Compensating algorithm for use with measurement type current transformers for protection

The requirements to avoid current transformer (CT) saturation result in impractically large CTs. This paper describes a compensating algorithm for the distorted secondary current of the CT caused by saturation. The algorithm can compensate the distorted current irrespective of the level of the remanent flux and is suitable for use with measurement type CTs for protection. A second-difference function detects when a CT first starts to saturate. At the start of first saturation, the negative value of the second-difference function corresponds to the magnetizing current; which is used in conjunction with the magnetization curve to obtain the core flux. This value is then used as an initial flux to calculate the core flux. The magnetizing current is determined by inserting the calculated core flux into the magnetization curve and added to the secondary current. The resultant compensated current is equal to the secondary referred primary current. Test results indicate that the algorithm can compensate the severely distorted currents due to the small size of measurement CTs and is not affected by the remanent flux. The paper concludes by describing the experimental test results on a small CT using a prototype compensation system based on a digital signal processor.