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
71 for the same system, once without SC and once
with 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.
en06000588.vsd
0 C EA
A
ACC
A
DEC
A
SM
CR
P
Mech
P
Mech
P
[
p
u
]
P
[
p
u
]
with SCwithout SC
A
ACC
A
DEC
A
SM
0 C EA CR
IEC06000588 V1 EN
Figure 71: Equal area criterion and first swing stability without and with series
compensation
This means that the system is stable if A
ACC
≤ (A
DEC
+ A
SM
). The stability margin is given
by the difference between the available decelerating energy (area between the P (δ) and
P
Mech
and the angular difference between δ
C
and δ
CR
) and the accelerating energy. It is
represented in figure
71 by the area A
SM
. Notice that a substantial increase in the stability
margin is obtained by installing a series capacitor. The series compensation can improve
the situation in two ways, it can decrease the initial angle difference δ
0
corresponding to
a certain power transfer and it also shifts the P – δ curve upwards.
Improve reactive power balance
A series capacitor increases its output of reactive power instantaneously, continuously
and automatically with increasing line load. It is thus a self-regulating device, which
improves voltage regulation and reduces the need for other means of voltage control for
example, shunt compensation. The reactive power balance of a series compensated line is
shown in figure 72 as an example for 500 km long 500 kV transmission line with 50%
compensation degree.
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 175
Application manual
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