapor and droplets during the arcing process, polluting the inner wall of the insulating shell, and avoiding causing the insulation strength of the vacuum interrupter shell to decrease or flashover.
• Improving the electric field distribution inside the VI is beneficial to the miniaturization of the insulation shell of the vacuum interrupter, especially for the miniaturization of the medium-voltage vacuum interrupter.
• Absorb part of the arc energy and condense the arc product. When the vacuum interrupter breaks the short-circuit current, most of the heat energy generated by the arc is absorbed by the shielding system, which is beneficial to improving the dielectric recovery strength between contacts. The greater the amount of arc products absorbed by the shielding system, the greater the energy absorbed by the shielding system, which plays a good role in increasing the breaking capacity of the vacuum interrupter.

1.4 contact

• Contacts are the parts where the arc is generated and extinguished.

The general material used for contacts is copper-chromium alloy. A copper-chromium alloy sheet is welded on the butt surfaces of the upper and lower contacts, generally 3mm thick each, and the rest is called the contact seat, and the material is oxygen-free copper. The contacts must have strong resistance to electrical corrosion and less metal evaporation.

• Contact material selection and features

The material selection of the contact directly affects the performance and reliability of the low current vacuum interrupter. Common contact materials include copper, copper alloys, silver, silver alloys, etc. Different materials have different characteristics and applicable scenarios:

1. 1. Silver and silver alloys: Silver has excellent electrical and thermal conductivity and is suitable for some scenarios that require high conductivity. Silver alloy reduces costs and improves durability while maintaining the advantages of silver.
   2. Copper and copper alloys: Copper has good electrical conductivity and is suitable for scenarios that need to transmit large currents. Copper alloy can improve wear resistance and corrosion resistance while ensuring electrical conductivity.

Other special materials: In some special applications, other materials can also be used, such as tungsten copper alloy, tungsten silver alloy, etc., to meet specific requirements, such as high temperature environment, high frequency current, etc.

1.5 Contact structure

Contact structure has great influence on the breaking capacity of vacuum interrupter tube. At present, spiral contact, cup-shaped contact with chute and longitudinal magnetic field cup-shaped contact are commonly used.

• Contact precautions and maintenance

1. Cleaning and maintenance: Clean the contacts regularly to avoid the accumulation of dust and dirt affecting the contact contact.
2. Replacement: If the contacts are damaged, corroded or worn, they should be replaced in time to ensure stable current transmission.
3. Prevent overload: Avoid exceeding the rated current of the contacts to avoid overload damage.

The correct selection of suitable contact types and materials and proper maintenance can ensure the safe operation and long-term stable performance of low-current vacuum interrupters. I hope the introduction of this article can help you better understand the types of commonly used contacts and their characteristics.

1.6 corrugated pipe

Bellows are thin-walled components made of stainless steel pipes with a thickness of 0.1 mm-0.2 mm. It mainly ensures that the moving electrode moves within a certain range and maintains the function of high vacuum for a long time, and ensures that the vacuum interrupter has a high electrical life.

In the process of opening and closing the vacuum switch, the bellows of the vacuum interrupter is subject to expansion and contraction, and the cross-section of the bellows is subject to variable stress. Therefore, the life of the bellows should be determined according to the amount of repeated expansion and contraction and the operating pressure.

The bellows can ensure the complete sealing of the vacuum interrupter shell when the moving contact reciprocates. Mechanically, it is the weakest element in a vacuum interrupter.

Every time the moving and static contacts are split into one, the corrugated thin wall of the corrugated pipe will undergo a large mechanical deformation. Long-term frequent and severe deformation can easily cause the fatigue and damage of the bellows trapped material, resulting in the air leakage of the vacuum interrupter and being unusable. Therefore, the mechanical life of the vacuum interrupter is mainly determined by the bellows.

There are 2 processes for bellows processing, one is hydraulic forming and the other is welding.

• Hydro formed bellow is made of 0.1 mm-0.2 mm thick stainless steel pipe after hydraulic forming to become the main blank of the corrugated pipe, and then processed after processing, correction and heat treatment. The corrugated pipe can’t be made very long, the working stroke is short, the fatigue life is only tens of thousands of times, the processing technology is relatively simple, and the price is relatively cheap.
• Sheet welded bellows are stamped into annular sheets with non-magnetic stainless steel sheets with a thickness of 0.10-0.15 mm and then formed by argon arc welding one by one. The length of the bellows can be increased arbitrarily according to needs, and the working stroke is long. At the same time, the fatigue life is long, generally up to millions of times, which is much longer than the life of hydraulic forming bellows. It is usually used in vacuum switches with long mechanical life requirements, high rated voltage level and large contact stroke

2.What is the old current smelting process of vacuum interrupter

The old current smelting process of vacuum interrupter refers to pulling the contacts at the same time with a large current of hundreds of amps, causing a diffusion arc between the contacts. The burning arc strips off the dirt, oxides and gases covering the contact surface, making the contact surface a completely fresh active surface, which plays a role in purifying the contact surface. Therefore, it can be said that compared with voltage aging, current aging has a more complete degassing effect.

High-current aging is divided into AC current aging and DC current aging. The DC aging method is adopted. The DC voltage is about 100V, the arc current is adjusted to 100 A to several hundred A, and the contacts are closed 10 ~ 20 times. Make the arc have a long enough arcing time during each closing and opening operation, and after accumulating a certain time, the purpose of purifying and degassing the contact surface can be achieved.

The ablation effect of the old current smelting process of vacuum interrupter on the contact surface mainly comes from the bombardment of cathode by positive ions, which has a good ablation effect on the cathode surface. In order to age the other contact, the polarity of DC power supply must be changed and the previous operation must be repeated.

The performance of breaking short-circuit current and opening and closing capacitor bank of vacuum interrupter aged by high current is more stable than that of vacuum interrupter aged by high current, mainly because the contact surface is purified and the gas absorbed by the surface layer is removed, and the arc burning contact releases less gas during breaking, which is very effective in restoring the dielectric strength between contacts.

3. Use conditions and acceptance criteria of vacuum interrupter:

3.1 Conditions of use

3.1.1 When used indoors, the ambient temperature shall not be higher than + 40 °C and not lower than-25 °C. When used outdoors, the ambient temperature shall not be higher than + 40 °C and not lower than-40 °C.

3.1.2 The altitude of the use site shall not exceed 1000 meters. If the altitude is high, the product needs special treatment.

3.1.3 The humidity of the indoor environment is not more than 95%. When used outdoors, waterproofing, snow-proof and dew-proof measures should be taken.

3.1.4 There should be no serious pollution and chemical corrosion in the place of use, and it should not be used in places with severe vibration. Fire and explosion hazards should also be eliminated after long night use.

3.1.5 The seismic intensity of the place of use cannot exceed magnitude 8.

3.1.6 When the product is used to switch capacitive or inductive loads such as capacitor banks, high-voltage motors and electric arc furnace transformers, over voltage protection measures should be taken.

3.1.7 When taking voltage protection measures for inductive loads, resistance-capacitance protection should be limited.

3.1.8 It should also meet the use conditions of the product in GB/T11022-1999.

3.1.9 When the above normal use conditions cannot be met, users need to negotiate with the technical personnel of the product manufacturer.

3.2 Acceptance criteria

3.2.1 When receiving the goods, you need to check whether the packaging is intact. After unpacking, check whether there are defects in the appearance of the product and whether the product is consistent with the certificate of conformity.

3.2.2 Check the power frequency withstand voltage. Before use, a power frequency withstand voltage test is required. Before testing, the surface of the product needs to be wiped clean, and the power frequency withstand voltage test results of the product should meet the standards.