ABB REL670 2.2 IEC, Applications Manual

Page: 380 / 908

loadability of transmission corridors. The principle is based on compensation of
distributed line reactance by insertion of series capacitor (SC). The generated
reactive power provided by the capacitor is continuously proportional to the square
of the current flowing at the same time through the compensated line and series
capacitor. This means that the series capacitor has a self-regulating effect. When
the system loading increases, the reactive power generated by series capacitors
increases as well. The response of SCs is automatic, instantaneous and continuous.
The main benefits of incorporating series capacitors in transmission lines are:
• Steady state voltage regulation and raise of voltage collapse limit
• Increase power transfer capability by raising the dynamic stability limit
• Improved reactive power balance
• Increase in power transfer capacity
• Reduced costs of power transmission due to decreased investment costs for
new power lines
8.13.3.1 Steady state voltage regulation and increase of voltage collapse limit
GUID-0AB9B4B5-92DF-4A3B-9046-FC3A460DA658 v1
A series capacitor is capable of compensating the voltage drop of the series
inductance in a transmission line, as shown in figure
195 . During low loading, the
system voltage drop is lower and at the same time, the voltage drop on the series
capacitor is lower. When the loading increases and the voltage drop become larger,
the contribution of the series capacitor increases and therefore the system voltage at
the receiving line end can be regulated.
Series compensation also extends the region of voltage stability by reducing the
reactance of the line and consequently the SC is valuable for prevention of voltage
collapse. Figure
196 presents the voltage dependence at receiving bus B (as shown
in figure 195 ) on line loading and compensation degree K
C
, which is defined
according to equation 358 . The effect of series compensation is in this particular
case obvious and self explanatory.
=
C
C
Line
X
K
X
EQUATION1895 V1 EN-US
(Equation 358)
A typical 500 km long 500 kV line is considered with source impedance
1
0
=
SA
Z
EQUATION1896 V1 EN-US
(Equation 359)
Section 8
1MRK 506 369-UEN B
Impedance protection
374 Line distance protection REL670 2.2 IEC
Application manual