GB804915A - Apparatus for testing current or voltage transformers - Google Patents

Abstract

804,915. Testing voltage and current transformers. METRA BLANSKO, NARODNI PODNIK. March 25, 1955, No. 8757/55. Class 37. A method of determining phase-angle and ratio errors of voltage of a current transformer by comparison with a standard transformer and using a differential circuit is characterized in that: (i) two circuit elements (3, 4) in the differential circuit produce voltages in quadrature with each other; and (ii) each of said voltages, either one of which may be selected, is compensated separately by two voltages in quadrature with each other and one compensation voltage is controlled manually and the other by automatic means. In the embodiment for testing current transformers shown in Fig. 1 a current equivalent to the difference between the secondary currents of the standard and test transformers 1, 2 flows in the differential circuit comprising resistor 3 and mutual inductor 4. The voltage drop across resistor 3 is in phase with the differential current and the voltage induced in the secondary of inductor 4 is 90 degrees phase displaced from the differential current. The voltage components in resistor 3 and the secondary of inductor 4 which are in phase with the current in the secondary of standard transformer 1 are proportional respectively to the ratio and phase-angle errors in test transformer 2. A resistance slide wire 6 is fed from an auxiliary transformer 7 in the secondary circuit of standard transformer 1 and with switch 5 in position a the stator coils of an electrodynamometer zero indicator 9 receives the voltage across resistor 3 together with tapped-off opposition voltage from slide wire 6. When indicator 10 reads zero the position of the tapping along slide wire 6 indicates the ratio error of the test transformer 2. The stator coils of indicator 10 are in series with the stator coils of an electrodynamometer instrument 9 and a phase shifter 13 is adjusted so that the voltage fed to the moving coil of instrument 9 is displaced by 90 degrees with respect to the voltage fed to the moving coil of indicator 10. The moving coil of instrument 9 is ganged to the moving coil of a variable ratio transformer 8 associated with slide wire 6 and for each position of the tapping along slide wire 6 the inductance 8 is automatically varied so that the quadrature component of the voltage across resistor 3 is nullified and the indicator 10 responds only to the voltage component across resistor 3 which is in phase with the current in the secondary of standard transformer 1. To measure phaseangle error the switch is put in position b so that the voltage across inductor 4 feeds indicator 10 and the adjusted position of the tapping along slide wire 6 coincident with zero on indicator 10 gives the phase-angle error of test transformer 2. In this case also the arrangement 8, 9 automatically compensates for the quadrature component of inductor 4. A modification is described which is arranged to compare voltage transformers on a similar basis, and also a further arrangement covering either of the voltage and current measurement arrangements by means of a changeover switch.