ABB REG670 - Page 288

operate in this state for a long time. Loss of excitation increases the generation of heat
in the end region of the synchronous machine. The local heating may damage the
insulation of the stator winding and even the iron core.
A generator connected to a power system can be represented by an equivalent single
phase circuit as shown in figure
145. For simplicity the equivalent shows a generator
having round rotor, (X
d
≈X
q
).
+
E
-
I, (P, Q)
j X
d
j X
net
+
V
-
+
E
net
-
en06000321.vsd
IEC06000321 V1 EN
Figure 145: A generator connected to a power system, represented by an
equivalent single phase circuit
where:
E represents the internal voltage in the generator,
X
d
is the stationary reactance of the generator,
X
net
is an equivalent reactance representing the external power system and
E
net
is an infinite voltage source representing the lumped sum of the generators in the system.
The active power out from the generator can be formulated according to
equation 168:
sin
net
d net
E E
P
X X
d
×
= ×
+
EQUATION1540 V1 EN
(Equation 168)
where:
The angle δ is the phase angle difference between the voltages E and E
net
.
If the excitation of the generator is decreased (loss of field), the voltage E becomes
low. In order to maintain the active power output the angle δ must be increased. It is
obvious that the maximum power is achieved at 90°. If the active power cannot be
reached at 90º static stability cannot be maintained.
The complex apparent power from the generator, at different angles δ is shown in
figure
146. The line corresponding to 90° is the steady state stability limit. It must be
Section 8 1MRK 502 071-UEN -
Impedance protection
282 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual