Cause Analysis of Short Circuit Fault of Power Transformer

Due to the short circuit of the power transformer outlet, there are many reasons for the internal faults and accidents of the transformer. It is also complicated. It is related to the structural design, the quality of the raw materials, the process level, the operating conditions and other factors, but the selection of the electromagnetic wire is the key. From recent years, the electromagnetic wire selected by the anatomical transformation based on the static theoretical design of the transformer has a large difference from the stress acting on the electromagnetic wire during actual operation.

(1) At present, the calculation procedures of various manufacturers are based on an idealized model such as uniform distribution of leakage magnetic field, same diameter of turns, and equal phase force. In fact, the leakage magnetic field of the transformer is not evenly distributed. In the relatively concentrated part of the iron yoke, the mechanical force of the electromagnetic wire in this area is also large; the transposition wire is in the transposition, because the climbing will change the direction of the force transmission, and the torque is generated; due to the factor of the elastic modulus of the block, the axis If the spacers are not equidistantly distributed, the alternating force generated by the alternating leakage magnetic field will be delayed and resonated. This is why the wire cake at the corresponding part of the core yoke, the transposition, and the voltage regulation tap is first deformed. The root cause.

(2) The effect of temperature on the bending and tensile strength of the electromagnetic wire is not considered in the calculation of the short-circuit resistance. The short-circuit resistance designed according to normal temperature can not reflect the actual operation. According to the test results, the temperature of the electromagnetic wire has a great influence on its yield limit of 0.2. With the increase of the temperature of the electromagnetic wire, its bending, tensile strength and elongation are both Decreased, the flexural tensile strength at 250 ° C is lower than that at 50 ° C, and the elongation is reduced by more than 40%. In the actual operation of the transformer, under the rated load, the average winding temperature can reach 105 ° C, and the hottest temperature can reach 118 ° C. Generally, the transformer has a reclosing process when it is running. 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 after being shocked by the first short-circuit current, The winding temperature increases sharply. According to the provisions of GBl094, the maximum allowable temperature is 250 °C. At this time, the short-circuit resistance of the winding has been greatly reduced. This is why the short-circuit accident occurs after the transformer is reclosed.

(3) The use of ordinary transposition conductors, the mechanical strength is poor, and it is prone to deformation, stranding and copper exposure when subjected to short-circuit mechanical force. When the ordinary transposition wire is used, due to the large current, the transposition climbing steeply, the part will generate a large torque, and the wire cake at the two ends of the winding will also be generated due to the joint action of the axial direction and the axial leakage magnetic field. Larger torque, resulting in distortion. For example, the A-phase common winding of the Yanggao 500kV transformer has 71 transpositions. Due to the use of thicker ordinary transposition conductors, 66 of them have different degrees of deformation. In addition, the main change of Wusong No. 1l is also due to the use of ordinary transposition conductors, and the high-voltage winding two-end line cake in the core yoke portion has different flipping lines.

(4) The use of flexible wires is also one of the main reasons for the poor resistance of transformers to short circuits. Due to lack of understanding in the early days, or difficulties in winding equipment and technology, manufacturers are reluctant to use semi-rigid wires or design without such requirements. From the perspective of faulty transformers, they are all soft wires.

(5) The winding of the winding is loose, the misalignment is improperly handled, and it is too thin, causing the electromagnetic wire to hang. From the point of view of the accident damage, the deformation is often seen at the position of the change, especially the transposition of the transposed conductor.

(6) The winding wire or the wire is not cured, and the short circuit resistance is poor. None of the early immersion treated windings were damaged.

(7) Improper control of the pre-tightening force of the winding causes the wires of the ordinary transposed conductor to be misaligned with each other.

(8) If the gap of the set is too large, the support acting on the electromagnetic wire is not enough, which increases the hidden danger of the short-circuit resistance of the transformer.

(9) The pre-tightening force acting on each winding or each gear is uneven, and the jump of the wire cake is caused by the short-circuit impact, so that the bending stress acting on the electromagnetic wire is excessively deformed.

(10) External short-circuit accidents are frequent. The accumulation effect of electric power after multiple short-circuit current surges causes the electromagnetic wire to soften or internal relative displacement, eventually leading to insulation breakdown.