Measures to improve the short-circuit resistance of power transformers

The power transformer is the hub of transmission and distribution of electric energy, and 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 to the safety of the entire system. The reliability of a power transformer is determined by its health status, not only depends on the design and manufacture, structural materials, but also closely related to the overhaul and maintenance. The problem of improving the short-circuit resistance of transformers in power systems is discussed.

July 19, 2022

 An overview of power transformers




The electronic power transformer is mainly realized by using power electronic technology. into a power frequency signal, that is, frequency reduction. By adopting an appropriate control scheme to control the operation of the power electronic device, the electrical energy of one frequency, voltage and waveform can be converted into electrical energy of another frequency, voltage and waveform. Since the volume of the intermediate isolation transformer depends on the saturation magnetic flux density of the iron core material and the maximum allowable temperature rise of the iron core and winding, and the saturation magnetic flux density is inversely proportional to the operating frequency, increasing its operating frequency can improve the utilization of the iron core efficiency, thereby reducing the size of the transformer and increasing its overall efficiency.




2. Measures to improve the short-circuit resistance of power transformers




The safe, economical, reliable operation and output of transformers depend on their own manufacturing quality, operating environment and maintenance quality. This chapter attempts to answer the measures to effectively prevent the sudden failure of the transformer during the operation and maintenance of the transformer.




The power grid is often short-circuited due to lightning strikes, malfunctioning or refusal of relay protection, etc. The strong impact of short-circuit current may damage the transformer, so efforts should be made to improve the short-circuit withstand capability of the transformer from all aspects. The statistical results of transformer short-circuit impact accidents show that the manufacturing reasons account for about 80%, while the operation and maintenance reasons account for only about 10%. The measures related to design and manufacture have been discussed in Chapter 2, and this chapter focuses on the measures that should be taken during operation and maintenance. In the process of operation and maintenance, on the one hand, short-circuit faults should be minimized, thereby reducing the number of times the transformer is impacted; on the other hand, the deformation of the transformer winding should be tested in time to prevent problems before they occur.




(1) Standardize the design and pay attention to the axial compression process of coil manufacturing. When designing, the manufacturer should not only consider reducing the loss and improving the insulation level of the transformer, but also consider improving the mechanical strength and short-circuit fault resistance of the transformer. In terms of manufacturing process, since many transformers use insulating pressure plates, and the high and low voltage coils share a pressure plate, this structure requires a high level of manufacturing technology, and the pads should be densified. Dry a single coil at constant pressure, and measure the height of the coil after compression; after each coil of the same pressure plate is processed by the above process, it is adjusted to the same height, and the hydraulic device is used to apply the specified pressure to the coil during final assembly. Reach the height of design and process requirements. In the general assembly, in addition to paying attention to the compression of the high-voltage coil, special attention should be paid to the control of the compression of the low-voltage coil.




(2) Carry out a short-circuit test on the transformer to prevent it from happening before it happens. The operation reliability of large transformers depends first on its structure and manufacturing process level, and secondly on various tests of the equipment during operation to grasp the working conditions of the equipment in time. To understand the mechanical stability of the transformer, it is possible to improve its weak links through short-circuit tests to ensure that the structural strength of the transformer is designed with certainty.




(3) Use reliable relay protection and automatic reclosing system. The short-circuit accident in the system is an accident that people try to avoid but cannot absolutely avoid, especially the 10KV line is very likely to cause a short-circuit accident due to misoperation, entry of small animals, external force and user responsibility. Therefore, for the transformer that has been put into operation, it should firstly be equipped with a reliable DC power supply for the protection system, and ensure the correctness of the protection action. Combined with the current situation that the external short-circuit strength of the transformer is poor in operation, the unfavorable factors should be seen for the automatic reclosing or forced commissioning after the system short-circuit tripping, otherwise it will sometimes aggravate the damage of the transformer and even lose the possibility of re-repair. . At present, some operating departments have canceled the use of reclosing for near-area overhead lines (such as within 2km) or cable lines according to the probability of whether the short-circuit fault can be eliminated automatically in an instant, or appropriately extend the interval between closings to reduce the problems caused by failure to reclose. The hazard of short-circuit tripping should be carried out as far as possible.




(4) Actively carry out deformation testing and diagnosis of transformer windings. Usually, after the transformer is impacted by the short-circuit fault current, the winding will be partially deformed, and even if it is not damaged immediately, it may leave a serious hidden trouble. First, the insulation distance will change and the solid insulation will be damaged, resulting in partial discharge. When encountering lightning overvoltage, it is possible to have inter-turn and inter-cake breakdown, resulting in sudden insulation accidents. Even under normal operating voltage, insulation breakdown accidents may be caused by the long-term effect of partial discharge.




Therefore, actively carrying out the diagnosis of transformer winding deformation, timely detection of faulty transformers, and planned hood verification and maintenance can not only save a lot of manpower and material resources, but also play an extremely important role in preventing transformer accidents.




The zero and pole distribution of the transfer function H(jw) (that is, the frequency response characteristic) is closely related to the components and connection methods in the two-port network. A large number of experimental research results show 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 change 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 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 requires high quality of personnel, it is not easy to be widely used in production and operation. Therefore, in actual work, the method of judging whether the winding is deformed according to the change of the transformer winding capacitance can be used as a useful supplement to the frequency response method. Especially when the frequency response method does not have the conditions, the working state of the transformer winding can be grasped in time by comparing the accumulated measured capacitance horizontally and vertically, so as to reduce the probability of accidents and ensure the safe and stable operation of the power grid.




(5) Strengthen the inspection in 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, the quality must be strictly controlled, and corresponding measures must be taken to eliminate the hidden dangers found. The operation and maintenance personnel should strengthen the inspection and maintenance and 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 at the outlet and near areas. In order to avoid the short-circuit fault of the system as much as possible, for the transformers that have been put into operation, firstly equip a reliable DC system for the protection system to ensure the correctness of the protection action; The frequency response method test technology measures the condition of the transformer after being impacted by short-circuit tripping, and purposefully checks the hood according to the test results, which can effectively avoid the occurrence of major accidents.




Whether the transformer can withstand various short-circuit currents mainly depends on the structural design and manufacturing process of the transformer, 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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