Maintenance Technology of on Load Tap Changer of Power Transformer

Maintenance technology of on load tap changer of power transformer

1 basic principle and structure of on load tap changer on load tap changer is an electrical device that can change tap position under excitation state. The basic principle of voltage regulation of on load tap changer is to change the number of effective turns, that is, to change the voltage ratio of the transformer, so as to achieve the purpose of voltage regulation by switching from one tap to another without interrupting the load current. Therefore, during the operation of on load tap changer, first, 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, the on load tap changer must use resistance to realize the transition, so as to limit the circulating current during the transition. Resistance 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 switching switch, selector switch and operation structure. Any of these problems will directly affect the normal operation of the transformer.

2 problems in the operation of on load tap changer 2.1 problem 1 2.1.1 frequent voltage regulation of contact heating in the change-over switch 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 spring, reduce the contact pressure between dynamic and static contacts and increase the contact resistance, It also increases the heat generation 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 of 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. Rotate each gear position of the tap changer several times a 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 2.2.1 the disconnection and looseness of transition resistance will cause the whole transformer to burn out. If the transition resistance is switched with load when it is burnt out, not only the load current will be interrupted, but also all phase voltages will appear on the fracture of the transition resistance and between the breaking openings of dynamic and static contacts.

This voltage will not only break through the fracture of the resistance, but also produce a strong arc when the dynamic and static contacts are disconnected, resulting in a short circuit between the transformed two taps and a short circuit burning of the tap section of the high-voltage winding. At the same time, the arc quickly decomposes the oil in the switch oil chamber and produces 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. (1) before the transformer leaves the factory, before operation and after overhaul, check whether the fastening of transition resistance is loose and whether the resistance wire is mechanically damaged, so as to avoid local overheating and burning during switching. (2) for on load switches with switching times of more than 20000 times or operating for more than 2 years, check whether the material of transition resistance becomes brittle, whether the resistance changes value and whether the fastening is loose. (3) when the transformer is switched at a large current of more than twice the rated current during operation, check whether the transition resistance is burned. (4) when the on load switch does not switch, that is, the main spring of the fast mechanism is tired or broken, the transmission system is damaged, the fasteners are loose, the machinery is stuck, the limit failure, etc., so that the switch cannot be switched, the switch fails halfway, and the time extension of the switching procedure exceeds the specified value, it is necessary to check whether the transition resistance is burned. 2.3 question 3   2.3.1 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, because a certain arc is generated during the operation of the change-over switch, resulting in the deterioration of the oil quality in the oil chamber.

This 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, that is, the judgment of transformer internal fault. The causes of oil leakage in the oil chamber of the transfer switch are as follows: (1) 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. (2) the sealing gasket between the two oil tanks has poor material or poor assembly process. During on-site treatment, it was found that most of the rubber pads did not work due to improper selection of rubber pad size and no allowance 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 on load tap changer oil tank was fractured, resulting in oil leakage. (3) the central transmission shaft oil seal is not tight. 2.3.2 preventive measures     Pay close attention to the oil level of the tap changer oil conservator during operation.

When the oil level of the transformer oil conservator is abnormally increased or decreased, check whether the oil chamber of the switch leaks oil. Take oil samples from the transformer regularly. If the contents of hydrogen, acetylene and total hydrogen in the chromatographic analysis of the main transformer exceed the standard, check whether the oil chamber of the change-over switch leaks oil for timely treatment. 2.4 question 4 2.4.1 the oil quality of on load tap changer is deteriorated. Each time the switch is operated, the oil quality is deteriorated due to arc, and the insulation level of the switch decreases. Transformer oil is the most basic insulating material of tap changer.

As 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 free carbon, hydrogen, acetylene and other gases and oil dirt are separated. 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 accumulate on the surface of the insulating parts of the switch. In addition, a small amount of metal particles sputtered after melting the contact material also remain on the surface of the insulator. The increase of these deposits will increase the leakage current and reduce the insulation resistance, which will eventually lead to oil discharge along the insulation surface and damage the switch. 2.4.2 preventive measures after 6-12 months of operation or 2000-4000 times of switching, the oil sample in the switch box shall 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.