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    2021-10-21 17:37:30
    What is Vacuum Circuit Breaker?
    Topics covered in this article:
    Ⅰ. What is a vacuum circuit breaker?
    Ⅱ. The characteristic of vacuum circuit breaker
    Ⅲ. How does vacuum circuit breaker work?
    Ⅳ. What is vacuum circuit breaker used for?
    Ⅴ. Analysis and reason of common faults of vacuum circuit breaker
    Ⅵ. What are the inspection test items for vacuum circuit breaker?


    Ⅰ. What is a vacuum circuit breaker?

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    Vacuum Circuit Breaker

    1.The definition of vacuum circuit breaker

    The name "vacuum circuit breaker" comes from the fact that both the arc extinguishing medium and the insulating medium of the contact gap after arc extinguishing are high vacuum; it has the advantages of compact size, lightweight, frequent operation, and no arc extinguishing maintenance. The use in the electricity grid is more common. A vacuum circuit breaker is an interior power distribution device in a three-phase AC system with a voltage of 3-10kV and a frequency of 50Hz. In industrial and mining organizations, power plants, and substations, can be utilized to protect and control electrical equipment. Circuit breakers can be configured in intermediate cabinets, double-layer cabinets, and fixed cabinets to manage and protect high-voltage electrical equipment in maintenance and frequent operation areas.

     

    2. The structure of vacuum circuit breaker

    Vacuum circuit breakers are made up of three basic parts: a vacuum interrupter, an operational mechanism, a bracket, and additional components.

    2.1Operating structure

    The vacuum circuit breaker uses an established and dependable electric energy storage spring working mechanism with electric closing, electric braking, manual energy storage, manual closing, manual breaking, and automatic tripping and breaking for overcurrent (or short circuit). There are six functions in all. Ratchet, cam, closing spring, opening spring, manual trip link, trip device, and auxiliary switch make up the mechanism.


    2.2What is a vacuum?

    The gas condition in a specific space that is much below the atmospheric pressure of an environment is referred to as vacuum. The degree of vacuum, which is expressed by the absolute pressure value of the gas, is used to determine the vacuum's quality. The higher the degree of vacuum, the lower the pressure.


    2.3What is a vacuum interrupter?

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    Vacuum Interrupter

    The essential component of a vacuum circuit breaker is the vacuum interrupter, often known as vacuum interrupter. Its major purpose is to employ the vacuum in the tube's exceptional insulation performance to swiftly extinguish the arc and suppress the current when the medium and high voltage circuits are switched off, thereby preventing accidents and accidents.

     

    2.4What are the basic structures of the vacuum interrupter?

    An airtight insulating shell, a conducting loop, a shielding system, contacts, bellows, and other pieces make up the vacuum interrupter.

    1) Airtight insulation system

    The airtight insulation system consists of an airtight insulating shell, moving end cover plate, fixed end cover plate, and stainless steel bellows made of glass or ceramics. In order to achieve effective airtightness between glass, ceramics, and metals, in addition to using strict sealing techniques, the material's air permeability must be as low as feasible, and the internal outgassing volume must be kept to a minimum.The stainless steel bellows can not only isolate the vacuum state in the vacuum interrupter from the external atmosphere, but they can also move the moveable contact and movable conductive rod within the defined range to perform vacuum switch on and off operations.

     

    2) Conductive system

    The conductive system of the arc extinguishing chamber is made up of the fixed conductive rod, the fixed arc surface, the fixed contact, the moveable contact, the movable arc surface, and the movable conductive rod. The fixed electrode is made up of the fixed conducting rod, the fixed running arc surface, and the fixed contact; the moving electrode is made up of the moving contact, the moving arc surface, and the moving conductive rod. The operational mechanism closes the two contacts through the movement of the movable conductive rod and completes the circuit connection when the vacuum circuit breaker, vacuum load switch, and vacuum contactor assembled by the vacuum interrupter are closed. The vacuum switch is equipped with a guide sleeve at one end of the moving conductive rod and uses a set of compression springs to maintain A rated pressure between the two contacts in order to make the contact resistance between the two contacts as small and stable as possible, and the arc extinguishing chamber has good mechanical strength when it bears dynamic and stable current.The two contacts of the arc extinguishing chamber are separated when the vacuum switch cuts the current, and an arc is formed between them until the arc is extinguished when the current naturally crosses zero, and the circuit breaking is completed.

    3) Shielding system

    A shielding cylinder, a shielding cover, and other pieces make up the vacuum interrupter's shielding system. The shielding system's main functions are to: (1) prevent the contacts from producing a large amount of metal vapor and droplets splashing during the arcing process, polluting the inner wall of the insulating shell, and causing the insulation strength to decrease or flashover;

    (2)Miniaturization of the vacuum interrupter insulation shell is aided by improving the electric field distribution inside the vacuum interrupter, particularly for high-voltage vacuum interrupters.

    (3) Capture a portion of the arc's energy and condense the arc products. Most of the heat created by the arc is absorbed by the shielding system, which helps to improve the recovery strength of the medium between the contacts, especially when the vacuum interrupter interrupts the short-circuit current. The bigger the number of arc products absorbed by the shielding system, the more energy it absorbs, which helps to increase the vacuum interrupter's breaking capacity.

    4) Contact system

    The contact is where the arc is formed and extinguished, and it has a lot of requirements in terms of materials and construction.

    (1) Contact material

    The contact material has the following requirements:

    a, high breaking capacity

    High electrical conductivity, low thermal conductivity, huge heat capacity, and low thermionic emission capabilities are all required of the material.

    b. High breakdown voltage

    When the breakdown voltage is high, the medium's recovery strength is also strong, which helps with arc extinguishing.

    c. High electrical corrosion resistance

    That is, it can endure arc ablation while evaporating a minimal amount of metal.

    d. Resistance to fusion welding.

    e. Low intercept current value, which is required to be below 2.5A.

    f, low air content

    All materials utilized inside the vacuum interrupter must have a low air content. Copper material, in particular, must be oxygen-free copper processed using a low-gas concentration technique. Silver and copper alloys are necessary for soldering.

    g. The vacuum interrupter for circuit breakers' contact materials are predominantly copper-chromium alloy, with copper and chromium each accounting for 50% of the total. On the butt surface of the upper and lower contacts, a copper-chromium alloy sheet with a thickness of 3mm is welded. The contact seat, which can be manufactured of oxygen-free copper, is the final component.

    (2) Contact structure

    The contact structure has a significant impact on the isolated room's breaking capacity. The arc extinguishing effect caused by different structures' connections varies. There are three types of spiral contacts: a cup with a chute, a cup with a longitudinal magnetic field, and a cup with a spiral structure. A longitudinal magnetic field cup is one of them. The contact is primarily in the shape of a structure.

    5) Bellows

    The vacuum interrupter's bellows are primarily responsible for ensuring that the movable electrode moves within a particular range and that the vacuum interrupter's function of high vacuum is maintained for a long period, as well as for maintaining the vacuum interrupter's mechanical life. The vacuum interrupter's bellows are a thin-walled stainless steel part with a thickness of 0.10.2mm. The bellows of the arc extinguishing chamber expand and contract during the opening and shutting of the vacuum switch, and the cross section of the bellows is subject to varying tension. As a result, the bellows' life should be calculated based on the amount of repetitive expansion and contraction as well as the operating pressure. The heating temperature of the operating conditions affects the bellows' service life. The leftover heat of the conductive rod is passed to the bellows when the vacuum interrupter breaks the huge short-circuit current, raising the temperature of the bellows. When the temperature rises to a certain point, the bellows will become fatigued and their service life will be shortened.

     

    Ⅱ. The characteristic of vacuum circuit breaker

    1. The advantages of vacuum circuit breaker

    1) The arc is extinguished in a sealed container, preventing the arc and hot gas from escaping. The arc extinguishing chamber is simple to set up and troubleshoot as a stand-alone component.

    2) The contact gap is tiny, typically around 10mm, the closing force is low, the mechanism is simple, and the service life is long.

    3) The arc extinguishing time is short, the arc voltage is low, the arc energy is low, the contact loss is low, and there are a lot of breaking times.

    4) The movable guide rod has a low moment of inertia, making it suited for repeated use.

    5) The operating mechanism is small, as is the overall volume and weight.

    6) When the switch is turned on, the control power is low, and the action noise is low.

    7) There is no risk of fire or explosion since the arc extinguishing medium or insulating medium does not contain oil.

    8) The contact head is segregated into a totally sealed construction that will not be affected by moisture, dust, toxic gases, or other external influences, and it has a dependable operation and stable on-off performance.

    9) The medium between the fractures recovers fast after the vacuum circuit breaker is opened, and the medium does not need to be replaced.

    10) The contact component of the vacuum switch tube does not require maintenance or inspection during its service life, which is typically around 20 years. The workload for maintenance is little, and the expense of maintenance is minimal.

    11) It is ideal for the application requirements in the distribution network due to its numerous reclosing function.

     

    2.The disadvantages of vacuum circuit breaker

    1) When breaking small inductive currents, circuit breaker contact materials with a high arc extinguishing ability are likely to cause current interception and overvoltage, so appropriate overvoltage protection should be used.

    2) The price is greater, which is mostly due to the vacuum interrupter's professional production and the mechanism's dependability requirements.

     

    Ⅲ. How does vacuum circuit breaker work?

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    Schematic Diagram of Vacuum Circuit Breaker

    The vacuum interrupter, or vacuum circuit breaker, is an electric vacuum device that achieves the on-off function of the power circuit by using a high-vacuum degree of insulating arc-extinguishing medium and a pair of contacts sealed in a vacuum. When it disconnects a particular amount of current, the current drops to a single point or a few points shortly after the moving and static contacts are separated, resulting in a sharp increase in resistance between the electrodes and a rapid rise in temperature until the electrode metal is destroyed. At the same time as evaporation, a very strong electric field is created, resulting in severe electric field emission and gap breakdown, resulting in a vacuum arc. The ionic body of the vacuum arc quickly diffuses around when the operating current is close to zero and the contact distance increases. The medium in the contact gap quickly converts from a conductor to an insulator after the arc current reaches zero, and the current is broken. During arcing, an adequate longitudinal magnetic field will be formed in the contact gap due to the contact's unique structure. This magnetic field can evenly disperse the arc on the contact surface while maintaining a low arc voltage, resulting in a greater back arc for the vacuum interrupter. Recovery time is medium, arc energy is low, and corrosion rate is low. The vacuum interrupter's current breaking capacity and service life are improved in this way.

     

    Ⅳ. What is vacuum circuit breaker used for?

    In a 3-10kV, 50Hz three-phase AC system, the vacuum circuit breaker is an interior power distribution device. In industrial and mining organizations, power plants, and substations, it can be utilized to protect and control electrical equipment. Circuit breakers can be built in center cabinets, double-layer cabinets, and fixed cabinets for control and protection of high-voltage electrical equipment in maintenance and frequent operation areas.

     

    Ⅴ. Analysis and reason of common faults of vacuum circuit breakers

    1. The circuit breaker refuses to close or open

    The closing (opening) electromagnet activates once the circuit breaker receives the closing (opening) order, the iron core push rod pulls the closing (opening) catch open, and the closing (opening) spring releases energy. Close (open) the circuit breaker, however the circuit breaker's interrupter cannot be closed (open).

    Reason:

    When the operational mechanism refuses to move, determine the cause, whether it is a malfunction in the secondary circuit or a mechanical part, and then address the problem. After confirming that the secondary circuit is working properly, it is discovered that the space between the universal joint shaft head and the operating mechanism's main crank arm is too great. Although the operational mechanism functions normally, it is unable to drive the circuit breaker's opening and closing linkages, leading the circuit breaker to fail to open regularly. Close.

    Treatment:

    Examine the gaps between all of the operating mechanism's connecting parts, and replace any unqualified parts with new high-hardness qualified parts.

     

    2. The circuit breaker is mis-opened

    The circuit breaker appears to be in normal operation until it trips for unknown reasons.

    Reason: 

    When there is no external running power supply and no mechanical opening action, the circuit breaker cannot be opened under normal operating conditions. Check the secondary circuit and operational mechanism after establishing that there is no malfunction. The operating mechanism box's auxiliary switch connections are found to be short-circuited, and the opening power supply is linked to the opening coil via the short-circuit point, resulting in erroneous opening. The reason for this is that the top of the circuit breaker mechanism box is leaking rain, which runs down the output arm and into the mechanism auxiliary switch, causing the contact to short circuit.

    Treatment:

    Check for and effectively block all probable rain leaks; install a sealing rubber sleeve on the output arm link; open the mechanism box's heating and damp-driving device.

     

    3. After the circuit breaker mechanism is stored, the stored energy motor does not stop

    The operational mechanism's energy storage motor starts to work when the circuit breaker is closed, but the motor continues to run after the spring energy is depleted.

    Reason:

    The operational mechanism's energy storage motor starts working once the circuit breaker is closed, and when the spring energy is full, it gives out a signal indicating the spring has been stored energy. In the energy storage circuit, a pair of typically open circuit breaker auxiliary contacts and a pair of normally closed travel switch contacts are connected in series. The normally open contact of the auxiliary switch is turned on after the circuit breaker is closed, and the energy storage motor begins to run. When the spring is fully charged, the mechanism's rocker arm unlocks the normally closed contact of the travel switch, de-energizing the energy storage circuit and turning off the energy storage motor. The energy storage motor has been functioning because the mechanism's rocker arm fails to open the typically closed contact of the travel switch after the spring has been charged, resulting in the energy storage circuit being charged and the energy storage motor being unable to cease working.

    Treatment:

    When the rocker arm is in the highest position, adjust the installation position of the travel switch so that the normally closed contact of the travel switch can be opened.

     

    4. The DC resistance of the circuit breaker increases

    It demonstrates that after the circuit breaker has been engaged for a specific period of time, the contact resistance of the contacts of the arc extinguishing chamber increases continually.

    Reason:

    The contact resistance of the vacuum interrupter is too high because it is a butt-connected type, and the contact is prone to heat when carrying current, which is not favorable to conducting and breaking the circuit, thus the contact resistance value must be smaller than the manufacturer standard. The contact spring pressure has a significant impact on contact resistance, and it must be assessed when the over-travel is qualified. The steady increase in contact resistance may also be a reflection of the contacts' electrical deterioration, which is complimentary. The rise in the DC resistance of the circuit breaker is caused by electrical wear of the contacts and a change in the contact distance of the circuit breaker.

    Treatment:

    Adjust the arc extinguishing chamber's contact opening distance and overtravel. The DC voltage drop method can be used to determine the contact resistance (the current must be greater than 100A); otherwise, the arc extinguishing chamber should be changed.

     

    5. Increased bounce time of circuit breaker closing

    The shutting bounce time of the circuit breaker continues to increase after it has been operational for a particular period of time.

    Reason: 

    The connections will always bounce when the vacuum circuit breaker is closed, but if it is too large, the contacts will be easily scorched or welded. The contact bounce time of a vacuum circuit breaker must meet a technical standard of 2ms. The decrease in the elastic force of the contact spring and the rise in the clearance of the crank arm and the shaft pin are the main reasons for the increase in the closing bounce time as the circuit breaker's working time increases.

    Treatment:

    (1) Properly raise the contact spring's starting pressure or replace the contact spring.

    (2) The crank arm and shaft pin can be replaced if the clearance between them is greater than 0.3mm.

    (3)Adjust the transmission mechanism to reduce contact bounce. Take advantage of the fact that the transmission ratio of the mechanism is minimal when the closure position exceeds the active arm's dead center, and adjust the mechanism toward the dead center.

     

    6. The surface of the CT of the circuit breaker middle box discharges to the support

    The performance is that the current transformer's surface discharges to the intermediate box support during circuit breaker operation.

    Reason:

    In the circuit breaker's middle box, a current transformer is inserted. The surface of the current transformer produces an uneven electric field when the circuit breaker is activated. To avoid this, the transformer maker covers the surface of the transformer with a coating of semiconducting adhesive to make the surface electric field uniform. Due to space constraints, the semiconductor glue around the transformer fixing bolts is scraped off during the circuit breaker assembly process, and the uneven electric field on the surface of the transformer during circuit breaker operation causes the surface of the transformer to discharge to the support.

    Treatment:

    To make the surface electric field homogeneous, apply a coating of semiconducting adhesive equally to the transformer's surface.

     

    7. The arc extinguishing chamber of the circuit breaker cannot be disconnected

    The performance is that the circuit breaker cannot be opened once it has been opened, or that not all phases of the circuit breaker have been opened.

    Reason:

    The circuit breaker can properly disconnect the circuit and turn off the current under typical circumstances, whether it is manual opening operation or protective action tripping. The superiority of vacuum circuit breaker over other types is not only oil-free equipment, but also manifested in its long electrical life, mechanical life, large breaking insulation capacity, and strong continuous breaking capacity, Small size, light weight, frequent operation, avoiding fire, less operation and maintenance, etc., and it will soon be operated, overhauled, and technical personnel by the electric power department.

    Treatment:

    If the vacuum interrupter's vacuum falls below the required value during testing, it should be replaced. Specific steps

    (1) The vacuum test on the vacuum interrupter that needs to be changed must pass.

    (2) Disconnect the original vacuum interrupter and replace it with a new one. It's best if the installation is vertical. It's important to keep the moving conductive rod and the arc extinguishing chamber coaxial, and they shouldn't be torn during operation.

    (3) Measure the open distance and overtravel (contact stroke) after installing the new vacuum interrupter. If it doesn't match the standards, make the following changes: 1 To control the overtravel, adjust the bolt on the insulated pull rod.② To modify the opening distance of the arc extinguishing chamber, alter the length of the movable conductive rod.

    (4) Measure mechanical parameters such as opening and closing speed, three-phase consistency, and closing bounce with a power switch comprehensive tester. Make modifications if it is unqualified.

     

    Ⅵ. What are the inspection test items for vacuum circuit breaker

    The following inspection and test items should be performed when the vacuum circuit breaker is handed over, overhauled, or after 1 to 3 years of operation:

     

    1. The insulation resistance test is the same for circuit breakers of various rated voltage levels as it is for oil circuit breakers.

     

    2. In the closed and separated states, the circuit breaker performs the main circuit to ground, fracture, and inter-phase AC resist voltage (must be done after replacing the insulation lifting rod).

     

    3. With an oil circuit breaker, test the auxiliary circuit, control circuit AC withstand voltage, and conductive circuit resistance.

     

    4. The circuit breaker's mechanical characteristics at the rated working voltage: The varied opening durations should not be greater than 2ms, and the different closing periods should not be greater than 3ms; the closing time, opening time, and closing and opening speeds should all conform with the manufacturer's specifications. bounce at the end For 12kV, the time is no more than 2ms, and for 40.5kV, the time is no more than 3ms: the opening rebound amplitude is no more than 20% of the contact opening distance.

     

    5. The arc extinguishing chamber's contact spacing and overtravel should be in accordance with the manufacturer's specifications.

     

    6. The closing contactor of the working mechanism, as well as the closing and opening electromagnets, have the same minimum operating voltage as the oil circuit breaker.

     

    7. The closing contactor and the closing and opening solenoid coils have the same DC resistance and insulation resistance as the oil circuit breaker.

     

    8. When practicable, the interrupter should be vacuum tested, and the results should fulfill the manufacturer's specifications.

     


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