ABB RELION REX640, Техническое руководство, Страница: 1021 / 1960

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.
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 567: 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. To avoid unwanted operation of the UEXPDIS function, a restrain area can be
defined which is enabled with
Restraint enable and parameterized with Rst offset X
and
Rst offset X angle setting values.
1MRS759142 F Protection functions
REX640
Technical Manual
1021