SYMMETRICAL COMPONENTS

Symmetrical Components, first developed by C.L.Fortescue in 1918, is a powerful technique for analysing unbalanced 3f systems. Fortescue defined a linear transformation from 3f components to a new set of components called symmetrical components. The advantage of this transformation is that for balance threephase networks the equivalent circuit obtained for the symmetrical components, called sequence networks, are separated into three uncoupled networks. Further more, for unbalanced threephase systems, the three sequence networks are connected only at the points of unbalance. As a result, sequence networks for many cases of unbalanced threephase systems are relatively easy to analyse.

The symmetrical component method is basically a modelling technique that permits systematic analysis and design of threephase systems. Decoupling a detailed threephase network into three simpler sequence networks reveals complicated phenomena in more simplistic terms. Sequence network results can then be superimposed to obtain threephase results.

The application of symmetrical components to unsymmetrical fault studies is indespensable.

DEFINITION OF SYMMETRICAL COMPONENTS:

In accordance with Fortescue, the threephase voltages, VR , VY , and V B are resolved into three sets of sequence components:

1. Zerosequence, consisting of three phasors with equal magnitudes and with zero phase displacement.

2. Positivesequence, consisting of three phasors with equal magnitudes, ±120^o phase displacement and positive sequence.

3. Negativesequence, consisting of three phasors with equal magnitudes, ±120^o phase displacement and negative sequence.



NOTES:
1. The sequence components do not exist as physical quantities in the network.
2. A balanced system has no negative or zero sequence components therefore:
actual balanced system = positive sequence system
3. The generated emf is balanced, and therefore positive phase sequence only.
4. In a 3phase 3wire

system, there are no zero sequence components, because there is no neutral connection.

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