ABB RELION 620 Series Technical Manual page 728

Section 4
Protection functions
722
The underexcitation also causes the generator to operate in the asynchronous mode.
This increases the rotor speed, which causes heating in the rotor iron and damps the
windings. A high intake of the reactive power from the network during
underexcitation causes problems in the network, for example voltage dip, stability and
power swings. Power swings stress the prime mover, causing for example turbine
blade cavitation and mechanical stress in the gearbox.
The capability curve of a synchronous generator describes the underexcitation
capability of the machine. An excessive capacitive load on the synchronous machine
causes it to 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 366: 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.
Generator
B
o
37
o 1.0
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
1MRS757644 F
P (Active power) p.u.
X =0.2
e
Technical Manual
620 series