Patent GB-424810-A (GB)
Title: Method of testing current transformers
IPC: G01R35/02
Filed: 1934-07-27
Abstract: 424,810. Inductance and phasedifference measurements. HOHLE, W., 13, Fraunhoferstrasse, Charlottenburg, Berlin. July 27, 1934, No. 22038. Convention date, July 28, 1933. [Class 37] In order to measure the current and phaseangle errors of a current transformer X, Fig. 2, the primary winding is connected in series with the primary winding of a standard transformer N and the secondary windings are so connected that the difference between their currents passes through an impedance, such as the primary winding of a mutual inductance M, the voltage drop across which is compensated by means of two regulable voltages, one in phase with the primary current and the other in quadrature therewith. The in-phase component is derived from a resistance r, for example a slide wire, and the quadrature component from a variable mutual inductance m. These voltages are in opposition to the voltage induced in the secondary winding of the mutual inductance M and are adjusted until no current passes through a zero instrument G, which may be a vibration galvanometer. The voltage tapped off from the resistance r is then proportional to the angle error and the secondary voltage of the mutual inductance m is proportional to the current error. The mutual inductance m and resistance r may be calibrated in percentages of current error and minutes of angle error. The mutual inductance M may be variable in order to extend the range of readings. A modification is shown in Fig. 3 in which the difference current is passed through a resistance R, the voltage across which is compensated by a variable mutual inductance m and a resistance r, of which one half is connected in the secondary circuit of the standard transformer N and the other half in the secondary circuit of the transformer X under test so that positive and negative current errors may be measured. On balancing, the setting of the mutual inductance m measures the angle error and that of the slider on the resistance r measures the current error. The compensating voltages may be supplied by a third transformer K, Fig. 4, which need not have the same transformation ratio as the transformers N, X. The variable mutual inductance m may have air-cored coils, the relative position of which is adjustable, or a fixed mutual inductance with secondary voltage divisions may be employed. In place of the mutual inductance m, the arrangements shown in Figs. 5<a> and 5<b> may be employed for obtaining the quadrature component of the compensating voltage. The primary winding P, Fig. 5<a>, of an iron-cored transformer, is traversed by the secondary current of the standard transformer N. In series with the secondary winding S are a high resistance R<1> and a slide wire resistance D, the resistance R<1> is shunted by a condenser C of such a capacity that the voltage across the resistance D is in quadrature with the primary current. The core E of the transformer may be of an alloy having a constant permeability over its working range, or may have an airgap and a graduated iron cross-section. In the modification shown in Fig. 5<b> the primary winding P is replaced by a resistance A, the voltage drop across which is applied to a voltage transformer T, to the secondary winding of which there is connected a condenser C and slide-wire resistance D.