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 str ess similarity in oil ducts. In order to investigate equivalent levels of other insulation tests to polarity reversal tests, dielectric stre ngth of transformer at polarity reversals was examined by use of a mod el of typical oil/paper composite insulation. From comparison of polar ity reversal and other insulation tests, it was found that dielectric strength of transformer insulation at polarity reversals is phenomenal ly similar to that under switching impulse stresses. Equivalent AC tes t voltage to polarity reversal test was also estimated from the experi mental results. The breakdown characteristics of polarity reversal tes ts showed that partial discharges at polarity reversals impose less ha rmful stress on insulation barriers than those at usual AC or impulse tests.