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 balanced windings are more 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, pulling, bending, etc. The faults caused by them are highly concealed and are not easy to inspect and repair. Therefore, after a short-circuit fault, the winding condition should be checked.
transformer
■ Measurement of transformer DC resistance: According to the measured value of the transformer DC resistance, to check the DC resistance unbalance rate of the winding and compare it with the previous measurement value, which 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 judged that the winding may have new strands. 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 winding capacitance 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 "S" 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 after the hood: After the hood of the transformer, 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. , The damage degree of the winding can also be judged from the displacement, falling off of the winding pad, the position of the pressure plate, and the displacement of the pressure nail.
■ Inspection of iron core 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 clamps on the iron core and its 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 conditions of the iron core, clips, pull plates and their connecting parts should be carefully checked, and check the situation: ① Check whether the iron yoke chip on the iron core is moving 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 cause of the accident. nature.
■ Matters needing attention in transformer short-circuit fault handling: (1) When replacing insulating parts, the performance of the insulating parts should be guaranteed, and the performance of the replaced insulating parts should be tested during processing and can be used 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 spread to the surface of the insulation. Therefore, the insulation defect is often checked after oiling. from. 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.
transformer
(3) The sharp corners of the iron core should be paid attention to when reassembling the iron core. When the iron core is re-installed 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 chip from overlapping and causing the core to be grounded at multiple points. 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 because the sharp corner of the iron core chip shorted the oil passage.
(4) Replacing the winding material with strong short-circuit resistance and improving the structure The mechanical strength of the transformer winding is mainly determined by the following two aspects: 1. The mechanical strength of the winding is determined by the factors of the winding's own structure; 2. The inner diameter of the winding The mechanical strength determined by the side support and 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.
transformer
(5) Drying of the transformer, 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) Before finishing work every day, the transformer should be buckled and a vacuum pump should be used to clean the transformer. Vacuuming is carried out to remove free water on the surface of the transformer body. When starting the next day, use dry nitrogen or dry air to release the vacuum. Generally, the transformer can be directly put into operation after 24 hours of hot oil circulation after maintenance. 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 that should be paid attention to, after the short-circuit accident of the transformer, in addition to the test of the transformer according to the conventional items, the test of the transformer oil, the gas in the gas relay, the DC resistance of the winding, the capacitance of the winding, and the measurement of the winding deformation should be combined. As a result, the nature of the fault was judged and analyzed, and the deformation of the winding, the displacement and looseness of the iron core and the clamp were checked, and then the treatment plan for the transformer and the preventive measures to be taken were determined. 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.