Seminar Topic on:-
SYMMETRICAL COMPONENT OF POWER
OUTLINE OF THE PRESENTATION
NEED FOR THE SYMMETRICAL
TECHINQUES USED TO ANALYSIS
A three-phase system is said to be symmetrical when
the system viewed from any phase is similar.
Thus, in a three-phase symmetrical system the
impedances of all the three phases are equal and the
mutual impedances, if any, between the three phases are
The three phase voltages(or currents) are said to be
balanced if the magnitude of the three voltages(or
currents) are equal and they separated from each other
by 120º(electrical) in phase.
When the system is unbalance the voltages, current and
the phase impedances are in general unequal. Such a
system can be solved by a symmetrical per phase
technique known as the method of symmertical
This method was proposed by Fortescue in 1981
and is often called the three-component method.
According to Fortescue, An unbalance set of n
phasors may be resolved in to (n-1) balanced n-
phase system of different phase sequence and one
zero-phase sequence system.
A zero-phase sequence system is one in which all
phasors are of equal magnitude and identical angle,
Phase-Sequence of the phasors is the order in which
they pass through a positive maximum.
Thus, a phase sequence a b c implies that the
maxima occur in order a, b, c.
If a, b, c is taken as the positive phase sequence then
a, c, b represents the negative phase sequence.
It should be noted that for both positive & negative
phase sequence, the direction of rotation of phaser is
taken to be anticlockwise
The phasor representation of three phase system
Need for the symmetrical
Negative sequence relay in generator has helped in
protecting the generator from over heating in the
event of unbalanced loads.
The positive sequence network is used to supply the
sensing voltage to generator voltage regulators
Certain connections of CT and PT develop zero
sequence components that are used in protective
ground relaying scheme
Method of adopted for
symmetrical component analysis
Va= Va0+ Va1+ Va2
Vb= Vb0+ Vb1+ Vb2
Vc= Vc0+ Vc1+ Vc2
Va Vb Vc are unbalance set of phasor
Va1, VbVc1 are set of positive sequence phasor
Va2, Vb2Vc2 are negative sequence phasor
Va0 Vb0 Vc0 are zero sequence phasor
Power system analysis using symmetrical
component is very helpful in improving the
reliability of the power system.
The principle adopted for the analysis of
unbalanced fault system is symmetrical component