OPERATON & Location of
TCSC
1.
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
fundamental-frequency 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.
- 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
parallel-connected 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.
2.
DIFFERENT MODES OF OPERATION
1. Bypassed Thyristor
mode:
- In this
bypassed mode, the thyristors are made to fully conduct with a
conduction angle of 180. Gate
pulses are applied as soon as the voltage across the thyristors
reaches zero and becomes positive, resulting in a continuous
sinusoidal of flow current through the thyristor
valves.
- The TCSC module behaves
like a parallel capacitor–inductor combination. However, the net
current through the module is inductive, for the susceptance of the
reactor is chosen to be greater than that of the
capacitor.
- Also known as the
thyristor-switched-reactor (TSR) mode, the bypassed
thyristor mode is distinct from the bypassed-breaker mode,
in which the circuit breaker provided across the series capacitor
is closed to remove the capacitor or the TCSC module in the event
of TCSC faults or transient over voltages across the
TCSC.
- This mode is employed
for control purposes and also for initiating certain protective
functions.
- Whenever a TCSC module
is bypassed from the violation of the current limit, a finite-time
delay, T delay, must elapse before the module can be
reinserted after the line current falls below the specified
limit.
2.Blocked – Thyristor
Mode:
- In this mode, also
known as the waiting mode, the firing pulses to the
thyristor valves are blocked.
- If the thyristors are
conducting and a blocking command is given, the thyristors turn off
as soon as the current through them reaches a zero
crossing.
- The TCSC module is thus
reduced to a fixed-series capacitor, and the net TCSC reactance is
capacitive.
- In this mode, the
dc-offset voltages of the capacitors are monitored and quickly
discharged using a dc-offset control without causing any harm to
the transmission-system transformers.
3 Partially
Conducting Thyristor Mode or Vernier
Mode:
- This mode allows the
TCSC to behave either as a continuously controllable capacitive
reactance or as a continuously controllable inductive
reactance.
- It is achieved by
varying the thyristor-pair firing angle in an appropriate range.
However, a smooth transition from the capacitive to inductive mode
is not permitted because of the resonant region between the two
modes.
- A variant of this mode
is the capacitive-vernier-control mode, in which the
thyristors are fired when the capacitor voltage and capacitor
current have opposite polarity.
- This condition causes a
TCR current that has a direction opposite that of the capacitor
current, thereby resulting in a loop-current flow in the TCSC
controller.
- The loop current
increases the voltage across the FC, effectively enhancing the
equivalent-capacitive reactance and the series-compensation level
for the same value of line current.
- To
preclude resonance, the firing angle α of the forward-facing
thyristor, as measured from the positive reaching a zero crossing
of the capacitor voltage, is constrained in the range α
min ≤ α ≤ 1800.
- This
constraint provides a continuous vernier control of the TCSC module
reactance. The loop current increases as α is decreased from
1800
to α min.
- The
maximum TCSC reactance permissible with a c α min is typically
two-and-a-half to three times the capacitor reactance at
fundamental frequency.
- Another variant is the
inductive-vernier mode, in which the TCSC can be operated
by having a high level of thyristor conduction.
- In this mode, the
direction of the circulating current is reversed and the controller
presents a net inductive impedance.
- Based on the three
modes of thyristor-valve operation, two variants of the TCSC
emerge:
1.
Thyristor-switched series
capacitor (TSSC),
which permits a discrete control of the capacitive
reactance.
2.
Thyristor-controlled
series capacitor (TCSC), which offers a continuous control of
capacitive or inductive reactance.