■ Inspection and test of winding

When the transformer is short circuited, under the action of electrodynamic force, the winding is simultaneously subjected to pressure, tension, bending and other forces, resulting in a strong concealment of the fault, which is not easy to check and repair, so the winding should be checked after the short circuit fault.

■ Measurement of DC resistance of transformer

According to the measured value of the DC resistance of the transformer, the imbalance rate of the DC resistance 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 the short circuit accident of a transformer, the DC resistance in direction C at the low voltage side increased by about 10%, so it was judged that there might be new strands in the winding. Finally, the winding was lifted out for inspection, and it was found that one strand of phase C winding was broken.

■ Measurement of transformer winding capacitance

The capacitance of the winding is composed of winding turn to turn, layer to layer and cake to cake capacitance and winding capacitance. This capacitance is related to the gap between windings and iron core and ground, the gap between windings and iron core, the gap between windings, the gap between layers and the gap between 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 a larger capacitance of the winding to the ground. The smaller the gap, the greater the capacitance change. Therefore, the capacitance of the winding can indirectly reflect the degree of winding deformation.

■ Inspection after lifting the cover

After hanging the cover of the transformer, if it is found that there are molten copper slag, aluminum slag or fragments of high-density cable paper inside the transformer, it can be judged that the winding has undergone a large degree of deformation and strand breakage. In addition, the damage degree of the winding can also be judged from the displacement and fall of the winding cushion block, the position of the pressing plate, the displacement of the pressing pin, etc.

■ Inspection of iron core and clamp

The iron core of the transformer shall have sufficient mechanical strength, which is guaranteed by the strength of all clamping pieces on the iron core and their connectors. When the winding generates electric force, the axial force of the winding will be offset by the reaction force of the clamp. If the strength of the clamp and pull plate is less than the axial force, the clamp, pull plate and winding will be damaged. Therefore, it is necessary to carefully check the condition of iron core, clamp, pull plate and its connectors, and check the following conditions:

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

② The insulation resistance between the core piercing screw and the iron core shall be measured, the core piercing screw jacket shall be checked for damage, and the pull plate and pull plate connector shall be checked for damage.

③ When the transformer is short circuited, there may be displacement between the pressing plate and the clamping piece, which may cause the grounding connection piece of the pressing plate and the iron yoke on the pressing pin to be pulled off or burned by overcurrent. Therefore, for the winding pressing plate, in addition to checking the damage of the pressing pin and pressing plate, check whether the grounding connection between the winding and the pressing pin and the upper yoke is reliable.

■ Analysis of transformer oil and gas

After the transformer suffers 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 in the transformer can be taken for chemical analysis to judge the nature of the accident.

■ Matters needing attention in handling transformer short circuit fault

(1) Ensure the performance of insulating parts when replacing insulating parts

During treatment, the performance of the replaced insulating parts shall be tested, and they can be used only after meeting the requirements. Special attention shall be paid to the insulation of wood block of lead support. The wood block shall be placed in thermal transformer oil at about 80 ℃ for a period of time before installation to ensure the insulation of wood block.

(2) The transformer insulation test shall be carried out after the transformer is oil filled and still for 24 hours

As some damped insulating parts will diffuse to the insulating surface after being soaked in hot oil for a long time, the insulation defects can not be detected after oil injection. For example, a support wood block of 10 kV copper bar was replaced at the low voltage side of a 31.5 MVA 110 kV transformer during treatment. After the transformer was filled with oil, all tests were normal. The insulation resistance of 10 kV 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 wood block of the 10kV copper bar was very low. Therefore, the insulation test shall be carried out reliably 24 hours after the transformer is still filled with oil.

(3) Attention shall be paid to the sharp corners of the iron core during reassembly

When installing the iron core back on the yoke, pay attention to the sharp corner of the iron core chip, and timely measure the insulation between the oil ducts, especially the sharp corner of the chip at the oil duct, to prevent the iron core from multi-point grounding due to chip bonding. For example, for a 220kV transformer with 120MVA, when replacing the winding at the low-voltage side and reinstalling the yoke, the insulation between the oil channels was measured to be 0 after the installation because the chip sharp corner was not noticed during reinstallation and the insulation between the oil channels was not measured in time. It took a long time to find out that the oil channel was shorted at the core chip sharp corner.

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

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

① First, the mechanical strength of the winding is determined by the factors of its own structure;

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 pull plates and clamps. At present, most transformer manufacturers use semi hard copper wire or self-adhesive transposed conductor to improve the anti short circuit capability of the winding itself, use cardboard cylinder with better quality or increase the number of braces to improve the ability of the winding to withstand radial force, and use pull plates or spring studs to improve the ability of the winding group to withstand axial force.

As the technical department of the power transformer manufacturer, during the technical demonstration before the signing of the transformer sales contract and the replacement of the transformer winding, the anti short circuit capability of the winding should be fully investigated and paid enough attention.

(5) Drying of transformer

Since the transformer needs to be overhauled for a long time after being impacted by short circuit, two measures can be taken to prevent the transformer from getting damp:

① First, before the end of each day's work, the transformer shall be covered, and the vacuum pump shall be used to vacuum the transformer to remove the free water on the surface of the transformer body. When the work starts the next day, dry nitrogen or dry air shall be used to remove the vacuum. Generally, the transformer can be directly put into operation after 24 hours of hot oil circulation after maintenance.

② Second, after the completion of the work every day, rain proof measures shall be taken for the transformer. After the work is completed, the transformer shall be dried by hot oil spraying method, which generally takes 7 to 10 days.

(6) Other precautions

After the transformer has a short circuit accident, in addition to testing the transformer according to conventional items, the nature of the fault shall be judged and analyzed by combining the test results of transformer oil, gas relay gas, winding DC resistance, winding capacitance and winding deformation measurement, and the deformation of windings, the displacement and looseness of iron cores and clamps shall be checked, and then the treatment scheme and preventive measures shall be determined for the transformer. When the winding needs to be replaced due to serious deformation caused by transformer short circuit accident, attention shall be paid to the reassembly of iron core chips, drying of all insulating parts, treatment of transformer oil and overall drying of the transformer.

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