Download Comparison of Voltage Stability by Using Various Fact

DOI 10.4010/2016.907
ISSN 2321 3361 © 2016 IJESC
Research Article
Volume 6 Issue No. 4
Comparison of Voltage Stability by Using Various Fact Devices
B.T.Ramakrishna Rao1, K.Murali2, K.S. Manoj Varma3, D.Sarath Chandra4
Associate Professor 1, UG Scholar2, 3, 4
Department of EEE
Lendi Institute of Engineering and Technology, Jonnada, Vizianagaram, Andhra Pradesh, India
Abstract:
The development of the modern system has led to an increasing complexity in the study of power systems, and also
presents new challenges to power system stability, and in particular, to the aspects of transient stability and small-signal stability.
So power system engineers are currently facing challenges to increase the power transfer capabilities of existing transmission
system. This is where the Flexible AC Transmission Systems (FACTS) technology comes into effect with relatively low
investment, compared to new transmission or generation facilities. Flexible AC transmission (FACTS) devices use power
electronics components to maintain controllability and capability of electrical power system. Comparison of voltage stability of
power system using various facts devices Fixed Capacitor Thyristor Controlled Reactor (FC-TCR), Static synchronous
compensator (STATCOM), Thyristor controlled Series Capacitor (TCSC), Static synchronous Series Compensator (SSSC) and
Unified Power Flow Controller (UPFC) for power system stability enhancement and improvement of power transfer capability
have been presented in this project. First, power flow results are obtained and then power (real and reactive power) profiles have
been studied for an uncompensated system and then compared with the results obtained after compensating the system using those
five FACTS devices. The simulation results demonstrate the performance of the system for each of the FACTS devices in
improving the power profile and thereby voltage stability of the same. All simulations have been carried out in
MATLAB/SIMULINK environment.
Keywords: FACTS, real and reactive power, FC-TCR, STATCOM, Voltage stability, SSSC, TCSC, power profile, UPFC
INTRODUCTION TO POWER SYSTEM
Modern power system is a complex network comprising of
numerous generators, transmission lines, variety of loads and
transformers. As a consequence of increasing power demand
some transmission lines are more loaded than was planned
when they were built. With the increased loading of long
transmission lines, the problem of transient stability after a
major fault can become a transmission limiting factor.
The stability of a system determines whether the system can
settle down to the original or close to the steady state after
the transient disappear. Transient stability refers to the
capability of a system to maintain synchronous operation in
the large disturbances such as multi-phase short-circuits
faults or switching lines. The resulting system response
involves large excursions of generator rotor angles and is
influenced by the nonlinear power angle relationship.
Stability depends upon both the initial operating conditions
of the system and the severity of the disturbance. Recent
development of power electronics introduces the use of
flexible ac transmission system (FACTS) controllers in
power systems. FACTS controllers are capable of
controlling the network condition in a very fast manner and
this feature of FACTS can be exploited to improve the
voltage stability, and steady state and transient stabilities of a
complete power system.
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This allows increased utilization of existing network closer
to its thermal loading capacity, and thus avoiding the need to
construct new transmission lines.
INTRODUCTION TO FACTS DEVICES
Most of the world’s electric supply systems are
widely interconnected. This is done for economic reasons,
to reduce the cost of electricity and to improve its reliability,
it must however be kept in mind that these inter connections
apart from delivering the power pool power plants and load
centers in in order to pool power generation and reduce fuel
cost . Thus they reduce the overall number of generating
sources, but saying goes a coin has two slides, like wise as
the power transfer grows.
The power system becomes increasingly complex
to operate and system can became less secure for riding
through major outages. It may lead to large power flows with
inadequate control, excessive reactive power, and large
dynamic swings between different parts of the system. Thus
the full potential of a transmission connection cannot be
utilized. It is very difficult to control such transmission of
power in such systems. Most of the controllers designed in
the past were mechanical in nature. But mechanical
controllers have been designed to supplement the potentially
faulty mechanical controllers. These power electronic
controllers are all grouped in a category called FACTS
controllers.
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Classification of FACTS devices:
1. SHUNT CONTROLLER:FC-TCR,
STATCOM
2. SERIES CONTROLLER:TCSC,
SSSC
3. SERIES-SHUNT CONTROLLER:UPFC
1. FIXED
CAPACITOR
THYRISTOR
CONTROLLED REACTOR (FC-TCR):
power grid, reduce system power loss and harmonics,
increase both transmission capacity and limit for transient
voltage. It also has advantage of smaller in dimension.
The control objective of SVC is to maintain the desired
voltage at a high voltage bus. In steady state, the SVC will
provide some steady- state control of the voltage to maintain
it the highest voltage bus at the pre-defined level.
STATIC SYNCHRONOUS COMPENSATOR
STATCOM uses three phases powerful Voltage Sourced
Converter as its core. Its voltage output connects system by
through reactor or transformer. And regulates AC voltage
amplitude and phase of inverter to absorb or produce
reactive power for system. As sourced compensation device,
STATCOM not only monitoring and compensates current
for impact load but also compensate and monitoring
harmonic current.
2.
Basic FC-TCR type static var compensator
If the voltage bus begins fall below its set point range, the
SVC will inject reactive power (Q net) into the system
(within its control limits), thereby increasing the bus voltage
back to its desired voltage less (or TCR will absorb more)
reactive power (within its control limits), and the result will
be to achieve the desired bus voltage. The Fixed Capacitor
Thyristor-Controlled Reactor (FC-TCR) is a var generator
arrangement using a fixed (permanently connected)
capacitance with at thyristor controlled reactor as shown in
Fig.
2. STATIC SYNCHRONOUS COMPENSATOR
(STATCOM):
STATCOM (Static Synchronous Compensator, also known
as SVG). It is an important device for Flexible AC
Transmission System (FACTS), which is the third
generation of dynamic VAR compensation device after FC,
MCR, and TCR type of SVC (Static VAR Compensator). Its
appearance represents the application of most advanced
technology for dynamic VAR compensation. It is also
known as STATCOM when apply in power distribution.
STATCOM is connected parallel in power grid and works as
reactive current source. Its reactive current can be flexibly
controlled and compensate reactive power for system
automatically. It solves problem of harmonics interfere
switching parallel capacitor banks. In another hand, it can
restrain harmonics and improve power quality according to
customers’ needs. STATCOM has superior performance in
lots of aspect such as responding speed, stabilize voltage of
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THYRISTOR
CONTROLLED
SERIES
CAPACITOR (TCSC):
The basic conceptual TCSC module comprises a series
capacitor, C, in parallel with a thyristor-controlled reactor,
LS, as shown in Fig. 3.3. However, a practical TCSC module
also includes protective equipment normally installed with
series capacitors. A metal-oxide varistor (MOV), essentially
a nonlinear resistor, is connected across the series capacitor
to prevent the occurrence of high-capacitor over- voltages.
Not only does the MOV limit the voltage across the
capacitor, but it allows the capacitor to remain in circuit even
during fault conditions and helps improve the transient
stability.
Basic model of TCSC
Also installed across the capacitor is a circuit breaker, CB,
for controlling its insertion in the line. In addition, the CB
bypasses the capacitor if severe fault or equipmentmalfunction events occur. A current-limiting inductor, Ld, is
incorporated in the circuit to restrict both the magnitude and
the frequency of the capacitor current during the capacitorbypass operation. An actual TCSC system usually comprises
a cascaded combination of many such TCSC modules,
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together with a fixed-series capacitor, CF. This fixed series
capacitor is provided primarily to minimize costs.
4.
STATIC
SYNCHRONOUS
SERIES
COMPENSATOR(SSSC):
The Voltage Sourced Converter (VSC) based series
compensators Static Synchronous Series Compensator
(SSSC) was proposed by Gyugyi in 1989. The single line
diagram of a two machine system with SSSC is shown in
Figure. The SSSC injects a compensating voltage in series
with the line irrespective of the line current. From the phasor
diagram, it can be stated that at a given line current, the
voltage injected by the SSSC forces the opposite polarity
voltage across the series line reactance. It works by
increasing the voltage across the transmission line and thus
increases the corresponding line current and transmitted
power.
STATIC SYNCHRONOU SERIES CAPACITOR
In series compensation the capacitor which is connected in
series compensates the inductive reactance of the
transmission line. SSSC output voltage (Vc) is in quadrature
with the line current (I). The voltage across series capacitor
is –jXcI (where Xc is the capacitive reactance of the series
capacitor) and voltage drop across line inductance (XL) is
+jXLI cancel each other thus reducing the effect of line
inductance. Due to this, power transfer capability is
increased [5].The symbolic representation of SSSC using
voltage source converter is shown in figure 4. Supply
voltage from a dc source is converted into ac voltage using
VSC (voltage source converter). Quadrature voltage is
injected into the line through a coupling transformer. This
injected voltage (Vc) lags the line current (I) by 90ºand
series compensation is done. SSSC control flow of real and
reactive power through the system
5.
UNIFIED
POWER
FLOW
CONTROLLER(UPFC):
The UPFC can provide simultaneous control of all basic
power system parameters (transmission voltage, impedance
and phase angle). The controller can fulfill functions of
reactive shunt compensation, series compensation and phase
shifting meeting multiple control objectives. From a
functional perspective, the objectives are met by applying a
boosting transformer injected voltage and an exciting
transformer reactive current. The injected voltage is inserted
by a series transformer. Besides transformers, the general
structure of UPFC contains also a "back to back" AC to DC
voltage source converters operated from a common DC link
capacitor, Figure 1. First converter (CONV1) is connected in
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shunt and the second one (CONV2) in series with the line.
The shunt converter is primarily used to provide active
power demand of the series converter through a common DC
link. Converter 1 can also generate or absorb reactive power,
if it is desired, and thereby provide independent shunt
reactive compensation for the line. Converter 2 provides the
main function of the UPFC by injecting a voltage with
controllable magnitude and phase angle in series with the
line via voltage source.
UNIFIED POWER FLOW CONTROLLER
UPFC is the most modernised device among all the FACTS
devices which can be used to enhance steady-state stability,
dynamic stability, real and reactive power flow and so on.
UPFC consists of two converters. One converter (SSSC) is
connected in series with the transmission line and other
converter (STATCOM) is connected in parallel with the
transmission line. The two converters are coupled through a
common dc link which provides bidirectional flow of real
power between series o/p SSSC and shunt output
STATCOM respectively. For balancing of power between
series and shunt controller it is necessary to maintain
constant voltage across the dc link. Series branch (SSSC) of
the UPFC injects variable magnitude voltage and phase
angle. This improves power flow capability and transient
stability .Shunt branch (STATCOM) maintains the balance
between the real power absorption from or injection into the
system.
PERFORMANCE ANALYSIS OF FACTS DEVICES
1. UNCOMPENSATED SYSTEM
Below figure shows the basic transmission (11kV) model of
an uncompensated system. This model consists of current
measurement block, voltage measurement block, real and
reactive power block and scopes. 11kv voltage is supplied
from the AC voltage source to the system. Source impedance
(0.01+0.001) Ω, Line impedance (5+0.023) Ω and load is
kept constant at 25MW and 50MVAR for the above
transmission line model. Simulation is done using
MATLAB/SIMULINK. Current measurement block is used
to measure the instantaneous source and load current flowing
through the transmission line, Voltage measurement block is
used to measure the source and load voltage. Real and
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reactive power in load side is measured using active and
reactive power measurement block. Scopes display results
after simulation. Above model provides three scopes: one
displays the source voltage (V) and source current (I),
second one displays real (P) and reactive (Q) power and
third one displays load voltage (V1) and load current (I1)
after simulation. Real and reactive power flows obtained
after simulation are shown in below:
Line impedance is kept at (0.01+0.001) Ω and load is fixed
at 25MW and 50MVAR. Results obtained after simulation of
FC-TCR model is shown below:
STATCOM compensated system:
Simulation and results:
The below figure shows the compensated model of static
synchronous compensator.
Uncompensated system
Uncompensated system
Simulating results:
The model is compensated for various capacitance values.
For a particular value of capacitance (350μF) plots for real
power (P), reactive power (Q), load voltage (V1) and load
current (I1) are shown below:
Fixed
capacitor
compensation:
thyristor
controlled
reactor
Simulation and results:
The SIMULINK model of FC-TCR (SVC) with line voltage
of 11KV is shown below:
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Thyristor controlled series capacitor compensated
system:
Simulation and results:
The circuit model used for simulation is shown below:
The above model shows a Thyristor Controlled Series
Capacitor connected to the system. In TCSC simulation
model, inductor is fixed at 100mH and results are obtained
for different capacitor values. Results obtained after
simulation is shown below:
UPFC compensated system:
Simulation and results:
The below circuit shows the basic model of UPFC (unified
power flow controller) connected to the system. Graphs
obtained after simulation are shown.
Static synchronous series compensated system:
Simulation and results:
The model of the SSSC compensated system is shown
below:
The below configuration shows the compensated model for
Static Synchronous Series Compensator (SSSC) connected
to the system. Real and reactive powers are obtained by
varying the value of capacitance connected in series with the
line.
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The below graphs show real, reactive and receiving end
voltage improvement using compensation. Graphs obtained
for a particular value of capacitor rating (350uF) are shown
below.
Comparison of all FACTS devices:
From the below table, it is seen that reactive power
improvement will vary with change in capacitance in all the
five cases. At a capacitor value of 350μF UPFC is seen to
give best performance and at capacitor value 1500μF,
STATCOM gives better performance. Since, increased
rating of capacitor means increase the cost of equipment. So,
from the above comparison table we can conclude that
UPFC FACTS controller will give optimum performance at
capacitor rating of 350μF.
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Conclusion:
MATLAB/SIMULINK environment is used for this
comparative study to model and simulate FC-TCR type
SVC, STATCOM, TCSC, SSSC, and UPFC connected to a
simple transmission line. This project presents performance
analysis of all the above FACTS devices and an elaborate
comparison between their performances. Power flow and
voltage profile are seen to improve with all the compensating
devices. Results show that in case of FC-TCR and
STATCOM compensation, reactive power flow improves
proportionally with increasing capacitance and is maximum
at maximum value of capacitance (1500 μF here).In case of
TCSC a fixed inductance of 100mH and capacitor value of
1500μF gives best result. For SSSC compensation a
capacitor rating of 350μF yields best result. For UPFC, a
capacitor rating of 350μF gives best results. Voltage
compensation using all the FACTS devices have been
studied. UPFC, SSSC and STATCOM, all are found to give
desirable performances under given operating conditions.
UPFC and SSSC gives their best performance at a capacitor
value of 350μF, but UPFC gives the best performance
amongst these two. But STATCOM fails to give any
impressive performance at this point. However, its
performance continues to improve with increase in
capacitance and it starts giving better performance than
UPFC and SSSC only after its capacitor value is kept around
1200μF. It gives optimum performance at the maximum
capacitor value, i.e. 1500μF. FC-TCR type SVC provides
compensation from a capacitor value as low as 50μF but
gives better performance only at a high value of capacitance.
Its best performance is achieved at the maximum capacitor
value i.e. 1500μF. TCSC behaves in a similar way as SVC
and gives best performance at 1500μF. If rating of capacitor
is increased then cost of the equipment is also increased.
Hence, it can be concluded that UPFC provides most
desirable performance when connected to the system as
compared to other FACTS devices.
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