In practice, the setting of fault resistance for both phase-to-ground (RFPG) and phase-to-
phase (RFPP) must be as high as possible without interfering with the load impedance to
obtain reliable fault detection.
8.1.2.8 Series compensation in power systems
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
A series capacitor is capable of compensating the voltage drop of the series inductance in
a transmission line, as shown in figure
68. 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
69 presents the voltage dependence at receiving bus B (as shown in figure 68) on line
loading and compensation degree K
C
, which is defined according to equation 48. The
effect of series compensation is in this particular case obvious and self explanatory.
EQUATION1895 V1 EN
(Equation 48)
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 173
Application manual
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