Positive Sequence

Positive sequence is the balanced set of three-phase quantities that has the normal phase rotation of the system. It represents normal operating conditions, so most steady-state network studies, including power flow, are effectively positive-sequence analyses.

Key Aspects of Positive Sequence:

• Normal Rotation: The three phasors have equal magnitude, are displaced by 120 electrical degrees, and follow the standard phase order, typically ABC. This is the sequence produced by a healthy generator feeding a balanced three-phase system.
• Base for Standard Studies: Power flow, economic dispatch network constraints, and balanced three-phase fault calculations all rely primarily on positive-sequence quantities. For many transmission studies, it is the dominant representation of the network.
• Machine Behavior: Synchronous generators are designed to operate naturally with positive-sequence currents and voltages. Under normal conditions, stator and rotor magnetic fields align with this forward rotating system.
• Impedance Use: Positive-sequence impedance is the standard impedance used for lines, transformers, and machines in balanced studies. It is also the sequence network used alone for symmetrical fault calculations.
• Reference for Unbalance: In symmetrical component analysis, the positive-sequence component is compared with the negative and zero sequences to judge how far the system has moved away from balanced operation.