Patent GB-558534-A (GB)
Title: Improvements in or relating to the measurement of ratio and phase angle errors in voltage transformers
IPC: G01R1/28
Filed: 1942-10-29
Abstract: 558,534. Measuring phase and angle errors. JOHNSON & PHILLIPS, Ltd., and STENNING, B. F. Oct. 29, 1942, No. 15193. [Class 37] Apparatus for measuring ratio and phase angle errors of a voltage transformer comprises means for producing a standard voltage which would be equal in magnitude and phase to the secondary voltage of the transformer under test if the latter were perfect, means for opposing the standard and secondary voltages to each other, and means for applying to the opposed voltages adjustable inphase and in-quadrature voltage components which bear a known relation to the standard voltage, and for adjusting these components until a state of balance is attained, wherein the voltage components are obtained from a three-phase transformer, one of the components being an adjustable proportion of the voltage across two phases in series of an output winding of this transformer and the other component being an adjustable proportion of the voltage across the third phase. A three-phase testing transformer has four windings per phase, viz. a primary winding 1, secondary winding 2, tertiary winding 3, and quaternary winding 4. The primary winding 1 is connected in delta to a three-phase supply R, Y, B. The secondary winding 2 is connected in star and supplies the primary winding 5 of the voltage transformer to be tested at its working voltage. The tertiary winding 3 is also star connected and supplies a voltage equal to the nominal voltage supplied by the secondary winding 6 of the transformer under test. The quaternary winding 4 is wound for a few volts per phase and has all ends brought out. The source of supply R, Y, B is a variable voltage source of standard phase sequence and is adjusted so that the secondary winding 2 supplies the required voltage to the primary winding 5 of the transformer under test, the secondary winding of which is loaded by a burden. 7. The connections shown are for testing R to neutral. The voltage of the R-phase of the secondary 6 is balanced against the voltage of the same phase of the tertiary winding 3 and to the difference voltage is added a small in-phase voltage from the R-phase of the quaternary winding 4 and also a small quadrature voltage from the other two phases of this winding. These in-phase and quadrature voltages are adjusted by means of slide wires 8, 9 until balance is attained, as determined by a vibration galvanometer 10 with a variable shunt. The position of the slider on the slide wire 8 connected across the R-phase of the winding 4 is then a measure of the ratio error and the position of the slider on the slide-wire 9 is a measure of the phase-angle error. For testing phase-to-phase, e.g. R to Y, the secondary 6 on the Y-phase is connected to the corresponding tertiary winding 3, instead of joining the neutrals. The slide wire 9 is connected across the B-phase of the winding 4 and the slide wire 8 is connected across the other two phases R, Y of the same winding in series. The slide wires are calibrated in terms of percentage error and minutes of phase-angle error. A resistor 11 is inserted in series with the slide wire 9-when testing phase-to-neutral errors, and has twice the resistance of this wire so that the same scale is applicable for phase-to-neutral errors. The tertiary winding 3 of the testing transformer may be dispensed with and the standard voltage supplied by the secondary of a standard or substandard voltage transformer the primary of which is connected in parallel with the transformer under test. The primary winding 1 of the testing transformer may be dispensed with and the voltage to the winding 2 supplied, either from a separate transformer or from high voltage mains. For testing the errors of successive phases a multiple switch may be provided for changing the connections in cyclic rotation. The apparatus apart from the testing transformer may be assembled in the form of a desk which can be placed on a bench.