overview
The contactor that uses vacuum as insulation and arc extinguishing medium is a vacuum contactor, vacuum is a general noun / Any state where the absolute pressure is lower than the normal atmospheric pressure can be called vacuum, and the space where the absolute pressure is equal to zero is called absolute vacuum, otherwise it is a relative vacuum.
The vacuum is expressed as the absolute pressure of the gas, and the lower the pressure, the higher the vacuum. The unit is in Pa (PA), which is the force of 1N/㎡.
An atmosphere of 0.1 megapascal (MPa) is also equal to 1 kgf/cm2, and 1MPa = 106Pa.
In the vacuum interrupter, the vacuum (pressure value) is between 10-2~10-5pa, and millimeters of mercury (mmHg) are used in the engineering community, a standard atmospheric pressure is 760 mmHg, and 1mmHg is called 1 乇, 1 乇 ==133.3pa, and the vacuum degree of the vacuum interrupter is between 10-4~10-7 乇, and the difference between the two representation methods is about 2 orders of magnitude.
Under such vacuum conditions, the gap insulation is very high, so the opening distance of the interrupter contact of the vacuum contactor is small, and the corrugated pipe made of metal is used to link the telescopic operation with the operating mechanism, and its structural principle is described as follows:
The arc extinguishing chamber shell of the vacuum contactor is basically made of a cylindrical tubular ceramic sleeve made of aluminum oxide, and the sealing of the moving contact relies on the bellows, which expands and contracts within the allowable elastic deformation. A metal shield with suspended potential is provided around the periphery of the dynamic and static contacts, which acts as a condensation to absorb metal vapor and protect the inner wall of the shell from insulation. The air pressure inside the qualified vacuum interrupter should be lower than 10-3Pa and should be above 10-4Pa. The opening distance is small, the operating energy is small, and the arc energy is also small, so the mechanical life and electrical life of the vacuum contactor are very high, the mechanical life can reach more than 500,000 times, and the electrical life can reach more than 300,000 times, and it is difficult for the contactor of other media to achieve this situation.
The vacuum contactor is safe to use, simple to maintain, and the arc extinguishing process has nothing to do with the external atmospheric conditions, so the structural parts involved in the arc extinguishing are sealed in a vacuum ceramic container, which is neither affected by environmental pollution nor polluted by the outside world.
Section 2 Structure and working principle of vacuum arc extinguishing
Vacuum interrupter is the key component of vacuum switch, vacuum switch includes vacuum contactor, vacuum interrupter performance basically determines the main performance of vacuum contactor. It can be seen from the structure of the vacuum interrupter that it is composed of four major parts (shell, contact, main shield cover and bellows), and the differences in the structure of these parts form a vacuum interrupter for different purposes.
1. Shell
Most of the shells of modern vacuum interrupters are cylinders made of ceramics, and the two ends are sealed with metal covers to form a closed container, and the contacts of the vacuum interrupter are provided inside, such as the shielding cover, the conductive rod of the dynamic and static contacts, etc.
In order to ensure that the vacuum interrupter has enough vacuum in operation, the shell must be sealed with no breathability, and in fact, the finished vacuum interrupter can not maintain the vacuum degree when the original vacuum interrupter was just made, and over time, the vacuum degree of the vacuum interrupter will be reduced, the main reason is the slow air leakage between the seal such as the insulated shell and the weld, as well as the degassing of the internal metal parts.
There is no absolute vacuum in the world, as long as the chronic air leakage is within the allowable range, the life of the vacuum can be guaranteed, according to the vacuum gap breakdown theory, the working conditions of the vacuum interrupter are safe to work in the vacuum degree of more than 10-2Pa. The arc extinguisher standard stipulates that its vacuum life is 18 years, and according to the calculation, the vacuum degree of the finished product from the manufacturer can be guaranteed at more than 10-5Pa. In addition to the sealing requirements, the shell of the vacuum interrupter must also have a certain mechanical strength in order to withstand the difference of close to one atmosphere between the inside and outside of the arc extinguishing chamber.
2. Shielding cover
The vacuum interrupter for vacuum contactors should be provided with two kinds of shielding covers, the main shielding cover around the contact and the shielding cover for protecting the bellows.
The function of the main shielding cover, when the contactor is broken, there is a large amount of metal vapor when the arc is generated between the contacts, and the main shielding cover blocks the metal vapor and is condensed on the surface of the main shield, so that the metal vapor will not return to the contact gap, which is helpful for the recovery speed and breaking ability of the medium strength after the arc.
The main shield of modern vacuum interrupter adopts suspension potential, which can eliminate the possibility of arc transfer to the shield during breaking, and also play a role in equalizing pressure, so that the electric field distribution inside the vacuum interrupter is more uniform.
The function of the protective bellows shielding cover is to prevent the metal vapor generated by the vacuum arc and the molten metal generated by the vacuum arc from burning and burning out the bellows when the breaking, and the main shielding cover should absorb about 70% of the total arc during the breaking process, and the material of the main shielding cover should be made of oxygen-free copper sheet.
3. Contacts
When the vacuum interrupter is broken, a large amount of metal vapor will evaporate from the contact surface, the larger the breaking current, the more metal vapor is generated, and after a large amount of metal vapor arc is extinguished, it is easy to cause the re-ignition of the vacuum gap, so from the breaking performance, it is hoped to use a contact material with little metal vapor generated in the process of breaking the current, so the contact should meet the following requirements:
1. High compressive strength and high resistance to fusion welding;
2. High electrical conductivity, high thermal conductivity, high mechanical strength:
3. Small contact resistance, small cut-off value;
4. Low oxygen content and low thermal electron emission capacity;
5. It has strong breaking capacity and low electrical erosion rate.
4. Bellows
The quality of the bellows determines the mechanical life of the vacuum interrupter, which is an indispensable element of the interrupter, the vacuum interrupter
Only the use of bellows can make the mechanical force change into contact movement, and ensure the sealing of the arc extinguisher housing. The actual expansion and contraction of the bellows is equal to the opening distance of the contact, so the bellows with different expansion and contraction are equipped with arc extinguishing chambers with different opening distances, which are generally made of stainless steel.
The diameter of the bellows determines the self-closing force of the vacuum interrupter, and the vacuum interrupter produces a force that makes the moving contact close to the static contact under the action of the external atmospheric pressure, which is called the self-closing force.
5. Conductive system
It is composed of a moving guide rod and a static guide rod, one end of which is connected with the dynamic and static contacts respectively, and the other end extends out of the interrupting chamber.
Section 3 Basic parameters of high-voltage vacuum contactor
The national standard GB/T14808-2001 has clear provisions on the requirements for high-pressure vacuum contactors, which is equivalent to the IEC470 standard.
For example, the rated voltage is 10 kV
1. Rated voltage: 12KV
2. Rated current: 160A~630A
3. Rated frequency: 50Hz
4. Lightning impulse withstand voltage: 75KV
5. Rated closing capacity: 10Ie 100 times
6. Rated breaking capacity: 8Ie 25 times
7. Rated short-time withstand current: 10Ie
8. Rated peak withstand current: 10Ie
9. Dynamic and thermal stability: 10Ie (thermally stable) 25Ie (dynamically stable)
10. Overload withstand current: 15Ie
11. Rated short-circuit breaking current: 10Ie O-180S-CO-180S-CO 1 time
12. Rated short-circuit closing current: 25Ie O-180S-CO-180S-CO 1 time
13. Rated opening distance: 5.5±0.5mm
14. Overstroke: ≥1mm
15. Closing synchronicity: ≤2Ms
16. Closing bounce: ≤3Ms
17. Loop resistance: ≤180μΩ
18. Interphase center distance: ≥165mm
19. Mechanical life: ≥ 500,000 times
Section 4 The influence of the control mechanism on the performance of the vacuum contactor
A complete vacuum contactor design should be matched with the parameters of the vacuum interrupter to achieve the breaking and closing performance.
1. Opening and closing speed
The opening process of the contactor can be broken down into two stages, the return of the overtravel at the beginning of the opening, followed by the travel phase of the rated opening distance. This requires the speed of the control mechanism to be sufficient at the beginning of the splitting to ensure that the accumulation arc during breaking becomes the diffusion arc as soon as possible. This requirement is mainly achieved by overtravel, so an appropriate increase in overtravel can increase the arc extinguishing performance. The arcing time of the vacuum interrupter is generally less than 10M, in order to ensure the reliability of the breaking, the moving contact is required to complete 70% of the opening distance within 10M, so the vacuum contact device is required to have a certain opening speed, which can meet the requirements within 0.8m/s. If the closing speed is too low, the pre-breakdown time will be prolonged, and the pre-breakdown arc will be caused to aggravate the erosion of the dynamic and static contacts.
2. Contact pressure
The vacuum interrupter of the contactor does not end when the contact between the dynamic and static contacts ends during the closing process. After the contact contact, there is also a process of contact spring compression, and this contact spring compression is called overtravel. Under normal circumstances, under the action of self-closing force, the force when the contact spring is just compressed is called the initial pressure, and the pressure when the spring is compressed is called the final pressure.
According to the theory of electrical contact, the contact resistance of the dynamic and static contacts is proportional to the final pressure, and the contact resistance of the arc extinguisher is also called the loop resistance of the arc extinguisher, and the size of the loop resistance determines the three important parameters of the contactor.
a. The contact resistance determines the size of the long-term loading current of the vacuum interrupter;
b. The contact resistance also determines the ability of the vacuum interrupter to withstand short-term current and peak current, also known as the ability of dynamic and thermal stable current;
c. The contact resistance also determines the rated short-circuit current closing ability of the contactor.
If there is a problem with the temperature rise and dynamic and thermal stability of the arc extinguisher during the type test, it should be recalculated and reviewed to check whether the contact pressure meets the design requirements. The accurate final pressure should take into account the reduction of the actual final pressure caused by the influence of the electrodynamic force of the contact, that is to say, the net pressure after deducting the electrodynamic force is the real final pressure. The reasons for the analysis are as follows: when the dynamic and static contacts are in contact, the point contact is the mainstay, then the current shrinkage phenomenon will occur at the contact through which the current flows, and the shrinkage electrodynamic force will always be in the direction of pushing the contact, and the size is proportional to the square of the current, so when the final pressure required for the dynamic and thermal stability current is less than the electrodynamic force, the contact will be pushed up, resulting in arc causing contact burning and welding, and when determining the parameters of the final pressure of the contact, the influence of the electrodynamic force repulsion due to the shrinkage phenomenon must be considered.
In general, when a current of 1 KA flows through a contact of φ25, an electric repulsion of 10 N is generated.
For example, the thermal stability current of a 630A contactor is 6300A, and the dynamic stability current is 16KA and the electric repulsion can be 160N, so the existence of electric repulsion cannot be ignored when designing the terminal pressure.
3. Contact material and terminal pressure of vacuum interrupter
Modern vacuum interrupter contacts can be divided into two main categories, Cu w and Cu Cr. The cut-off value of the Cu w contact material is low, and the electrical wear rate is small under the rated current operation of more than one million times, and it can be operated appropriately and frequently. This type of material is suitable for the interrupter of vacuum contactors. Cu Cr is suitable for arc extinguishing chambers for vacuum circuit breakers, and this contact material has high breaking capacity and high voltage resistance. Under the action of arc, it has a certain resistance to fusion welding, and is generally used in various arc extinguishing chambers above 630A.
Based on the experience of type testing, the relationship between the contact material and the final pressure can be roughly determined. Cu w class contacts, 35N per 1000A thermally stable current, plus electric repulsion.
Example 1: Taking a 250A contactor as an example, the thermal stability current is 2500A, the electric repulsion is about 25N, and the final pressure should be
35×2.5＋25=112N
Example 2: Taking a 630A contactor as an example, the thermal stability current is 6300A, the electric repulsion is about 63N, and the final pressure should be
35×6.3＋63=283N
4. In addition to the above factors, whether the size of the final pressure meets the requirements of the circuit resistance and final pressure depends on whether the circuit resistance meets the requirements of dynamic and thermal stable current.
The main requirements for the loop resistance between the dynamic and static contacts of the vacuum interrupter are:
a. In long-term work, the temperature rise does not exceed the specified level through the rated current;
b. When the short-circuit current is passed in a short time, the contact is required to be not welded and damaged;
c. In the closing process, a certain short-circuit current can be closed, and no fusion welding occurs;
d. When the test is dynamic and thermally stable, no fusion welding occurs.
When the contact flows through the short-circuit current, the contact part is strongly heated, and the local melting can occur due to overheating within a few seconds, at this time the current is called the beginning of the melting current, when the current exceeds 20-30% of the beginning of the melting current, the contact begins to weld, if this happens, it will seriously affect the reliable work of the contactor.
When the moving contact and the static contact are closed, there is contact resistance, and the vacuum interrupter contact used in the vacuum contactor is generally made of Cu w material, and the voltage drop of the contact resistance when it begins to melt is 0.75V, in order to verify that the terminal pressure is reasonable, it is feasible and necessary to calculate the loop resistance of the interrupter.
