Phase Segregated Soft Calibration of Instrument Transformers Using Synchronised Phasor Measurements

We consider the problem of ‘soft’ calibration of instrument transformers (ITs) i.e., current transformer (CT) and capacitive voltage transformer (CVT) using PMU data. In this paper, we derive phase-wise models for IT calibration within the framework of Open Circuit (OC) tests. Closed-form expressions are derived for calibration under the assumption of a 3-phase balanced source at one end of the line. The proposed method is validated on a 400 kV, 67 km transmission line using real field data. A detailed error analysis is presented using simulations to study the effect of small and large unbalances in voltage magnitudes at the source end and it is shown that the proposed open circuit tests give meaningful and accurate results even under such unbalances.