Power transformer is the main equipment of power system, which is prone to short-circuit fault in long-term operation. Outlet short circuit will cause insulation overheating fault and winding deformation fault, in which the deformation of transformer winding has become the direct cause of other faults and accidents.
Short circuit fault of power transformer mainly refers to the fault caused by short circuit at the outlet of transformer, short circuit to ground between leads or windings in transformer, and short circuit between phases of transformer. According to statistics, the impact of export short-circuit fault on the normal operation of power transformer is the most frequent and serious. Even more than half of transformer accidents in some areas are directly caused by short-circuit fault current impact, and this situation has an increasing trend in many areas. The transformer outlet short-circuit fault will bring very serious losses to people, especially the fault or accident caused by the transformer low-voltage outlet short-circuit, which usually needs to replace some or all of the windings.
The insulation overheating fault caused by short-circuit current is mainly because the short-circuit current of the power transformer may cause dozens of times of the rated value when the short-circuit occurs suddenly. At the same time, it will concentrate through the high and low-voltage windings, resulting in great heat in an instant, resulting in serious heating of the power transformer. Once the overload short-circuit current exceeds the bearing capacity of the power transformer, its thermal stability will deteriorate and the insulation material of the transformer will be seriously damaged, resulting in the breakdown or damage of the engineering transformer in the next year. The outlet short circuit of power transformer mainly includes three-phase short circuit, two-phase short circuit, two-phase grounding short circuit and single-phase grounding short circuit. Among them, the failure rate of single-phase grounding short circuit accounts for more than half of all short circuit faults, followed by two-phase short circuit faults. The other two short circuit faults have a small probability, but the instantaneous short-circuit current generated by three-phase short circuit, Therefore, the factor of three-phase short-circuit current should be fully considered in the design of power transformer.
The winding deformation fault caused by short-circuit electrodynamic force is mainly due to the excessive short-circuit current generated by the power transformer when it is impacted by short-circuit. This situation will even cause the delay action of relay protection of the transformer, resulting in serious deformation or damage to the winding. If the short-circuit current is relatively small, the relay protection can carry out the protection action correctly, and the resulting winding deformation is also very slight. Such slight deformation should be repaired in time. If slight winding deformation is ignored, the position of the cushion block is not restored in time, and the pressure nails, iron yoke pull plates and pull rods of the winding are fastened to strengthen the clamping force of the lead, the power transformer will be damaged due to the accumulation of winding deformation after multiple short-circuit shocks. When the winding is suddenly short circuited, the current carrying conductor is subjected to strong electrodynamic force under the action of magnetic flux leakage. We decompose this electrodynamic force into vertical axis and horizontal axis.
The vertical axis magnetic field causes the winding to generate radial force, while the horizontal axis magnetic field causes the winding to receive axial force. Therefore, when the outlet short circuit occurs in the winding of power transformer, it needs to bear great axial and radial electric force. The axial electric force will compress the winding in the middle. It is a kind of mechanical stress, which greatly affects the insulation characteristics between turns of the winding, damages the insulation between turns of the winding, and the axial force makes the whole winding under tension and generates tensile stress in the conductor. The axial force of the winding mainly has two parts: one is caused by the interaction between the radial component of the magnetic flux leakage bending part at the end of the winding and the current carrying conductor, which acts on the inner and outer windings in the form of pressure; The other part is generated by the interaction between radial flux leakage and current carrying conductor caused by the imbalance of ampere turns inside and outside the winding. This force mainly makes the inner winding squeezed and the outer winding pulled. The greater the ampere turn imbalance, the greater the axial force. Radial electric power mainly expands the whole winding outward, making it lose stability and causing phase to phase insulation damage. If the electric force is too large, it will lead to more serious consequences such as winding distortion or wire fracture.
Based on the above, in order to prevent the winding deformation caused by short-circuit fault and reduce the transformer short-circuit accident rate, the academic circles have discussed the remedial measures for power transformer short-circuit fault, and also put forward some technical improvement measures.
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