1Variable impedance type series compensators a GTO ThyristorControlled Series Capacitor GCSC b ThyristorSwitched Series Capacitor TSSC c ThyristorControlled Series Capacitor TCSC ID: 674074
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Slide1
UNIT-VII
Static Series CompensatorsSlide2
(1)Variable impedance type series compensators.
(a) GTO Thyristor-Controlled Series Capacitor (GCSC)
(b) Thyristor-Switched Series Capacitor (TSSC)
(c) Thyristor-Controlled Series Capacitor (TCSC)
(2)Switching converter type series compensators.
Static synchronous series compensator (SSSC)Slide3
GTO
Thyristor
-Controlled Series Capacitor (GCSC)
It consists of a fixed capacitor in parallel with a GTO
thyristor (or equivalent) valve (or switch) that has the capability to turn on and off upon command.
Fig. (a) Basic
GTO-Controlled Series
Capacitor, (b) principle
of turn-off
delay angle control,
and
(c) attainable
compensating voltage
waveformSlide4
The
objective
of
the GCSC scheme is to control the ac voltage vc across the capacitor at a given line
current i. Evidently, when the GTO valve, sw, is
closed
, the voltage across the
capacitor
is zero, and
when the valve
is open, it is maximum
. For
controlling the
capacitor
voltage
,
the closing and opening of
the
valve
is
carried out
in
each half-cycle in synchronism with the ac
system
frequency
.
The GTO valve
is stipulated to close
(
through
appropriate control
action
)
whenever
the
capacitor voltage crosses zero. (Recall
that the thyristor valve of the TCR opens whenever the current crosses zero.)Slide5
When the valve
sw
is opened at the crest of the (constant) line current (
γ = 0), the resultant capacitor voltage vc will be the same as that obtained in steady state with a permanently open switch. When the opening of the valve is delayed by the angle γ
with respect to the crest of the line current, the capacitor voltage can be expressed with a defined line current, i(t) = I
cos
ωt, as follows:
The amplitude of fundamental capacitor voltage can be expressed as a function of
γ
where
γ
is
the amplitude of the line current,
C
is the capacitance of the GTO
thyristor
controlled
capacitor, and
ω
is
the angular frequency of the ac
system.Slide6
Fundamental component of the series capacitor voltage vs. the turn-off delay angle
γ
.Slide7
This
impedance can
be
written as
In a practical application the GCSC can be operated either to control the compensating
voltage,
V
CF(γ
), or the compensating reactance,
XC(γ).
In the voltage compensation mode
, the GCSC is to maintain the rated compensating voltage in face of decreasing line current over a defined interval
I
min
<=
I
<=
I
max
as illustrated in Figure
(a1).
In this compensation mode the capacitive reactance
X
C
, is
selected so as to
produce the
rated compensating voltage with
I
=
I
min
, i.e
.,
V
Cmax
=
X
C
I
min
. As
current
I
min
is increased
toward
I
max
,
the turn-off delay angle
γ
is
increased to reduce the duration of the capacitor injection and thereby maintain the compensating voltage with increasing line current
.Slide8
In the impedance compensation mode, the GCSC
is to maintain the maximum rated
compensating reactance
at any line current up to the rated maximum. In this compensation mode the capacitive
impedance is chosen so as to provide the maximum series compensation at rated current,
X
C
= Vcmax/
Imax, that
the GCSC can vary in the 0 <= XC
(γ
) <=
X
C
range by controlling
the
effective capacitor voltage
V
CF
(
γ
), i.e
.,
X
C
(
γ
) =
V
CF
(
γ
)/
I
. Slide9
Thyristor-Switched Series Capacitor (TSSC)
The operating
principle: the degree of series compensation is controlled in a step-like manner by increasing or decreasing
the number of series capacitors inserted. A capacitor is inserted by turning off, and it is bypassed by turning on the corresponding thyristor valve. A thyristor valve commutates "naturally," that is, it turns off when the current crosses zero. Thus a capacitor can be inserted into the line by the thyristor
valve only at the zero crossings of the line current. Slide10
Since the insertion takes place at line current zero, a full half-cycle of the line current will charge the capacitor from zero to maximum and the successive, opposite polarity half-cycle of the line current will discharge it from this maximum to
zero.
As can be seen, the
capacitor insertion
at line current zero, necessitated by the switching limitation of the thyristor valve, results in a dc offset voltage which is equal to the amplitude of the ac
capacitor voltage
. In order to minimize the initial surge current in the valve, and the corresponding circuit transient, the
thyristor valve should be turned on for bypass only when the capacitor voltage is zero. With the prevailing dc offset, this requirement can cause
a delay of up to one full cycle, which would set the theoretical limit for the
attainable response time of the TSSC.Slide11
Thyristor-Controlled Series Capacitor (TCSC)
It
consists of the
series compensating capacitor shunted by a TCR. In
a practical TCSC implementation, several such basic compensators may be connected
in series to
obtain the desired voltage rating and operating
characteristics. This arrangement is similar in
structure to
the TSSC and, if the impedance of the reactor,
X1
,
is
sufficiently
smaller than that of
the capacitor
,
X
C
, it can
be
operated in an
on/off manner like
the TSSC
.
Basic TCSC SchemeSlide12
However, the basic idea behind the TCSC scheme is to provide a continuously variable capacitor by means of partially canceling the effective compensating capacitance by the TCR
.Slide13
Damping effects of TCSCSlide14
Applications of variable series compensation -TCSC
Power flow control
Enhancing
transient stability
Damping of power swings Sub-synchronous resonance damping Slide15
TCSC at a Substation Slide16
Static Synchronous Series Compensator
(SSSC)
The
SSSC is one of the most recent FACTS devices for power transmission series compensation. It can be considered as
a synchronous voltage source as it can inject an almost sinusoidal voltage of variable and controllable amplitude and phase
angle, in
series with a transmission line. The
injected voltage is almost in quadrature with the line current. A small part of
the injected voltage that is in phase with the line current provides the
losses in the inverter.
Most of the injected voltage, which is
in quadrature
with the line current, provides the effect of
inserting an
inductive or capacitive reactance
in series
with
the transmission
line. The variable reactance influences the
electric power
flow in the transmission line. The basic configuration of
a SSSC
is shown in Fig.Slide17
SSSC (a)WITH OUT STORAGE and (b)WITH STORAGESlide18
A static synchronous Series Compensator operated without an external energy source as Reactive Power with output voltage is in quadrature with and fully controllable independently of the transmission line current for the purpose of increasing or decreasing the overall reactive voltage drop across the transmission line and thereby controlling the electric power flow.
The SSSC FACTS device can provide either capacitive or inductive injected voltage compensation, if SSSC-AC injected voltage, (Vs), lags the line current IL by 90º, a capacitive series voltage compensation is obtained in the transmission line and if leads IL by 90º, an inductive series compensation is achieved.Slide19
Theory of the SSSC
Figure 1 shows a single line diagram of a simple Transmission line with an inductive transmission reactance, XL, connecting a sending
end voltage source, and a receiving end voltage source, respectively.Slide20Slide21
The expression of power
flow is given bySlide22
Where
Xeff
is the effective total transmission line reactance between its sending and
Receiving power system ends, including the equivalent “variable reactance” inserted by the equivalent injected voltage (Vs) (Buck or Boost) by the SSSC-FACTS Compensator.