When dealing with short-circuit accidents of transformer in daily work, it is necessary to identify the nature of the problem through inspection and testing. When a transformer experiences a sudden short circuit, both the high and low voltage sides will be subjected to a large short-circuit current. In the short time period before the circuit breaker disconnects, an electrodynamic force proportional to the square of the current will act on the winding of the transformer. This electrodynamic force can be divided into radial and axial forces.
During a short circuit, the radial force acting on the winding will cause tension and pressure on the high and low voltage windings, respectively. Because the winding is circular, it is more susceptible to deformation under pressure than under tension. Therefore, the low voltage winding is more prone to deformation. The axial force generated during a sudden short circuit will cause compression of the winding and axial displacement of the high and low voltage windings. The axial force also acts on the core and clamps. Thus, the low voltage winding and the balanced winding are the most prone to deformation, followed by the high and medium voltage windings, the core, and the clamps. In addition to checking the main windings, core, clamps, and other parts after a short circuit accident of transformer, attention should also be paid to a series of related issues:
Inspection and testing of windings
Since a transformer's windings are subjected to multiple forces, such as tension, compression, and bending, under the action of electrodynamic force during a short circuit, the resulting faults are highly concealed and difficult to inspect and repair. Therefore, the condition of the windings should be carefully checked after a short circuit fault.
Measurement of transformer's DC resistance
The measurement of transformer DC resistance can be used to check the imbalance rate of the winding's DC resistance and to compare it with previous measurement values, which can effectively examine the damage to the transformer winding. For example, if the DC resistance of the low voltage side C increases by about 10% after a transformer’s short circuit accident, it can be inferred that there may be a new situation in the winding. As a result, after the winding is hoisted out for inspection, part of the C-phase winding can be found to be broken.
Measurement of winding capacitance of transformer
The capacitance of a winding consists of the capacitance between turns, layers, discs, and the winding generating capacitance. This capacitance is related to the gaps between the winding and the iron core, the winding and the ground, the winding turns, layers, and discs. When the winding deforms, it bends in an "S" shape, which causes the distance between the winding and the iron core to decrease and the capacitance between the winding and the ground to increase. Additionally, the smaller the gap, the greater the change in the capacitance. Therefore, the capacitance of the winding can indirectly reflect the degree of deformation of the winding.
Inspection after hoisting the cover
After the transformer cover is hoisted, if there are melted copper slag, aluminum slag, or fragments of high-density cable paper inside the transformer, it can be judged that the winding has undergone significant deformation and breakage. In addition, the damage to the winding can be inferred from the displacement of the winding pads, detachment of the press plate, and displacement of the press nails.
Inspection of the core and clamps
The core of the transformer should have sufficient mechanical strength, and this strength is ensured by the strength of all clamping elements and their connecting components. When the winding generates electrodynamic force, the axial force of the winding will be counteracted by the reaction force of the clamping elements. If the strength of the clamping elements and pulling plates is less than the axial force, the clamping elements, pulling plates, and winding will be damaged. Therefore, the condition of the core, clamps, pulling plates, and their connecting components should be carefully checked, and the following conditions should be examined:
Check whether the iron yoke chip on the iron core has upward and downward movement.
Measure the insulation resistance between the core-penetrating screw and the iron core, check whether the outer covering of the core-penetrating screw is damaged, and inspect whether the pulling plates and their connecting components are damaged.
When a transformer is short-circuited, the press plate and clamping elements may displace, causing the grounding connection sheet of the press plate to break or burning because of the overcurrent. Therefore, in addition to checking the damage to the press nails and press plates, the reliability of the grounding connection of the winding to the press nails and the upper yoke should be checked.
Analysis of transformer oil and gas
After a transformer is impacted by a short circuit, a large amount of gas may accumulate in the gas relay, so it is possible to analyze the gas in the gas relay and the oil inside the transformer to determine the nature of the accident.
