Background of research and development of electric wear estimation function for vacuum circuit breaker contacts:
With the oil-free process of power system switching equipment, vacuum circuit breakers have been widely used in power system rapidly by virtue of their own advantages. In the field of medium voltage in China, vacuum circuit breakers have accounted for more than 80%. In 2000, the output of vacuum circuit breakers in China accounted for 87.36% of the total output of medium-voltage circuit breakers, and the share was 98.85% in 2004.
While the number of vacuum circuit breakers is growing rapidly, due to the characteristics of small maintenance and long maintenance cycle, it is easy for users to ignore the electrical life problem, and the main factor affecting the electrical life of vacuum circuit breakers is the electrical wear of contacts. With the passage of time, the first batch of vacuum circuit breakers used have gradually entered the aging, and the problem has become increasingly prominent, which should be paid enough attention.
According to the concept of prevention and treatment, prevention is the focus, our company carefully listen to the feedback of customers (in the vacuum circuit breaker with current opening or fault opening, there is a certain damage to the circuit breaker contacts, can you quantify the damage), referring to the application of relay protection at home and abroad and related technical literature, our company independently developed the contact electrical wear estimation function, In view of the vacuum circuit breaker vacuum tube contacts in the event of failure, there will be certain damage to the contacts, the invisible, invisible things are quantified with a mathematical formula model, and an alarm is made to notify the operation duty personnel to check the circuit breaker, and the hidden danger of the accident is eliminated in the bud.
The following quotes Wang Min's "synchronous motor failure caused by poor contact of vacuum circuit breaker contacts" in the article fault analysis to illustrate the necessity of contact electrical wear estimation method in application.
Fault process:
During the inspection of the equipment, the operation duty officer of a factory found that the 250KW/6KV synchronous motor equipped with the No. 1 oxygen generator ran abnormally, the motor vibration was large, and the ammeter pointer of the control cabinet stator swung up and down greatly, and the machine was stopped immediately. The maintenance electrician found that the secondary outlet line of 6KV high voltage cabinet No. C411 current transformer was seriously burned out, which was treated, and the fixed rotor air gap of synchronous motor was measured, and no sweeping phenomenon was found, but the vacuum circuit breaker was not checked. The operation duty officer starts the synchronous motor again, and after less than two minutes of operation, the unit appears the aforementioned fault phenomenon again, and the fault trip stops, so engineering and technical personnel are invited to analyze and deal with the faults.
Troubleshooting:
According to the above fault phenomenon reflected by the operator, the technician conducted a comprehensive inspection of the 6KV high-voltage circuit breaker (ZN9-630A/10KV-20KA vacuum circuit breaker).
1) Use 2500V megohm meter to measure the insulation status of synchronous motor and 6KV high voltage vacuum circuit breaker, and the dielectric voltage between phases and to the ground is in the normal range.
2) Check the closing synchronization of the vacuum circuit breaker three-phase contact with a three-phase synchronous light, and find that the three-phase inconsistency is more than 1mS, of which phase C is the worst, and the C-phase synchronous light is slightly bright under the closing condition.
3) The contact negative value of the three-phase main contact is measured by the double-arm bridge, and the result is 160 microohms for phase A, 90 microohms for phase B, and 25 ohms for phase C, which seriously exceeds the standard (the specification requires no more than 50 microohms). Adjust and increase the overrange, the C-phase contact resistance does not change, so it is judged that the contact of the C-phase vacuum interrupter is poor.
4) Replace the C-phase vacuum chamber, adjust the three-phase synchronization, so that the three-phase inconsistency is less than 1mS, and test the three-phase contact resistance is less than 50 microohms.
After the above treatment, the operator restarts the operation, the motor fault phenomenon is eliminated, and the unit operation returns to normal.
Vacuum circuit breaker contact failure cause analysis and harm:
Based on the comprehensive analysis of the above inspection and treatment, the direct cause of the synchronous motor current change, unit sound abnormal, and large vibration is that the C-phase main contact resistance of the 6KV vacuum circuit breaker seriously exceeds the standard, resulting in poor contact and heat, resulting in the three-phase current imbalance of the synchronous motor, resulting in overcurrent trip failure.
The technician inspected the replaced vacuum interrupter and found that the vacuum circuit breaker dynamic conductor rod and the vacuum interrupter outer shell were overheating. The vacuum arc extinguishing chamber was further disassembled, and it was found that the arc surface of the active and static contacts in the arc extinguishing chamber had been seriously burned and the surface was uneven, and the three Archimedes spiral grooves on the arc surface of the dynamic and static contacts had been burned and blocked, and a lot of contact welding slag had fallen in the arc extinguishing chamber.
According to the above disassembly inspection results, the technical personnel believe that the reason for the serious melting of the dynamic and static contacts of the vacuum circuit breaker is that the contact pressure decreases, the contact wear causes the over-range to decrease, and the contact resistance increases during operation and heats up. Vacuum circuit breaker contacts are made of fusible multi-material, the contact hair heat will burn, destroy the arc surface of the contact, so that the arc extinguishing ability of the contact is reduced, and the vacuum arc generated when the circuit breaker is opened and closed accelerates the melting of the contact, such a vicious cycle, the vacuum arc extinguishing room of the static contact burned out. If the above faults are not dealt with in time, the vacuum arc extinguishing chamber may explode due to high temperature heating and can not effectively extinguish arc.
In SUMMARY:
The arc extinguishing chamber of the vacuum circuit breaker is a non-disassembly sealed whole, in which the good contact of the contact is directly related to the quality of the power consumption of the user (the poor contact of the contact will cause the three-phase current imbalance, overcurrent tripping, and the contact resistance will lead to high temperature heating can not effectively extinguisher the arc and explosion and other faults).
The electrical wear of vacuum circuit breaker mainly refers to the electrical loss of arc extinguishing chamber, arc extinguishing medium and contact. It is generally believed that the determining role is the electrical wear of the contact. The electrical wear of contacts is mainly manifested as the net loss of contacts, metal transfer of contact materials and chemical corrosion. The main factors affecting the electrical wear of the contact are the size of the current, the length of the arc burning time, the material of the contact, the magnetic field structure, the bouncing performance and so on.
However, the contact can not be seen or touched in the arc extinguishing room. So how do we estimate the wear of the contact? Grasp the main factors that affect the electrical wear of the contact, the size of the current and the time of arc burning.
Our company refers to the application of relay protection at home and abroad and the research literature of vacuum circuit breaker contact wear, in the controller to increase the contact electrical wear estimation function, the algorithm is used for each phase, each time the wear amount will be superimposed with the previously stored value, when the operating count of any phase in the three phases exceeds the set value, the system will issue a warning signal. Inform the operator to check the contact resistance, stroke, opening distance and other parameters of the contact of the vacuum circuit breaker to eliminate the hidden trouble in the bud.
Contact electrical wear estimation method:
As we all know, arc burning is closely related to arc energy, and arc energy breaking current is closely related to arc burning time, our company adopts the cumulative amount of electrical wear (arc energy) per breaking, when the amount of electrical wear reaches the set value, the controller sends an alarm signal to notify the operating duty personnel to check the circuit breaker.
The total electrical wear is equal to the accumulation of the arc energy of N breaks, in which the effective value of the breaking current is easy to obtain through the high-speed sampling chip, but the arc time is difficult to obtain. In the conventional application, the arc time is generally replaced by the breaking time, but the practice and the breaking experiment prove that the method is relatively rough to replace the arc time with the breaking time. To this end, our company has made a certain optimization of the arc time, referring to a large number of literature on contact wear, arc time analysis, combined with foreign electrical wear estimation methods for relaying protection devices, the controller adds the breaking time count function, opening and closing statistical function, combined with practical experience (zero time of on-off flow, contact bounce time, over-range running time, etc.). The arcing time variable is transformed into a relatively reasonable time constant, which makes the electric wear estimation method more practical.
