Principles of Symmetrical Components

Principles of symmetrical components allow an unbalanced system to be

considered as three separate, balanced subsystems. These balanced

subsystems may then be analyzed as single phase quantities.

These quantities are the positive, negative, and zero sequence components

of current and voltage.

The positive sequence component of current (I1) represents the portion of

the total current which has normal phase rotation and produces no adverse

effect on the system. An ideally balanced system contains only positive

sequence phase currents and voltages.

The zero sequence component of current (I0) also has no adverse effect

on a three-phase, three-wire (no neutral connection) power system because

it produces no appreciable magnetic flux and causes no excessive heating

in the generator rotor or windings.

The negative sequence component of current (I2) produces a magnetic flux

in the stator that has the same rotational speed as the rotor flux, but in the

opposite direction. This causes the stator magnetic flux to rotate at twice

the system frequency and induce eddy currents into the rotor. These eddy

currents create excessive heat in the rotor iron and windings, and, if allowed

to persist, could result in severe damage to the system.