ABB REG670 Applications Manual page 291

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
Q [pu]
Motor Generator
A
0.8
0.6
Overexcited
37
0.2
Underexcited
-0.3
E
-0.5
D
IEC06000451 V1 EN
Figure 148: Capability curve of a generator
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
X = Source impedance of connected power system
s
Loss of excitation protection can be based on directional power measurement or
impedance measurement.
The straight line EF in the P-Q plane can be transferred into the impedance plane by
using the relation shown in equation 169.
* 2
×
V V V V V
= = = =
Z
* *
I ×
I V S S
EQUATION1723 V1 EN
The straight line in the PQ-diagram will be equivalent with a circle in the impedance
plane, see figure 149. In this example the circle is corresponding to constant Q, that is,
characteristic parallel with P-axis.
B
S
o
0.8 1
0.4 0.6
o
18
F
X
C
H
F
X =0
s
en06000451.vsd
2 2 2
× × ×
S V P V Q
= +
j
* 2 2 2
× + +
S P Q P Q
Section 8
Impedance protection
P [pu]
=0.2
e
= +
R jX
2
(Equation 169)
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