Electrical Power Systems

232 Electrical Power Systems
In a synchronous machine, no zero sequence voltage is induced. Zero sequence impedance
of the machine is due to the flow of zero-sequence current. The flow of zero sequence currents
creates three mmfs which are in time phase but are distributed in space phase by 120º.
Therefore, resultant air gap flux would be zero and there is no reactance due to armature
reaction. Hence, machine offers a very small reactance due to the leakage flux. Therefore, the
rotor windings present leakage reactance only to the flow of zero sequence currents, i.e.,
Z0 = jxl ...(9.40)
9.6 SEQUENCE NETWORKS O A LOADED SYNCHRONOUS MACHINE
igure 9.3 shows a synchronous machine with neutral grounded through an impedance Z n . The
machine is supplying balanced three-phase load.
ig. 9.3: Three phase synchronous m/c supplying balanced three phase load.
Balanced three phase of synchronous machine is represented as a positive-sequence set of
phasors, i.e.,
E a = E a , E b = b E a , E c = b 2 E a
or
L
NM
O
Q
Ea
Eb
E P c
=
rom ig. 9.3, we can write,
L
NM
O
Q
1
b
2
b P Ea Va = Ea – ZsIa – Zn In
Vb = Eb – ZsIb – Zn In
Vc = Ec – ZsIc – Zn In
U
V|
W|
...(9.41)
...(9.42)