Type Tests and Routine Tests of Transformer Oil and Their Purpose

Type Tests:

• Specific Gravity of Oil at 20 deg. C
• This test has special significance when the transformer is operated in very low temperature zone.
• The max. value of density is fixed at 27 deg. C ensures that water in the form of ice remains at the bottom and does not tend to float on the oil up to a temperature of about 10 deg. C.
• Oxidation Stability
• This test is the measure of  neutralization  value and sludge after oil is aged by simulating the actual service conditions of a transformer. 
• The oxidation stability test is very important for new oil but not for oil in service and shows presence of natural inhibitors which impart anti oxidation characteristics to oil.
• Sludge Value should be less than equal to 0.10
• Acidity after oxidation KOH/gm should be less than equal to 0.4
• Dielectric Dissipation Factor
• DDF is magnetically equal to sine of the loss angle and is good tool to indicate the quality of an insulation.
• A high value of DDF is an indication of the presence of contaminants of deterioration products such as water, oxidation products, metal soaps, soluble varnishes and resins.

Routine Tests

• Viscosity
• It is the measure of oil resistance to  continuous  flow without the effect of external forces.
• The oil must be mobile, as heat transfer in transformers occurs mainly by convection currents.
• Since viscosity increase with decrease in temperature.
• It is necessary that viscosity be as low as possible at low temperature.
• Viscosity @40 deg. C should be less than equals to 16.5
• Viscosity @-15 deg. C should be less than equals to 800
• Flash Point
• It is the temperature at which oil gives so much vapour that this vapour, when mixed with the air, forms an ignitable mixture and gives a momentary flash on application of flame under prescribed conditions
• Flash point of oil should be greater than equal to 140 deg. C.
•  Acidity Value
• It is the measure of free organic and inorganic acids present in the oil and is expressed in terms of milligrams of KOH required to neutralize the total free acids in one gram of oil.
• Total Acid KOH/gm should  be less than equal to 0.03.
• Di-electric strength
• Breakdown Voltage is the voltage at which breakdown occurs between two electrodes when oil is subjected to an electric field under prescribed conditions.
• Electric strength is the basic parameter for insulation system design of transformer.
• It serves to indicate the presence of contaminating agents like moisture, fibrous materials, carbon particles and sediments.
• Di-electric strength should be greater than equal to 30kV.
• Pour Point
• The temperature at which oil will just flow under the prescribed conditions  is known as the pour point.
• If the oil becomes two  viscous  or solidifies, it will hinder the formation of convection currents and thus cooling of equipment will be  severely  affected. 
• Pour point should be less than equal to -30 deg. C .
• Moisture Content
• The amount of free and dissolve water present in the oil is known as moisture content and is expressed in ppm.
• Presence of moisture is harmful since it adversely affects the electrical characteristics of oil and accelerates deterioration of insulation paper.
• Corrosive Sulphur
• This test is designed to detect any traces of free corrosive sulphur that may be present in oil.
• Presence of corrosive sulphur in oil results in pitting and black deposit on the surface of bare copper used in transformer, which will adversely affects the dissipation of heat and consequently the performance of the equipment.