Compensation of the Distortion in the Secondary Current Caused by Saturation and Remanence in a CT

Abstract

Current-transformer (CT) saturation may cause the maloperation of a protection relay. This is particularly onerous when the remanent flux in the core of the CT adds to the flux change caused by the fault. The CT is forced into deep saturation and the waveshape of the secondary current is severely distorted. An algorithm for compensating the distortion in the secondary current caused by saturation and remanence in a CT is described in this paper. A second-difference function detects when the CT first starts to saturate. At this instant, the negative value of the second-difference function corresponds to the magnetizing current which, in conjunction with the magnetization curve, is used to estimate the core flux. This is then used as an initial value to calculate how the flux changes during the fault. The magnetizing current is estimated by inserting the estimated core flux into the magnetization curve and added to the secondary current; the result, the compensated secondary current, is equal to the secondary referred primary current. Various test results indicate that the proposed algorithm can accurately compensate a severely distorted secondary current and is not affected by remanence. The paper concludes by describing the hardware implementation of the algorithm on a prototype compensation unit based on a digital signal processor.