In generation system, transformer is one of the important and the main parts in high voltage system. The heat and insulate problem are consuming some problem and can cause transformer failure. In order to cope with this problem, liquid dielectric as insulating material due to its function as heat transfer fluid and electrical insulation and able to adapt to the problem. In order to make sure the transformer work in good condition, use the transformer insulating oil or petroleum base mineral oil that fulfils all the requirements, specifications and characteristic of good dielectric.
Good transformer oil acts as a liquid insulation in an electrical power transformer and dissipates heat from the transformer as a cooling medium. The insulator helps preserve the core and windings of the transformer for they dipped in it. Due to the oxidation susceptibility of the cellulose paper insulation made for the both windings, primary and secondary, the transformer oil prevents direct contact of atmospheric oxygen.
Petroleum based mineral oil has been used since 1891 as transformer oil. In addition, the oil is not only used as a liquid insulation, but also as a coolant of the transformer. The petroleum based mineral oils have been used for insulation because of their excellent dielectric properties. This oil has been used since long time ago due to ability to self-cured to the situation leading to partial or total discharge. Dielectric fluid acts as cooling medium and insulation of appliances. The insulating oil fills up pores in fibrous insulation as well as the gaps between the coil conductors and the spacing between windings and the tank. Thus, the dielectric strength of insulation is increased. The disadvantages of mineral oil are non-biodegradable and non-environmentally friendly. The mineral oil is low flash point, low breakdown voltage and difficulty of disposal after usage. Thus, many researches have been carried to find the alternative of liquid insulating material as they are friendly to the environment.
Characteristics of Transformer Oil
• Appearance: The oil shall be clear transparent and free from suspended matter of sediments.
• Density: 0.89gm/cm3, at 29.5C maximum, – at 20C, 0.89589gm/ cm3
• Corrosive: Sulphur, (In terms of classification of copper strip+paper), Not corrosive (1500C for 72 hrs)
• Electric Strength: New Untreated Oil, After treatment, 30 kV minimum (rms),70 kV minimum (rms)
• Oxidation Stability: after 500 Hrs.@ 120C, Max, Total Acidity in mg KOH/gm maximum-0.3, Total sludge per cent by weight maximum-0. 05, DDF at 90C- 0.02.
• Kinematic Viscosity, in CST: at 40 0C, maximum-12, at -20 0C, maximum-1800 sqmm/sec
• Flash Point: 135C minimum, Penskey-Marten (Closed)
• Interfacial Tension: at 27C, minimum, 0.04 Newton/M
• Pour point: maximum – 30C
• Neutralisation Value: Total acidity, mg KOH/gm, maximum,0.01
• Total Sulphur Content % w/w: < 0.15
• Dielectric Dissipation Factor: (Tan delta) at 90 0C 0.002 maximum
• Presence of Anti-Oxidation I n h i b i t o : ( A d d i t i v e ) , Minimum-0.25 per cent, Maximum-0.4 per cent
• Water Content PPM
(maximum-As delivered): < 30 (in bulk) ;< 40 (in drum)
• Poly Chlorinated Biphenyls (PCB) Content: Not detectable, (< 2 PPM)
• Poly Cyclic (PCA) content: maximum 3% BS 2000 part 346
• 2 FAL/ Furans, ppm: < 0.1
• Impulse Breakdown Voltage Test: > 145 kVp ASTM method
Testing of Transformer Oil
The first part of a preventative maintenance program requires establishing when remedial action is necessary. With transformer oil, this is done through testing of the oil. The following 5-part tests are a minimal requirement of a yearly maintenance program.
Dielectric breakdown: Dielectric strength is a measure of the voltage which the oil will conduct. Many contaminants conduct electricity better than oil therefore, lowering the dielectric breakdown voltage.
Neutralisation or Acid Number: Oxidation occurs in the oil causing the build-up of acid which will lead to the formation of sludge. The test indicates the level of acid present in the oil.
Interfacial tension: This test points to the presence of polar compounds, which indicate oxidation contaminates or deterioration from the transformer materials i.e. paint, varnish, paper.
Colour: Indicates the quality, aging, and the presence of contaminants.
Water: Is measured in parts per million. The presence of water will decrease the dielectric breakdown voltage.
Key Players in the Global Transformer Oils Market
• Nynas AB (Sweden)
• Ergon Inc. (U.S.)
• Calumet Specialty Products (U.S.)
• APAR Industries Limited (India)
• PetroChina Company (China)
• Sinopec Group (China)
• Hydrodec Group Plc. (U.K)
• Cargill Inc. (U.S.)
• Engen Petroleum Limited (South Africa)
• Valvoline Inc. (U.S.)
• San Joaquin Refining (U.S.)
• Gandhar Oil Refining (India).
Types of Transformer Oil
There are two main types of transformer oil in use today: Paraffinbased transformer oil and naphthabased transformer oil.
Paraffin-based oil: It is not as easily oxidised as naphtha-based oil, in theory producing less sludge. However, whatever sludge naphthabased oil generates is more easily removed than the sludge from paraffin-based oil, because it is more soluble.
Naphtha-based oil: It does not contain dissolved wax, as does the paraffin-based type. This wax can increase the pour point and potentially cause issues, but in warmer climates where the temperature never gets very low, this is not an issue.
Despite the apparent superiority of naphtha-based oil, paraffin oil remains the most commonly used type of oil in transformers worldwide.
Mineral oils have been used as insulating and coolant material for almost over a century in transformers due to their thermal and insulating features. The increasing demand of high voltage rate and small size for transformers, the development of transformer oil with favourable dielectric and thermal characteristics is extensively required. Therefore, dielectric nanofluids have been produced to meet the above-mentioned necessary attributes. Nanofluids, a well-known and a prominent term now-a-days in dielectric society, have been the subject of huge research work over the past decade. In fact, the term “nanofluid” was first presented by Choi at Argonne National Laboratory of the USA in 1995. A liquid with nanosized particles homogeneously suspended at just a few weight percentage (wt%) is called nanofluid or nano liquid. However, for the scope of high voltage liquid electrical insulation research, the terms “nanofluids” and “nano-liquids” are used mutually to refer to transformer oil or nanoparticle combination for insulating and cooling interest.
The advantages of transformer oil-based nanofluids as compared to transformer oil are given as follows:
• Nanofluids have better AC and impulse breakdown performance as compared to the mineral oils. So, it is favourable to be used in HVAC and HVDC application.
• The AC breakdown strength of transformer oil-based nanofluids is less influenced by moisture as compared to mineral oils. So, it is helpful in improving insulation life and hence, transformer life.
• Nanofluids have better partial discharge characteristics as compared to the mineral oil.
• Nanofluids have a better antiaging characteristic as compared to mineral oil. So, it can improve the operational reliability and lifetime of high voltage transformers.
• Nanofluids have a higher thermal conductivity than the transformer oil and they are helpful in better cooling of transformers.
Dr. Sarat Kumar Sahoo
of Electrical Engineering,
Sadhana Rani Satapathy
of Electrical Engineering,