A Circuit Model in a Wide Frequency Range for Power Transformer and Analysis of its Characteristics

In order to predict VFTO (very fast transient overvoltage) distributions in the windings during transformer design stage, it is important to acquire the frequency range of the surge wave that could enter the transformer winding, and then model the windings in that frequency range. In GIS (gas insulated station), transformer is connected to bus through bushing and lead wire. The bushing capacitance to ground will bypass the high frequency components, so there should be a upper limit of the frequency components of the surge wave at the entrance of the transformer. In this paper, the transformer windings are modeled using MTL (multitransmission-line) model firstly, then the lumped equivalent circuit is formulated with the -type circuits, and finally the driving-point impedance and voltage transfer functions of the equivalent circuit are calculated using nodal analysis. The driving-point impedance of a model winding is calculated and compared with the measured result, and the validation of the equivalent circuit of transformer winding in the frequency range below 20 MHz is verified. Then the driving-point impedances of two real transformer windings are calculated with the equivalent circuit. Taking the lead impedance into consideration, the impedance of the winding is compared with the one of the bushing capacitance. The results show that most of the energy of the frequency components above 10 MHz in the surge wave will pass through the bushing capacitance and will not cause damage to transformer insulations.