1.
Paper presented in the National conference on Energy
Management 2009, at
Engineering Collage Bikaner held on 20-21 Nov. 2009
2.
LabVIEW Based Simulation of TCSC FACTS
Controller
Arun Kumar Swami (Student, M E Instrumentation and Control)
Mrs. Lini Mathew (Assistant Professor)
Dr. S. Chatterji (Professor and Head)
Department of Electrical Engineering
N.I.T.T.T.R., Chandigarh.
ABSTRACT
Power is an essential requirement of our life. The demand of power in India is
enormous and is growing steadily. However, because of lack of available investments,
the development of new transmission systems in the country does not follow the increase
in power demand. Power system engineers are currently facing challenges to increase the
power transfer capabilities of existing transmission system. As such, it is necessary to
utilize the existing power transmission system at its maximum capacity to meet
increasing demand of electrical energy.
Flexible AC Transmission System (FACTS) controller can balance the power
flow and there by using the existing power system network most efficiently. Thyristor
controlled series compensator (TCSC), is the first generation of FACTS controller, can
continuously control line impedance through introduction of a thyristor controlled
capacitor in series with the transmission line. TCSC is used as series compensator in
transmission system, it can be designed to control the power flow in order to increase the
power transfer limits or to improve the transient stability.
The TCSC controller can provide a very fast action to increase the
synchronization power through quick changing of the equivalent capacitive reactance to
the full compensation in first few cycles after a fault, hence subsequent oscillations are
damped. In the present work author simulated the TCSC using LabVIEW software, the
design of TCSC controller has been done using transfer function model. The advantage of
using transfer function model is that the change in rotor speed can be directly converted
into compensation required by the TCSC.
3.
I INTRODUCTION
Power is an essential requirement for all facets of our life and has been
recognized as a basic human need. It is the critical infrastructure on which the socio-
economic development of the country depends. The demand of power in India is
enormous and is growing at a very high pace. By the year 2012, India s peak demand
would be 157107 MW. It is essential to raise the amount of transmitted power preferably
with existing transmission facilities in order to meet the increasing demand.
Because of lack of available infrastructures, the development of new
transmission systems in the country does not follow the increase in power demand.
Hence, there is a gap between transmission capacity and actual power demand, and as a
consequence, some transmission lines are loaded more than were planned when they
were built. With increased load on transmission lines, the problem of transient stability
due to a major fault can very well become a transmission power-limiting factor. The
power system must adapt to momentary system conditions, or in other words, power
system should be flexible in nature. This concept has given birth to the term called the
Flexible AC Transmission System (FACTS). The first generation FACTS devices namely
Static Var Compensator (SVC) and Thyristor Controlled Series Compensator (TCSC)
have been introduced in1980s.
II POWER SYSTEM STABILITY
Stability of power system has been a major concern in electrical power system
operation. This arises from the fact that in steady state, the angular speed of all the
generators must remain the same, anywhere in the system. This is termed as synchronous
operation of a system. Any disturbance small or large can affect the synchronous
operation.
Stability as per IEE can be defined as: That attribute of a system which
enables it to develop restoring forces between elements there of equal or greater than
disturbing forces so as to restore a state of equilibrium between elements.
A power system is said to be in steady state stable condition (for a particular
operating condition) if following any small and gradual disturbance, it reaches a steady
state operating condition which is identical or close to the pre disturbance operating
4.
condition. The transient stability can be defined as the capability of the power system to
maintain synchronism when subjected to a severe disturbance. The transient state stability
refers to the maximum flow of power possible through a point without losing the stability
with sudden and large changes in the network conditions brought about by faults, or by
sudden large increment of loads. The maximum power, which can be transmitted within
transient stability, is known as the transient stability limit.
III FACTS TECHNOLOGY FOR TRANSIENT STABILITY ENHANCEMENT
The loss of transient stability in a power system is due to overloading of some
of the lines or due to severe line faults, as a consequence of tripping off of the other lines
after faults or heavy loss of loads. By means of rapid and flexible control over the ac
transmission parameters and network topology, FACTS technology can facilitate power
control, enhance the power transfer capacity, decrease the line losses, increase power
system damping and improve the stability and security of the power system.
The TCSC controller can provide a very fast action to increase the
synchronization power through quick changing of the equivalent capacitive reactance to
the full compensation in first few cycles after a fault, hence subsequent oscillations are
damped.
IV THYRISTOR CONTROLLED SERIES COMPENSATOR
A Thyristor controlled series capacitor (TCSC) improves the overall
performance of the series compensated electrical power system due to fast and flexible
control of the effective reactance produced by the TCSC. In addition, ability of TCSC to
effectively damp sub-synchronous oscillations makes it a versatile electrical power
systems stability controller. In conjunction with series capacitors, it can generate reactive
power that increases with line loading, thereby aiding the regulation of local network
voltages. During events of high short circuit current, the TCSC can switch from the
controllable-capacitance to the controllable-inductance mode, there by restricting the
short circuit currents.
Thyristor controlled series compensator (TCSC) is an effective and
economical means of solving problems of transient stability, dynamic stability, steady
5.
state stability and voltage stability in long transmission lines. Many relevant benefits such
as better utilization of transmission capability, efficient power flow control, transient
stability improvement, power oscillation damping, and fault current limitation etc can be
achieved by adjusting the reactance of the TCSC flexibly and quickly.
TCSC can control the line impedance through the introduction of a thyristor-
controlled capacitor in series with the transmission line and there by continuously control
the power flow by quickly changing the effective reactance of line. The basic module of a
TCSC is shown in Fig.1. It has a series capacitor C, in parallel with a thyristor-controlled
reactor, Ls as shown in Fig.1, whereas a simplified TCSC circuit is shown in Fig.2 for
analysis purpose.
Fig. 1. Basic Module of a TCSC
Fig. 2. Simplified TCSC circuit
Transmission line current is assumed to be the independent variable and is
represented as variable current source, is(t). For the analysis purpose, the line current is
assumed to be sinusoidal. The equivalent TCSC reactance is represented by:
XC2
s + sin s 4XC2
cos 2 (s 2 ) (k tan k (s 2 ) - tan (s 2 ))
X TCSC = X C - +
(X C - X P ) p (
(X C - X P ) k 2 - 1 ) p
(1)
where, XC is the reactance of capacitor
XP is the reactance of inductor connected in parallel with capacitor
is conduction angle
6.
V SYSTEM M ODELING WITH TCSC CONTROLLER USING LABVIEW
Fig. 3. shows the single-line diagram of SMIB power system with TCSC controller.
The swing equation of a synchronous machine may be represented in state space form as:
dw 1
= ( PM - Pe - Pd ) pu/sec. (2)
dt M
= 0( -1 ) elec rad/s (3)
,
E Vt
where, Pe = ,
sin d pu (4)
Xd
E ' = Vt + jX d I pu
'
(5)
dw
Where, M is the accelerating power.
dt
VS Vt V0
Xdd
ZL TCSC
Generator
Fig. 3 SMIB System with TCSC Controller
Electrical power S =Pe +jQe (6)
also S =vt.i
vt - V0
i= (7)
ZL
Fig. 4 LabVIEW Simulation Model of TCSC Controller
The TCSC controller model is shown in Fig. 4.in which rotor angle is input and
deviation in conduction angle is output. Initial conduction angle is added with it and the
7.
output that is sigma is given as input along with XC and X P to sigma to XTCSC sub VI to
find out corrected value of XTCSC as shown in Fig. 4.
The SMIB power system with TCSC controller is simulated as shown in Fig.5.
The TCSC controller model is incorporated in this model. The complete simulation
system has been developed using LabVIEW software is shown in Fig. 5.
Fig. 5 LabVIEW Simulation of SMIB Power System with TCSC Controller
VI SIMULATION RESULTS
The effectiveness of a TCSC controller for enhancing the transient stability
performance of a power system has been analyzed for different conditions. For the
different values of damping constant k, the rotor angle (delta) versus time curve has been
analyzed at various instants of time.
Fig. 4 represents the rotor angle (delta) vs time curve for damping constant k=5.
From this curve, it has been analyzed that at t = 5 sec., rotor angle reaches its maximum
value at 80 degrees. At this point, system becomes unstable. To bring back the system to
8.
stability, the TCSC controller injects a particular value of voltage in the transmission line.
With the result of this voltage, rotor angle decreases to 25 degrees at t =30 sec., and
finally between instants 65 to 70 sec. rotor angle is stable at around 28 degrees.
Fig. 6 Rotor Angle Variation with Time for k = 5
As the values of damping constant is increased to 10, 15, 20, 25, 30 and 40 respectively,
the system attains stability at a faster rate and oscillations are also reduced
Table 1 provides the tabulated results which show that the system attains stability
after 15 sec. when the value of damping constant k = 35 as compared to 70 sec for k = 5.
This is shown in Fig.5.
Table 1 Time taken to Attain Stability of Rotor Angle with Damping Constant k
Damping Time taken to attain Final sigma value
Constant stability (deg)
k (sec)
5 68-72 60
10 40-42 60
20 24 60
30 18 60
35 15 60
9.
Fig. 7 Rotor Angle Variation with Time for k = 35
CONCLUSION
The analysis shows that if compensation is provided through TCSC
controller, then system attains stability at faster rate. The Stability of the system depends
upon reactance of TCSC controller. The value of TCSC controller reactance changes with
the change in conduction-angle of the thyristor in TCSC controller which is governed by
the rotor-angle.
The analysis shows that with changes in the value of damping constant (k)
keeping the controller gain (K) at a constant value the time taken by the system to attain
stable state reduces significantly. This is due to the fact that by increasing damping
constant (k) the oscillations in the system are reduced and the rotor attains stability faster.
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