Negative Sequence

Negative sequence is the balanced set of three-phase voltages or currents that rotates in the reverse phase order relative to normal system rotation. It appears when the system is unbalanced, such as during line-to-line faults, open conductor conditions, or severe phase loading imbalance.

For rotating machines, negative-sequence current is especially important because it creates a magnetic field that moves opposite to the rotor. That reverse field induces double-frequency currents in rotor parts and can produce rapid heating if the condition is not cleared or limited.

Key Aspects of Negative Sequence:

• Reverse Rotation: The three phasors have equal magnitude and 120 degree spacing, but the phase order is reversed, typically ACB instead of ABC. This distinguishes negative sequence from the positive-sequence quantities that represent healthy balanced operation.
• Indicator of Unbalance: Negative sequence appears only when the network is not electrically balanced. Protection engineers monitor it to detect asymmetrical faults, broken conductors, unequal loads, or phase loss conditions.
• Machine Heating: Synchronous generators and large motors are sensitive to negative-sequence current because it causes additional rotor losses. Relay settings often include an inverse-time negative-sequence thermal function to protect the machine from sustained unbalance.
• Fault Analysis Role: In symmetrical component studies, the negative-sequence network is connected with the positive and zero sequence networks depending on the fault type. This allows engineers to calculate currents and voltages during unsymmetrical disturbances.
• Protection Use: Negative-sequence elements are widely used where phase overcurrent alone may not detect all abnormal conditions with enough sensitivity. They are common in generator protection, motor protection, and ground fault schemes.