Use of a Current Source for Calibrating an Automatic Transformer Test Set

Abstract- This paper describes an innovative procedure for calibrating sets for evaluating instrument current transformer (ICT) errors. The innovation lies in the use of a current source for generating a measured current. There is an adjustable current source at 50 Hz mains frequency with a current transformer on its output. The internal impedance must be sufficiently high to eliminate the methodic error due to calibration. Calibration is carried out by means of a measuring system controlled by a PC through an IEEE 488 bus. Known values of the simulated ratio error and phase displacement are compared with the values measured by the device that is being tested. A software tool determines the simulated and measured error at each calibration point and determines whether the instrument that is being tested fulfils the parameters stated by the manufacturer. I. Function and calibration of an automatic transformer test set Ratio error and phase displacement are determined by a comparative method, the principle of which is shown in Figure 1. Calibrated and tested instrument current transformers (ICTs) with identical primary and secondary currents are fed by a common current I1. The secondary circuit of the ICT under test is loaded by an electronic or passive burden B. The deviation ΔI2 between secondary currents I2X and I2N expressed as differences in the ratio error ΔεI and phase displacement ΔδI is evaluated by means of an automatic transformer test set (ATTS). ICT errors can be expressed a