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An exciting current resulted from the hysteresis characteristics of the core causes an error of a measurement current transformer (CT). To produce the high accuracy measurement CT, it is designed to minimise the exciting current. This requires a large cross-section of the core and the high permeability core. If the exciting current can be estimated exactly and compensated, the accuracy of the measurement CT can be improved significantly. This study describes the development of a compensation algorithm for a measurement CT. The proposed compensation algorithm decomposes the exciting current into the core-loss current and the magnetising current. The algorithm uses not the hysteresis curve but the flux-magnetising current curve to minimise the number of required curves for interpolation. The exciting current at every sampling interval is obtained by summing the core-loss current and the magnetising current and then added to the measured secondary current to obtain the correct primary current. The test results using Electromagnetic Transients Program (EMTP)-generated data indicate that the proposed algorithm can improve the accuracy of the measurement CT significantly even with a couple of the curves. The experimental test results indicate that the compensator can improve the accuracy of the CT significantly from 2.5 to 0.1%.
Date of Publication: May 2011