ABB REL670 2.2 IEC, Applications Manual

Page: 170 / 908

69 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 SC without SC
A
ACC
A
DEC
A
SM
0 C EA CR
IEC06000588 V1 EN-US
Figure 69: 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 69 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
SEMOD168320-43 v2
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
70 as an example for 500 km long 500
kV transmission line with 50% compensation degree.
Section 8
1MRK 506 369-UEN B
Impedance protection
164 Line distance protection REL670 2.2 IEC
Application manual