During continuous valve bypass the TCSC represents an inductive impedance of about
20% of the capacitor impedance. Both operation in capacitive boost mode and valve
bypass mode can be used for damping of power swings. The utilization of valve bypass
increases the dynamic range of the TCSC and improves the TCSC effectiveness in power
oscillation damping.
8.1.2.9 Challenges in protection of series compensated and adjacent power lines
System planning does not consider any more possible protection issues and difficulties,
when deciding for a particular, non conventional solution of certain operation and stability
problems. It is supposed that modern communication and state of the art computer
technologies provides good basis for the required solution. This applies also to protection
issues in series compensated networks. Different physical phenomena, which influence
conventional principles of IED protection, like distance protection, phase comparison
protection, are well known and accordingly considered in IED design. Some other issues,
like influence of controlled thyristors in series capacitor banks are getting increased
importance, although not as high as they would deserve.
The most important challenges, which influence the operation of different protection
functions in the greatest extent, are described in this chapter.
Voltage and current inversion
Series capacitors influence the magnitude and the direction of fault currents in series
compensated networks. They consequently influence phase angles of voltages measured
in different points of series compensated networks and this performances of different
protection functions, which have their operation based on properties of measured voltage
and current phasors.
Voltage inversion
Figure
81 presents a part of series compensated line with reactance X
L1
between the IED
point and the fault in point F of series compensated line. The voltage measurement is
supposed to be on the bus side, so that series capacitor appears between the IED point and
fault on the protected line. Figure 82 presents the corresponding phasor diagrams for the
cases with bypassed and fully inserted series capacitor.
Voltage distribution on faulty lossless serial compensated line from fault point F to the bus
is linearly dependent on distance from the bus, if there is no capacitor included in scheme
(as shown in figure
82). Voltage V
M
measured at the bus is equal to voltage drop D V
L
on
the faulty line and lags the current I
F
by 90 electrical degrees.
The situation changes with series capacitor included in circuit between the IED point and
the fault position. The fault current I
F
(see figure 82) is increased due to the series
capacitor, generally decreases total impedance between the sources and the fault. The
reactive voltage drop D V
L
on X
L1
line impedance leads the current by 90 degrees. Voltage
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 181
Application manual
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