Comparison of the dielectric strength of transformer oil under DC and repetitive multimillisecond pulses

A Short Review of Some of the Factors Affecting the Breakdown Strength of Insulating Oil for Power Transformers

“…What is reported in [50] is in line with the findings of [38]. The results reported in [38] and [50] generally agree with those of [51], where it was shown that with DC or pulses longer than 50 ms foreign particles can form a bridge that may initiate a breakdown. In [51], it is cautiously suggested that for pulsed conditions at frequencies of up to 100 Hz, one can afford to decrease the insulation thickness by 10% to 20% relative to those required at DC voltage.…”

“…The results reported in [38] and [50] generally agree with those of [51], where it was shown that with DC or pulses longer than 50 ms foreign particles can form a bridge that may initiate a breakdown. In [51], it is cautiously suggested that for pulsed conditions at frequencies of up to 100 Hz, one can afford to decrease the insulation thickness by 10% to 20% relative to those required at DC voltage. Further evidence in validation of [38,50,51] was offered in [52], where a distinct difference between DC and AC voltages regarding the movement of foreign particles in transformer oil was described.…”

“…In [51], it is cautiously suggested that for pulsed conditions at frequencies of up to 100 Hz, one can afford to decrease the insulation thickness by 10% to 20% relative to those required at DC voltage. Further evidence in validation of [38,50,51] was offered in [52], where a distinct difference between DC and AC voltages regarding the movement of foreign particles in transformer oil was described. According to [51], bridging of foreign particles under DC voltage is encouraged by dielectrophoretic (DEP) forces and as the voltage increases the velocity of particles also increases with the rate of bridge formation intensified.…”

“…Further evidence in validation of [38,50,51] was offered in [52], where a distinct difference between DC and AC voltages regarding the movement of foreign particles in transformer oil was described. According to [51], bridging of foreign particles under DC voltage is encouraged by dielectrophoretic (DEP) forces and as the voltage increases the velocity of particles also increases with the rate of bridge formation intensified. The associated conduction current increases and the bridges become wider but not denser.…”

“…It is evident that electrode geometry affects greatly the breakdown process and strength of transformer oil. Things become even more complicated because far too often there are bubbles and minute foreign particles in an oil gap [51,61]. Especially, non-uniform electrode arrangements may indicate the breakdown streamer mechanism in an oil gap [66,67].…”