To deal with transformer short-circuit accidents in daily operations, it is necessary to find out the essence of the problem through inspection and testing. When the transformer suffers from a sudden short circuit, both the high and low voltage sides will be subject to a large short-circuit current. In the short time before the circuit breaker can be disconnected, the short-circuit current will generate an electromotive force proportional to the square of the current that will act on the winding of the transformer. Electrodynamic force can be divided into radial force and axial force.

When short-circuited, the radial force acting on the winding will cause the high-voltage winding to be under tension and the low-voltage winding to be under pressure. Since the winding is circular, a circular object is more easily deformed by pressure than by tension, so low-voltage windings are more easily deformed. The axial force generated during a sudden short circuit compresses the winding and causes axial displacement of the high and low voltage windings, and the axial force also acts on the iron core and clamps. Therefore, when the transformer suffers from a sudden short circuit, the low voltage winding and balanced winding are most likely to be deformed, followed by the high and medium voltage windings, iron core and clamps. In addition to checking the main windings, iron core, clamps and other parts after the transformer short-circuit accident, a series of related problems should also be paid attention to during the processing process:
■ Inspection and test of winding
When the transformer is short-circuited, under the action of electromotive force, the winding is simultaneously subjected to various forces such as compression, tension, and bending, and the faults caused by them are relatively concealed and difficult to check and repair. Winding condition.
■ Measurement of Transformer DC Resistance
According to the measured value of the DC resistance of the transformer, the DC resistance unbalance rate of the winding is checked and compared with the previous measured value, which can effectively investigate the damage of the transformer winding. For example, after a transformer short-circuit accident, the C-to-DC resistance of the low-voltage side increased by about 10%. From this, it was judged that there might be a new strand in the winding. Finally, the winding was lifted out for inspection, and it was found that one strand of the C-phase winding was broken.
■ Measurement of transformer winding capacitance
The capacitance of the winding is composed of the winding inter-turn, interlayer and inter-cake capacitance and the winding capacitance. This capacitance is related to the gap between the winding and the core and the ground, the gap between the winding and the core, the gap between turns, layers and cakes. When the winding is deformed, it is generally bent in an "S" shape, which causes the gap distance between the winding and the iron core to become smaller, and the capacitance of the winding to the ground will become larger, and the smaller the gap, the greater the capacitance change, so the winding The capacitance can indirectly reflect the degree of deformation of the winding.
■ Inspection after hood
After the transformer is suspended, if there are melted copper slag, aluminum slag or high-density cable paper fragments inside the transformer, it can be judged that the winding has undergone a large degree of deformation and broken strands. , falling off, equal position of the pressing plate, displacement of the pressing nail, etc. can also judge the damage degree of the winding.
■ Inspection of iron core and clamp
The iron core of the transformer should have sufficient mechanical strength. The mechanical strength of the iron core is guaranteed by the strength of all the clamping parts on the iron core and their connecting parts. When the winding generates electromotive force, the axial force of the winding will be offset by the reaction force of the clip. If the strength of the clip and the pull plate is less than the axial force, the clip, pull plate and winding will be damaged. Therefore, the condition of the iron core, clips, pull plates and their connections should be carefully checked, and the following conditions should be checked:
① Check whether the iron yoke chips on the iron core move up and down.
② The insulation resistance between the core screw and the iron core should be measured to check whether the jacket of the core screw is damaged, and whether the pull plate and the connecting parts of the pull plate are damaged.
③ When the transformer is short-circuited, displacement may occur between the pressure plate and the clamp, causing the ground connection between the pressure plate and the iron yoke on the pressure nail to be broken or over-current burned. Therefore, for the winding pressing plate, in addition to checking the damage of the pressing nail and the pressing plate, it is also necessary to check whether the grounding connection between the winding, the pressing nail and the upper iron yoke is reliable.
■ Analysis of transformer oil and gas
After the transformer suffers a short-circuit impact, a large amount of gas may accumulate in the gas relay. Therefore, after the transformer accident, the gas in the gas relay and the oil inside the transformer can be taken for laboratory analysis to determine the nature of the accident.
■ Matters needing attention in transformer short-circuit fault handling
(1) When replacing the insulating parts, the performance of the insulating parts should be guaranteed
During processing, test the performance of the replaced insulating parts and use them only if they meet the requirements. In particular, attention should be paid to the insulation of the wood block of the lead bracket. The wood block should be placed in the thermal transformer oil at about 80°C for a period of time before installation to ensure the insulation of the wood block.
(2) The transformer insulation test should be carried out after the transformer is filled with oil and rests for 24 hours
Because some damp insulating parts are soaked in hot oil for a long time, the moisture will diffuse to the insulating surface, so the insulation defect cannot be detected after the test after oil filling. For example, a 31.5MVA 110kV transformer low-voltage side replaced a 10kV copper bar bracket block during processing. After the transformer was filled with oil, the test was normal, and the 10kV low-voltage side's insulation resistance to the iron core, clamps and ground was reduced to about 1 MΩ. After the inspection of the hanging hood, it was found that the insulation of the support wooden blocks of the 10 kV copper bar was very low. Therefore, the insulation test should be carried out after the transformer is filled with oil for 24 hours to be more reliable.
(3) Pay attention to its sharp corners when reassembling the iron core
When the iron core is reinstalled on the iron yoke, attention should be paid to the sharp corners of the core chips, and the insulation between the oil passages should be measured in time, especially the sharp corners of the chips at the oil passages should be paid attention to, so as to prevent the multi-point grounding of the iron core caused by the lapping of the chips. For example, for a 120MVA 220kV transformer, when replacing the winding on the low-voltage side and reinstalling the iron yoke, because the sharp corners of the chip were not paid attention to during reassembly, and the insulation between the oil passages was not measured in time, the insulation between the oil passages was measured as 0, it took a long time to find it in the end because the sharp corner of the core chip shorted the oil passage.(4) Replace the winding material with stronger short-circuit resistance
The mechanical strength of the improved structure transformer winding is mainly determined by the following two aspects:
① One is the mechanical strength of the winding determined by the structure of the winding itself;
② The second is the mechanical strength determined by the support on the inner diameter side of the winding, the axial compression structure of the winding, and the manufacturing process of drawing plates and clamps. At present, most transformer manufacturers use semi-hard copper wire or self-adhesive transposition wire to improve the short-circuit resistance of the winding itself, and use better quality cardboard tubes or increase the number of stays to improve the radial force of the winding. And use pull plate or spring pressure nail to improve the ability of winding to bear axial force.
As the technical department of a power transformer manufacturer, when conducting technical demonstrations before signing a transformer sales contract and when replacing transformer windings, the short-circuit resistance capability of the windings should be fully investigated and given sufficient attention.
(5) Drying of the transformer
After the transformer is impacted by a short circuit, it generally takes a long time to carry out maintenance. To prevent the transformer from getting wet, two measures can be taken:
① One is to buckle the cover of the transformer before closing work every day, and use a vacuum pump to evacuate the transformer to remove free water on the surface of the transformer body. When starting work the next day, use dry nitrogen or dry air to release the vacuum. It can be put into operation directly after 24 hours of hot oil circulation after maintenance.
② The second is to take rainproof measures for the transformer after work is over every day. After all the work is completed, the transformer is dried by spraying hot oil. This method generally takes 7 to 10 days.
(6) Other matters that should be paid attention to
After a transformer short-circuit accident occurs, in addition to testing the transformer according to routine items, the nature of the fault should be judged and analyzed based on the test results of transformer oil, gas in the gas relay, winding DC resistance, winding capacitance, and winding deformation measurement, and check The deformation of the winding, the displacement and looseness of the iron core and the clamp, and then determine the treatment plan for the transformer and the preventive measures that should be taken. When the winding is seriously deformed due to a transformer short-circuit accident and needs to be replaced, attention should be paid to the reassembly of the core chip, the drying of all insulating parts, the treatment of the transformer oil, and the overall drying of the transformer.
Reassembly of core chips, drying of all insulating parts, treatment of transformer oil and overall drying of transformers.