ABB RET670 - Page 213

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 74: 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
74 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
75 as an example for 500 km long 500 kV transmission line
with 50% compensation degree.
1MRK504116-UUS C Section 3
IED application
207
Application manual