First, the impact of the export short circuit on the power transformer
The short circuit fault of power transformer mainly refers to the short circuit at the outlet of the transformer, the short circuit between the lead or winding of the transformer and the short circuit between the phases of the transformer. According to statistics, the impact of export short-circuit fault on the normal operation of power transformers is the most frequent and the most serious, and even more than half of transformer accidents in some areas are directly caused by the impact of short-circuit fault current, and this situation has a growing trend in many areas. Transformer outlet short circuit fault will bring very serious losses to people, especially the fault or accident caused by transformer low-voltage outlet short circuit, usually need to replace part or all of the windings.
Short-circuit current causes insulation overheating fault, mainly because the power transformer in the sudden short-circuit, may lead to the rated value of tens of times of short-circuit current through the high and low voltage winding at the same time, so that the instant great heat, resulting in serious heating of the power transformer. Once the overload short-circuit current exceeds the capacity of the power transformer, its thermal stability deteriorates, and the insulation material of the transformer is seriously damaged, thus causing the breakdown or damage of the engineering transformer in the next year. Power transformer main export short circuit of three phase short circuit, two phase short circuit, two phase short circuit and several main forms such as one-phase ground fault, among them, the highest failure rate of one-phase ground fault, more than half of all fault, two phase short circuit fault, the other two were less likely to short-circuit fault happens, but three phase short circuit instantaneous short circuit current value resulting from the highest, Therefore, the three-phase short-circuit current should be fully considered in the design of power transformer.
Winding deformation fault caused by short circuit electrical power is mainly due to the power transformer in short circuit impact produced too large short circuit current, this situation will even cause the transformer relay protection delay action rejection, so that the winding has serious deformation or damage. If the short circuit current is relatively small, then the relay protection can correctly protect the action, so the winding deformation is very slight, the slight deformation of this kind of timely maintenance. If the slight winding deformation is ignored, the position of the pad block is not recovered in time, the windings of the pressure nail and iron yoke plate, rod fastening, strengthen the clamping force of the lead, after many short circuit impact, the power transformer will be damaged by the accumulation of winding deformation. When the winding is short-circuited suddenly, the current-carrying wire is subjected to strong electrodynamic action under the action of magnetic leakage, which is decomposed into two components of longitudinal axis and transverse axis for study. The longitudinal magnetic field causes the winding to produce radial force, while the transverse magnetic field causes the winding to receive axial force. Therefore, when the winding of the power transformer is short-circuited at the outlet, it needs to bear a lot of axial and radial electric power. Axial electrical force will compress the winding in the middle, it is a kind of mechanical stress, which greatly affects the insulation characteristics between the turns of the winding and damages the insulation between the turns of the winding, and the axial force makes the whole winding subjected to tension and produces tensile stress in the wire. The axial force of the winding is mainly composed of two parts: one is caused by the interaction between the radial component of the bending part at the end of the winding and the current-carrying conductor, acting on the inner and outer windings in the form of pressure; The other part is generated by the interaction between radial magnetic leakage caused by the imbalance between the inner and outer ampere-turns of the winding and the current-carrying conductor. This force mainly causes the inner winding to be squeezed and the outer winding to be pulled. And the greater the imbalance of the ampere-turns is, the greater the axial force will be. Radial electric power mainly expands the whole winding outwards, making it lose stability and causing insulation damage between phases. If this electrical force is too large, it will lead to winding distortion or wire fracture and other serious consequences.
Second, analysis of transformer winding deformation characteristics caused by short circuit fault
It is found that the deformation of transformer windings has become one of the main causes of other faults and accidents. In the operation process of power transformer, its windings appear serious deformation and continue to run, will cause power failure, even lead to the whole transformer burn. Analysis of the main reasons, winding winding process rough, mechanical structure strength is insufficient and other design and manufacturing reasons lead to its ability to withstand current impact and resistance to external mechanical interference force is poor, so the transformer winding is easy to deformation after being affected by internal electrical force and external mechanical force. In particular, the short-circuit shock current and electric force caused by the short circuit at the transformer outlet will make the windings instantly distorted or even damaged.
Electrical force is the main reason 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 pressure nails at the end of the winding, resulting in bending deformation of the tension plate and various fasteners, and winding relaxation. In the inner column or the middle part of the middle and low voltage winding is often affected by axial and radial compression force, so that the winding end fastening pin loose, pad block displacement, support tilt, line cake distortion or appear a large area of concave and upwarped phenomenon; The ampere-turns in the splicing area and the recessing area are not balanced, which is easy to produce transverse magnetic leakage, and the wire cake is easy to be deformed or damaged under the action of electric force in short circuit.
Mechanical force is also a major factor affecting the winding deformation, strong external mechanical force will cause the overall displacement deformation of the transformer winding.
Third, power transformer short circuit fault remedial measures and technical improvement
Based on the above, in order to prevent winding deformation caused by short circuit fault and reduce transformer short circuit accident rate, the academic circle has discussed the remedial measures for power transformer short circuit fault, and also put forward some technical improvement measures.
In order to reduce the occurrence of power transformer short circuit accident, first of all, we must do a good job in the selection of transformer, choose the transformer in accordance with the requirements of short circuit test, reasonable determination of transformer capacity and short circuit impedance; Second is to optimize the operating conditions of transformers, mainly to improve the insulation level of power lines, reduce the influence and harm of faults near the area; The third is to optimize the operation mode of the transformer, according to the precise accounting of the short-circuit current to determine its operation mode, while taking certain protection measures to limit the harm of the short-circuit current; Fourth, we should improve the level of operation management, avoid short-circuit caused by misoperation, and strengthen transformer point inspection and maintenance.
In terms of technology, advanced technology should be used to improve, including winding structure, body structure, core structure and so on.
