Power transformers are the hub for transmitting and distributing electric energy and are the core components of power grids. Their reliable operation is not only related to the quality of electric energy for the majority of users, but also to the safety of the entire system. The reliability of power transformers is determined by their health status, which not only depends on design, manufacturing, and structural materials, but is also closely related to overhaul and maintenance. This paper discusses the issue of improving the short-circuit resistance of transformers in power systems.

I. Overview of power transformers

Electronic power transformers are mainly realized by power electronics technology. The basic principle is to convert the power frequency signal into a high-frequency signal through a power electronic circuit on the original side, that is, frequency increase, and then couple it to the secondary side through an intermediate high-frequency isolation transformer, and then restore it to a power frequency signal, that is, frequency reduction. By adopting an appropriate control scheme to control the operation of power electronic devices, electric energy of one frequency, voltage, and waveform can be converted into electric energy of another frequency, voltage, and waveform. Since the volume of the intermediate isolation transformer depends on the saturation flux density of the core material and the maximum allowable temperature rise of the core and winding, and the saturation flux density is inversely proportional to the operating frequency, increasing its operating frequency can increase the utilization rate of the core, thereby reducing the volume of the transformer and improving its overall efficiency.

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

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

The power grid often causes short circuits due to lightning strikes, relay protection misoperation or refusal to operate, and the strong impact of short-circuit current may damage the transformer, so efforts should be made from all aspects to improve the transformer's ability to withstand short circuits. The statistical results of transformer short-circuit impact accidents show that manufacturing reasons account for about 80%, while operation and maintenance reasons account for only about 10%. The measures related to design and manufacturing have been discussed in Chapter 2. This chapter focuses on the measures that should be taken during operation and maintenance. During operation and maintenance, on the one hand, short-circuit faults should be minimized to reduce the number of shocks to the transformer; on the other hand, the deformation of the transformer winding should be tested in time to prevent accidents.

(i) Standardize the design and pay attention to the axial compression process of coil manufacturing. When designing, manufacturers should not only consider reducing the loss of the transformer and improving the insulation level, but also consider improving the mechanical strength and short-circuit fault resistance of the transformer. In terms of manufacturing technology, since many transformers use insulating pressure plates, and high and low voltage coils share a pressure plate, this structure requires a high level of manufacturing technology. The pads should be densified, and the single coil should be dried at constant pressure after the coil is processed, and the height of the coil after compression should be measured; after the coils on the same pressure plate are processed by the above process, they are adjusted to the same height, and the specified pressure is applied to the coil by the oil pressure device during assembly, and finally the height required by the design and process is reached. In the final 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.

(ii) Perform a short-circuit test on the transformer to prevent accidents. The operational reliability of large transformers depends first on their structure and manufacturing process level, and secondly on various tests of the equipment during operation to keep abreast of the equipment's working conditions. To understand the mechanical stability of the transformer, it is possible to improve its weak links by subjecting it to short-circuit tests to ensure that the structural strength of the transformer is well designed.

(III) Use reliable relay protection and automatic reclosing systems. Short-circuit accidents in the system are accidents that people try their best to avoid but cannot absolutely avoid, especially the 10KV line, which is very likely to cause short-circuit accidents due to misoperation, entry of small animals, external forces, and user responsibility. Therefore, for transformers that have been put into operation, a reliable DC power supply for the protection system should be equipped first, and the correctness of the protection action should be guaranteed. Combined with the current situation that the external short-circuit strength of the transformer in operation is poor, 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 degree of damage to the transformer and even lose the possibility of repairing it. At present, some operating departments have cancelled the use of reclosing for nearby overhead lines (such as within 2km) or cable lines, or appropriately extended the interval between reclosing to reduce the harm caused by the failure of reclosing, and should try to test and inspect the transformers that tripped on the short circuit.

(IV) Actively carry out deformation test diagnosis of transformer windings. Usually, after the transformer is hit by the short-circuit fault current, the winding will be partially deformed. Even if it is not immediately damaged, it may leave serious fault hazards. First, the insulation distance will change, the solid insulation will be damaged, and partial discharge will occur. When encountering lightning overvoltage, turn-to-turn and cake-to-cake breakdown may occur, leading to sudden insulation accidents. Even under normal operating voltage, the long-term effect of partial discharge may also cause insulation breakdown accidents.

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

The distribution of zeros and poles of the transfer function H(jw) (i.e., frequency response characteristics) 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 points are usually fewer, and it is less sensitive to changes in distributed capacitance; when the frequency exceeds 1 MHZ, the inductance of the winding is bypassed by the distributed capacitance, and the resonance points will also decrease accordingly, which is less sensitive to changes in inductance. Moreover, 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 general during production operation. Therefore, in actual work, the method of judging whether the winding is deformed based on the change in the capacitance of the transformer winding can be used as a beneficial supplement to the frequency response method. Especially when the frequency response method is not available, the working status of the transformer winding can be timely grasped 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.

(V) 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 must be strictly controlled. Appropriate measures must be taken to eliminate the hidden dangers found. 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 export and near-area short-circuit failures. In order to avoid short-circuit failures in the system as much as possible, for the transformers that have been put into operation, first of all, a reliable DC system for the protection system should be equipped to ensure the correctness of the protection action; secondly, the transformers that have tripped due to short circuit should be tested and inspected as much as possible. The frequency response method test technology can be used to measure the condition of the transformer after the short-circuit tripping impact, and the cover inspection can be carried out purposefully according to the test results, so as to effectively avoid major accidents.

Whether the transformer can withstand various short-circuit currents mainly depends on the transformer structural design and manufacturing process, and has a great relationship with operation management, operating conditions and construction process level. Transformer short-circuit accidents are extremely harmful to the operation of the power grid system. In order to avoid accidents, effective control measures should be taken from multiple aspects to ensure the safe and stable operation of the transformer and the power grid system.

XZT 7111 High Voltage Circuit Breaker Tester