Negativesequence

Negativesequence is a term used in power engineering to denote the negative-sequence component of a three-phase quantity, such as voltage or current. It is one part of the Fortescue symmetrical components decomposition, which also includes the zero-sequence and positive-sequence components. In a three-phase system with phase quantities Va, Vb, and Vc, the negative-sequence component consists of three phasors of equal magnitude that are 120 degrees apart and rotate in the opposite direction to the positive-sequence component.

In practical terms, a healthy, perfectly balanced system has zero negative-sequence (and zero sequence) components. When unbalance or faults occur—such as line-to-line faults, line-to-ground faults, or uneven loading—the negative-sequence component becomes nonzero. The presence of negativesequence indicates unbalance and is used to analyze how such conditions propagate through the network and affect equipment.

A key physical implication is that negative-sequence currents, especially in electrical machines like induction motors or generators, can produce torque pulsations and additional heating, since they create rotating fields that counteract the machine’s normal rotation. This makes the negativesequence component important for understanding fault severity, protective relaying, and stability analyses.

Computation and use: Symmetrical components are obtained by transforming phase quantities Va, Vb, Vc using a defined matrix (Fortescue transformation), yielding the zero-, positive-, and negative-sequence components V0, V1, and V2 (often denoted V2 for negativesequence). These components are analyzed separately to assess unbalance effects and to design protection schemes.

See also: symmetrical components, Fortescue transformation, positive sequence, zero sequence.