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Basic knowledge of high voltage vacuum circuit breaker


The influence of various mechanical characteristics on product performance The quality of the product's mechanical characteristics has an important relationship with the various electrical properties of the product, and affects the reliability of the product's operation. To measure the performance of the vacuum circuit breaker, the performance of the vacuum isolation chamber itself is important, but the mechanical characteristics also play a decisive role. The relationship between each mechanical characteristic parameter and product performance is described as follows:
1. Opening distance The opening distance of the contacts mainly depends on the rated voltage and withstand voltage requirements of the vacuum circuit breaker. Generally, when the rated voltage is low, the opening distance of the contacts is selected to be smaller. However, if the opening distance is too small, the breaking capacity and withstand voltage level will be affected. If the opening distance is too large, although the withstand voltage level can be increased, it will reduce the service life of the bellows of the vacuum interrupter. During design, the opening distance should be selected as small as possible under the condition of meeting the withstand voltage requirements of operation. The opening distance of a 10kV vacuum circuit breaker is usually between 8 and 12mm, and that of a 35kV vacuum circuit breaker is between 30 and 40mm.
2. When there is no external force on the contact pressure, the moving contact will generate a closing force on the inner cavity under the action of atmospheric pressure to make it close with the static contact, which is called self-closing force, and its size depends on the port of the bellows diameter. When the arc extinguishing chamber is in working condition, the force is too small to ensure good electrical contact between the moving and static contacts, and an external pressure must be applied. The sum of the applied pressure and the self-closing force is called the contact pressure of the contact. This contact pressure has the following effects:
(1) Ensure good contact between the dynamic and static contacts, and make the contact resistance less than the specified value.
(2) Satisfy the dynamic stability requirements in the rated short-circuit state. The contact pressure should be greater than the repulsive force between the contacts in the rated short-circuit state, so as to ensure complete closure and no damage in this state.
(3) Suppress closing bounce. The contact can be buffered when it collides, and the kinetic energy of the collision is converted into the potential energy of the spring, and the bouncing of the contact is restrained.
(4) Provide an acceleration force for opening. When the contact pressure is high, the moving contact gets a larger opening force, which is easy to break and will melt the solder joints, increase the initial acceleration of opening, reduce the arcing time, and improve the breaking capacity. The contact pressure of the contact is a very important parameter, and it is more suitable to be selected after many verifications and tests in the initial design of the product. If the contact pressure is too small, it cannot meet the requirements of the above aspects; but if the contact pressure is too high, on the one hand, it is necessary to increase the closing operation work, and on the other hand, the mechanical strength requirements of the arc extinguishing chamber and the whole machine also need to be improved. It's not economical.
3. Contact stroke (or compression stroke)
At present, the vacuum circuit breaker adopts the butt-type contact method without exception. After the moving contact hits the stationary contact, it cannot advance any further, and the contact contact pressure is provided by each pole contact compression spring (sometimes called a closing spring). The so-called contact stroke is the distance between the contact of the switch contact and the force end of the contact pressure spring continuing to move to the end of the contact, that is, the compression distance of the contact spring, so it is also called the compression stroke.
The contact stroke has two functions, one is to press the contact spring to provide contact pressure to the mating contact; the other is to ensure that a certain contact pressure is still maintained after running and grinding, so that it can be reliably contacted. Generally, the contact stroke can be about 20% to 30% of the opening distance, and the 10kV vacuum circuit breaker is about 3 to 4mm.
In the actual structure of the vacuum circuit breaker, the contact closing spring is designed to have a considerable amount of pre-compression and pre-pressure even in the opening position. This is to make the moving contact have considerable strength to resist the electric force and not shrink back when the moving contact has not touched the static contact during the closing process. When the contact touches the moment, The contact pressure suddenly increases to the pre-pressure value to prevent the closing bounce, which is enough to resist the electric repulsion and make the contact in a good state at the beginning; as the contact stroke advances, the contact pressure between the contacts gradually increases, and when the contact stroke ends , the contact pressure reaches the design value. The contact stroke does not include the pre-compression range of the closing spring, which is actually the second compression stroke of the closing spring.
4. Average closing speed The average closing speed mainly affects the electrical erosion of contacts. If the switching speed is too low, the pre-breakdown time will be long, the arc will exist for a long time, the contact surface will be greatly worn, and even the contacts will be welded and stuck, which will reduce the electrical life of the arc extinguishing chamber. However, if the speed is too high, closing bounce will easily occur, and the output power of the operating mechanism will also increase, which will have a large mechanical impact on the arc extinguishing chamber and the whole machine, and affect the reliability and mechanical life of the product. The average closing speed is usually about 0.6m/s.
5. Average opening speed The opening speed of the circuit breaker is generally as fast as possible, so that the first opening phase can break the fault current 2~3ms before the current approaches 0; otherwise, the first opening phase cannot be opened and Continuing to the next phase, the original first opening phase becomes the last opening phase, the arcing time is lengthened, the difficulty of breaking is increased, and even the breaking fails. However, if the opening speed is too fast, the rebound of the opening is also large. If the rebound is too large and the vibration is too severe, it is easy to cause re-ignition, so the opening speed should also consider this aspect. The opening speed mainly depends on the energy storage of the moving contact spring and the opening spring when closing. In order to increase the opening speed, the energy storage of the opening spring can be increased, and the compression of the closing spring can also be increased. This will inevitably increase the output power of the operating mechanism and the mechanical strength of the whole machine, reducing the technical and economic indicators. After years of tests, it is considered that the average opening speed of a 10kV vacuum circuit breaker can be guaranteed to be 0.95-1.2m/s.
6. Closing bounce time Closing bounce time is the time between when the circuit breaker is noisy, when the contact first touches, and then separates, may touch and leave again, and reaches its stable contact.
This parameter is not clearly stipulated in foreign standards. At the end of 1989, the Electric Power Department of the Ministry of Energy proposed that the closing bounce time of vacuum circuit breakers must be less than 2ms. Why is the closing bounce time less than 2ms? The main reason is that the moment of closing and bouncing will cause L.C high-frequency oscillation in the power system or equipment, and the overvoltage generated by the oscillation may cause damage or even damage to the insulation of electrical equipment. When the closing bounce is less than 2ms, no large overvoltage will be generated, the insulation of the equipment will not be damaged, and there will be no welding between the moving and static contacts when closing.
7. Closing and opening asynchronousness If the asynchronicity of closing is too large, it will easily cause the bounce of closing, because the movement impulse output by the mechanism is only borne by the contact of the first closing phase. If the asynchronicity of the opening is too large, the arcing time of the post-opening phase tube will be prolonged and the breaking capacity will be reduced.
The asynchronicity of closing and opening generally exists at the same time, so the asynchronicity of closing is adjusted, and the asynchronicity of opening is guaranteed. The product requires that the asynchrony of closing and opening is less than 2ms.
8. Closing and opening time
Opening and closing time refers to the period of time from the moment when the terminal of the operating coil is energized to the time when the three-pole contacts are all closed or separated.
Closing and opening coils are designed for short-time work. The power-on time of the closing coil is less than 100ms, and that of the opening coil is less than 60ms. The opening and closing times are generally adjusted when the circuit breaker leaves the factory, and there is no need to move it again.
When the circuit breaker is used in the power generation system and short-circuited near the power supply, the fault current decays slowly. If the opening time is very short, the fault current broken by the circuit breaker may contain a large DC component, and the breaking condition is even worse. , which is very detrimental to the opening of the circuit breaker. Therefore, it is advisable to design the opening time of the vacuum circuit breaker used in the power generation system as long as possible.
9. Loop resistance
The loop resistance value is a parameter that characterizes whether the connection of the conductive loop is good, and various types of products have specified values within a certain range. If the loop resistance exceeds the specified value, it is likely that a connection of the conductive loop is in poor contact. During high current operation, the local temperature rise at the poor contact will increase, and in severe cases, it will even cause a vicious cycle and cause oxidation and burning. Especially for circuit breakers used for high current operation, extra attention should be paid. It is not allowed to use the bridge method to measure the loop resistance, but the DC voltage drop method stipulated in GB763 must be used.
10. Contact system
The contacts of vacuum circuit breakers often adopt butt-type contacts.
Because the distance between the moving and static contacts of a general vacuum circuit breaker in the opening state is only 16mm, it is difficult to make contact surfaces of other shapes, and the damage of the instantaneous action arc on the flat contact surface is also small. One of the advantages of a vacuum circuit breaker is its small size, and the moving and static contacts must operate in an absolute vacuum space. If it is made into other docking methods, the volume of the circuit breaker itself will be increased and the circuit breaker will be smaller.

One of the advantages of a vacuum circuit breaker is its small size, and the moving and static contacts must operate in an absolute vacuum space. If it is made into other docking methods, it will also increase the volume of the circuit breaker itself!

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