Table of Contents
ToggleWhat is Function of Transformer Oil?
The purpose of use of transformer oil is dissipate heat by conduction through the solid insulation to the oil which by conservation effects either alone or aided by forced circulation flows over the cooling surfaces.
The efficiency of this process is essentially dependent upon the oil having low viscosity which falls with increase of temperature. The specification of the oil has been fixed in IS : 335 with amendment form time to time. The revised IS isIS : 335- 1983.
Properties of Transformer Oil
- It should be have a high dielectric strength i.e. not less than 30KV (rms) for the gap 4mm of the electrodes.
- It should be free from moisture contents & water vapour limiting value should not be more.
- Acidity contents should be very in the oil limiting value 0.4mg of KOH/g.
- It should have high flash point i.e. 1600C & lower point as per specification led down (1450C). The temperature at which the oil will ignite & continue burning should be about 2000C.
- The oil should be chemically stable.
- It should be not contain impurities such as sulpher & its compounds to avoid rusting & sludge formation.
- It should posses’ low viscosity.
- Sludge value of the oil after treating should be 1.2% (limiting value is less than 0.1%)
- Density of oil at 200C should be 0.89 gm/cm3.
- Relative permittivity should be 2.2
- The oil should be frequently clear & plane in colour, transparent & free from suspended matter sediments.
- Mineral oil grade B should be used.
- The properties of good transformer & switchgear oil are recommended by IS 335-1963.
- And for maintain the properties IS code of practice No. 1866-1961 to be refer.
Characteristic of transformer oil
(1) Appearance
The oil shall be clear and transparent and free form suspended matter and sediment. Light colour for petroleum products generally indicates degree of refining, whereas cloudy or foggy appearance indicates the presence of moisture. A greenish tinge indicates the presence of copper slats and acrid smell indicates the presence of volatile acids which cause corrosion.
(2) Density at 27 0 C
The value should be 0.89 gm/cm3 maximum which indicates a mass of substance per unit volume.
(3) Kinematic Viscosity at 27 0 C
The viscosity test is a measurement of the average rate at which an oil will flow under specified condition.
(4) Interfacial tension at 27 0 C
The interfacial is a measure of counter-action of polar compounds in an oil which may come form deterioration products or contaminants. The value is expressed in Newton per meter and a high IFT value is an indication that the oil is neither deteriorated not contaminated with solublepolar compounds.
(5) Flash point
The flash point of oil is the lowest temperature at which the vapors from oil will ignite momentarily on application of a flame under test condition. It is relevant to safety transformer and switch gears as regards.
(6) Pour point
It is the lowest temperature expressed as a multiple of 3 0C at which the oil is observed to flow when cooled. In transformer freezing of insulating oil may create trouble in Buchholz relay.
(7) Neutralization value
The acidity of neutralization value is defined as the number of milligrams of KOH required to neutralize completely the acids present in one gram of oil. It is the gauge of extent of deterioration of oil or contamination. Oil should be reconditioned if the value of acidity is 0.5 to 1mg KOH /gm and should be discarded if the value is above 1 mgKOH/gm.
(8) Corrosive sulphar
The test is designed to detect objectionable quantities of free sulphar and corrosive sulphar compounds and the presence of corrosive sulphar compound will result in deterioration of the metal in contact.
(9) Electric strength
It is the voltage at which the electrical failure occurs in a sample of oil tested under prescribed condition . It indicates the presence of free or suspended but not dissolved water and also determines whether oil is in safe operating condition or not.
(10) Dielectric Dissipation factor
Dielectric dissipation factor of an insulation oil is the tangent of the loss angle. Low power factor indicates poor quality of oil and its value is influenced by temperature, voltage and frequency. The oil should be reconditioned if the value is within 0.01 to 0.1 and should be discarded if it is above 0.01.
(11) Specific resistance
Specific resistance in ohm –cm of a liquid is the ratio of D.C. potential gradient in volts /cm paralleling the current flow within the specimens to the current density in ampere sq. cm at a given instant of time and under prescribed condition. The oil should be reconditioned if the value is between 1 x 10 12 and 10 x 10 12 ohm-cm and should be reclaimed or replaced if the value is below 1 x 10 12 ohm-cm at 27 0 Cs.
(12) Oxidation stability
Transformer oil in service undergoes change in physical, chemical and electrical properties and this is called ageing. With ageing beyond certain limit a coating is formed over the oil immersed parts and this is resistant to heat transfer. This is due to formation of acids sludge’s.
(13) Oxidation inhibitor
This is for uninhibited oil . But for inhibited oil treatment of oil with 0.3 % of anti-oxidant by weight will meet the requirement.
(14) Water Content
Water in transformer oil remains in soluble state as well as in free from. Solubility of water in oil is influenced by temperature and condition of oil as oil deteriorates its water solubility increases.