Symmetrical components

In electrical engineering, the method of symmetrical components simplifies the analysis of a three-phase power system exhibiting an electrical fault or other unbalanced condition. The symmetrical components corresponding to an asymmetrical set of three phasors are: Sequence 0 (also known as zero sequence or homopolar) is one-third the sum of the original three phasors. Sequence 1 (positive sequence) is one-third the sum of the original three phasors rotated counterclockwise by 0°, 120°, and 240°. Sequence 2 (negative sequence) is one-third the sum of the original three phasors rotated counterclockwise 0°, 240°, and 120°. The analysis of power systems is much simpler in the domain of symmetrical components, because the resulting equations are mutually linearly independent if the power system itself is balanced. In this case, each symmetrical component can be analyzed separately, similar to the per-phase analysis. Protective relays utilize symmetric components for fault detection. For example, during normal operation, the zero-sequence current is very small, so a high current value is a convenient and reliable indicator of a ground fault.