Analysis of High Frequency Oscillations in Voltage Transformer

The voltage transformers (VTs) used in medium voltage circuit have a large number of turns more than 10000. They are reported to be sometimes vulnerable to fast rising abnormal voltages due to lightnings or switchings. It has been regarded difficult to analyse such a high frequency phenomenon in the VT because of the large number of turns. A high frequency model of VT is obtained in a form of lumped constant circuit, and is analysed using the Fast Fourier Transform (FFT). The circuit constants of model are calculated from the winding specification data based on turn-to-turn basis, but the circuit is analysed in terms of multiple-turn groups reducing the order of equations to a manageable level. The dissipation related constants are introduced in the equivalent circuit considering the skin and proximity effects in the conductors as well as the dielectric loss in the insulation. Firstly, the state equation of VT model circuit is solved for a range of FFT frequencies. Then, the time domain analysis is performed using the inverse FFT technique. Both the frequency and time domain characteristics are verified by the experiment on an actual 6.6 kV VT, in which internal voltage oscillations are observed through taps. The general tendencies of high frequency oscillations are investigated using a simpler model. It is found the interlayer insulation near the input terminal is disproportionally highly stressed for impulses with fast rise time. The proposed analysis is considered to be useful since it can be performed when the geometrical specifications of VT are available.