Abstract
Abstract Traditionally, instrument transformers are calibrated using bridges. By definition, bridges use the null method of measurement. The traditional calibration programme for instrument transformer bridges characterise namely this null measurement. Many new commercial comparators for instrument transformer use a very different method. They sample the secondary signals of reference and device under test (dut) transformer independently. Based on the samples, magnitude and phase of both signals are determined. Ratio error and phase displacement are calculated. Consequently, the significance of their calibration using the traditional calibration programme is limited. Moreover, the operating range of modern comparators is much larger than that of bridges. The additional versatility cannot be used without an adapted calibration programme. This article analyses the calibration programmes for both technologies. An experimental study confirms both the suitability of the new calibration programme and the need to chose the calibration programme depending on the technology of the device to be calibrated. The conclusion is very general and applies to all measurement problems where an operating principle is replaced by another – when changing the operating principle, it is important to check the calibration programme and adapt it if necessary.
Highlights
Another example is the calibration of an inverting dut, where the amplitude of the difference signal would be as large as the sum of the amplitudes of the secondary signals of reference and dut, exceeding the operating range of a welldesigned bridge by far
It is possible and very common to maintain the traditional calibration programme when replacing bridges by comparators using an indirect method of measurement without giving it much thought
It is possible to use the calibration programme for bridges, which has been used for decades and is generally accepted, since the signal levels used during calibration are covered by the operating range
Summary
The traditional calibration programme for instrument transformer bridges characterises the difference measurement and the normalisation to reference amplitude. A practical case is the calibration of a Rogowski coil [7] as dut, where the rated phase displacement is 90° Another example is the calibration of an inverting dut, where the amplitude of the difference signal would be as large as the sum of the amplitudes of the secondary signals of reference and dut, exceeding the operating range of a welldesigned bridge by far. While commercial comparators do not include this feature yet, metas set up and calibrated such a comparator along with the appropriate sources It is possible and very common to maintain the traditional calibration programme when replacing bridges by comparators using an indirect method of measurement without giving it much thought.
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