In a three phase system, one set of phasors has the same phase
sequence as the system under study (ie. A B C). This is called the
positive phase sequence and produces a normal rotating field.
The second set has the reverse sequence (ie. B A C). This is
called the negative phase sequence and produces a field with
opposite rotation.
Finally, a third set exists which is in phase with each other
and is called the zero phase sequence and produces a field that
oscillates but does not rotate.
In a perfectly balanced system, no negative phase sequence
currents would exist. However, it is virtually impossible to
acheive this perfectly balanced system in practice and so these
negative phase currents need to be considered.
Line voltage imbalances caused by electrical faults or
imbalanced loads lead to current imbalances in each conductor.
Therefore, the magnetic coupling between windings becomes uneven. A
counter rotating field (in respect to the main field) will now
exist and the resultant field will cause undesirable eddy currents
to flow. The consequenes of this for generators will either be a
loss of torque, or depending on the load, will increase the current
for the same slip speed and hence raise the temperature of the
alternator.
The basics of the zero, positive, negetive sequence may be
explained as follows
Any unbalanced three phase currents(voltage) can be split to
three phasor components . positive , negetive and zero sequnce for
analysing the effect of unbalnce as there is no direct method
avilable for analysing the effect of unbalanced currents or
voltages
Now the positive and negetive sequence currents are balanced
phasors and the only difference is that their sequnce is opposit As
-ve and +ve sequnce currents are balanced , if you add it up at any
moment anywhre you will get zero Hence if you have a star network
with neutral earthed , there will not be any -ve or +ve sequence
through the neutral ( it add up to zero ! ) The Zero sequnce
phasors are equal in magnitude and there is no phase difference b/w
the sequnce components of three phases . Hence adding this
components( of three phases) at any moment will not give zero but
an added value Hence if you have star nework with neutral earthed ,
then the zero sequence of each phase will add up in neutral . Hence
the currents available in the neutral of a star network will be
only zero sequence and +ve and -ve sequnce can not exists in
neutral
