1.Functions of oil
The oil in a transformer has several main functions:
► Cooling
► Electrical insulation
► information carrier Besides, there are several secondary functional Properties expected from transformer oil such as compatibility with other equipment, materials, ageing stability. Lack of interference analysis and so on.
Furthermore, the oil shall fulfill all requirements of health, safety and environmental legislation s.
Oil as a cooling medium
Magnetic and electrical fields create losses in a transformer that we categorize as ” no load losses ” and ” load losses “. The energy of these losses is converted in the steel sheet core , the copper Windings and other conductors and parts to so called ” heating losses ” that leads to an increase of temperature in a transformer.
If the transformer is not cooled down, the heat would quickly lead to overheating of the equipment.
Oil as an insulating material
Electrical equipment design is guided by requirements for adequated insulating systems consisting of solid and liquid materials. The liquid portion of an insulating system plays a vital role and must be able to withstand the calculated and designed electrical stresses.
Oil as an information carrier
oil in a transformer not only serves as a cooling and insulating liquid, but also as a solvent that reacts to all normal and abnormal transformer operating conditions , the same as blood in a living organism . Together with the transformer, the oil ages and produces its Owen oxidation products. Moreover, the oil will dissolve some materials through normal aging of the transformers solid insulation materials. Among the degradation products are particles, furanic compounds, water and acids. The oil indicates abnormal transformer condition if it dissolves abnormal quantities of particles, furanic compounds, water and acids. The oil is degraded by both normal and abnormal operation mostly related to higher temperatures that break down oil molecules producing so called “dissolved gases”.
The production rate and distribution of these gases can be analyzed to provide an indication of the transformers condition.
2.Basic chemistry of transformer oil
A mineral transformer oil consists of mainly carbon and hydrogen in molecules with different structures .The letters which indicate the basic structure of a mineral transformer oil are P, N and A.
P for paraffinic structure. This group of molecules can either be straight or branched. The straight type Normal alkanes (N alkanes) are known as waxes.
N indicates naphthenic structure. Molecules of this group are known as sycloalkanes.
Finlay A indicates aromatic structure. All of transformer oil contains aromatics.
Variation of hydrocarbon type composition in transformer oil depends on the feed stuck, the processing type and degree. The oil manufacturer can use more or less refined oil for the intended use.
The raw material used for the production of insulating oil is mainly from naphthenic or paraffinic bases.
3.Transformer oil properties and their significance
Viscosity
The viscosity of oil is important for cooling of the transformer: the lower the viscosity, the greater is the cooling .it is never an advantage to use high viscosity oils, because the higher the viscosity, the worse the cooling properties.
Flash point
The flash point of oil is specified for safety reasons. fEC 60296 specifies the PM (pensky Marten) closed cup method. In ASTM D 3487 the COC (Cleveland Open Cup) is used, which gives a 5 -10 C° higher flashpoint value. The flashpoint depends on the light part of the oil and is extremely sensitive to contaminants form lighter oils such as gas oil or petrol. Even though both methods yield relatively poor reproducibility, the closed cup method is preferred because it provides better repeatability.
Density
In cold climates ,it is important to avoid the occurrence of ice floating in the oil at lower temperatures and to be able to calculate the total weight of a transformer. This can occur when there is free water present in non -energized transformers, which can cause failure during start- up.
Oxidation stability
There are two types of oils at the market, inhibited and uninhibited.
Actually all oils are inhibited – the inhibited ones with synthetic compounds, dominantly hindered phenol (DBPS and DBP) added (radical destroying) and the uninhibited with natural inhibitors (mainly peroxide destroying). The majority of all oils used in the world are inhibited with phenolic inhibitors at different levels.
Water
The water solubility in the oil depends on the temperature and the amount of polar / aromatic molecules in the oil, this amount depends on the refining processes and to what degree they are applied to the oil. Oils with higher water may foam excessively when they are treated in degassing equipment. This is due to the evaporation of water. However real , stable foams can be found in contaminated systems , for example oils contaminated with particles or other liquids incompatible with the oil ,e, g . Silicon oils. As a general rule, clean liquid do not foam.
Electrical breakdown (AC)
The property is very complex and the measured value depends on the particle content, type of particles, water content and the test method used. The common methods for measurement of AC breakdown voltage are 60156 and ASTM D 1816 with these methods the electrodes are spherically or hemi spherically placed at a distance of 2.5 mm and the voltage is increased by 2 KV/s until breakdown occurs. The result is as an average of six tests, due to the low repeatability of each test.
Dielectric dissipation (DDF/ tan delta)
This is a parameter that will always be found in the transformer oil specification. The tan delta depends on the amount of ionisable and polar molecules in the oil, which will make a small contribution to the temperature rise of the oil in service. Well -refined oil always gives a low value for this parameter, but it is very sensitive to contaminants during handling.
Interfacial tension
The interfacial tension test measures the strength of the interface between oil and water. The interfacial tension depends on the polar groups in the oil, while tan delta (90°C, 50 Hz) tells more about the content of ionisable contaminants. New oils should have a value of at least 40 mN/m.
Acidity
In new well- refined transformer oil, the acidity value is obtained in new oil ,this may affect metal parts in the equipment and indicates that poor aging can be expected.
Corrosion
In IEC 60296, this requirement is based on a method where a copper strip is immersed in the oil at 140°C for 19 hours. Its sensitivity is good versus corrosion will take place on sensitive metal parts in electrical equipment.
