drop out-of-step. The reason is the steady-state stability limit as defined by the load angle
being 90°, which can only be reached when the unit is underexcited.
Q (Reactive power) p.u.
P (Active power) p.u.
A
B
C
D
FE
0.0
0.8
0.6
0.5
0.3
1.0
0.9
GeneratorMotor
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
GUID-FD5EDD69-0D03-4A97-A1AB-46C755F531A5 V2 EN
Figure 348: Capability curve of a synchronous generator
40 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.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 701
Technical Manual
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