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How to improve the short-circuit resistance of power transformers

ZTELEC Insulation Materials | 18-10-29

The power transformer is the hub for transmitting and distributing electrical energy, and is the core component of the power grid. Its reliable operation is not only related to the power quality of the majority of users, but also related to the security level of the entire system. The reliability of a power transformer is determined by its health, not only by design and manufacturing, structural materials, but also closely related to maintenance. How to improve the short-circuit resistance of power transformers has become a problem for many people.

The power grid is often short-circuited due to lightning strikes, relay protection malfunctions or refusal. The strong impact of the short-circuit current may damage the transformer. Therefore, efforts should be made to improve the short-circuit resistance of the transformer from various aspects. The statistical results of transformer short-circuit shock accidents show that 80% of the causes are caused by manufacturing, and only about 10% are caused by operation and maintenance. During the operation and maintenance process, on the one hand, the short-circuit fault should be minimized, thereby reducing the number of times the transformer is subjected to impact; on the other hand, the deformation of the transformer winding should be tested in time to prevent it from happening.

(1) Standard design, pay attention to the axial compression process of coil manufacturing. When designing a manufacturer, in addition to considering the transformer to reduce losses and improve the insulation level, it is also necessary to consider improving the mechanical strength and short-circuit fault resistance of the transformer. In terms of manufacturing process, since many transformers use insulating press plates and high and low voltage coils share a press plate, this structure requires a high level of manufacturing technology, and the pad should be densified, after the coil is processed. The single coil is subjected to constant pressure drying, and the height of the coil after compression is measured; after the respective coils of the same pressure plate are processed by the above process, the coils are adjusted to the same height, and a predetermined pressure is applied to the coil by the hydraulic device during assembly. Meet the height of design and process requirements. In the total assembly, in addition to paying attention to the compression of the high-voltage coil, special attention should be paid to the control of the low-voltage coil compression.

(2) Short-circuit test the transformer to prevent it from happening. The operational reliability of large transformers depends first on the structure and manufacturing process level. Secondly, various tests are carried out on the equipment during the operation, and the working conditions of the equipment are grasped in time. To understand the mechanical stability of the transformer, it is possible to improve the weak link by bearing the short-circuit test to ensure that the structural strength of the transformer is designed with great care.

(3) Use reliable relay protection and automatic reclosing system. Short-circuit accidents in the system are accidents that people try to avoid and cannot be absolutely avoided. In particular, the possibility of short-circuit accidents caused by misoperation, small animal entry, external force and user responsibility of the 10KV line is extremely high. Therefore, for the transformer that has been put into operation, it should first be equipped with a reliable DC power supply for the protection system and ensure the correctness of the protection action. In combination with the current situation of poor external short-circuit strength of the transformer in operation, the automatic coincidence or forced operation after the system short-circuit trip should see its unfavorable factors, otherwise it will sometimes aggravate the damage degree of the transformer and even lose the possibility of repair. . At present, some operating departments have eliminated the use of coincidences for near-area overhead lines (such as within 2km) or cable lines based on the probability of short-circuit faults being instantaneously and automatically eliminated, or appropriately extending the inter-compartment interval to reduce the failure of reclosing. The hazard, and should try to test the short-circuit tripped transformer.

(4) Actively carry out deformation test and diagnosis of transformer windings. Usually, after the transformer is subjected to the short-circuit fault current, the winding will be partially deformed, and even if it is not immediately damaged, it may leave a serious fault. First, the insulation distance will change, and the solid insulation will be damaged, causing partial discharge to occur. When encountering lightning overvoltage, there may be breaks between the turns and between the cakes, resulting in sudden insulation accidents. Even under normal operating voltage, the long-term effects of partial discharge may also cause insulation breakdown accidents.

Therefore, the diagnosis of transformer winding deformation is actively carried out, the transformer with problems is found in time, and the lifting cover verification and overhaul are carried out in a planned manner, which not only saves a lot of manpower and material resources, but also plays an extremely important role in preventing the occurrence of transformer accidents.

The zero and pole distribution of the transfer function H(jw) (ie, the frequency response characteristic) is closely related to the components and connections in the two-port network. A large number of experimental studies have shown that transformer windings usually have more resonance points in the frequency range of 10KZ~1MHZ. When the frequency is lower than 10KHZ, the inductance of the winding plays a major role, the resonance point is usually less, and it is less sensitive to the variation of the distributed capacitance. When the frequency exceeds 1 MHZ, the inductance of the winding is bypassed by the distributed capacitance, and the resonance point is also It will be reduced accordingly, less sensitive to changes in inductance, and as the frequency increases, the stray capacitance of the test loop (lead) will also have a significant impact on the test results.

Because the transformer winding deformation tester is expensive and has high requirements on the quality of personnel, it is not easy to carry out in production and operation. Therefore, in actual work, the method of judging whether the winding is deformed according to the change of the capacitance of the transformer winding can be used as a useful supplement to the frequency response method. Especially in the case that the frequency response method does not have the conditions, the measured capacitance accumulated in the horizontal and vertical directions can be used to grasp the working state of the transformer winding in time, so as to reduce the probability of accidents and ensure the safe and stable operation of the power grid.

(5) Strengthen the inspection during on-site construction and operation and maintenance, and use a reliable short-circuit protection system. When installing the transformer on site, the construction must be carried out in strict accordance with the manufacturer’s instructions and specifications, and the quality should be strictly controlled. Corresponding measures must be taken to eliminate the hidden dangers. Operation and maintenance personnel should strengthen the inspection and maintenance warranty management of the transformer to ensure that the transformer is in good operating condition and take corresponding measures to reduce the probability of short-circuit faults in the exit and near-area. In order to avoid the short-circuit fault of the system as much as possible, the transformer that has been put into operation is first equipped with a reliable DC system for the protection system to ensure the correctness of the protection action. Secondly, the transformer due to the short-circuit trip should be tested as much as possible. The frequency response test technique measures the condition of the transformer after being subjected to a short-circuit trip, and the cover is inspected purposefully according to the test result, so that the occurrence of a major accident can be effectively avoided.

Whether the power transformer can withstand various short-circuit currents mainly depends on the transformer structure design and manufacturing process, and has a great relationship with the operation management, operating conditions and construction technology level. The transformer short-circuit accident is extremely harmful to the operation of the power grid system. To avoid accidents, effective control measures should be taken from various aspects to ensure the safe and stable operation of transformers and power grid systems.

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