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.^[1]

The symmetrical components corresponding to an asymmetrical set of three phasors are:^[1]

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 system is much simpler in the domain of symmetrical components, because the resulting equations are mutually linearly independent if the power system itself is balanced.^[2] In this case, each symmetrical component can be analyzed separately, similar to the per-phase analysis.

The protective relays utilize the symmetric components for fault detection. For example, during the normal operation, the zero-sequence current is very small, so a high current value is a convenient and reliable indicator of a ground fault.^[3]

^ ^a ^b Amberg & Rangel 2020, p. 1.
^ Blackburn, J. Lewis (1993-06-07). Symmetrical Components for Power Systems Engineering (1st ed.). New York: CRC Press. ISBN 978-0-8247-8767-7.
^ Anderson 1998, pp. 271–272.

[FOOTNOTEAmbergRangel20201-1] Amberg & Rangel 2020, p. 1.

[2] Blackburn, J. Lewis (1993-06-07). Symmetrical Components for Power Systems Engineering (1st ed.). New York: CRC Press. ISBN 978-0-8247-8767-7.

[FOOTNOTEAnderson1998271–272-3] Anderson 1998, pp. 271–272.

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