Failure analysis of transformer instability in ladle refining furnace

Failure analysis of transformer instability in ladle refining furnace

In recent years, with the continuous increase of system capacity and single transformer capacity, under the premise of certain transformer impedance, the threat of short-circuit electromagnetic force to the transformer has become more serious. Therefore, in order to ensure the safe and reliable operation of the transformer, we must try to improve the ability of the transformerto withstand short-circuit impact.

Main reason analysis of winding fault

The direct cause of the failure is the loosening and instability of the transformer's axial direction. This type of fault is more common in transformers with impulsive loads. Due to its strong potential for failure, especially the looseness and instability of the inner coil is difficult to detect in advance, which seriously affects the safe and stable operation of the equipment. 

Therefore, it is particularly necessary to conduct in-depth research on transformer instability and how to take targeted measures in transformer manufacturing and operation and maintenance to avoid the recurrence of similar accidents. The brief analysis is as follows:

1) Product design and manufacturing process reasons

Including ampere-turn balance and short-circuit force checking, coil structure selection, insulation material selection and pretreatment, coil winding process quality, coil compression device strength, coil drying assembly pre-tightening control, etc. If the above factors are not well controlled, it will cause insufficient pre-tightening force or failure of the pre-tightening force during operation.

2) The residual (permanent) deformation of the insulating pad is another cause of axial instability

The insulating pad is made of fiber cardboard, which is a compressible material. Under the action of pressure, it will deform. When the pressure is removed, residual (permanent) deformation will be left, that is, the cushion will shrink. The occurrence of residual (permanent) deformation means that the cushion will lose part of its thickness, which will result in a decrease in the axial preload.

Therefore, underthe continuous action of the axial dynamic short-circuit force, the axial preload of the winding will be significantly reduced. According to some information, in the most unfavourable case, the use of ordinary insulating cardboard spacers for windings can reduce the axial preload by about 70%. However, the axial pre-tightening force of windings with pre-compressed insulating cardboard spacers can also be reduced by about 40%.

When the axial preload is reduced to a certain extent, and the axial natural frequency of the winding is close to the frequency of the dynamic short-circuit force, resonance will occur. At this time, the amplitude of the axial dynamic short-circuit force will increase sharply. In the process of short-circuit vibration, the gaps between the pads appeared and disappeared many times, causing the pads and the turn insulation of the wire to produce violent mechanical impacts, which eventually resulted in damage to the turn insulation and the formation of a short-circuit fault between turns. In addition, due to the existence of gaps between the pads, some pads are in a free state and no longer bear axial pressure. Under the impact and vibration of the short-circuit force, the pads are very easy to loosen and shift, resulting in axial instability.

3) Improper use and operation and maintenance are another reason for transformer damage

The electric furnace transformer belongs to the shock load due to the production process. After the new transformer is put into operation for a period of time, under the continuous action of thermal and mechanical stress, the internal stress of the transformer can be released, especially the residual deformation of the insulating pad, and the transformer will inevitably produce a certain looseness. When the prestress of the transformer coil is small to a certain value, the transformer will produce loose and unstable faults. The above-mentioned problems cannot be discovered through oil chromatographic analysis and routine electrical testing.

According to the loose deformation of transformer windings (changes in axial and radial dimensions, body displacement, winding distortion, bulging, etc.), it will cause changes in transformer impedance and distributed inductance. According to this phenomenon, two test methods are commonly used at present, namely the short-circuit impedance method and the frequency response method. However, the above methods have to be performed offline and are not intuitive, so further analysis and judgment are needed.