Insulation Characteristics of Oil-immersed Power Transformer under Lightning Impulse and AC Superimposed Voltage
ABSTRACT
The insulation performance of an oil-immersed power transformer against lightning surges is verified by applying a lightning impulse (LI) voltage alone. However, the surge voltage generated at the transformer terminal in an actual system is superimposed over the ac operating voltage. Particularly for UHV- and 500 kV-class, where the ratio of ac operating voltage to the LI withstand voltage is high, the insulation characteristics for this LI and ac superimposed voltage are crucial factors to study the insulation design. Accordingly, this paper reviewed the research results of insulation characteristics of transformers for the superimposed voltage. Subsequently, these results were evaluated from perspectives of an insulation coordination and transformer insulation test. The insulation elements of transformers covered were the turn-to-turn and section-to section insulation, for which the LI test is key for insulation design. As a result, for the section-to-section insulation for a shell-type transformer and the turn-to-turn insulation, the breakdown (BD) voltage for the superimposed voltage remained unchanged from the result when the LI was applied alone. Conversely, for the section-to-section insulation of a continuous disk winding for a coretype transformer, the BD voltage attributable to the superimposed voltage declined by about 10% to 20% from that when the LI was applied alone if the oil-impregnated paper component ratio compared to the oil-gap length was low. This is because the oil gap was primarily responsible for the BD and flaws were generated there when the ac voltage was applied. On the other hand, when this ratio was increased, the oil-impregnated paper part was primarily responsible for BD and the decline in BD voltage could be suppressed. For the section-to-section insulation for the interleaved disk winding, the BD voltage attributable to the superimposed voltage remained constant from that when the LI was applied alone. This is because the BD was determined by the LI partial discharge inception at the turn-to-turn part of the section-to-section insulation, where the superimposed voltage made little impact on BD. Based on the above, the LI test is considered adequate to verify the insulation performance of the section-to-section insulation for a shell-type transformer and for an interleaved disk winding and the turn-to-turn insulation against surge overvoltage. Conversely, for the section-to-section structure of a continuous disk winding, the influence of the ac voltage must be taken into account. These results are key findings for rationalizing transformers while maintaining their insulation reliability.
Index Terms — Oil-immersed power transformer, lightning impulse and ac superimposed voltage, insulation characteristics, oil-impregnated paper, oil gap, turn-toturn insulation, section-to-section insulation.
THE insulation performance of a power transformer is verified by the lightning impulse (LI) voltage test and the ac voltage test [1, 2]. The LI test mainly verifies surge overvoltage whereas the ac test verifies the normal operating voltage and temporary overvoltage during operation [3, 4].
These tests are performed by applying the respective voltage waveforms alone. On the other hand, the surge voltage generated at the transformer terminal in an actual system is superimposed over the ac operating voltage to form a waveform that differs from the LI voltage waveform. Consequently, the insulation characteristics of a transformer for the voltage under combined conditions with the LI voltage superimposed over the ac voltage (hereinafter referred to as the “superimposed voltage”) are crucial factors to study the insulation design. For UHV- and 500 kV-class transformers in particular, with a high ratio of ac operating voltage to the lightning impulse withstand voltage (LIWV) [1, 2], it is important to clarify the insulation characteristics attributable to this difference in waveform. Where the insulation characteristics vary due to the presence of ac voltage, the influence of the ac voltage must be taken into consideration for the LI test.
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