I. Lifespan of Vacuum Circuit Breakers
The lifespan of a vacuum circuit breaker includes three categories: vacuum life, mechanical life, and electrical life.
1. Vacuum life: This measures whether the internal vacuum level of a vacuum interrupter remains within the allowable operating vacuum level during transportation, storage, installation, and operation from the date of manufacture. The end of the vacuum life is primarily determined by testing whether the vacuum pressure within the interrupter remains above the maximum allowable operating value.
2. Mechanical life: This refers to the number of mechanical operations of the circuit breaker from the date of manufacture. Each opening and closing operation is counted as one operation, and it can generally reach more than 10,000 operations. The mechanical life mainly depends on the life of the stainless steel bellows, and the end of the mechanical life is when the bellows breaks.
Besides manufacturing quality, the main factors affecting the lifespan of corrugated pipes include the following, which may accelerate pipe damage:
(1) The influence of the usage environment, such as chemical corrosion, high temperature, etc.
(2) Plastic deformation of the bellows may occur due to improper use or adjustment;
(3) The actual travel during operation is too large or the buffer force of the operating mechanism's damper is too large;
(4) The guiding device does not meet the requirements; the moving contact rod shakes or the bellows rubs when it moves.
(5) The bellows was damaged by excessive torque.
3. Electrical life: This refers to the number of times the rated short-circuit current or rated operating current can be interrupted. The end of the electrical life can be determined by the allowable wear thickness of the contacts; when the contact wear reaches the specified value, it indicates the end of the electrical life of the arc-extinguishing chamber.
II. Inspection and Maintenance of Vacuum Circuit Breakers
(I) Maintenance Cycle
The arc-extinguishing chamber of a vacuum circuit breaker itself requires no maintenance. Once installed and adjusted according to requirements, the vacuum circuit breaker can be put into operation, and maintenance during operation is very simple. During operation, when the number of operations reaches one-fifth of its mechanical life, a comprehensive inspection and adjustment should be performed by shutting down the power supply. If the mechanical or electrical life is nearing its end, the inspection and adjustment cycle should be appropriately shortened.
(II) Main contents of inspection and adjustment
The inspection and adjustment mainly include the following aspects:
(1) The tightness of the connection parts of the main circuit terminals.
(2) Clean the operating mechanism and the outer shell of the arc extinguishing chamber.
(3) Add lubricating oil to the moving parts and replace damaged or rusted parts.
(4) Inspection of contact wear.
(5) Vacuum degree check of vacuum interrupter.
(6) Adjustment of other parameters (mainly checking and adjusting contact gap and compression stroke).
(III) Identification and Replacement of Vacuum Degree in Arc-Extinguishing Chamber
1. Vacuum degree assessment of the arc-extinguishing chamber
The vacuum level of a vacuum circuit breaker is directly related to its insulation and arc-extinguishing performance. In practice, it is difficult to accurately determine the vacuum level of the arc-extinguishing chamber. The commonly used method is to use the power frequency withstand voltage method to qualitatively determine whether the vacuum level is qualified.
The power frequency withstand voltage test at the fracture surface is the most effective method for checking the vacuum level of the arc-extinguishing chamber and is currently widely used.
Pull the two contacts of the arc-extinguishing chamber to the rated opening distance, and apply the rated power frequency withstand voltage between the contacts. If flashover occurs, it indicates that the vacuum level has decreased.
During power frequency withstand voltage tests, various forms of luminescence often occur within the vacuum interrupter chamber. These include flash points, moving sparks, or a pale blue glow on the inner glass wall. These luminescence phenomena are normal and characteristic of vacuum devices and are unrelated to the vacuum level. Only when a glow fills the entire space within the interrupter chamber can a vacuum level be considered unsatisfactory. The color of this glow depends on the gas composition: air is purplish-red, hydrogen is orange-red, alcohol is white, and water is slightly yellowish-white. If this glow occurs within the interrupter chamber, and flashover occurs within 1 minute of withstanding the rated power frequency withstand voltage, then the vacuum level of the vacuum interrupter chamber is deemed unqualified.
When no specific regulations are specified, a 42kV power frequency voltage is generally applied to a 10kV vacuum circuit breaker for 1 minute. During the test, the voltage should be slowly increased from zero to 70% of the rated voltage, stabilized for 1 minute, and then evenly increased to the rated power frequency voltage over 0.5 minutes. If the test equipment can maintain this voltage for 1 minute without tripping or sudden current changes, it is considered合格 (qualified).
Current setting of the test transformer:
When testing a single vacuum interrupter, the high-voltage side current should be set to 20mA. If testing 2 to 6 interrupters at a time, the high-voltage side current should be set to 40mA.
Vacuum devices emit X-ray radiation when the power frequency withstand voltage is above 42kV. Therefore, radiation protection measures should be in place during testing, or a sufficient distance should be maintained between the tester and the test object to prevent harm from X-rays.
2. Replacement of the arc-extinguishing chamber
The vacuum interrupter should be replaced if it is in any of the following conditions.
(1) The vacuum level drops significantly or the power frequency withstand voltage test fails.
(2) The mechanical life of the arc-extinguishing chamber has reached the specified value.
(3) The wear of the moving and stationary contacts is greater than or equal to 3mm, which is the specified value.
(4) The arc-extinguishing chamber is damaged and can no longer work properly.
Replacing the arc-extinguishing chamber is relatively simple and can generally be carried out according to the procedures specified in the manufacturer's instruction manual. After replacement, the assembly dimensions of the equipment, the circuit breaker's stroke, overtravel, and opening distance should be measured, and adjustments should be made if they are not up to standard. Then, a power frequency withstand voltage test should be performed.
3. Precautions for replacing the vacuum interrupter
(1) When replacing the arc-extinguishing chamber, the same model of arc-extinguishing chamber must be used. Other models of arc-extinguishing chambers are not allowed to be used as substitutes.
(2) During assembly, attention must be paid to the coaxiality of the conductive rod and the arc-extinguishing chamber axis. The coaxiality error is generally required to be no more than 2mm, so as to prevent the arc-extinguishing chamber from being subjected to shear and shear forces.
(3) When the circuit breaker is in operation, the bellows should not be subjected to resistance and should not rub against any part.
(4) The movement trajectory of the moving conductive rod should be straight, and sparks should not be generated around the bellows at any time. The compression and stretching of the bellows should not exceed the limit of the contact opening.
(5) The pressure ring on the end face of the arc extinguishing chamber should be subjected to uniform force in all directions.
(6) The guide sleeve can be made of polytetrafluoroethylene or high-temperature resistant nylon. If a metal guide sleeve is used, the distance between the guide sleeve and the bellows should be controlled to prevent the voltage drop formed under strong current from generating sparks between the bellows and the guide sleeve.
(iv) Parameter checking and adjustment
1. Adjustment of total stroke and contact opening distance
The total stroke or contact opening distance of the arc-extinguishing chamber of a vacuum circuit breaker varies depending on the voltage level, and these specifications are provided in the product manual.
(1) The total stroke of the conductive rod is generally adjusted to the specified value by adjusting the height of the limit screw of the circuit breaker.
(2) Contact Spacing and Adjustment. The contact spacing is determined by the rated voltage of the vacuum circuit breaker, and also by the breaking capacity and withstand voltage requirements under operating conditions. There are technical specifications for the contact spacing of a particular type of vacuum circuit breaker. Too small a spacing will cause a decrease in breaking capacity and withstand voltage level. The following table shows the selection range for the contact spacing of vacuum circuit breakers under different rated voltages.
Contact gap selection range: Vacuum circuit breaker: 10kV is 10-15mm, 35kV is 20-35mm. Different manufacturers have different specifications.
The contact gap can be measured by the actual stroke of the moving conductive rod in the arc-extinguishing chamber. If the contact gap is not up to standard, it can be adjusted by adjusting the thickness of the buffer pad. Generally, increasing the thickness of the buffer pad will decrease the contact gap, and vice versa.
2. Contact itinerary and adjustments
The function of contact stroke:
(1) Ensure that the contacts can maintain a certain contact pressure and make reliable contact even after a certain degree of electrical wear.
(2) Provide a buffer when the contacts close to reduce bounce.
(3) When the contacts are open, the moving contact is given a certain initial speed (energy) to break the weld point, reduce the arc time, and improve the medium recovery speed.
The contact stroke is typically 15% to 40% of the contact opening distance. The required contact stroke is usually achieved by adjusting the connecting thread between the insulating rod and the arc-extinguishing chamber's conductive rod. During adjustment, a wrench can be used to operate the circuit breaker, pull out the metal pin at the end of the insulating rod, and rotate the connector to the moving conductive rod of the arc-extinguishing chamber to adjust the contact stroke.
a. For a connector with a 1.5mm pitch, the adjustment distances when rotating 900, 1800, 2700, and 3600 degrees are 0.375mm, 0.75mm, 1.125mm, and 1.5mm, respectively.
For connector b with a 1mm pitch, the adjustment distances are 0.25mm, 0.5mm, 0.75mm, and 1mm when rotated 900, 1800, 2700, and 3600 degrees, respectively.
3. Adjustment of opening and closing speeds
The opening and closing speeds are adjusted using a spring. When the spring is extended, the opening speed is faster, while the closing speed is slower; when the spring is shortened, the opening speed is slower, while the closing speed is faster.
4. Adjustment of the three identical periods
The maximum error in the synchronicity of the three-phase opening and closing of a vacuum circuit breaker should not exceed 1ms. The adjustment method is the same as that for the contact travel.
5. Inspection of contact wear
Changes in the contact stroke directly reflect the wear of the arc-extinguishing chamber contacts. After each adjustment of the contact stroke, the adjustment amount must be recorded. When the cumulative adjustment amount exceeds (or reaches) the contact wear thickness, the arc-extinguishing chamber should be replaced.
