3.1Thyristor Controlled Series Capacitor (TCSC)
Intrroduction to series compensation
 TCSC is a power electronic based system and Thyristor
Switched Capacitor is connected in series with a
bidirectional thyristor valve.
 The TCSC can control power flow, mitigate subsynchronous resonance, improve transient stability, damp
out power system oscillations resulting increase of power
transfer capability.
 A single diagram of TCSC shows two modules
connected in series and there can be one or more module
depending on the requirement to reduce the costs and
TCSC may be used in conjunction with fixed series
capacitors.
 Nowadays TCSC is being included in some of the
transmission systems and the basic circuit of a TCSC in
one of the phase is shown in the fig.controls the current
through the reactor.
0
 The forward-looking thyristor has firing angle 90 –
0
180 and firing the thyristors at this time results in a
current flow through the inductor that is opposite to the
capacitor current and in this loop current increases
the voltage across the capacitor.
0
 Further the loop current increases as firing angle decreases from 180 .
 The different compensation levels are obtained by
varying the firing angle of the reactor-circuit-thyristor.
 TCSC is a power electronic based system and Thyristor Switched Capacitor is
connected in series with a bidirectional thyristor valve.
 The TCSC can control power flow, mitigate sub-synchronous resonance, improve
transient stability, damp out power system oscillations resulting increase of power
transfer capability.
 A single diagram of TCSC shows two modules connected in series and there can be
one or more module depending on the requirement to reduce the costs and TCSC may
be used in conjunction with fixed series capacitors.
 Nowadays TCSC is being included in some of the transmission systems and the basic
circuit of a TCSC in one of the phase is shown in the fig.controls the current through
the reactor.
 The forward-looking thyristor has firing angle 900 – 1800 and firing the thyristors at
this time results in a current flow through the inductor that is opposite to the capacitor
current and in this loop current increases the voltage across the capacitor.
 Further the loop current increases as firing angle decreases from 1800.
 The different compensation levels are obtained by varying the firing angle of the
reactor-circuit-thyristor.
Basic Principle
 A TCSC is a series-controlled capacitive reactance that can provide continuous
 control of power on the ac line over a wide range.
 The principle of variable-series compensation is simply to increase the fundamentalfrequency voltage across an fixed capacitor (FC) in a series compensated line through
appropriate variation of the firing angle.
 This enhanced voltage changes the effective value of the series-capacitive reactance.
A simple understanding of TCSC functioning can be obtained by analyzing the
behavior of a variable inductor connected in parallel with an FC, as shown in Fig.
 The equivalent impedance, Zeq, of this LC combination is expressed as
The impedance of the FC alone, however, is given by −j(1/ ωC). If ωC−(1/ ωL) > 0 or,
in other words, ωL > (1/ ωC), the reactance of the FC is less than that of the parallelconnected variable reactor and that this combination provides a variable-capacitive reactance
are both implied. Moreover, this inductor increases the equivalent-capacitive reactance of the
LC combination above that of the FC.
If ωC − (1/ ωL) c 0, a resonance develops that results in an infinite-capacitive
impedance is obviously unacceptable condition. If, however, ωC − (1/ ωL) < 0, the LC
combination provides inductance above the value of the fixed inductor. This situation
corresponds to the inductive-vernier mode of the TCSC operation.
In the variable-capacitance mode of the TCSC, as the inductive reactance of the
variable inductor is increased, the equivalent-capacitive eactance is gradually decreased.
The minimum equivalent-capacitive reactance is obtained for extremely large inductive
reactance or when the variable inductor is open-circuited, in which the value is equal to the
reactance of the FC itself.
The behavior of the TCSC is similar to that of the parallel LC combination.
The difference is that the LC-combination analysis is based on the presence of pure sinusoidal
voltage and current in the circuit, whereas in the TCSC, because of the voltage and current in
the FC and thyristor-controlled reactor (TCR) are not sinusoidal because of thyristor
switchings.
A Variable inductor connected in shunt with an FC