Power factors of askarel and askarel-filled transformers

Abstract

Power factor measurements are one of a series of tests performed during periodic Preventative Main tenance shutdowns to determine the serviceability of transformers. Measurements have been made on units insulated with mineral oil and askarel (PCBfs) and on dry-type units. Wide variations in power factor values have been recorded for service-aged askarel liquid and askarel-filled transformers. Various organizations have suggested power factor limits — sane high, sane low.3'4,5,6 what power factor limits should be used to determine if a unit is still serviceable? The power factor guidelines suggested by Monsanto Company, the manufacturer of the liquid, are quite broad while electrical insulation consultants such as Doble Engineering Company have published very low limits but have also suggested higher values. Transformer manufacturers have a different set of values. Our experience indicates that power factor limits should be at more reasonable values, more in line with actual data being recorded for today's transformers. What are the determining factors that may help establish practical upper limits for safe operation? Present and proposed standards for acceptable power factor values are reviewed and suggested tolerances are presented for consideration. As a basis for carparison, power factors for oil and askarel liquid and for oil and askarel immersed transformers are presented.