ABB RELION 670 Series Applications Manual page 164

Section 7
Impedance protection
500
400
300
200
100
0
IEC06000586 V1 EN-US
Figure 67: Voltage profile for a simple radial power line with 0, 30, 50 and 70% of
Increased power transfer capability by raising the first swing stability limit
Consider the simple one-machine and infinite bus system shown in figure 68.
P
Mech
E
A
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
output power from the generator (P
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
input power and the generator deaccelerates. The generator decelerates as long as equal area
condition A
is that the angular displacement after fault clearing and during the deceleration does not
exceed its critical limit δ
the synchronism is lost. The first swing stability and the stability margin can be evaluated by
studying the different areas in figure
series compensation. The areas under the corresponding P – δ curves correspond to energy
and the system remains stable if the accelerating energy that the generator picks up during
the fault is lower than the decelerating energy that is transferred across the transmission line
during the first system swing upon fault clearing.
158
200 400 600 800 1000 1200
P[MW]
compensation
A
-jX
C
Z +jX
SA1 Line
~
. After reclosing of the system, the transmitted power exceeds the mechanical
C
=A has not been fulfilled. The critical condition for post-fault system stability
ACC DEC
, because if it does, the system cannot get back to equilibrium and
CR
U
limit
1400 1600 1800
en06000586.vsd
B
System
en06000587.vsd
) and the generator angle is d
E
69 for the same system, once without SC and once with
1MRK 504 152-UEN B
SEMOD168320-32 v2
) is equal to the electrical
Mech
. If a 3-phase fault occurs
0
Application manual