1、Influence of outlet short circuit on power transformer

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 frequent and serious. Even more than half of the transformer accidents in some areas are directly caused by the impact of short-circuit fault current, and this situation has an increasing trend in many areas. The short-circuit fault at the outlet of transformer will bring very serious losses to people, especially the fault or accident caused by the short-circuit at the low-voltage outlet of transformer, 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-voltage 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 fault rate of single-phase grounding short circuit is high, accounting for more than half of all short-circuit faults, followed by two-phase short circuit faults, and the other two short-circuit faults are less likely to occur, However, the instantaneous short-circuit current generated by three-phase short-circuit is high, so 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 and refusal 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, the pressure nails of the winding, the iron yoke pull plate and pull rod are fastened, and the clamping force of the lead is strengthened, 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 two components: 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. 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 part 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 is, the greater the axial force is. 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.

2、Analysis of transformer winding deformation caused by short circuit fault

After analyzing the transformer with fault or accident, we find that the deformation of transformer winding has become one of the main causes of other faults and accidents. During the operation of power transformer, the winding is seriously deformed and continues to operate, which will lead to power failure and even the burning of the whole transformer. The main reasons are analyzed. The design and manufacturing reasons such as rough winding process and insufficient mechanical structure strength of the winding lead to its poor ability to withstand current impact and resist external mechanical interference force. Therefore, the transformer winding is easy to deform after being affected by internal electric force and external mechanical force, In particular, the short-circuit impulse current and electric force caused by the short-circuit at the outlet of the transformer will instantly distort, deform and even damage the winding.

Electrodynamic force is the main cause of winding deformation. In the winding structure, the high-voltage winding is in the outer layer, and the axial tensile stress and radial expansion stress will loosen the nail at the end of the winding, resulting in the bending deformation of the pull plate and various fasteners and the relaxation of the winding; The medium and low voltage windings in the inner column or middle part are often affected by axial and radial compression forces, resulting in loose fastening nails at the end of the winding, displacement of cushion blocks, inclination of braces, distortion of wire cakes, or large-area concave and upturned phenomena; The ampere turns in the winding tapping area and correction area are unbalanced, which is easy to produce transverse leakage magnetic field. In case of short circuit, the wire cake is easy to deform or damage under the action of electrodynamic force.

Mechanical force is also a major factor affecting winding deformation. Strong external mechanical force will cause the overall displacement and deformation of transformer winding.

3、Remedial measures and technical improvement of short circuit fault of power transformer

Based on the above, in order to prevent winding deformation caused by short-circuit fault and reduce the rate of transformer short-circuit accident, the academic circles have discussed the remedial measures for power transformer short-circuit fault, and put forward some technical improvement measures.

In order to reduce the occurrence of power transformer short-circuit accidents, we must first select the transformer, select the transformer that meets the requirements of short-circuit test, and reasonably determine the capacity and short-circuit impedance of the transformer; Secondly, it is necessary to optimize the operation conditions of the transformer, mainly to improve the insulation level of the power line and reduce the impact and harm of the fault in the near area; The third is to optimize the operation mode of transformer, determine its operation mode according to the accurate accounting of short-circuit current, and take certain protective measures to limit the harm of short-circuit current; Fourth, we should improve the operation management level, avoid short circuit caused by misoperation, and strengthen the spot inspection and maintenance of transformer.

Technically, we should use advanced technology to improve, including winding structure, body structure, core structure and so on.