Discussion on maintenance of on load tap changer o

2022-08-05
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On the maintenance of on load tap changer of power transformer

in view of the on load voltage regulating transformer widely used in Ulanqab power system, several aspects affecting the normal operation of on load tap changer of transformer are put forward, and targeted suggestions are put forward

on load tap changer plays an important role in power system. It can not only stabilize the load center voltage, but also is an indispensable equipment for connecting power, adjusting load flow and improving reactive power distribution. At present, Wulanchabu Power Bureau has 69 power transformers, of which 54 35kV and above transformers are on load voltage regulating transformers

the on load tap changer installed on the on load voltage regulating transformer will inevitably have faults during operation. These faults may be caused by the manufacturer's quality problems or improper operation and maintenance. According to statistics, among the unplanned outages of 220kV and above transformers nationwide in 2004, the number of unplanned outages caused by on load tap changer was 8, accounting for 10.343% of the total unplanned outage time. Therefore, the safe operation and maintenance of transformer on load tap changer is very important

1 basic principle and structure of on load tap changer

on load tap changer is an electrical device that can change the tapping position under excitation. The basic principle of on load tap changer voltage regulation is to change the number of effective turns by switching from one tap to another without interrupting the load current, that is, to change the voltage ratio of the transformer, so as to achieve the purpose of voltage regulation. Therefore, during the operation of on load tap changer, firstly, the continuity of load current should be ensured; Second, it should have good arc breaking performance in the action of switching and tapping

in the process of changing tap changer, the on load tap changer must use resistance to realize transition, so as to limit the circulating current during transition. Resistance type combined on load tap changer is usually used. In practice, the structure of resistance current limiting on load tap changer can be divided into three parts, namely, change-over switch, selector switch and operation structure. Any of these problems will directly affect the normal operation of the transformer

2 problems of on load tap changer in operation

2.1 problem I

2.1.1 the contacts in the transfer switch are heated

frequent voltage regulation will cause serious mechanical wear, electrical corrosion and contact pollution between contacts. Especially for transformers with large load current, the thermal effect of current will weaken the elasticity of springs, reduce the contact pressure between dynamic and static contacts, and increase the contact resistance, It also increases the heat output between the contacts. Heating accelerates the oxidation corrosion and mechanical deformation of the contact surface, and forms a vicious cycle, resulting in the damage of the change-over switch

2.1.2 preventive measures

before maintenance and operation, the DC resistance at each tapping position of the switch shall be tested respectively. During maintenance of the hanging cover, the contact resistance of the contact shall be measured to check whether the contact coating and contact are good. The tap changer shall be rotated several times every year in combination with maintenance or test to remove the influence of oxide film or oil stain and make it in good contact

2.2 problem 2

2.2.1 disconnection and looseness of transition resistance

disconnection and looseness of transition resistance will cause burnout of the whole transformer. If at this stage, as long as the transition resistor is not monopolized in the market and switched with load when it has been burnt out, not only the load current will be interrupted, but also all phase voltages will appear on the fracture of the transition resistor and between the breaking opening of the dynamic and static contacts. This voltage will not only break through the fracture of the resistance, but also generate a strong arc when the dynamic and static contacts are disconnected, resulting in a short circuit between the two tap changers, resulting in a short circuit and burning of the tap section of the high voltage winding. At the same time, the arc rapidly decomposes the oil in the switch oil chamber, producing a large amount of gas. If the safety protection device cannot discharge these gases immediately, the switch will be damaged. The energy of the arc can also burn out the insulating cylinder of the switch, so that the switch cannot be repaired

2.2.2 preventive measures

strengthen the inspection of transition resistance

⑴ before the transformer leaves the factory, before operation and after overhaul, it is necessary to check whether the transition resistance is loose and whether the resistance wire is mechanically damaged, so as to avoid local overheating and burnout during switching

⑵ for on load switches that have been switched for more than 20000 times or operated for more than 2 years, it is necessary to check whether the material of transition resistance becomes brittle, whether the resistance changes value and whether the fastening is loose

⑶ when the transformer is switched at a large current of more than 2 times the rated current during operation, it is necessary to check whether the transition resistance is burnt out

⑷ if the on load switch does not switch, i.e. the main spring of the fast mechanism is tired or broken and does not work, the transmission system is damaged, the fasteners are loose, the machinery is stuck, the limit fails, etc., which makes the switch unable to switch or the switch fails midway, and the switching procedure time is extended beyond the specified value, it is necessary to check whether the transition resistance is burned

2.3 question 3

2.3.1 oil leakage in the oil chamber of the change-over switch

the oil chamber of the change-over switch is an independent oil tank. During operation, the oil in the oil chamber of the change-over switch is absolutely not allowed to enter the transformer body. This is because the change-over switch generates a certain arc during operation, resulting in the deterioration of the oil quality in the oil chamber. This can be tested and provided with data according to relevant domestic standards such as GB, ISO, DIN, ASTM, JIS, etc. the oil can only be used in the oil chamber of the change-over switch, but cannot enter the transformer body. Once entered, it will seriously affect the chromatographic analysis of transformer internal oil, i.e. transformer internal fault judgment. The causes of oil leakage in the oil chamber of the transfer switch are:

⑴ the oil drain valve at the bottom of the oil tank of the on load tap changer is not fastened, resulting in the mixing of oil in the oil tank of the transformer body and the oil tank of the on load tap changer

⑵ the material of the sealing gasket between the two oil tanks is poor or the assembly process is poor. During field treatment, it was found that most of the rubber pads did not work because the size of the rubber pads was improperly selected and there was no margin after compression. In addition, the upper edge of the on load tap changer oil tank is misaligned with the reserved hole of the main oil tank, making it more difficult to seal. In addition, the upper edge of the insulating cylinder of the oil tank of the on load tap changer was fractured, resulting in oil leakage

⑶ the central transmission shaft oil seal is not tight

2.3.2 preventive measures

pay close attention to the oil level in the oil conservator of the tap changer during operation. When the oil level in the oil conservator of the transformer is abnormally increased or decreased, check whether there is oil leakage in the oil chamber of the switch. Oil samples shall be taken from the transformer regularly. If the contents of hydrogen, acetylene and total hydrogen in the chromatographic analysis of the main transformer are found to be abnormally over standard, the oil chamber of the change-over switch shall also be checked for oil leakage for timely treatment

2.4 problem 4

2.4.1 on load tap changer oil quality deterioration

every time the switch is operated, the oil quality deterioration is caused by the arc, and the insulation level of the switch decreases. Transformer oil is the most basic insulating material of tap changer. As the insulation and arc extinguishing medium, it also has the functions of cooling, lubrication and corrosion prevention. During tapping and switching, due to the action of electric arc, the insulating oil in the switch oil chamber is decomposed and the free carbon, hydrogen, acetylene and other gases are separated. Our new production line will realize one feed and one discharge body and oil dirt. The gas will generally be discharged from the insulating oil, but part of the free carbon particles and oil dirt are mixed in the insulating oil, and part of them are accumulated on the surface of the insulating parts of the switch. In addition, a small amount of metal particles sputtered after melting of contact materials also remain on the surface of insulating parts. The increase of these deposits will increase the leakage current, reduce the insulation resistance, and eventually cause the oil to discharge along the insulation surface and damage the switch

2.4.2 preventive measures

"xiaoyaqing said that after running for months or switching for 2000 ~ 4000 times, the oil sample in the switch box should be taken for test. After 5000 ~ 10000 times of switching or when the breakdown voltage of insulating oil is lower than 25kV, the insulating oil of switch box shall be replaced and the surface of insulating parts shall be cleaned

3 conclusion

the operation and maintenance unit shall pay special attention to and strengthen the maintenance and overhaul of the on load tap changer, so as to keep the on load tap changer in good condition and ensure the safe and stable operation of electricity

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