Experimental investigation of high frequency voltage oscillation in transformer windings

Very fast transient overvoltages (VFTO) comprising oscillating waveforms generate a voltage oscillation in transformer windings. The behaviour of the voltage oscillation in the winding is investigated experimentally on 500 kV class transformers using a pulse and a sinusoidal voltage. The interturn voltages are directly measured using an optical fiber transmission system. In the case of a pulse application, it is found that an interturn traveling wave is generated at either end of the coil and propagates along the winding. In the case of a sinusoidal voltage application, a resonance phenomenon, probably due to the superposition of the traveling waves, is found in the interturn voltage, which could reach 0.25 times the applied voltage. In the tested transformers, the estimated interturn voltage level at incoming VFTO can be 3 times higher than that in the lightning impulse voltage test.