Power-factor or dissipation factor ( tangent of the loss angle) testing of insulating oil shall performe in accordance with American Society for Testing and Materials (ASTM) D 924, Standard Test Method for Dissipation Factor (or Power Factor) and Relative Permittivity (Dielectric Constant) of Electrical Insulating Liquids as below:
The overall PF test results of oil-filled power and distribution transformers indicate the insulation condition of the solid windings, oil, barriers, bushings, etc. The overall PF value for individual windings-to-ground and inter windings insulation of modern oil-filled transformers should be 0.5% or less, corrected to 20°C. Service-aged power transformers will have somewhat higher PF values due to normal aging, loading (heat), and voltage stress.
A PF value as high as 1% is considered acceptable for older transformers when previous history or knowledge of PF value of the transformer, or similar transformers is not available. However, when the PF value of one insulation system of the transformer is higher than the others, for example, HV winding insulation PF is higher than the LV and inter winding insulation PF, then causes of the higher PF should be investigated. A PF value of 2% for extremely old power transformers may be considered acceptable. In the case of older transformers that utilized varnished-cambric or varnish insulation, these transformers may have normal PF values in the range of 4% to 5% at 20°C.
Generally the PF of good, new oil is expected to be 0.05% or less at 20°C. Used oil in good condition should have a PF of 0.5% or less at 20°C, and if the PF exceeds
0.5% then the oil is considered questionable for continued use. Oil having PF greater than 1.0% at 20°C should be investigated or reconditioned, or replaced.