Stevenson J. Power system analysis

CPTER

SYMMETRICAL

COMPONENTS

D SEQUENCE

NETWORKS

One of the most powerful tools for dealing with unbalanced polyphase circuits

is the method of mmetrical components introduced by C. L. Fortescue.

Fortescue

s work proves that an unbalanced system of n related phasors can be

resolved into n systems of balanced phasors called the symmetrical components

of the original phasors. The n phasors of each set of componen ts are equal in

length, and the angles between adjacent phasors of the set are equal. Although

the method is applicable to any unbalanced polyphase system, we conne our

discussion to three-phase systems.

In a three-phase system which is normally balanced, unbalanced fault

conditions generally cause unbalanced currents and voltages to exist in each of

the phases. If the currents and voltages are related by constant impedances, the

system is said to be linear and the principle of superposition applies. The

voltage response of the linear system to the unbalanced currents can be

determined by considering the separate responses of the individual elements to

the symmetrical components of the currents. The system elements of interest

are the machines, transformers, transmission lines, and loads connected to . or

Y congurations.

Ie. L. Fortescue, "Method of Symmetrical Coordinates Applied to the Solution of Polyphase

Networks," Tra