ABB RET670 Applications Manual page 213

1MRK504116-UUS C
Application manual
without SC
A
DEC
A
P
SM
Mech
A
ACC
0 C EA CR
IEC06000588 V1 EN
Figure 74: Equal area criterion and first swing stability without and with series
compensation
This means that the system is stable if A
given by the difference between the available decelerating energy (area between the P
(δ) and P and the angular difference between δ
Mech
energy. It is represented in figure
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 corresponding to a certain power transfer and it also shifts the P – δ curve
0
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 75
with 50% compensation degree.
with SC
A
SM
A
DEC
P
Mech
A
ACC
0 C EA
≤ (A
ACC DEC
74 by the area A
as an example for 500 km long 500 kV transmission line
Section 3
IED application
CR
en06000588.vsd
+ A ). The stability margin is
SM
and δ ) and the accelerating
C CR
. Notice that a substantial increase
SM
207