Operation and maintenance of the hottest 10kV drop

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Use and operation maintenance of 10kV drop fuse

drop fuse is the most commonly used short-circuit protection switch for 10kV distribution line branch lines and distribution transformers. It has the characteristics of economy, convenient operation and strong adaptability to outdoor environment. It is widely used in 10kV distribution lines and the primary side of distribution transformers as protection and equipment switching operations. It is installed on the branch line of 10kV distribution line, which can reduce the scope of power failure. Because it has an obvious disconnection point and has the function of isolating switch, it creates a safe working environment for the lines and equipment in the maintenance section and increases the sense of safety of maintenance personnel. Installed on the distribution transformer, it can be used as the main protection of the distribution transformer. Therefore, it has been popularized in 10kV distribution lines and distribution transformers. Its working principle is: the moving contacts at both ends of the fuse tube are fastened by the fuse (melt). After the upper moving contact is pushed into the protruding part of the "duck's beak", the upper static contact made of phosphor copper sheet is against the upper moving contact, so the fuse tube is firmly stuck in the "duck's beak". When the short-circuit current is fused through the fuse, an arc is generated, and the steel paper tube lined with the fuse tube generates a large amount of gas under the action of the arc. Because the upper end of the fuse tube is sealed, the gas is ejected to the lower end, blowing out the arc. Due to the fuse fusing, the upper and lower moving contacts of the fuse tube lose the fastening force of the fuse. Under the gravity of the fuse tube itself and the action of the upper and lower static contact spring sheets, the fuse tube quickly falls, breaking the circuit and cutting off the faulty line or faulty equipment

in the actual rural 10kV line system and the fuse on the distribution transformer can't operate correctly, one of the reasons is that the rural electrician's quality is poor, his heart is not strong, and he doesn't carry out the maintenance and overhaul of drop fuse all the year round; The second reason is that the product quality of drop fuse is poor, and it cannot be pulled and closed flexibly. Two reasons reduce the function of the drop fuse. In reality, there are often cases of lack of melt pipe, lack of melt or the use of copper wire, aluminum wire or even iron wire to replace the melt. The trip rate of the line and the fault rate of the distribution transformer remain high

1 Analysis of the device defects of drop fuse

(1) the product process is rough, the manufacturing quality is poor, and the elasticity of contact spring is insufficient, resulting in poor contact of contact and spark overheating

(2) the rough and inflexible manufacturing of the rotating shaft of the fuse tube makes the angle of the fuse tube fail to meet the requirements of the specification, especially the size of the equipped fuse tube fails to meet the requirements of the specification. The long fuse tube will push the duck beak to death, resulting in the fuse tube cannot fall quickly after the melt melts, and the arc will be cut off and extinguished in time, causing the fuse tube to burn or explode; The size of the fuse tube is short, it is difficult to close, and the contact is poor, resulting in electric sparks. The "6.7" accident of Guangling electric power bureau belongs to this kind of reason, which led to the staff's impatience and flashed out the dynamic of holding the fuse tube with their hands, resulting in electric shock and death

(3) the rated breaking capacity of the fuse is small, and its lower limit is less than the three-phase short-circuit capacity of the protected system. At present, 10kV outdoor drop out fuses are divided into three types, namely 50a, 100A and 200A. The upper limit of breaking capacity of 200A drop fuse is 200mva and the lower limit is 20MVA. According to its interrupting capacity, it is not difficult to see that in case of short-circuit fault, the fuse cannot be extinguished in time after the fuse is fused, and it is also easy to burn or explode the fuse tube

(4) the fuse tube size of some new switches does not match the size of the fixed contact part of the fuse, which is very easy to loosen. Once encountering external force, vibration or windy weather during operation, it will automatically move by mistake and fall, which not only reduces the power sales, but also makes it difficult to ensure the reliability index of power supply

the existence of the defects in the above aspects not only increases the workload of maintenance personnel, but also promotes the improper use of maintenance personnel (especially agricultural electricians), and completely loses the protection function of the drop fuse. A short circuit in the circuit can only expand the scope of power failure, bypassing the main circuit breaker of 10kV outgoing line of the substation to trip, resulting in power failure of the whole line

2 selection of drop fuse (switch)

10kV drop fuse is suitable for outdoor places where the ambient air is free of conductive dust, corrosive gas, flammable, explosive and other hazardous environments, and the annual temperature difference ratio is within ± 40 ℃. The selection is based on the two parameters of rated voltage and rated current, that is, the rated voltage of the fuse must match the rated voltage of the protected equipment (line). The rated current of the fuse shall be greater than or equal to the rated current of the melt. The rated current of the melt can be 1.5 ~ 2 times of the rated load current. In addition, the selected fuse shall be checked according to the three-phase short-circuit capacity of the protected system. Ensure that the three-phase short-circuit capacity of the protected system is less than the upper limit of the rated breaking capacity of the fuse, but it must be greater than the lower limit of the influence capacity of the rated breaking superimposed Spring Festival on production. If the rated breaking capacity of the fuse (generally refers to its upper limit) is too large, it is likely that the three-phase short-circuit capacity of the protected system will be less than the lower limit of the rated breaking capacity of the fuse, which will make it difficult to extinguish the arc when the melt melts, and eventually cause accidents such as fuse tube burning and explosion. At present, some power supply units are still at the peak of agricultural transformation. When selecting this kind of fuse, we must strictly control the product quality, protect the qualified equipment, and pay attention to the upper and lower limits of its rated breaking capacity

3 installation of drop out fuse

(1) the melt should be tensioned during installation (so that the melt is subject to about 24.5n tension), otherwise it is easy to cause contact heating

(2) the fuse shall be installed on the cross arm (frame) firmly and reliably without any shaking or shaking

(3) the fuse tube should have an inclination of 25 ° ± 2 ° downward, which doesn't sound high, so that when the melt melts, the fuse tube can fall quickly by its own weight

(4) the fuse should be installed on the cross arm (frame) with a vertical distance of not less than 4m from the ground. The single arm electronic universal testing machine is mainly suitable for relevant experiments on materials below 5000n. If it is installed above the distribution transformer, a horizontal distance of more than 0.5m should be maintained from the outermost contour boundary of the distribution transformer, in case of other accidents caused by the falling of the fuse tube

(5) the length of the fuse tube should be adjusted appropriately. It is required that the duck beak tongue can buckle more than two-thirds of the length of the contact after switching on, so as to avoid the misoperation of falling by itself during operation. The fuse tube should not be pushed against the duck beak, so as to prevent the fuse tube from falling in time after the fuse is fused

(6) the melt used must be the standard product of a regular manufacturer and have a certain mechanical strength. Generally, the melt is required to withstand a tensile force of at least 147n

(7) 10kV drop out fuse is installed outdoors, and the interphase distance is required to be greater than 70cm

4 operation of drop fuse

generally, it is not allowed to operate the drop fuse with load, but only the unloaded equipment (line). However, branch lines of 10kV distribution lines and distribution transformers with rated capacity less than 200KVA in rural areas are allowed to operate with load according to the following requirements:

(1) the operation is carried out by two people (one person is responsible for supervision and one person is responsible for operation), but they must wear qualified insulating gloves, insulating boots and goggles, and operate with qualified insulating rods with matching voltage levels. It is forbidden to operate in lightning or heavy rain

(2) during switching operation, it is generally stipulated that the intermediate phase is pulled off first, then the leeward side phase, and finally the windward side phase. This is because the distribution transformer is changed from three-phase operation to two-phase operation, and the arc spark generated when breaking the intermediate phase is the smallest, which will not cause phase to phase short circuit. The second is to pull off the leeward side phase, because the intermediate phase has been pulled away, and the distance between the leeward side phase and the windward side phase has doubled. Even if there is overvoltage, the possibility of causing phase to phase short circuit is very small. When the windward side phase is finally pulled off, there is only capacitive current to the ground, and the electric spark generated is very slight

(3) when closing, the operation sequence is opposite when opening. First close the windward side phase, then the leeward side phase, and finally the intermediate phase

(4) operating the melting tube is a frequent project. If you don't notice it, it will cause contact burns and poor contact, overheat the contact, and anneal the spring, which will make the contact worse and form a vicious circle. Therefore, when pulling and closing the fusion tube, use appropriate force. After closing, carefully check that the duck's beak tongue can tightly buckle more than two-thirds of the length of the tongue. You can hook the upper duck's beak with a brake rod and press it down a few times, and then gently try to pull it to check whether it is closed well. When closing, it is not in place or firmly closed, and the pressure of the static contact on the fuse is insufficient, which is very likely to cause contact burns or fuse tube falling by itself

5 operation and maintenance management of drop out fuse

(1) in order to make the fuse operate more reliably and safely, in addition to strictly selecting qualified products and accessories (including fuses, etc.) produced by regular manufacturers according to the requirements of the regulations, special attention should be paid to the following items in the operation and maintenance management:

① whether the rated current of the fuse matches the melt and load current value properly, and adjustment must be made if it does not match properly

② each operation of the fuse must be careful and careful, not careless, especially the closing operation, and the dynamic and static contacts must be in good contact

③ the standard melt must be used in the melting tube. It is forbidden to replace the melt with copper wire and aluminum wire, and it is not allowed to bind the contact with copper wire, aluminum wire and iron wire

④ for newly installed or replaced fuses, the acceptance process must be strict, and the quality requirements must be met. The installation angle of the fuse tube must reach a downward angle of about 25 °

⑤ after the melt is fused, it should be replaced with a new melt of the same specification. It is not allowed to connect the fused melt and then put it into the fuse tube for further use

⑥ the fuse should be inspected regularly, at least once a month at night, to check whether there are discharge sparks and poor contact. If there is discharge, there will be a hissing sound, which should be handled as soon as possible

(2) during the spring inspection and power-off maintenance, the fuse should be checked as follows:

① whether the static and moving contacts are consistent, tight and intact, and whether there are burn marks

② whether the rotating part of the fuse is flexible, whether there are abnormalities such as rust and poor rotation, whether the parts are damaged and whether the spring is rusted

③ whether the melt itself has been damaged, and whether it has been heated and stretched too much after long-term power on to become loose and weak

④ whether the arc suppression tube for gas production in the melting tube is burned after multiple actions, whether it is damaged and deformed after exposure to the sun and rain, and whether the length is shortened

⑤ clean the insulator and check whether there is damage, crack or discharge trace. After disassembling the upper and lower leads, use a 2500V megger to test that the insulation resistance should be greater than 300m Ω

⑥ check whether the upper and lower connecting leads of the fuse are loose, discharged and overheated

the defects found in the above items must be carefully repaired and handled. (end)

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