ABB RELION RET670, Applications Manual

Page: 419 / 1016

In practice, the setting of fault resistance for both phase-to-ground RFPE and phase-to-
phase RFPP should be as high as possible without interfering with the load impedance
in order to obtain reliable fault detection.
8.13.3 Series compensation in power systems
GUID-7F3BBF91-4A17-4B31-9828-F2757672C440 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 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
199 . 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
200 presents the voltage dependence at receiving bus B (as shown in
figure 199 ) on line loading and compensation degree K
C
, which is defined according to
equation 328 . The effect of series compensation is in this particular case obvious and
self explanatory.
1MRK 504 163-UUS A
Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 413
Application manual