Generally speaking, the transformer requires less care compared to other electrical equipment. But, as I already stated in some of the earlier technical articles, transformer failures can cause a huge problem in power system, since it’s one of the most critical link and it can take a while to replace if it fails.
Let’s discuss now about what are the main causes of the transformer failures and how to discover them on time //
- Causes of the failure of a transformer
- Types of failures
- Early discovery of the failures
- Internal defects of the transformer
- How to detect internal failures?
To track the cause of the failure is the first step to formulate its solution. The origin of the defects is not simple. Generally, it is the combination of many factors that can be classified in the following way //
- Imperfection on the specifications
- Mistake in the selection of the type of insulation.
- Not appropriate capacity.
- Lack of attention to the conditions in the place of installation (dampness, temperature, dangerous gases, etc)
- Imperfection on the facilities
- Wrong installation.
- Wrong capacity and protection range of lightning rods
- Switch and relay for protection is wrong.
- Imperfections on the operation and maintenance of the equipment
- External conducting parts loose and heating up of the same.
- Deterioration of the insulating oil
- Excessive load or mistakes in the connection of the cables.
- Mistake in the operation, and carelessness in the arrangement of the protection circuits.
- Insufficient inspection of the gaskets and valves.
- Poor maintenance of the accessories. Abnormal voltage
- Normal wear and tear
- Natural disasters
Transformer failures produced by the causes mentioned above, create secondary failures and of a third type, hindering their tracking.
However, the operating conditions in the moment of the failure, the inspection records of the relays of protection of the various parts, as well as the transformer maintenance and the regular inspection, will help to detect the cause in many occasions.
The defects of a transformer can be classified in the following way //
- Internal failures of the transformer: in core and coil
- Dielectric interruption
- Rupture and twist of the winding
- Mistake on the grounding
- Open connection of tap changer
- Insulating oil
- External defects of the transformer: In the tank
- Due to oil leaks in the gasket, valve, or weld cord
- Due to the bushings of the breathers, over pressure valve, thermometers, oil level gauge, etc.
- Defects on the forced cooling fans, Buchholz Relay, exit of the current transformers of the bushings, etc.
It is unnecessary to say that the sooner a failure is detected, the better for the transformer, and it requires a careful and detailed maintenance and inspection. There are procedures made for the regular inspection and of routine.
Some defects are caused by reasons beyond human control, such as sudden transformer failures and defects that develop slowly. Let’s describe each of these non-human related reasons.
3.1 Sudden transformer failures
Most of the dielectric interruptions occur suddenly, especially due to lightning or to an abnormal tension, causing a direct failure. Excessive current by an external short circuit or by a mechanical hit also happen suddenly.
Disturbances by earthquakes and fires can accidentally damage the transformer.
3.2 Defects that develop slowly
3.2.1 Sudden defects are related generally to totally external or foreign factors to the transformer of such form that it is outside of our scope the power to foresee them and to prepare us to face these.
The objective of transformer maintenance and inspection is to discover the defects that occur and that may develop slowly. These defects are described below //
3.2.2 Deformation of the insulation materials and of the transformer windings, due to mechanical blows caused by an external short circuit. The transformer generally is designed and is manufactured to resist the heat and the mechanical blows. However, if it is exposed to frequent and intensive mechanical blows, even a small deformation can be converted into a serious internal defect.
3.2.3 Insulation of the core. There can be poor insulation between the sheets of the core, between the tightening screw of the core and the insulation pipe, etc. The poor insulation cause a short circuit in the magnetic flow, producing a constant short circuit current flow in this place and generating excessive heating up which can lead to serious defects.
3.2.4 Poor insulation due to a harsh operational condition such as excessive load. According to what was mentioned in the instruction manual, the insulation of the transformer deteriorates by the increase in the temperature and this deterioration over the years worsens and is converted into a serious failure when the transformer suffers an excessive load.
3.2.5 Deterioration of the insulation materials such as oil, bushings, etc. due to moisture absorption, to oxidation and to formation of a partial discharge, etc.
3.2.6 Deterioration of the external insulation of the transformer due to wind, snow, salt and dust. This can be prevented with the correct inspection and maintenance.
3.2.7 Defects in the accessories, oil leak, gas leak, etc.
4.1 Defects in the winding
4.1.1 Short circuits – There are short circuits between the turns, between the phases and between the windings. Most short circuits failures are caused by abnormal tensions on the surge arresters and others because of the deterioration of the insulation oil and to the penetration of rain.
Also, some short circuits are caused by the deterioration because of heat, caused by an electromagnetic mechanical force or by an abnormal excessive load.
4.1.2 Breakage of the terminals of the winding – The terminals of the spooled suffer damages by an excess of current (external short circuit, etc) or by a lighting strike. Also, the short circuit accidents of the system that accumulate, cause damages on the supports of the windings, by their repetitive mechanical destructive force, that finally break the terminals.
4.1.3 Short circuit to ground – The impulse voltage or the deterioration of the insulation can cause a short circuit to the grounding of the winding or of its terminal to the core or to the tank. The mentioned defects can be detected easily through an external diagnosis or an electrical monitoring.
4.2 Defects in the core
There are defects due to poor insulation of the tightening screws of the core, or an oil-cooling duct obstructed, that cause excessive heating of the core. The defects on the core develop slowly.
A poor or loose tightening between the core and the clamps of the windings can cause damaging vibrations.
As additional protection, use the different relays that the transformer has to detect and be protected of failures. Next you can find which parts are used to protect the transformer from internal failures:
- Those devices that are attached directly to the transformer, and detect failures mechanically:
- Buchholz relay,
- Pressure rise relay,
- Pressure relief device.
- Those devices that are indirectly joined to the control cabinet which detect failures electrically:
- Differential relay,
- Overcurrent relay,
- Ground current relay.
Reference // Operation and Maintenance for Power Transformers – ABB’s User’s Manual