A Y-connected synchronous generator, grounded through a neutral impedance, is designed to produce balanced internal phase voltages with only positive-sequence components. The generator's sequence networks include a source voltage that is exclusively in the positive-sequence network. The sequence components of line-to-ground voltages at the generator terminals illustrate this configuration.
Zero-sequence current induces a voltage drop across the generator's neutral impedance and other zero-sequence impedances in the circuit. This necessitates an additional impedance in the zero-sequence network, in series with the generator's zero-sequence impedance.
In power stations, the stator of a synchronous generator under steady-state conditions carries balanced three-phase, positive-sequence currents. These currents generate a net magnetomotive force (MMF) that aligns with the rotor's direction. On the other hand, balanced three-phase, negative-sequence currents in the stator produce a rotating MMF that opposes the rotor's rotation, resulting in a negative-sequence impedance that is typically smaller than the positive-sequence synchronous impedance.
Zero-sequence currents, which have identical magnitude and phase, produce an almost zero net MMF. Consequently, the generator's zero-sequence impedance is typically the smallest among the sequence impedances.
Sequence impedances in rotating machines are categorized into synchronous, transient, or subtransient impedances. These classifications help in simplifying the analysis of three-phase synchronous and induction motors by using sequence networks.
