In daily operations to deal with transformer short circuit accidents, are to find out the essence of the problem through inspection and test. Transformer in the sudden short circuit, high and low voltage side will be subject to a lot of short circuit current, in the short time of the circuit breaker too late to break off, short circuit current and current square proportional to the electrical force will act on the transformer winding, the electrical force can be divided into radial force and axial force.
In short circuit, the radial force acting on the winding will make the high voltage winding under tension, low voltage winding under pressure. Because the winding is round, a round object is easier to deform under pressure than under tension, so the low-voltage winding is easier to deform. The axial force generated in the sudden short circuit compresses the windings and makes the high and low voltage windings have axial displacement. The axial force also acts on the iron core and clamp. Therefore, transformer in the sudden short circuit, the most prone to deformation is the low-voltage winding and balance winding, and then the high voltage winding, iron core and clamp.
In addition to checking the main winding, core, clamp and other parts after transformer short circuit accident, a series of related problems should also be paid attention to in the process of treatment:
Because the transformer short circuit, under the action of electric power, winding is also under the action of pressure, pull, bending and other forces, the fault caused by the hidden is strong, is not easy to check and repair, so the short circuit fault should focus on checking the winding situation.
According to the measured value of dc resistance of the transformer, the unbalance rate of dc resistance of the winding is checked and compared with the previous measured value, which can effectively inspect the damage of the transformer winding. For example, after the short-circuit accident of a transformer, the c-dc resistance of the low-voltage side increased by about 10%, so it was judged that the winding might have new shares. Finally, the winding was hung out for inspection, and it was found that the C-phase winding was broken by 1 strand.
The capacitance of the winding is composed of the capacitance between turns, layers and cakes of the winding and the generating capacity of the winding. The capacitance and winding are related to the gap between the core and the ground, the gap between the winding and the 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 curved in the shape of "S", which leads to the gap distance between the winding and the iron core becoming smaller, and the electrical capacity of the winding to the ground will become larger, and the smaller the gap is, the greater the change of electrical capacity, so the electrical capacity of the winding can indirectly reflect the degree of deformation of the winding.
Transformer core should have enough mechanical strength, the mechanical strength of the core is by the strength of all the clamping parts on the core and its connector to ensure. When the winding produces electrical force, the axial force of the winding will be offset by the reaction force of the clamp, if the strength of the clamp and the tension plate is less than the axial force, the clamp, the tension plate and the winding will be damaged.
Therefore, it is necessary to carefully check the condition of the iron core, clamps, drawplates and their connectors, and check the situation:
① Check whether the yoke chip on the iron core moves up and down.
② The insulation resistance of the core threaded screw and the iron core should be measured, and the core threaded screw coat should be checked for damage, and the pull plate and the pull plate connector should be checked for damage.
③ In the transformer short circuit, the pressure plate and the clamp between the displacement may occur, so that the pressure plate and the pressure nail on the iron yoke of the ground connection piece broken or overcurrent burn loss. Therefore, for the winding pressure plate, in addition to checking the damage of the pressure nail, the pressure plate, but also check whether the grounding connection between the winding and the pressure nail and the iron yoke is reliable.
After the transformer is hit by short circuit, 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 in the transformer can be analyzed to determine the nature of the accident.
█ Precautions for troubleshooting transformer short circuit faults
（1） When replacing insulating parts, the performance of insulating parts should be guaranteed
During processing, test the performance of the insulating parts to be replaced, and meet the requirements before use. In particular, attention should be paid to the insulation of the lead support wood block. The wood block should be placed in the heat transformer oil at about 80℃ for a period of time before installation to ensure the insulation of the wood block.
(2) Transformer insulation test should be carried out in the transformer oil static 24 hours
Because some damp insulation in hot oil soaked for a long time, moisture will spread to the surface of the insulation, so the test after oil is often insulation defects can not be checked out. For example, a 31.5mVA 110kV transformer's low-voltage side replaced a support block of 10kV copper bar during processing. After the transformer was oiling, all the tests were normal, and the insulation resistance of 10kV low-voltage side to iron core, clamp and ground was reduced to about 1 M ω. After the inspection of the hanging cover, it was found that the insulation of the support block of the 10 kV copper bar was very low. Therefore, insulation test should be more reliable after 24 hours of static transformer oil filling.
(3) Replace the winding material with strong short-circuit resistance
The mechanical strength of the improved structure transformer windings is mainly determined by the following two aspects:
① The winding mechanical strength determined by its own structure;
② The second is the winding inner diameter side of the support and winding axial compression structure and drawing plate, clamp and other manufacturing processes determined by the mechanical strength. At present, most transformer manufacturers use semi-hard copper wire or self-adhesive transposition wire to improve the winding's own short-circuit resistance, use better quality cardboard tube or increase the number of support to improve the winding's radial force capacity, and use tension plate or spring nail to improve the winding's axial force capacity.
As the technical department of the power transformer manufacturer, the anti-short-circuit ability of the windings should be fully investigated and paid enough attention to in the technical demonstration before signing the transformer sales contract and when the transformer windings are replaced.
(4) drying of transformer
Because the transformer is affected by short circuit impact, it generally needs a long time to repair, in order to prevent the transformer from being damp, two measures can be taken:
① One is before the end of each day, the transformer buckle cover, the use of vacuum pump to vacuum the transformer, in order to extract the free water on the surface of the transformer body, the next day when the start, the use of dry nitrogen or dry air to remove the vacuum, the general transformer in the maintenance of hot oil circulation 24 hours can be directly put into operation.
②The second is every day after work, the transformer to take rain protection measures, after the completion of the work, the transformer using hot oil spray method for drying, this method generally takes 7 to 10 days.
(5) Other matters needing attention
After the transformer short circuit accident, besides according to the conventional project was carried out on the transformer test, should focus on combining with gas relay in transformer oil, gas, winding dc resistance, winding, winding deformation measurement of capacitance test results to judge the nature of the failure analysis, and examine the deformation of the winding and iron core and displacement and loosening of a situation, Then determine the transformer treatment scheme and preventive measures should be taken. When the winding needs to be replaced due to serious deformation caused by transformer short circuit accident, attention should be paid to the reassembly of iron core chip, drying of all insulating parts, processing of transformer oil and overall drying of transformer.
