Using symmetrical components for internal external fault discrimination in differential protection schemes

Classical differential protection schemes are subject to ghost differential currents due to CT saturation and magnetization currents. Several methods are used to counter the impact of these un-wanted differential currents. This paper will focus on using the symmetrical component techniques to stabilize differential protection schemes without sacrificing the speed and sensitivity of the protection. Negative sequence quantities are inherently present in any power system disturbances. Even in the case of a three phase symmetrical fault, negative sequence currents are present during the first few cycles of the fault as a result of the dc component. By accurately measuring the magnitudes and relative phase angles of the negative sequence currents leaving and entering into the differential zone can provide a very reliable indication of whether the fault is inside or outside of the protection zone. In addition, it can supplement the traditional restrained differential protection to achieve high levels of speed with very high sensitivity for high impedance faults. In transformer protection, turn to turn faults at the neutral points are typically difficult to detect. By using negative sequence quantities, these faults can be detected reliably.