ABB Relion 670 series, Руководство по применению, Страница: 163 / 760

In practice, the setting of fault resistance for both phase-to-earth (
RFPE) and phase-to-phase
(
RFPP) must be as high as possible without interfering with the load impedance to obtain
reliable fault detection.
7.1.2.8 Series compensation in power systems
SEMOD168320-4 v2
The main purpose of series compensation in power systems is virtual reduction of line
reactance in order to enhance the power system stability and increase 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 transient stability limit
• Improved reactive power balance
• Increase in power transfer capacity
• Active load sharing between parallel circuits and loss reduction
• Reduced costs of power transmission due to decreased investment costs for new power
lines
Steady state voltage regulation and increase of voltage collapse limit
SEMOD168320-24 v2
A series capacitor is capable of compensating the voltage drop of the series inductance in a
transmission line, as shown in figure 81 . 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 82
presents the voltage dependence at receiving bus B (as shown in figure 81 ) on line loading and
compensation degree K
C
, which is defined according to equation 66 . 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 66)
A typical 500 km long 500 kV line is considered with source impedance
1
0
=
SA
Z
EQUATION1896 V1 EN-US
(Equation 67)
~
E
A
Z
SA1 Power line
A B
Seires
capacitor
Load
en06000585.vsd
IEC06000585 V1 EN-US
Figure 81: A simple radial power system
1MRK 505 343-UEN B Section 7
Impedance protection
157
Application manual