0.2 0.4 0.6
-0.3
0.6
-0.5
0.8
Q [pu]
0.8
1
E
D
X
s
=0
B
A
R
a
t
e
d
M
V
A
p
.
f
.
0
.
8
l
a
g
g
i
n
g
R
a
t
e
d
M
V
A
p
.
f
.
0
.
9
5
l
e
a
d
i
n
g
S
37
o
F
X
e
=0.2
C
H
18
o
GeneratorMotor
Overexcited
Underexcited
F
P [pu]
en06000451.vsd
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
s
= Source impedance of connected power system
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 2 2 2
* * * 2 2 2 2
V V V V V S V P V Q
Z j R jX
I
I V S S S P Q P Q
× × × ×
= = = = = + = +
× × + +
EQUATION1723 V1 EN
(Equation 169)
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.
1MRK 502 071-UEN - Section 8
Impedance protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 285
Application manual
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