Sequence Networks of Rotating Machines

A Y-connected synchronous generator, grounded via neutral impedance, is designed to generate balanced internal phase voltages with only positive-sequence components.

The generator's sequence networks include a source voltage only in the positive-sequence network, and the sequence components of line-to-ground voltages at the terminals are depicted.

The voltage drop in the neutral impedance is caused solely by zero-sequence current, neutral impedance in series with the generator's zero-sequence impedance.

In power stations, the synchronous generator stator produces balanced three-phase positive-sequence currents under steady-state conditions, generating a net magnetomotive force (MMF) aligned with the rotor.

With balanced three-phase negative-sequence currents, the net MMF opposes the rotor, resulting in lower negative-sequence impedance than positive-sequence impedance.

Zero-sequence currents, identical in magnitude and phase, create near-zero net MMF, making the zero-sequence impedance the smallest.

Sequence impedances vary across rotating machines and are classified as synchronous, transient, or subtransient.

These networks simplify the operation of three-phase synchronous and induction motors.