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Analysis and pre control of an explosion accident of a capacitor bank vacuum circuit breaker
Addtime:2012-07-12 14:43:56

Abstract: vacuum circuit breaker is widely used in the 10 kV voltage level of substation because of its superior arc extinguishing and maintenance free advantages. It mainly plays the role of separating the load current of the line and cutting off the fault current. Because the vacuum circuit breaker is more difficult to extinguish the capacitive current than the inductive current, the capacitor bank switch caused by the failure to extinguish the arc occurs. By analyzing the explosion accident of a 10 kV capacitor bank switch, the reasons are analyzed, and relevant pre control measures are put forward.


0 Introduction


Dongguan is a developed city with large electricity base and fast load growth. So large electrical energy demand has caused great pressure to the reactive power equipment of the substation. In order to meet the changing demand of reactive power in the grid, capacitors need to be switched frequently. In summer peak hours, the number of switching is increased to 6 times a day. Such frequent operations, such as vacuum drop and so on, especially the equipment that has been put into operation for more than 10 years poses a great threat to the safe and stable operation of the power grid [1].


1 introduction of the accident


At 7:39 on May 7, 2011, when the 110kV Jinzhou substation of Dongguan Power Supply Bureau was switched on the 527 switch of the 4# capacitor bank, the switch was switched on the switch immediately. The 527 switch protection device sent the "time limit current fast break protection action ABC phase, the Ib=112.95A" signal, which reacted the B phase to the fault and jumped the 527 switch. Then, the secondary 502 B switch tripped, and the 502 B switch protection device sent "IV segment repressurized blocking over current protection action AC phase, Ia=35.19A", "bus protection action AC phase, Ia=35.19A" signal, reflecting the fault in the range of the parent difference and jumping 502 B switch. The accident resulted in the loss of 10kVII B busbar. Subsequently, after the duty inspector found that the fault point was in the 527 switch vacuum bubble, it quickly isolated the 527 switch and resumed the operation of II B bus in time.


2 site inspection of accident


The switchgear cabinet of 4# capacitor group which has an accident is XGN type, switch type ZN28 - 10Q, which is equipped with CT19 operation mechanism and has been put into operation since June 1998. Check and analyze the fault site, and find out:


1) there are obvious three phase short circuit marks on the vacuum circuit breaker, and there are different degrees of damage on the outer wall of the vacuum bubble, of which A and C phase burst, and the B phase has cracks, as shown in Figure 1.


2) after removing the switch and connecting to the bus bar, it is found that there are obvious traces of burn on the connection surface of the vacuum bubble support, and the following contact surfaces are intact, as shown in Figure 2. It can be seen that the short circuit point is connected to the connecting point of the bracket on the vacuum bubble. When the short circuit is switched off, the huge short-circuit current only burns the upper bracket contact surface.

3) breaking the outer wall of the vacuum bubble to carry out internal inspection, it is found that the internal contact and vacuum mask burn of C phase is serious, as shown in Figure 3. And A and B are in good condition. It can be seen that the C phase arc chamber is burned by the arc, while the outer wall of the A and B vacuum bubbles burn outside.


4) the switch mechanism indicator switch is divided in the position, by measuring the opening point, breaking the vacuum bubble to observe the contact position (see Figure 3) also confirms the switch in the partition, thus it can be seen that the switch tripping is successful.


5) check the capacitor bank, find fuse fuse of B phase B14, measure capacitance and withstand voltage test are all qualified.


3 cause analysis of accident


When the capacitor bank is in power supply, the differential current is generated due to the fuse of the B14 fuse. The differential current protection action adopted by the capacitor bank causes the switch to trip [2] immediately. In the process of sluice, the C phase vacuum bubble can not extinguish the arc (it is known from the internal severe burn), which causes the explosion of the arc extinguishing chamber and causes the three phase breakage at the upper bracket. From the fault recording, it is known that the current circuit current was 24 kA, and the huge circuit breakers caused the contact surface burn of the upper bracket, and the external burns A and B were really vacuoles, but failed to destroy them.


It is clear that the accident starts from the failure of the C phase true bubble to effectively extinguish the arc, and according to the switch in the division, it is known that the vacuum bubble can not be extinguished is the arc during the trip. The possible reasons for the arc extinguishing failure are as follows: (1) the vacuum degree decreases; (2) the amplitude of the contact rebound is too large when the gate is broken. The switch was tested in 2009 and qualified. Therefore, it is obvious that the large amplitude of the contact rebound causes the greater probability. In addition, the capacitor passes through capacitive current. According to the characteristics of arc extinguishing, the capacity of the vacuum switch to extinguish capacitive current is much more difficult than that of inductive current [3].


4 pre control measures


The switch has been put into operation for 13 years since 1998, and no technical modification or renovation has been carried out. Because the capacitor is the 10 kV equipment of the same kind, the most frequent switching and switching of the most switching times of the device [4], and the vacuum bubble each extinguished capacity current is much more difficult than the inductive current of the line. Therefore, the perennial harsh operation has not been found in time after its performance decline, which is the cause of the accident. To avoid similar accidents, we consider it necessary to take the following preventive measures for capacitor bank switches.


1) conduct a comprehensive assessment of the current operation of the capacitor switch, especially if it has been put into operation for a certain number of years. The mechanical life and electrical life of the normal operation of the vacuum circuit breaker are 10000 times [5]. This capacitor is operated for 13 years and is calculated on an average of 3 times per day. The number of split times has reached 336513 = 14235 times, far exceeding the specified number of times. In addition, many registers are damaged and need to be investigated and processed to obtain accurate data for reference.


2) in addition to the regular items, we should also focus on the gate rebound project while doing the mechanical characteristic test. At present, our handover and pre test items only have closing time and no rebound amplitude. And according to DL /T 402 - 2000, "6. kV 4.1 40.5 kV high voltage vacuum circuit breaker ordering technical conditions" 6., 4.1 provides [6], "vacuum circuit breaker... The mechanical characteristics tests include the time of closing, closing time, closing bounce time and the amplitude of the brake rebound. "According to the regulations, the amplitude of the vacuum bubble brake rebound of different models and manufacturers is not the same, but generally the amplitude should not exceed 30%[7] of the contact opening distance. For the switch of the capacitor bank, because the vacuum bubble is disconnected by the capacitive current, it is much more difficult to extinguish the inductive current of the general line. If the contact is too large and exceeds the specified value at the time of the switch, the arc failure caused by the insufficient drawing arc or the arc reignition will result in the explosion of the vacuum bubble [8]. Therefore, we must have the vacuum bubble explosion. We need to pay attention to the test of the sluice rebound to ensure this qualification.


3) check the performance of the buffer. The function of the buffer is to absorb the excess energy when the switch is sluice, otherwise the brake arm will have a hard collision with the mechanism, which is the main cause of the rebound of the contact [9]. Due to the long running time and lack of maintenance, partial buffer is invalid, so we must carefully check and maintain it to ensure its good effect.


4) pay attention to the vacuum test of the arc extinguishing chamber. In Section 7.3 of South Grid's "preventive test procedure for power equipment" [10], it is clearly stipulated that the measurement period of vacuum of capacitor switching interrupter is 3 years. Although the pressure test can be replaced by pressure test at present, the pressure test is only qualitative, and the vacuum degree can not be reflected quantitatively, that is, when the vacuum degree is near disqualification, the pressure test can also pass, but it can not reflect the condition that the vacuum degree is near the disqualification, [11], at this time, the operation of the brake is induced to rebound too much. It will cause an accident of failure in arc extinguishing.


5) measure the contact wear amount, mainly for capacitor switch to extinguish capacitive current, difficult to burn contacts, and prevent contacts from heating when switching on.


6) measuring the resistance value of the loop, mainly measuring the contact situation of the static and dynamic contacts.


5 Conclusion


At present, we have maintained the same requirements as ordinary circuit switches for the maintenance and maintenance of the capacitor switch and the acceptance test, and there are no different treatment in various relevant regulations. Considering the frequent switching conditions of capacitor banks in the heavy load area of Dongguan, combined with the characteristics of the vacuum switch to extinguish the capacitive flow, we put forward a series of pre control measures aimed at the frequent failures of the capacitor banks, in order to improve the safety and reliability of the operation of the capacitor banks.