Symmetrical Components, first developed
by C.L.Fortescue in 1918, is a powerful technique for analyzing
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 three phase networks the equivalent circuit obtained
for the symmetrical components, called sequence networks, are
separated into three uncoupled networks.
Further more, for unbalanced three
phase systems, the three sequence networks are connected only at the
points of unbalance. As a result, sequence networks for many cases of
unbalanced three phase systems are relatively easy to analyze.
The symmetrical component method is
basically a modeling technique that permits systematic analysis and
design of three phase systems. Decoupling a detailed three phase
network into three simpler sequence networks reveals complicated
phenomena in more simplistic terms.
Sequence network results can then be
superimposed to obtain three phase results. The application of
symmetrical components to unsymmetrical fault studies is
indispensable.
DEFINITION OF SYMMETRICAL COMPONENTS:
In accordance with Fortescue, the three
phase voltages, VR , VY , and V B are resolved into three sets of
sequence components:
1. Zero sequence, consisting of three
phasors with equal magnitudes and with zero phase displacement.
2. Positives equence, consisting of
three phasors with equal magnitudes, ±120 o phase displacement and
positive sequence.
3. Negative sequence, 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.
No comments:
Post a Comment