Dielectric strength of transformer insulation at DC polarity reversal

Among special insulation tests for DC converter transformer, polarity reversal test can be substituted by applied AC voltage test from a stress similarity in oil ducts. In order to investigate equivalent levels of other insulation tests to polarity reversal tests, dielectric strength of transformer at polarity reversals was examined by use of a model of typical oil/paper composite insulation. From comparison of polarity reversal and other insulation tests, it was found that dielectric strength of transformer insulation at polarity reversals is phenomenally similar to that under switching impulse stresses. Equivalent AC test voltage to polarity reversal test was also estimated from the experimental results. The breakdown characteristics of polarity reversal tests showed that partial discharges at polarity reversals impose less harmful stress on insulation barriers than those at usual AC or impulse tests.