A NOVEL METHOD TO MEASURE RATIO AND NON-STANDARD PHASE DISPLACEMENTS OF SPECIAL TRANSFORMERS USING A SINGLEPHASE SUPPLY AND MERITS OF USING HIGHER TEST VOLTAGE

摘要

Three-phase voltage supplies together with phase vector meters have been traditionally used for measuring voltage ratio of transformers having non-standard phase displacements. Non-standard phase displacements refer to phase-shifts that do not follow 30° clock steps (example, -7.5°, 22.5°, etc). Some examples being: phase-shifters, rectifier, arc-furnace, quad-boosters and traction transformers. In the new method, instead of using a 3-phase voltage supply, 3-phase equivalent voltages are simulated at different terminals of a transformer by energising a pair of the terminals and interconnecting certain other terminals in a custom defined manner to generate the necessary 3-phase equivalent voltages. This permits determination of the ratio and phase displacement using common ratio bridges, which use single-phase voltage supplies. In the first part of the paper, theoretical background of this method and results obtained from the field to validate this approach are presented. In the second part of the paper, it is shown analytically as to how the test voltage level can affect the flux distribution in the transformer and ultimately the measured ratio accuracy. This is followed by a presentation of a few field results to show how the test voltage plays a significant role on the measurement accuracy, which hitherto is unknown to many in the industry.