A New Method for the Measurement of Surge Voltages in Windings

The increasing demand for electrical windings to be impulse tested has led to the study of the factors affecting the distribution of impulse voltages along the conductors. To obtain the essential test information it is necessary to record oscillograms of the voltage at various points along the winding when an impulse voltage is applied to the line terminal. In the normal way this would require removal of the conductor insulation to make connections to the windings. The paper describes two methods of utilizing a capacitance tapping on the conductor whereby any damage to the winding insulation is avoided. In the first method a simple capacitance voltage-divider circuit is used between the deflection plates of the cathode-ray oscillograph and the winding, the connection to the winding being obtained by wrapping a piece of foil round the outside of the conductor insulation. Better results are obtained, however, by using a suitably designed cathode-follower circuit between the tapping foil and the cathode-ray oscillograph. Consistent results were obtained for impulse waves chopped in times as short as about 0.07 microsec, corresponding to a frequency of about 4 Mc/s when a capacitance greater than 300¿¿F was used between the tapping foil and the winding.