Using the light emission measurement in assessment of electrical discharge development in different liquid dielectrics under lightning impulse voltage

This article presents the results of studies concerning the assessment of propagation modes of electrical discharges developing in different liquid dielectrics. This assessment was performed on the basis of measurements of light emitted during discharge development. For liquid dielectrics such as natural ester, synthetic ester and mineral oil tested under standard lightning impulse voltage of both polarities, two propagation modes of discharges were identified: slow developing discharges of propagation velocities of few mm/μs and fast developing discharges of velocities of tens mm/μs. Comparing the results concerning the liquids tested in synthetic ester and natural ester fast discharges appeared at much lower testing voltage than in the case of mineral oil. Simultaneously, at the same voltage level, the frequency of light pulses corresponding with the discharges developing in esters was higher than frequency of pulses observed when discharges developed in mineral oil. Thus the results showed that both esters are less resistant to the appearance of fast and energetic discharges, which means that esters may have a lower ability to protect against the lightning impulses in real insulation systems. It is suggested that this disadvantage of the ester liquids should be necessarily taken into account during the design process of transformers filled by them.