In general, a transformer requires less maintenance compared to other electrical equipment; however, a failure can cause a big problem in the power system, since it is an important element that can take some time to replace.
Causes of faults in a transformer
Finding the cause of a transformer failure is the first step in solving the problem. Generally, it is the combination of many factors that can be classified as follows:
1. Imperfections in specifications:
Error in choosing the type of insulation.
Lack of attention to the conditions of the installation site.
2. Imperfections in the installations:
Inappropriate capacity and protection range of the lightning rod.
Inadequate switch and protection relay.
3. Imperfections in operations and maintenance in power transformers:
Loose parts of the external conduit.
Deterioration of the insulating oil.
Excessive load or errors in the connection of the cables.
Errors in operation and negligence in the arrangement of circuit protection.
Insufficient inspection on valves and gaskets.
Poor maintenance of accessories
4. Normal wear.
5. Natural disasters.
Transformer failures due to the aforementioned causes create secondary and even tertiary faults that can be difficult to detect.
However, the operating conditions at the time of the fault, the inspection records of the protection relay of various parts and the regular maintenance of power transformers can help to detect the cause.
Transformer defects can be classified as follows:
1. Internal, core and coil failures:
Open tap changer connection
2. External defects, in the tank:
Due to oil leaks in the joints, valves or weld seam.
Due to the bushings in the breathers, excess pressure in the valves, thermometers, oil level indicators, etc.
Defects in the forced cooling fans, Buchholz relay, bushing current transformer output.
Early fault finding
The faster the fault is found, the better for the transformer. Detailed and careful maintenance and inspection is required.
There are several routine procedures that can be carried out. Some defects are beyond human control, such as those that are sudden or develop gradually. Some of them are listed below:
Sudden transformer failures
Most dielectric interruptions occur from lightning or abnormal voltage, causing direct failure. Excessive current from an external short circuit or mechanical shock are also some of the causes.
Earthquakes and fires can also accidentally damage the transformer.
Defects that develop slowly
- These are related to external factors and it is difficult to anticipate them.
- The deformation of the insulating materials and the transformer windings can be caused by mechanical shocks resulting from an external short circuit. The transformer is designed to withstand heat and shock like this; however, if it is exposed to a large number of them, even a small deformation can turn into a serious internal defect.
- Core insulation. There may be poor insulation between the core sheets, such as the tightening screw and the insulation tube. This can cause a short circuit in the magnetic current, which would result in more of these inflowing and excessive heating.
- Poor insulation due to harsh operating conditions, such as excessive load. According to what the instruction manual says, the insulation of the transformer deteriorates due to the increase in temperature. This is accentuated over time and can become a very serious failure.
- Deterioration of insulating materials, such as oil, bushings, etc., due to moisture absorption, oxidation and the formation of a partial discharge.
- Deterioration of the external insulation of the transformer caused by wind, snow, salt and dust. This can be prevented with the correct inspection and maintenance on power transformers.
- Defects in accessories and gas and oil leaks.
Internal defects in the transformer
- short circuits. These can occur between turns, phases and windings. Most of the short-circuit faults are caused by abnormal voltage on the surge arrester due to the deterioration of the insulating oil and the penetration of rain. Other shorts are due to heat deterioration resulting from electromagnetic mechanical force or excessive load.
- Breakage of the winding terminals. Terminals are damaged by excessive current or lightning. Similarly, short-circuit accidents in the system that accumulate cause damage to the winding support due to its repetitive and destructive mechanical force, which ends up breaking the terminals.
- Short circuit on the floor. Impulse voltage or insulation deterioration can lead to a short circuit to the ground of the winding, core terminal, or tank. These defects can be easily detected through external diagnostics or electrical monitoring.
There are defects due to poor insulation of the core tightening screw or a clogged cooling oil passage that can cause excessive heating in the core. These develop little by little.