Insulation Level and Test Technology of 1000kV Power Transformer

Abstract: The insulation coordination for the first 1000kV UHVAC system in China is studied and determined under the optimization principle. It is not the simple extrapolation of 500kV system and is not in accordance with the standards of GB 311.1-1997 and IEC60071-1-1993. The insulation level of 1000kV power transformer adheres to following items: the lightning impulse voltage is 2250kV; the switching impulse withstand voltage is 1800kV; the power frequency withstand voltage is 1100kV (5min). Due to the differences among the insulation levels and insulation test voltage for different windings of UHV power transformer, the power frequency and impulse test voltage are transmitted among these windings according to transformer ratio. Hence, it is bound to bring about the condition that the insulation design for some conductor terminals cannot be checked by the test voltage specified in the technical specification. Besides, owing to the high voltage class, large capacity and super size of UHV power transformer, the size of its test circuit is correspondingly enlarged, and the influence of stray inductance and capacitance will be more intense. It makes the wave front time of voltage waveform in lightning impulse test extend. However, the design and calculation are generally conducted according to standard waveform. Thus, in the design of UHV power transformer the attention should be paid to the impact of long wave front time test voltage. In this paper, the structural features and insulation level of transformer for 1000kV power transmission project in China as well as special problems during the insulation test are presented in detail. 

Key words: UHV, power transformer, insulation level, test technology