There was no oil used in the earlier Transformers. But it was the necessity of using some effective insulating medium in the Transformers. In the year of 1892, the General Electric Company first used insulating oil like Pennsylvania Paraffinic in the Transformer for the purpose.
The Transformer oil serves as the insulating as well as the cooling medium of transformers. Also, it preserves the core and coil assembly and creates a barrier between atmospheric oxygen and cellulose and other materials inside the transformer to prevent them from oxidation.
At the very beginning of using transformer oil, the gassing problem of the oil was ignored. But in 1919 the gassing of the transformer oil during its service was first recognized. Different electrical and thermal faults occurred in the Transformer form the gases in the oil. Then Buchholz relay came into use. The purpose of the Buchholz relay is to collect the gases inside the oil and initiates an alarm or trip of the transformer.
The main problem with this relay is that the gases only accumulate in the relay container when the rate of production of the gases is high enough and when the oil gets saturated with the gases. Because the gases evolved due to falts mostly dissolve in the oil before these reach to the relay. So to analyze the faulty condition of the transformer it became necessary to analyze the dissolved gases in the oil.
The technique used to analyze the gases dissolved in oil by gas chromatography is known as Dissolved Gas Analysis (DGA). DGA was first introduced for transformer oil in the year 1965 in the UK. The success and correctness of detected faults in transformers led a decision to undergo a routing dissolved gas analysis of all 400 KV transformers in the year 1967 in the UK. After that, the DGA detects 90% of the internal transformer faults before any major problem occurs in the transformer.
Types of Transformer Oil
Transformer oil is the mineral hydrocarbon oil obtained from raw petroleum. There are mainly three types of Transformer oil in general use for the purpose. These are paraffin, naphthene, and aromatic types of mineral oil.
The paraffin and naphthene based oil are mostly used for the purpose. Among these two types of oil naphthene based oil is more preferred. Although the oxidation rate of naphthene oil is much more than that of paraffin oil. Because the byproducts created during oxidation are highly soluble in the naphthene oil. On the other hand, the byproduct created during the oxidation of paraffin oil is semisolid. Therefore it penetrates in the bottom of the transformer. As a result, it obstructs the convectional circulation of oil during cooling. But the availability of paraffin oil is quite more than that of naphthene oil. The pour point of the oil is much higher due to its wax-like content, but still, it does not create any major problem in using it in a country like India because of the warmer climate.
On the other hand, naphthene based oil has a very low pour point. That is the reason, naphthene base oil becomes the only means for the purpose in cold climate countries.
Some synthetic insulating oils are also available in the market but we generally avoid to use them as transformer oil. Because these oils contain polychlorinated biphenyls. These are carcinogenic and highly biodegradable. Thus the oil may cause health and environmental hazards.
Composition of Transformer Oil
Transformer oil is nothing but the mineral hydrocarbon oil extracted from raw petroleum. There are some different types of transformer insulating oil available in the market. The basic chemical structures of all the oil are more or less the same. The only differences are in their molecular structure and molecular weight. The most popularly used transformer oil is either paraffin or naphthene or aromatic based.
Formation of Gases in Transformer Oil
Under normal conditions, the transformer oil deteriorates very slowly. Since the formation of different gases due to the deformation of the molecular structure of the transformer oil is quite small. Most of the gases are easily dissolved in the oil. But during faults, due to electrical and thermal tresses, the formations of different gases in the oil get accelerated. These gases are also dissolved in the oil but when there is a sufficient amount of gas formed in the oil it starts accumulated in the Buchholz relay.
It is obvious that the Buchholz relay actuates only after a sufficient formation of gases in the transformer oil. Therefore only observing the behavior of the Buchholz relay is not sufficient to judge the healthy condition of a transformer. This is the reason, we need to analyze the dissolved gas in the oil from time to time to monitor the condition of the transformer.
What is the purpose of transformer oil?
The oil used in different pieces of switchgear equipment like circuit breakers, current transformers, potential transformers, etc. has two main purposes. The oil behaves as the dielectric medium as well as the coolant.
What are the different tests performed on the transformer oil?
There are some essential tests to be performed on an insulating oil before selecting it for the purpose of insulation and coolant. Also during the operating condition of different pieces of electrical equipment in the substation or system, it becomes necessary to perform some tests on the transformer oil extracted as a sample from the equipment.
In India, transformer oil is tested on the basis of IS 335.
Oil has three different basic properties that identify the suitability of the oil for the purpose of insulation and cooling. These properties are physical, electrical, and chemical properties. To confirm these properties, the tests performed on the transformer oil can also be divided into three different categories.
Physical Tests: – specific gravity, viscosity, flash point, pour point, color, interfacial tension.
Chemical Tests: – neutralization number, saponification value, copper strip corrosion, oxidation stability, inorganic chlorides and sulphates, steam emulsion number.
Electrical Tests: – dielectric strength, power factor, resistivity.
Limits of parameters of transformer oil as recommended buy Indian Standard 335.
Appearance: – The color of the oil must be clear and transparent. Also, it should be free from any suspended matters.
Density: – The density of healthy transformer oil should be 0.89 gram per cubic centimeter at the temperature of 29.5 centigrade.
Viscosity: – The kinetic viscosity of the oil should be under 27 CST at 27-degree centigrade.
Interfacial Tension: – That should be 0.04 Newton per meter at 27-degree centigrade.
Flashpoint: – The minimum flash point of the oil should be 140-degree centigrade.
Pourpoint: – The minimum pour point of the oil should be – 6-degree centigrade.
Neutralization Value: – The acidity of the oil is measured by its neutralization value and its maximum value is 0.03 milligram of KOH per gram of oil for new oil and it is 0.05 milligram of KOH per gram of oil for used oil.
Dielectric Strength: – For new untreated oil that should be a minimum 30 KV RMS. And for new treated oil that should be a minimum of 60 KV RMS.
Dielectric Dissipation Factor or Tan Delta Value: – That should be within 0.02% at 90-degree centigrade for new oil. That should be within 2% at 90-degree centigrade for used oil.
Resistivity: – The minimum resistivity of a new transformer oil should be 35X1012 ohm-cm 90 degree centigrade and the same should be 1500XX1012 ohm-cm at 27 degree centigrade. The minimum resistivity of a used transformer oil should be 0.2X1012 ohm-cm 90 degree centigrade and the same should be 2.5XX1012 ohm-cm at 27 degree centigrade.
Sludge Content: – That should be of a maximum of 0.1 percent by weight for new oil and it is 0.5 percent by weight for old and used oil.
Water Content: – The water content of transformer oil should be within 50 PPM.