The power transformer is the basic part that provides energy consumption in the power system, and is also an important induction device to ensure the safe operation of electricity. Its composition structure is composed of the primary coil, the secondary coil and the iron core. The principle of electromagnetic induction is used to change the AC voltage. After a long-term technical transformation, the reliability and stability of the entire power supply work have been continuously improved, but there are still hidden hidden dangers. Some transformer devices are prone to short-circuit phenomena due to insufficient short-circuit shock resistance. In order to effectively judge the fault The reason and location of the transformer must be increased research on transformer faults and diagnostic techniques, so as to effectively solve the transformer fault diagnosis efficiency by using corresponding technologies.
The hazard of short circuit of power transformer
1. Impact of inrush current: The sudden short circuit of the transformer will produce a large short-circuit current. Although the duration is short, this hidden danger may have been formed before the main circuit of the transformer has been cut off. The problem that may arise after the formation is the transformer Internal damage, reduced insulation, etc.
2. The influence of electromotive force: when short circuit, the overcurrent will produce great electromotive force, which will affect the stability. In severe cases, it will have a certain impact on the winding of the transformer, such as: deformation of the winding, damage to the insulation strength of the winding, other components It will also be damaged. In severe cases, it may cause electrical safety accidents such as the burning of power transformers.
Causes of power transformer short circuit
1. The current calculation program is based on an idealized model with a uniform distribution of leakage magnetic fields, the same turn diameter, and equal-phase forces. In fact, the leakage magnetic field of the transformer is not uniformly distributed. Relatively concentrated, the electromagnetic force in this area is also subject to a greater mechanical force; the transposition conductor at the transposition will change the force transmission direction due to climbing, and generate torque; due to the factor of the elastic modulus of the pad, the axial pad does not The equidistant distribution will make the alternating force generated by the alternating leakage magnetic field delay resonance, which is the fundamental reason why the wire cake at the core yoke, transposition, and the corresponding part of the voltage regulation tap is first deformed.
2. The use of ordinary transposition wire, poor mechanical strength, easy to deform, loose strands, copper exposed when subjected to short-circuit mechanical force. When using common transposed conductors, due to the large current and steep transposition climbing, this part will generate a large torque, and the wire cake at the two ends of the winding will also be generated due to the combined effect of the amplitude and axial leakage magnetic field. Larger torque causes distortion. For example, there are 71 transpositions in the A-phase common winding of the 500kV transformer of Yanggao. Due to the thicker transposition conductors, 66 of them have different degrees of deformation. In addition, the main transformer of Wujing No. 11 is also due to the use of ordinary transposed conductors, and the two ends of the high-voltage winding wire cake at the core yoke part have different phenomenon of flipping the exposed wire.
3. The short-circuit resistance calculation does not consider the influence of temperature on the bending and tensile strength of the electromagnetic wire. The short-circuit resistance designed at room temperature does not reflect the actual operating conditions. According to the test results, the temperature of the electromagnetic wire limits its yield? 0.2 has a great influence. As the temperature of the electromagnetic wire increases, its bending strength, tensile strength and elongation decrease. At 250℃, the bending tensile strength is lower than that at 50℃, and the elongation decreases by 40%. the above. The actual operating transformer, under the rated load, the average temperature of the winding can reach 105 ℃, the hottest point temperature can reach 118 ℃. Generally, the transformer has a reclosing process during operation. Therefore, if the short-circuit point cannot disappear for a while, it will be subjected to the second short-circuit shock in a very short time (0.8s), but due to the first short-circuit current shock, The temperature of the winding increases sharply. According to the regulations of GBl094, the maximum allowable temperature is 250°C. At this time, the short-circuit resistance of the winding has greatly decreased. This is why most short-circuit accidents occur after the transformer recloses.
4. The winding of the winding is loose, improper transposition treatment, too thin, causing the electromagnetic wire to hang. Judging from the location of the accident damage, the deformation is more common in the transposition, especially the transposition of the transposition conductor.
5. The use of flexible wires is also one of the main reasons for the poor short-circuit resistance of transformers. Due to insufficient understanding of this early, or difficulties in winding equipment and technology, manufacturers are reluctant to use semi-rigid wires or no design requirements at all in this regard. From the perspective of faulty transformers, they are all soft wires.
6. The set gap is too large, resulting in insufficient support acting on the electromagnetic wire, which adds hidden danger to the transformer’s ability to withstand short circuits.
7. The preloading force acting on each winding or each file is not uniform, and the wire cake jumps during short-circuit impact, resulting in excessive bending stress on the electromagnetic wire and deformation.
8. The winding turns or the wires are not cured, and the short circuit resistance is poor. None of the early windings treated by dipping paint were damaged.
9. Improper control of the winding preload force causes the wires of the common transposed wires to be misaligned with each other.
10. Frequent external short-circuit accidents, the cumulative effect of electromotive force after multiple short-circuit current impacts causes the electromagnetic wire to soften or the internal relative displacement, which eventually leads to insulation breakdown.
Improving measures to improve short circuit resistance of power transformers
1. Perform a short-circuit test on the transformer to prevent it from happening.
The operational reliability of large transformers depends firstly on its structure and manufacturing process level, followed by various tests on the equipment during operation, and timely grasp the working conditions of the equipment. To understand the mechanical stability of the transformer, the short circuit test can be used to improve its weak links to ensure that the structural strength of the transformer is well understood.
2. Standard design, pay attention to the axial compression process of coil manufacturing.
In the design of the manufacturer, in addition to reducing the loss of the transformer and improving the insulation level, it is also necessary to consider improving the mechanical strength of the transformer and the ability to withstand short-circuit failure. In terms of manufacturing process, because many transformers use insulating platens, and the high and low voltage coils share a platen, this structure requires a high level of manufacturing process, the pads should be densified, and after the coil is processed Constant pressure drying is performed on a single coil, and the height of the coil after compression is measured.
After the coils of the same pressure plate are processed by the above process, they are adjusted to the same height, and the oil pressure device is used to apply the prescribed pressure to the coil during assembly, and finally reach the height required by the design and process. In the general assembly, in addition to paying attention to the compression of the high-voltage coil, special attention must be paid to the control of the compression of the low-voltage coil.