INFLUENCE OF BURDEN ON THE ACCURACY OF CURRENT TRANSFORMER AT HIGH FREQUENCY

Abstract

Poor quality of measurement happened when current transformer (CT) operated at high frequency, because it is typically constructed to operate only at the industrial frequency (50/60 Hz) and is not designed and tested for harmonic frequencies. Moreover, no standard provided the recommend practice for the application of CT over wide frequency range. In order to guarantee the accuracy of CT, IEC 61869-2-2012 and IEEE C57.13-2008 provided the limits of current error with the specified burden which shall have power factor of 0.8 and 0.9 lagging, respectively. Therefore, the influence of burden on the accuracy of CT over wide frequency range is investigated. This work used the several burden based on both IEC and IEEE standard burdens. The single frequency current of 10 A at frequency from 50 Hz to 1 kHz and the harmonic current up to 19th order were injected to CT. Because the value of burden was proportional to the test frequency, the author proposed two cases of burden to calculate the output current of CT. From the test results, for the first case, the ratio error corresponded to the limits of current error according to IEC 61869-2-2012 at frequency of 50 Hz. Unfortunately, the ratio error dramatically increased with increasing frequency. In second case, the high ratio error at high frequency was reduced, especially when the standard burden as manufacturer declared was the burden of CT under test. Although the ratio error in second case was acceptable according to IEC 61869-2-2012 at all test frequencies when standard burden was used, for other burden, the ratio error was slightly high. Therefore, this work used the ratio error from the single frequency current test as the correction factor (CF) to correct the ratio error. As results, the ratio error was slightly reduced.