GB439661A - Method of testing voltage transformers - Google Patents

Abstract

439,661. Inductance and phase-difference measurements. HOHLE, W., 13, Fraunhoferstrasse, Charlottenburg, Berlin. Dec. 28, 1934, No. 37197. Convention date, Dec. 30, 1933. [Class 37] The ratio and angle errors of a voltage transformer are measured by connecting the primary winding in parallel with the primary winding of a standard voltage transformer of the same transformation ratio, connecting the secondary windings in opposition, and compensating the difference between the secondary voltages, or a fraction thereof, by a zero method employing two regulable voltages in quadrature. As shown, the primary windings of the standard transformer N and the transformer X to be tested are connected in parallel and supplied from an alternating current source, the secondary windings being connected through the primary winding of a transformer HT so that their voltages oppose one another. In series with the secondary winding of the transformer HT is a zero instrument, such as a vibration galvanometer VG, the secondary winding of a regulable mutual inductance m and part of a slide-wire resistance s. The primary winding of the mutual inductance m is connected, in series with the slide wire resistance s and a resistance RV, across the secondary winding of the standard transformer N. A condenser C connected across the resistance RV causes a displacement in phase between the voltage at the slide wire and the secondary voltage. The mutual inductance and the slider are adjusted until the instrument VG gives zero indication, under which conditions the resistance tapped off from the slide wire is a measure of the ratio error, and the value of the mutual inductance is a measure of the angle error. A series resistance may be included in the primary circuit of the transformer HT, in which case the secondary voltage of this transformer is practically in quadrature with the difference voltage. The resistance tapped off from the slide wire then measures the angle error and the mutual inductance measures the ratio error. In a modification the circuit comprising the primary winding of the mutual inductance m and the resistances s, RV is connected across the secondary winding of an auxiliary transformer, of which the primary winding is connected across the secondary winding of the standard transformer N. In another modification the slide wire and the primary winding of the mutual inductance are fed from the secondary winding of a current transformer, the primary winding of which is connected in series with the resistance RV across the secondary winding of the standard transformer N. Instead of compensating the whole of the difference voltage, a portion may be tapped off from a voltage divider. In all cases the secondary coils of the mutual inductances carry no current when compensation is effected, so that their resistance and inductance do not enter into the measurement. The mutual inductance m may comprise air coils, of which the mutual inductance is varied by adjusting their relative position. Fixed mutual inductances with a secondary voltage division may be employed. Other means for supplying a variable voltage in quadrature to a circuit current or voltage may be employed, for example iron-cored transformers, to the secondary winding of which a circuit consisting of resistances and condensers is connected, this circuit including a variable resistance from which a variable voltage is derived.