Dejan Vuković

Typical insulation system of power transformers consists of insulating liquid (e.g. mineral oil, esters) and liquid impregnated solid insulation. As solid insulation often is used cellulose based material in form of thin paper, single pressboard sheet or laminated multiple pressboard sheets. The requirements for pressboard that can be used in power transformers are define by internation standards, e.g. IEC 60641-2, 60243-1 or ASTM D149 & D3394. This presentation focuses on test methos as defined by IEC 60243-1 and IEC 60763-2. The relevance of samples and test setup preparation/preconditioning is being analyzed. Parameters like the influ-ence of drying and impregnation of samples, high voltage connections and application as well as post processing of measured values is evaluated involving some examples of bad and good practices. The dielectric stresses generated by these test methods are calculated and compared with typical stresses that appear inside power transformers. The study which is here presented indicates that existing test methods need to be improved to achieve better reproducibility of measurements. Furthermore, there is a need to develop new test methods which would replicate more realistically the dielectric stresses in solid insulation as it can be found within power transformer insulation system.

Keywords: testing, solid insulation materials, power transformers