MODULE 3
Static shunt Compensator - Objectives of shunt compensations, Methods of controllable VAR
generation - Variable impedance type VAR Generators -TCR , TSR, TSC, FC-TCR Principle of
operation, configuration and control .
Static Series compensator - Objectives of series compensations, Variable impedance type
series compensators - TCSC - Principle of operation, configuration and control.
2.1 OBJECTIVES:
Steady state transmittable power can be increased and the voltage profile along the line
can be controlled by suitable reactive compensation
Reactive compensation change the natural electrical characteristics of the transmission
line to make it more compatible with the prevailing load demand.
Thus, shunt connected, fixed or mechanically switched reactors are applied to
minimize line over voltage under light load conditions, and shunt connected, fixed or
mechanically switched capacitors are applied to maintain voltage levels under heavy
load conditions.
The ultimate objective of applying reactive shunt compensation in a transmission
system is to increase the transmittable power, improve the transient stability and
reduce loss.
2.2 METHODS OF CONTROLLABLE VAR GENERATION
Capacitors generate and reactors (inductors) absorb reactive power when connected to
an AC power source. They have been used with mechanical switches for controlled
VAR generation and absorption
Continuously variable VAR generation or absorption was originally provided by over-
or under-excited rotating synchronous machines and later, by saturating reactors in
conjunction with fixed capacitors.
High power, line-commutated thyristors in conjunction with capacitors and reactors
have been employed in various circuit configurations to produce variable reactive
output.
These in effect provide a variable shunt impedance by switching shunt capacitors
and/or reactors "in" and "out" of the network.
Using appropriate switch control, the VAR output can be controlled continuously from
maximum capacitive to maximum inductive output at a given bus voltage.
More recently gate turn-off thyristors and other power semiconductors with internal
turnoff capability have been used in switching converter circuits to generate and
absorb reactive power.
All of the different semiconductor power circuits, with their internal control enabling
them to produce VAR output proportional to an input reference, are collectively
termed as static var generators (SVG).
The static var generator is a device that draws controlled capacitive or inductive
current from the power system and thus generates or absorbs reactive power.
A static var compensator (SVC) is, by the IEEE CIGRE co-definition, a static var
generator whose output is varied so as to maintain or control specific parameters (e.g.,
voltage, frequency) of the electric power system.
According to the IEEE-CIGRE definition, a static var generator becomes a static var
compensator when it is equipped with special external controls, to execute the desired
compensation of the transmission line.
The different types of var generator can be operated with the same external control to
provide substantially the same compensation functions.
Modern static var generators are based on high-power semiconductor switching
circuits.
2.3 VARIABLE IMPEDANCE TYPE VAR GENERATORS
1) Thyristor controlled reactor (TCR)
2) Thyristor switched reactor(TSR)
3) Thyristor switched capacitor(TSC)
4) Fixed capacitor - Thyristor controlled reactor (FC-TCR)
1. Thyristor controlled reactor (TCR):
A basic single-phase Thyristor-Controlled Reactor (TCR) consist of a bidirectional
thyristor valve in series with a linear air-core reactor of inductance L.
Thyristor valve T1 conducts in positive half-cycles and thyristor valve T2 conducts in
negative half-cycles of the supply voltage.
The current in the reactor can be controlled from maximum (thyristor valve closed) to
zero (thyristor valve open) by the method of firing delay angle control
The closure of the thyristor valve is delayed with respect to the peak of the applied
voltage in each half-cycle.
When = 0, the thyristor T1 closes at the crest of the applied voltage
The resulting current in the reactor will be the same as that obtained in steady state
with a permanently closed switch
When the gating of the valve is delayed by an angle (0 90) with respect to
the crest of the voltage, the current in the reactor can be expressed with
v(t ) V cost as follows:
t
1 V
iL (t )
L v(t )dt =
L
(sin t sin )
The magnitude of the current in the reactor can be varied continuously by this method
of delay angle control from maximum ( = 0) to zero ( = 90).
The amplitude of the fundamental reactor current iLF(α) can be expressed as a
function of angle α:
( ) ( )
Where V is the amplitude of the applied AC voltage, L is the inductance of
the thyristor controlled reactor and ω is the angular frequency of the
applied voltage.
In a three-phase system, three single-phase thyristor-controlled reactors are used,
usually in delta connection.
Under balanced conditions, the triple-n harmonic currents (3rd, 9th, 15th, etc.)
circulate in the delta connected TCRs and do not enter the power system. The
magnitudes of the other harmonics generated by the thyristor-controlled reactors can
be reduced by various methods.
For high power applications, employs m (m 2) parallel-connected TCRs, each with
of the total rating required.
The reactors are "sequentially" controlled, that is, only one of the m reactors is delay
angle controlled, and each of the remaining m - 1 reactors is either fully "on" or fully
"off," depending on the total reactive power required.
2. Thyristor switched reactor(TSR)
The TSR is a special case of a TCR in which the variable firing-angle control option is not
exercised.
Instead, the device is operated in two states only : either fully ON or fully OFF.
If the thyristor valves are fired exactly at the voltage peaks corresponding to α = 90° for
the forward-thyristor valve T1 and α = 270° (90 + 180) for the reverse-thyristor valve T2,
The maximum inductive current flows in the TCR as if the thyristor switches were
replaced by short circuits.
However, if no firing pulses are issued to the thyristors, the TSR will remain in a
blocked-off state, and no current can flow.
When a large magnitude of controlled reactive power, Q, is required, a part of Q is usually
assigned to a small TSR of rating, say, Q/2; the rest is realized by means of a TCR also of a
reduced rating Q/2. This arrangement results in substantially decreased losses and
harmonic content as compared to a single TCR of rating Q.
The TSR provides a fixed inductive admittance
If the TSRs are operated at = 0, the resultant steady-state current will be sinusoidal.
3. Thyristor switched capacitor(TSC)
It consists of a capacitor, a bidirectional thyristor valve, and a relatively small surge
current limiting reactor.
The reactor is needed primarily to limit the surge current in the thyristor valve under
abnormal operating conditions
Under steady-state conditions, when the thyristor valve is closed and the TSC branch
is connected to a sinusoidal ac voltage source, v V sin t , the current in the branch is
given by:
The amplitude of the voltage across the capacitor is:
n2
Vc V
n2 1
The TSC branch can be disconnected at any current zero by prior removal of the gate
drive to the thyristor valve.
n2
At the current zero crossing, the capacitor voltage is at its peak value Vc V .
n2 1
The disconnected capacitor stays charged to this voltage and, consequently, the voltage
across the non conducting thyristor valve varies between zero and the peak-to-peak
value of the applied ac voltage.
If the voltage across the disconnected capacitor remained unchanged, the TSC bank could
be switched in again, without any transient, at the appropriate peak of the applied ac
voltage.
Normally, the capacitor bank is discharged after disconnection. Thus, the reconnection of
the capacitor may have to be executed at some residual capacitor voltage between zero
n2
and Vc 2 V
n 1
Case-1: If the residual capacitor voltage is lower than the peak ac voltage (Vc < V), then
the correct instant of switching is when the instantaneous ac voltage becomes equal to
the capacitor voltage
Case-2 : If the residual capacitor voltage is equal to or higher than the peak ac voltage
(Vc > V), then the correct switching is at the peak of the ac voltage at which the thyristor
valve voltage is minimum
4. Fixed capacitor - Thyristor controlled reactor (FC-TCR)
A basic var generator arrangement using a fixed (permanently connected) capacitor
with a thyristor-controlled reactor (FC-TCR).
The constant capacitive var generation (Qc) of the fixed capacitor is opposed by the
variable var absorption (QL) of the thyristor-controlled reactor, to yield the total var output (Q)
required. At the maximum capacitive var output, the thyristor-controlled reactor is off (α
90°).
To decrease the capacitive output, the current in the reactor is increased by decreasing
delay angle α.
At zero var output, the capacitive and inductive currents become equal and thus the
capacitive and inductive vars cancel out.
With a further decrease of angle α, the inductive current becomes larger than the
capacitive current, resulting in a net inductive var output.
Circuit configuration
Control scheme for FC-TCR
STATIC SERIES COMPENSATOR
Objectives:
AC power transmission over long lines was primarily limited by the series reactive
impedance of the line.
Series capacitive compensation cancels a portion of the reactive line impedance and
thereby increase the transmittable power.
Variable series compensation is highly effective in both controlling power flow in the line
and in improving stability.
Controllable series line compensation is a cornerstone of FACTS technology.
It can be applied to achieve full utilization of transmission assets by controlling the power
flow in the lines,
With the use of fast controls system disturbances can be minimised.
Fixed or controlled series capacitive compensation can also be used to minimize the
end-voltage variation of radial lines and prevent voltage collapse.
Series compensation, appropriately controlled to counteract prevailing machine swings,
can provide significant transient stability improvement for post-fault systems and can be
highly effective in power oscillation damping.
Variable impedance type series compensators
1. Thyristor-Controlled Series Capacitor (TCSC)
It consists of the series compensating capacitor shunted by a Thyristor-Controlled Reactor.
In a practical TCSC implementation, several such basic compensators may be connected in
series to obtain the desired voltage rating and operating characteristics
The steady-state impedance of the TCSC is that of a parallel LC circuit, consisting of a fixed
capacitive impedance, Xc, and a variable inductive impedance, XL( ), that is;
The TCSC thus presents a tunable parallel LC circuit to the line current.
As the impedance of the controlled reactor, XL( ), is varied from its maximum (infinity)
toward its minimum ( L ), the capacitive impedance increases from minimum until
parallel resonance at Xc = XL( ),is established and XTCSC,max theoretically becomes
infinite.
Decreasing XL( ) further, the impedance of the TCSC, XTCSC( ) becomes inductive,
reaching its minimum value of XL XC / XL -XC at = 0, where the capacitor is in effect
bypassed by the TCR.
Fig: Impedance Vs delay angle chara
