ABB 615 series Technical Manual page 707

1MAC059074-MB A
615 series ANSI
Technical Manual
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.
Motor
A
0.9
0.6
Over-Excitation
0.0
Under-Excitation
0.3
E
0.5
D
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.
Generator
B
o
37
P (Active power) p.u.
1.0
o 0.8
18
C
H
F
X =0.0
s
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
Section 4
Protection functions
X =0.2
e
701