Q (Reactive power) p.u.
P (Active power) p.u.
A
B
C
D
F
E
0.0
0.8
0.6
0.5
0.3
1.0
0.9
Generator
Motor
Over-Excitation
Under-Excitation
H
X
e
=0.2
X
s
=0.0
37
o
18
o
Where,
AB= Field current limit
BC= Stator current limit
CD= End region heating limit of stator. Due to leakage flux
BH= Possible active power limit due to turbine output power limitation
EF= Steady -state limit without AVR
Figure 376: Capability curve of a synchronous generator
UEXPDIS protects the synchronous machines against an unstable operation due to
loss of excitation. A partial or total loss of excitation causes a reactive power intake
from the network to the machine, and the reactance of the system viewed from the
machine terminals turns negative. This kind of drop-of-reactance condition can be
detected by measuring the impedance of the system.
The operating characteristic is an offset-mho circle in the impedance plane, and the
circle is parameterized with the
Offset, Diameter and Displacement setting values.
Table 683: Parameters of the circle
Setting values
Description
Offset
Distance of the top of the circle from the R-axis. This is usually set equal
to - x
d
’/2, where x
d
’ is the transient reactance of the machine. The sign
of the setting value determines the top of the circle regarding the R-axis.
If the sign is negative, the circle lies below the R-axis.
Diameter
Normally set equal to the machine's synchronous reactance x
d
, which
determines the size of the impedance circle.
Displacement
Displacement of the center of the circle from the reactance axis or the R-
coordinate of the center. The setting can be used to adjust the sensitivity
of the underexcitation protection. If the sign of the setting is positive,
the circle is shifted to the right, that is, closer to the normal operating
point. Respectively, if the sign is negative, the circle is shifted to the left
and thus moves away from the normal operating point.
Protection functions
1MRS757644 H
724
620 series
Technical Manual
