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Power Transformer Oil Characteristics

IN GENERAL :‑


The transformer oil basically functions as a liquid dielectric medium and as a coolant in transformer and in switchgears. This is a pure hydrocarbon mineral oil and is characteristic are generally in accordance with IS : 335. The electrical apparatus and equipment operate under conditions of high electrical stress and also are exposed to conditions of ageing within them. The transformer oil acts as an electrical divider between core and the tank and also as a medium to dissipate heat. The safety and safe operation of such equipment together with uninterrupted operation is of paramount importance. The presence of air and the effect of temperature gives rise to slugging and acidity and thereby deteriorates the transformer oil requiring periodic filtration / replenishment.


CHARACTERISTIC 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 is IS : 335- 1983.


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.


Density at 27 0 C


The value should be 0.89 gm/cm3maximum which indicates a mass of substance per unit volume.


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.


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 soluble polar compounds.


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.


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.


Neutralization value


The acidity of neutralization value is the number of milligrams of KOH needed 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 1 mg KOH /gm and should be discarded if the value is above 1 mg KOH/gm.


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.


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.


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.


Specific resistance


Specific resistance in ohmcm 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.

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