Causes of power transformer short circuit
(1) At present, the calculation program is based on the idealized model of uniform distribution of leakage magnetic field, the same wire turn diameter and equal phase force. In fact, the leakage magnetic field of transformer is not evenly distributed, which is relatively concentrated in the yoke part, and the mechanical force on the electromagnetic line in this area is also large; At the transposition point, the climbing of transposition conductor will change the transmission direction of force and produce torque; Due to the factor of the elastic modulus of the cushion block and the unequal distribution of the axial cushion block, the alternating force generated by the alternating leakage magnetic field will delay resonance, which is also the fundamental reason why the wire cake at the iron core yoke, transposition and the corresponding parts with voltage regulating tapping deforms first.
(2) Ordinary transposition conductor is adopted, which has poor mechanical strength and is prone to deformation, loose strand and copper exposure when bearing short-circuit mechanical force. When using ordinary transposition conductor, due to large current and steep transposition climbing, this part will produce large torque. At the same time, the wire cake at the two ends of the winding will also produce large torque due to the joint action of amplitude and axial leakage magnetic field, resulting in distortion and deformation. For example, there are 71 transpositions of phase a common winding of Yanggao 500kV transformer. Due to the thick ordinary transposition conductor, 66 transpositions have varying degrees of deformation. In addition, Wujing 1L main transformer is also due to the use of ordinary transposition conductor, and the wire cakes at the two ends of high-voltage winding at the iron core yoke have different overturning and exposed wires.
(3) The influence of temperature on the bending and tensile strength of electromagnetic wire is not considered in the calculation of short-circuit resistance. The short-circuit resistance designed under normal temperature can not reflect the actual operation. According to the test results, the temperature of the electromagnetic wire has an impact on its yield limit? 0.2 has a great impact. With the increase of the temperature of the electromagnetic wire, its bending strength, tensile strength and elongation decrease. The bending tensile strength decreases at 250 than at 50 , and the elongation decreases by more than 40%. For the actual transformer, under the rated load, the average winding temperature can reach 105 and the hottest spot temperature can reach 118 . Generally, the transformer has reclosing process during operation. Therefore, if the short-circuit point cannot disappear for a while, it will bear the second short-circuit impact in a very short time (0.8s). However, due to the sharp increase of winding temperature after the impact of the first short-circuit current, the maximum allowable temperature is 250 according to gbl094. At this time, the short-circuit resistance of the winding has decreased greatly, This is why most short circuit accidents occur after transformer reclosing.
(4) The winding is loosely wound, the transposition is not handled properly, and it is too thin, resulting in the suspension of the electromagnetic wire. From the accident damage location, the deformation is mostly seen at the transposition, especially at the transposition of transposition conductor.(5) The use of flexible conductor is also one of the main reasons for the poor short-circuit resistance of transformer. Due to insufficient understanding of this in the early stage, or difficulties in winding equipment and technology, manufacturers are reluctant to use semi-hard conductors, or there are no requirements in this regard in the design. From the perspective of faulty transformers, they are all soft conductors.(6) The clearance of the suit is too large, resulting in insufficient support on the electromagnetic line, which increases the hidden danger to the anti short circuit ability of the transformer.
(7) The preload acting on each winding or gear is uneven, resulting in the jumping of wire cake during short-circuit impact, resulting in excessive bending stress acting on the electromagnetic line and deformation.(8) The winding turns or wires are not cured, and the short-circuit resistance is poor. None of the windings treated by dipping paint in the early stage was damaged.(9) Improper control of the pre tightening force of the winding causes mutual dislocation of the wires of the ordinary transposition wires.
(10) External short-circuit accidents are frequent. The accumulation effect of electrodynamic force after multiple short-circuit current shocks causes electromagnetic wire softening or internal relative displacement, and finally leads to insulation breakdown.Measures to improve short circuit resistance of power transformer1. Short circuit test shall be conducted on the transformer to prevent accidents.
The operation reliability of large transformer first depends on its structure and manufacturing process level, and then carries out various tests on the equipment during operation to master the working conditions of the equipment in time. To understand the mechanical stability of the transformer, we can improve its weak links through short-circuit test, so as to ensure that we know well about the structural strength design of the transformer.2. Standardize the design and pay attention to the axial compression process of coil manufacturing.In the design, the manufacturer shall 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 pressing plates, and high and low voltage coils share one pressing plate, this structure requires a high level of manufacturing process. The cushion block shall be densified. After the coil is processed, a single coil shall be dried at constant pressure, and the height of the coil after compression shall be measured.
After the above process treatment, each coil of the same pressing plate shall be adjusted to the same height, and the specified pressure shall be applied to the coil with the oil pressure device during the final assembly, so as to finally reach the height required by the design and process. In the general assembly, in addition to the compression of the high-voltage coil, special attention should be paid to the control of the compression of the low-voltage coil.
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