What should I do if the transformer is short-circuited suddenly?

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 a sudden short circuit, both the high and low voltage sides will be subject to a large short circuit current. In a short period of time when the circuit breaker is too late to open, the short circuit current will generate an electromotive force proportional to the square of the current, which will act on the windings of the transformer. Electrodynamic force can be divided into radial force and axial force.

During a short circuit, 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. Because the windings are circular, circular objects are more easily deformed by pressure than by tension, so low-voltage windings are more easily deformed. The axial force generated during the sudden short circuit compresses the winding and causes the axial displacement of the high and low voltage windings, and the axial force also acts on the iron core and the clamp. Therefore, when the transformer suffers from a sudden short circuit, the low-voltage windings and balance windings are most likely to be deformed, followed by the high- and medium-voltage windings, iron cores and clips. In addition to checking the main windings, iron cores, clamps and other parts after a transformer short-circuit accident, a series of related issues should also be paid attention to in the process:

■ Inspection and testing of windings

When the transformer is short-circuited, under the action of electric power, the windings are simultaneously subjected to various forces such as compression, tension, bending, etc. The faults caused by them are highly concealed and are not easy to inspect and repair. winding condition.

■ Measurement of transformer DC resistance

According to the measured value of the DC resistance of the transformer, to check the DC resistance unbalance rate of the winding and compare it with the previous measured value, it can effectively investigate the damage of the transformer winding. For example, after a short-circuit accident of a transformer, the DC resistance of the low-voltage side C increased by about 10%. From this, it was determined that the winding may have new strands. 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 Capacity

The capacitance of the winding is composed of winding inter-turn, inter-layer and inter-cake capacitance and winding generating capacitance. This capacitor is related to the gap between the winding and the iron core and the ground, the gap between the winding and the iron core, the gap between the turns of the winding, the gap between the layers and the gap between the cakes. When the winding is deformed, it is generally bent in an shape, which leads to a smaller gap distance between the winding and the iron core, and the capacitance of the winding to the ground will become larger, and the smaller the gap, the larger the capacitance change, so the winding The capacitance can indirectly reflect the degree of deformation of the winding.

■ Inspection behind the hood

After the transformer is suspended, if there is molten copper slag, aluminum slag or fragments of high-density cable paper inside the transformer, it can be judged that the winding has been deformed to a large extent and broken strands, etc. In addition, the displacement from the winding spacer block , falling off, pressure plate equalization, pressure nail displacement, etc. can also judge the degree of damage to the winding.

■ Inspection of iron cores and clamps

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 electric power, 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, the pull plate and the winding will be damaged. Therefore, the condition of the iron core, clips, pull plates and their connecting parts should be carefully checked, and the conditions should be checked:

① Check whether the iron yoke chip on the iron core moves up and down.

② The insulation resistance of the grommet screw and the iron core should be measured, check whether the jacket of the grommet screw is damaged, and check 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 piece of the iron yoke on the pressure plate and the pressure nail to be broken or burnt due to overcurrent. Therefore, for the winding pressure plate, in addition to checking the damage of the pressure nail and the pressure plate, it is also necessary to check whether the ground connection between the winding, the pressure nail and the upper iron yoke is reliable.

■ Analysis of transformer oil and gas

After the transformer is hit by a short circuit, a large amount of gas may accumulate in the gas relay. Therefore, after a transformer accident, the gas in the gas relay and the oil in the transformer can be tested and analyzed 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

Test the performance of the replaced insulating parts during processing, and use them only if they meet the requirements. In particular, attention should be paid to the insulation of the lead bracket wood block, and the wood block should be placed in a thermal transformer oil of 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 when the transformer is still oiled for 24 hours

Since some damp insulating parts are soaked in hot oil for a long time, the water will diffuse to the surface of the insulation, so the test after oil injection often cannot detect insulation defects. For example, the low-voltage side of a 31.5MVA 110kV transformer was replaced with a wooden support block of 10 kV copper bar during treatment. After the transformer was filled with oil, everything was normal, and the insulation resistance of the 10kV low-voltage side to the iron core, clamps and ground was reduced to about 1 MΩ. After inspection of the hood, it was found that the insulation of the wooden block of the support of the 10 kV copper bar was very low. Therefore, the insulation test should be performed more reliably after the transformer has been oiled for 24 hours.

(3) Attention should be paid to the sharp corners of the iron core when it is reassembled

When the iron core is put back on the iron yoke, attention should be paid to the sharp corners of the iron 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, to prevent the iron core from being grounded at multiple points due to the overlapping of the chips. For example, for a 120MVA 220kV transformer, when the winding is replaced on the low-voltage side and the iron yoke is installed, because the sharp corners of the chip were not paid attention to during the reassembly, and the insulation between the oil passages was not measured in time, the insulation between the oil passages was measured after installation as 0, it took a long time to find it because the sharp corner of the iron core chip shorted the oil passage.

(4) Replace the winding material with strong short-circuit resistance

The mechanical strength of the improved structural transformer winding is mainly determined by the following two aspects:

① One is the mechanical strength of the winding, which is 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 the pull plate and the clip. At present, most transformer manufacturers use semi-hard copper wire or self-adhesive transposed wire to improve the short-circuit resistance of the winding, and use a better quality cardboard tube or increase the number of struts to improve the radial force of the winding. And the use of pull plates or spring pressure nails to improve the ability of the winding to receive axial force.

As the technical department of a power transformer manufacturer, when the technical demonstration before signing the transformer sales contract and when the transformer winding is replaced, the short-circuit resistance of the winding should be fully investigated and given sufficient attention.

(5) Drying of transformers

Since the transformer is impacted by a short circuit, it generally takes a long time for maintenance. In order to prevent the transformer from being damp, two measures can be taken:

① One is to buckle the transformer cover and use a vacuum pump to evacuate the transformer to remove the 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. After the maintenance, the hot oil can be directly put into operation after 24 hours of circulation.

② The second is to take rain-proof measures for the transformer after finishing work every day. After the work is completed, use the hot oil spray method to dry the transformer. This method generally takes 7 to 10 days.

(6) Other matters needing attention

After a short-circuit accident occurs in a transformer, in addition to testing the transformer according to conventional items, the test results of transformer oil, gas in the gas relay, winding DC resistance, winding capacitance, and winding deformation should be used to judge and analyze the nature of the fault, 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 to be taken. When the winding is seriously deformed due to the short-circuit accident of the transformer and the winding needs to be replaced, attention should be paid to the reassembly of the iron core chip, the drying of all insulating parts, the treatment of the transformer oil and the overall drying of the transformer.