Voltage-time and voltage-number characteristics of insulation elements with large scale oil-immersed transformers under field-use conditions

In order to attain reduction in insulation test voltages, voltage-time (V-t) characteristics for ac voltage and voltage-number (V-N) characteristics for lightning impulse voltage were experimentally investigated on the turn-to-turn insulation model, coil-to-coil insulation model and barrier-oil-duct insulation model of core- and shell-type transformers under non-standard of field-use conditions. The conditions used for experiments on insulation characteristics in view of field-use consisted of "clean oil" and "hot oil" as the conditions for insulating oil, "trapezoidal waves", "alternate applications of positive and negative voltages", and "steep front waves" as the conditions for applied voltages, and "low dielectric constant pressboards" as an insulating object in experiments. Although the measured V-t and V-N characteristics varied depending on the conditions, the total evaluation in terms of insulation coordination showed that results under standard conditions were satisfactory in almost all the cases and some cautionary points in a few cases were recognized