ABB REL670 2.2 IEC - Page 169

~
E
A
Z
SA1
Power line
A B
Seires
capacitor
Load
en06000585.vsd
IEC06000585 V1 EN-US
Figure 66: A simple radial power system
en06000586.vsd
0 200 400 600 800 1000 1200 1400 1600 1800
100
200
300
400
500
P[MW]
U
[
k
V
]
U
limit
P
0
P
3
0
P
5
0
P
7
0
IEC06000586 V1 EN-US
Figure 67: Voltage profile for a simple radial power line with 0, 30, 50 and
70% of compensation
Increased power transfer capability by raising the first swing stability
limit
SEMOD168320-32 v2
Consider the simple one-machine and infinite bus system shown in figure 68.
en06000587.vsd
~
E
A
Z
SA1
A B
P
Mech
+jX
Line
-jX
C
System
IEC06000587 V1 EN-US
Figure 68: One machine and infinite bus system
The equal-areas criterion is used to show the effectiveness of a series capacitor for
improvement of first swing transient stability (as shown in figure 69).
In steady state, the mechanical input power to the generator (P
Mech
) is equal to the
electrical output power from the generator (P
E
) and the generator angle is d
0
. If a
3-phase fault occurs at a point near the machine, the electrical output of the
generator reduces to zero. This means that the speed of the generator increases and
the angle difference between the generator and the infinite bus increases during the
fault. At the time of fault clearing, the angle difference has increased to δ
C
. After
reclosing of the system, the transmitted power exceeds the mechanical input power
and the generator deaccelerates. The generator decelerates as long as equal area
condition A
ACC
=A
DEC
has not been fulfilled. The critical condition for post-fault
system stability is that the angular displacement after fault clearing and during the
deceleration does not exceed its critical limit δ
CR
, because if it does, the system
cannot get back to equilibrium and the synchronism is lost. The first swing stability
and the stability margin can be evaluated by studying the different areas in figure
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 163
Application manual