Download UNIT-VII Static Series Compensators

UNIT-VII
Static Series Compensators
(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)
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 waveform
• 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.)
• 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:
1 t
1
sin t  sin  
vC (t )   i (t )dt 

C
C
The amplitude of fundamental capacitor voltage can be
expressed as a function of γ
I
2 1
VCF ( )  (1    sin 2 )
C  
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.
Fundamental component of the series capacitor voltage vs. the
turn-off delay angle γ.
This impedance can be written as
X C ( ) 
1
2
1
(1    sin 2 )
C


In a practical application the GCSC can be operated either to control the
compensating voltage, VCF(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 Imin<= I <=Imax
as illustrated in Figure (a1).
In this compensation mode the capacitive reactance XC, is selected so as to
produce the rated compensating voltage with I= Imin, i.e., VCmax = XC Imin. As
current Imin is increased toward Imax, 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.
• 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, XC = Vcmax/Imax,
that the GCSC can vary in the 0 <= XC(γ) <= XC
range by controlling the effective capacitor voltage
VCF(γ),
i.e.,
XC(γ) = VCF(γ)/I.
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.
• 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.
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, XC, it can be operated
in an on/off manner like the TSSC.
Basic TCSC Scheme
• 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.
X C X L ( )
X TCSC ( ) 
X L ( )  X C

X L ( )  X L
  2  sin 
Damping effects of TCSC
Applications of variable series
compensation -TCSC
•
•
•
•
Power flow control
Enhancing transient stability
Damping of power swings
Sub-synchronous resonance damping
TCSC at a Substation
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.
SSSC (a)WITH OUT STORAGE and (b)WITH STORAGE
• 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 SSSCAC 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.
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.
• The expression of power flow is given by
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.