Common problems and solutions in operation of on-load tap changer of transformer:
Common problem 1: leakage of oil in switch oil chamber
The switch oil chamber is separate from the fuel tank. In operation, the oil in the switching switch oil chamber is absolutely not allowed to enter the transformer body, this is because the switching switch operation produces a certain arc, resulting in poor oil quality in the oil chamber, this oil can only be used in the switching switch oil chamber, but not into the transformer body. Once entering will seriously affect the chromatographic analysis of transformer internal oil, that is, transformer internal fault judgment. The reasons for oil leakage in the oil chamber of the switch are as follows:
(1) The oil discharge valve at the bottom of the oil tank of the on-load tap switch is not fastened, resulting in the mixture of oil in the oil tank of the transformer body and the on-load tap switch oil tank.
The sealant pad between the two tanks is poor in material or assembly process. In the field processing, it was found that most of the rubber pad did not work because of improper size selection and no margin after compression. In addition, the upper edge of the on-load tap switch tank and the reserved hole of the body tank are misaligned, making the sealing more difficult. The upper edge of the insulation cylinder of the on-load tap switch oil tank is fractured and there is a gap, resulting in oil leakage.
⑶ The oil seal of the central drive shaft is not strict.
Solution: keep a close watch on the oil level in the tap switch oil storage tank during operation. When the oil level in the transformer oil storage tank is abnormally raised or lowered, check whether the oil leakage occurs in the oil chamber of the switch. The transformer regularly take oil samples, if it is found that the chromatographic analysis of hydrogen, acetylene and total hydrogen content of the main transformer is abnormal, it should also check whether the switch oil chamber is leaking oil, so as to timely processing.
Common problem 2: Switch inside touch hair hot
Frequent voltage regulation, will make the mechanical wear between contacts, electrical corrosion and contact pollution is serious, especially the load current of the transformer, the thermal effect of the current will make the spring elasticity is weak, the contact pressure between dynamic and static contacts is reduced, the contact resistance increases, and the heat between contacts increases. The heat accelerates the corrosion and mechanical deformation of the contact surface, creating a vicious cycle that leads to switch damage.
Treatment method: Before maintenance and operation, the DC resistance of each tap position of the switch should be tested respectively. During maintenance of the hanging cover, the contact resistance should be measured to check whether the contact coating and contact are good. Each year combined with maintenance or test on the tap switch position more than a few times to remove the influence of oxide film or oil, so that the contact is good.
Common problem 3: The transition resistor is disconnected and loose
When the transition resistance is disconnected or loose, the whole transformer will be burnt down. If the transition resistor is switched with load when it has been burned out, not only the load current will be interrupted, but the full phase voltage will appear on the break of the transition resistor and between the break of the static contact. The voltage will not only break down the fracture of the resistance, but also produce a strong arc when the static and static contact is disconnected, resulting in a short circuit between the two tap joints of the transformation, resulting in a short circuit of the wiring section of the high voltage winding. At the same time, the arc will switch oil chamber quickly decomposition, produced a large number of gas. If the safety guard does not immediately expel these gases, it will break the switch. The energy of the arc can also burn out the switch insulation cylinder, making the switch unrepairable.
Treatment method: strengthen the inspection of transition resistance
(1) In the transformer factory and operation before and after overhaul, it is necessary to check whether the fastening of the transition resistance is loose, and whether there is mechanical damage to the resistance wire, so as not to produce local overheating and burn off when switching.
(2) The switching times of more than 20,000 times or more than 2 years of operation of the load switch, it is necessary to check whether the material of the transition resistance becomes brittle, whether the resistance changes value, fastening is loose.
(3) In the operation of the transformer in more than 2 times the rated current of the large current switching, it is necessary to check whether the transition resistance burned.
(4) There has been a load switch does not switch, that is, the fast mechanism of the main spring fatigue or fracture does not work, the transmission system damage, fastener loosening, mechanical stuck, limit failure, so that the switch can not switch and switch failure and switch program time extension exceeds the specified value, must check whether the transition resistance is burned.
Common problem 4: oil deterioration of on-load tap switch
Each time the switch is operated, the insulation level of the switch decreases due to oil deterioration caused by arc. Transformer oil is the basic insulation material of tap switch, as insulation and arc extinguishing medium, but also has the function of cooling, lubrication and corrosion prevention. In tap switch, due to the action of arc, the insulation oil in the switch oil chamber is decomposed, and free carbon, hydrogen, acetylene and other gases and oil scale are precipitated, the gas is generally discharged from the insulation oil, but the free carbon particles and oil scale are mixed in the insulation oil, part of the surface of the switch insulation parts. In addition, there are a small amount of contact material after the sputtering metal particles also stay on the surface of the insulating parts. The buildup of these deposits will increase leakage current, reduce insulation resistance, and eventually cause oil to discharge along the insulation surface, causing switch damage.
Treatment method: After running for 6-12 months or switching for 2000 ~ 4000 times, the oil sample in the switching switch box should be taken for test. After switching for 5000 to 10000 times or when the breakdown voltage of the insulating oil is lower than 25kV, replace the insulating oil in the switch box and clean the surface of the insulating parts.
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