Theory

Voltage unbalance enhancement using SFCL in a Power feed
Network with Electric railway system

1. INTRODUCTION
Recently, Electric railway systems have gained great interest owing to depletion
of fossil fuels and limits on carbon emissions to prevent global warming. The use of
an electric railway causes a large-scale load owing to demand increase and
performance improvement this system uses a single-phase source supplied through a
Scott transformer from a 3-phase transmission system and has rapidly changing load
characteristics in time. These characteristics can cause voltage unbalances from the
utility viewpoint, single-phase loads that cause voltage unbalances in the transmission
line are constantly being increased. In response to the unbalances, flexible AC
transmission systems (FACTs) are applied to control transmission system power flow
and to improve system stability. A thyristor-controlled series capacitor (TCSC) is one
of the practical devices that can improve the implementation of FACTS. Actual line
voltage and current information is quite important TCSC control scheme.
Fig 1.1 Series compensation concept for a power system
However, voltage and current unbalance produced by an electric railway load causes
serious TCSC control errors. This problem can influence system stability. In
particular, a voltage and current unbalances after fault will cause further problems.
1.1 FACTS:
The power system is an interconnection of generating station to load centres through
high voltage electric transmission lines and in general is mechanically controlled. It
can be divided into three subsystems: generation, transmission and distribution
subsystems. Until recently all three subsystems have been under supervision of one
body within a certain geographical area providing power at regulated rates. In order to
provide cheaper electricity the deregulation of power system, which includes separate
generation, Transmission and distribution companies, is already being implemented.
At the same time electric power demand continues to grow and also the building of
Department of EEE Y.S.R Engineering college of Y.V.U Page 1

the new generating units and transmission circuits is becoming more difficult because
of economic and environmental reasons. Therefore, power utilities are forced to rely
on utilization of existing generating units and to load existing transmission lines close
to their thermal limits. However, stability has to be maintained at all times. Hence, it
is necessary to operate the power system effectively, without reduction in the systems
security and quality of supply, even in the case of contingency conditions. The
contingency may be such as loss of transmission lines and/or generating units, which
occur frequently, and will most probably occur at higher frequencies. So a new
control strategies need to be implemented, to take care of such expected situations. In
the late 1980s the Electric Power Research Institute (EPRI) has introduced a new
Technology program known as FACTS. The main idea behind this program was to
increase controllability and optimize the utilization of the existing power system
capacities by replacing mechanical controllers by reliable and high-speed power
electronic devices.
Power electronic devices have had a radical impact on the electric power systems
around the world. The availability and application of Thyristors have resulted in a
new breed of thyristor-based fast operating devices devised to control and switching
operations. Flexible AC Transmission System (FACTS) devices are new comings,
which have found a widespread application in the power industry for active and
reactive power control. FACTS devices enhance the capacity of the transmission
lines.
These controllers are fast and increase the stability operating limits of the
transmission systems when their controllers are properly tuned. These devices provide
control of the power system through appropriate compensation of network
parameters, such as line series impedance, line shunt impedance, current, voltage, and
real and reactive power. They help the operation of the power network closer to its
thermal limits.
The FACTS technology encompasses a combination of various controllers, each of
which can be applied individually or in a co-ordination with other devices to control
the interrelated parameters of the system. In recent years, the environment, right of
way and high cost problems have delayed the construction of new transmission lines.
This has highlighted the need to change the traditional system concepts and achieve
better utilization of existing lines.

Fig 1.2.Operational limits of transmission lines for different voltage levels
Constraints preventing the use of full thermal capability of conventional AC

circuits are
 Poor power sharing in parallel circuits under different network operating
conditions.
 Transient, dynamic and voltage instability.
 Voltage control and associated reactive power flow problems.
 Fault level constraints.

Technologies available for improving circuit utilization
 Changes to network configuration.
 HVDC.
 FACTS.
Advantages of FACTS technology over other solutions to network reinforcement

 Has potential to control flow as required.
 Less environmental impact than most alternative techniques of transmission

reinforcement.
 Flexible AC transmission system based on power electronics has been
developed to improve the performance of weak AC systems to enhance
transmission capabilities over long AC lines.
 FACTS controllers can be used to solve technical problems in interconnected
power systems
 To improve the synchronous operation of the interconnections and influence
the load flow conditions.
 Cost is also less than other alternatives.
 Due to the invention of modern power electronic components such as Gate
Turn Off (GTO) Thyristor, and Voltage Source Inverter (VSI) technology,
new generation of FACTS devices are developed.
Fig 1.3 conventional& FACTS devices
Numerous kinds of FACTS controllers have been specially made to place in various
parts of the world. They can be classified into four categories
 Series Controllers
 Shunt Controllers
 Combined series-series Controllers
 Combined series-shunt controllers

1.1.1 Series Controllers:
The series controller, as shown in fig 1.4 might be changeable impedance, such as
capacitor, reactor, etc., or a power electronics base variable source of major
frequency, sub synchronic and consonant frequencies to assist the requirement. In
standard, every sequence controllers infuse voltage in sequence with the line. Even
changeable impedance multiply by the current run through it, represent an inject
sequence voltage in the line. The series controller merely provides or consumes
changeable reactive power. Any other phase association will entail management of
real power as well.
1.1.2 Shunt Controllers:
The shunt Controllers, as shown in fig 1.5. Could be variable impedance, variable
source, or a grouping of these. In standard, all shunt Controllers introduce current into
the power system at the point of connection. Even changeable shunt impedance linked
to the line voltage causes a changeable current flow and therefore represent injection
of current into the line. The shunt controller only provides or consumes changeable
reactive power. Every other phase association will engage handling of real power too.
1.1.3 Combined Series-Series Controllers:
This can be a grouping of separate series controllers, which are controlled in a
synchronized way, in a multiline transmission system, or it might be a integrated
controller, in which series controllers provide autonomous series reactive power
compensation for each line but also relocate real power amongst the lines by means of
the power link. The real power transmit ability of the combined series-series
controller makes it achievable to wheel both the real and reactive power flow in the
lines and as a consequence maximize the use of the transmission system.
1.1.4 Combined Series-Shunt Controller:
This can be a mixture of different shunt and series controllers, which are administered
in a synchronized way or a UPFC with series and shunt elements. In theory, united
shunt and series controllers introduce current into the system with the shunt part of the
controller and voltage in series in the line with series part of the controller. on the
other hand, when the shunt and series controllers are combined, there can be a real
power swap over between the series and shunt controllers by the use of the power
link.

Fig 1.4: Series Controllers Fig 1.5: Shunt Controllers
Fig 1.6: Combined series-series controller Fig 1.7: Combine series-shunt

controller
1.2 STATIC VAR COMPENSATOR:
Fig 1.8: Static VAR Compensator
SVC is a general phrase for a thyristor-controlled or thyristor switch reactor And/or

thyristor-switched capacitor or combination as shown in fig1.8 From functioning
point of view, the SVC behaves similar to shunt-attached variable reactance, which
either produces or absorbs reactive power in turn to control the voltage magnitude at
the point of link to the AC network. So it is extensively used for fast reactive power
and voltage regulation support. In power flow analysis, the total susceptance of the
SVC may be taken as variable and additional voltage or reactive power control
equation should be included . The firing angle control of the thyristor enables the
Static VAR Compensator, to cover almost instant speed of response; based on the
operation of SVC it can be configured as variable shunt susceptance model and firing-
angle representation. Furthermore, a compound transformer and SVC representation
based on the SVC firing-angle representation is also available. Fig 1.9 shows the
steady-state and dynamic voltage-current characteristics of the SVC. In the active
control range, current/susceptance and reactive power is varied to regulate voltage
according to a slope (droop) characteristic.
Fig 1.9: V-I Characteristics of SVC Fig1.10: V-Q Characteristics of SVC
The slope value depends on the desired voltage regulation, the desired sharing of
reactive power production between various sources, and other needs of the system.
The slope is typically 1-5%.At the capacitive limit, the SVC becomes a shunt
capacitor. At the inductive limit, the SVC becomes a shunt reactor (the current or
reactive power may also be limited).Fig 1.10 gives the V-Q characteristics of the
SVC.
1.2.1 Shunt Variable Susceptance Model Of Svc:
The susceptance model represents the SVC as a flexible susceptance linked to the
high-voltage bus with susceptance. This model assumes a set voltage at the SVC’s
terminal bus when operating inside the restrictions. For that motive, it is like a PV
bus. Figure 1.11 Shows the shunt susceptance model SVC
Fig 1.11: Susceptance model of SVC

1.2.2 Firing Model of SVC
SVC firing angle model is shown in Fig 1.12.The equivalent reactance Xsvc, which is
function of altering firing angle alpha (α), is made up of the parallel arrangement of a
thyristor controlled reactor (TCR) equivalent admittance and a fixed capacitive
reactance. This representation provides information on the SVC firing angle essential
to accomplish a certain level of compensation.
Fig1.12.firing angle model of SVC
1.3. STATIC SYNCHRONOUS COMPENSATOR (STATCOM)
The STATCOM is a synchronous voltage source which generates a balanced 3-ph

sinusoidal voltage at the fundamental frequency with controllable magnitude and
phase angle. The con-figuration of a STATCOM is shown in Fig: 1.13. Basically it
consists of a voltage source converter (VSC), a coupling transformer and a dc
capacitor. Control of re-active current and hence the susceptance presented to power
system is possible by variation of the magnitude of output voltage (VVSC) with
respect to bus voltage ( VB ) and thus operating the STATCOM in inductive region or
capacitive region. There is a need to respond to dynamic (fast- changing) network
conditions.
Fig.1.13.configuration of STATCOM
The conventional solutions are normally less expensive than FACTS devices but
limited in their dynamic behaviour. It is the task of the planners to identify the most
economic solution. Shunt FACTS devices are classified into two categories, namely
variable impedance type (SVC) and switching converter type (STATCOM).
1.4. DYNAMIC VOLTAGE RESTORER (DVR)
A Dynamic Voltage Restorer (DVR) be the power-electronic-converter based device
that has been designed to protect critical loads from all supply-side disturbances other
than outages. It is connected in series with a distribution feeder and is capable of
generating or absorbing real and reactive power at its ac terminals. The basic principle
of a DVR is simple: by inserting a voltage of required magnitude and frequency, it
can be return the load-side voltage to the required amplitude and even when the
supply voltage of the following waveform has unbalanced or distorted. Usually a
DVR is connected to protect sensitive loads during faults in the supply system.
Fig.1.14.Schematic Diagram of a Typical DVR
Fig.1.14 shows the schematic diagram of a DVR is used as the voltage correction. By
changing the source voltage, the voltage is injecting at the required load voltage
magnitude to maintain them. The voltage source inverter is supplied to DVR which
produce output voltage to the booster transformer voltage is injected. The active
and/or reactive power injection is needed to correct the voltage drop. The reactive
power is generated in DVR, but the injected active power must come from the energy
storage part. Therefore it reduces the active power injection so as to increase the life
of the energy storage system.

1.5 UNIFIED POWER QUALITY CONDITIONER (UPQC)
The main principle of a UPQC is to compensate for contribute the voltage
flicker/imbalance, negative-sequence current, reactive power, and harmonics. During
former expression, the UPQC have the capacity for improving the power quality at the
distribution systems or industrial power systems the power is to be installed. The
UPQC is the common controlling solutions for big capability loads sensitive and to
voltage flicker/imbalance. Unified Power Quality Conditioner (UPQC) designed for
the non-linear loads and voltage sensitive loads such as follows as bellow:
 To improve of utility current quality at the nonlinear loads we should
eliminate the harmonic currents at the supply side
 UPQC provides the requirement of the VAR load, so that the supply voltage
and current are always in phase, therefore, no additional power factor
correction equipment is necessary.
 It maintains rated voltage value at the load end side still in the occurrence of
supply voltage sag.
 The UPQC has to maintain at the load end voltage is injected at the required
value has the same dc link, there is no other dc link voltage supported by the
series compensator.
Fig.1.15.Schematic Diagram of UPQC

The main function of the shunt compensator has to balance for the reactive power
demanded by the load, the harmonic components of nonlinear loads has to be
eliminated so that the source current was sinusoidal and balanced. This equipment is a
good solution for the case when the voltage source presents distortion and a harmonic
sensitive load is close to a nonlinear load.
1.6. THYRISTOR CONTROLLED SERIES COMPENSATOR:
TCSC is a typical series FACTS device comprising a thyristor controlled reactor
across a series capacitor, which is used to differ the reactance of the transmission line.
It reduces the electrical distance of the compensated transmission line. The TCSC
may be lone, big unit, or might consist of numerous equally or different-sized minor
capacitors in turn to attain a advanced performance. Fig1.16 shows the layout of one
phase of TCSC. Since TCSC works through the transmission system directly, it is
much more effectual than the shunt FACTS strategy in the use of power flow run and
power system fluctuation damping control (de Souza et al 1997; Lof et al 1992).
Fig 1.16: Thyristor Controlled Series Compensator model
Thyristor Controlled Series Capacitors (TCSC) is used to solve a specific problem in

transmission systems. large electrical systems are interconnected then it increases the
damping. Sub Synchronous Resonance (R) problem is also solved by using TCSC [3],
on interaction between large thermal generating units and series compensated
transmission systems this procedure involves. The TCSC having a high speed
switching capability and used for controlling power flow in line, in transmission lines,
to various contingencies the line power flow is adjusted rapidly. Steady-state power
flow is also regulated by using TCSC. From a basic technology point of view,
conventional series capacitor is resembles as TCSC [4]. in an isolated steel platform
all the power equipment is placed , the main capacitor bank behaviour is also
controlled by thyristor valve. on ground potential the control and protection is located
together with other auxiliary systems. Figure1.17 shows the structure and operational
diagram of TCSC.

From the boundaries of the operational diagram the firing angle and the thermal limits
of the Thyristors are determined.
Advantages:
 The desired compensation level is controlled continuously
 In the network the power flow is controlled by direct smooth control.
 Capacitor bank protection is also improved.
 Sub-synchronous resonance (SSR) is improved locally. The higher levels of

compensation are permitted in networks, here turbine-generator torsion
vibrations or with other control or measuring systems are interacted.
 In a large interconnected power network areas electromechanical (0.5-2 Hz)

power oscillations are damped. Due to the dynamics of inter area power
transfer these oscillations are occurred. And the total power transfer over a
corridor is high virtual to the transmission power then it exhibits poor
damping.
1.6.1. Variable Series Impedance Power Flow Model:
The TCSC power flow representation offered in this division is based on the trouble-
free concept of a changeable series reactance, the value of which is adjusted without
human intervention to restrict the power flow across the branch to a specific value.
The quantity of reactance is determined well using Newton’s technique.
The varying reactance XTCSC, shown in Figures 1.18 and Fig.1.19 represents the
equivalent reactance of all the series-connected modules making up the TCSC, whilst
functioning in either the inductive or the capacitive regions.

Fig1.18: TCSC equivalent circuit Fig1.19: TCSC equivalent circuit

(Inductive) (capacitive)
1.6.2. Static Model of TCSC:

The model of a transmission line with TCSC connected between bus –i and bus –j is
made known in Figure 1.20. In steady state, the TCSC can be considering as a static
reactance -jxc. The change in the line flow due to series capacitance can be
represented as a line without series capacitance with additional power (complex)
injections at the receiving Sjc and sending Sic ends as shown in Figure 1.21
Fig 1.20: Static model of TCSC Fig 1.21: Injection model of TCSC
1.7. POSSIBLE BENEFITS FACTS CONTROLLERS:

 The FACTS strategies facilitate the transmission system to acquire one or
more of the subsequent advantages
 Control of power flow as commanded. This is the key role of FACTS devices.
The use of power flow control may be to follow an agreement, meet the
utilities’ own requirements, make sure optimal power flow, walk through
emergency circumstances, or a mixture of them.
 Boost the use of low cost power generation. One of the major reasons for
transmission interconnections is to make use of the low expenditure
generation. When this cannot be complete, if follow that there is not sufficient
cost-effective transmission ability. Cost-effective improvement of capability
will consequently allow enlarged use of low cost generation.

 Dynamic stability enhancement. The additional function includes the
transitory steadiness enhancement, power fluctuation damping and voltage
stability control.
 Boost the loading potential of lines to their thermal capability, together with
short period and seasonal loads.
 Offer safe tie-line connections to neighbouring utilities and sectors in this
manner decrease overall generation reserve needs on both ends.
 Upgrading of transmission lines.
 Decrease reactive power flows, therefore permitting the lines to transmit more
active power.
 With balanced reactive power a contractual power is exchanging.
 Especially at industrial machines, metal melting plants, railway or

underground train systems having a huge demand fluctuations so the
compensation of consumers and improvement of power quality is essential.
1.8. APPLICATIONS AND TECHNICAL BENEFITS OF FACTS DEVICES
 Problems of voltage limit
 Addressing in steady state applications
 Problems of thermal limits
 Problems at short circuit levels and problems of sub synchronous resonance

2. TRANSMISSION LINE WITH TCSC AND
ELECTRIC RAILWAY
2.1. TCSC-COMPENSATED TRANSMISSION:
Power transmitted between a sending-end bus and a receiving-end bus in an AC

transmission system is dependent on the series impedance. Further, impedance of a
transmission line consists mainly of inductive reactance, with resistance accounting
for only 5–10% of impedance.
If a series capacitor is inserted into transmission line, the inductive reactance of

transmission line could be compensated by a capacitive supply. This concept of series
compensation is illustrated. Typical configuration of a TCSC from a steady-state
perspective involves a fixed capacitor (FC) with a thyristor controlled reactor (TCR).
Fig2.1. TCSC Closed-loop constant current control methodology

2.2 OPERATION AND CONTROL OF A TCSC SYSTEM:

The equivalent impedance XTCSC of TCSC is as follows:
Xtcsc = XcXTCR/XC−XTCR=( XCXL/XC) / (π (2(π−α)+sin 2α))−XL) (2.1)
Where XL is the reactance of the fixed reactor
α is firing angle of the thyristor measured from the zero crossing, and
XC is reactance of the fixed capacitor.
Control of α typically applies open-loop control or closed loop Control. Fig.2.1 details
a schematic of a constant current Closed-loop control. Iref is desired transmission Line
current, Im is actual current, and Ierror is difference between Iref and Im. In particular,
Ierror is an important quantity in this control loop. A current unbalance can cause
serious issues for TCSC control.

2.3 UNBALANCED LOAD IN AN ELECTRIC RAILWAY SYSTEM:
An electric railway characteristic that most utilities are concerned with is current
unbalance produced by large single-phase loads. These unbalanced currents cause 3-
phase voltage unbalance [6] [7]. To minimize voltage unbalances in 3-phase power
feed networks, Scott transformers are widely used. Nevertheless, an unbalance can be
generated owing to large rapidly changing single-phase loads. Fig.2.3 shows an
unbalance in a 3-phase transmission line voltage produced by a single-phase load in
T-phase of the railway.
Fig 2.3. Transmission system connected with a railway
TABLE 2.1 Load parameters and values
Parameter Zn(Ω) TF(s) a1 a2 b1 b2

Value 10-20 0.01 -40 -120 10-20 3

3. MODELLING OF SFCL AND FAULT OF THE
TRANSMISSION LINE IN AN ELECTRIC RAILWAY
3.1. INTRODUCTION
By using a superconducting technology in power grids result a vital advance
technology. A large number of different energy sources generate rapid changes in the
energy load so new procedure is necessary at power grids.
Now-a-day hierarchically structured grid receives power from a large centralized
power plants, here from the highest-voltage level to the medium-voltage level a
current flows and then to the low-voltage level and finally send to households and
businesses purpose. From producers to consumers to transmit and distribute reliable
electric power, by using grid elements like transformers the voltage levels are
separated from one another. In the course of Germany’s current energy transition,
distribution grids will have to increasingly handle small, decentralized and also
volatile renewable power plants without significant changes to the established
hierarchical grid structure.
However, the instability that can be produced by energy feeds into distribution grids
with these established structures may result in large-scale power cuts[8]. In the final
analysis, there are too many energy sources in the network – sources that feed too
much power into the grid and thus overload it. One solution is to change the
distribution grids’ network structure.
For example, in the future, (new) high-voltage lines will feed wind-generated power
from the north of Germany to the south, while a dense distribution grid will ensure
that this power is reliably distributed to consumers [9]. However, the new power
generation structure may lead to an increase in the amount of short-circuit current in
distribution grids. The grids will lack suitable mechanisms to protect them from this
increased current since the measures that have successfully secured and protected grid
operation to date – such as the use of transformers or similar measures to separate grid
structures – will no longer be effective. As a result, new technologies will be required
to ensure that short circuits do not permanently damage grids in practice.
The use of superconducting technology in short-circuit limiting reactors will support
the existing grid structure. The short-circuit currents occurring in power transmission
and distribution grids will be limited very quickly, effectively, automatically and,
thus, with a high degree of intrinsic safety. After a short cooling period,
superconducting fault current limiters can return to operation without additional
measures.
The special properties of high-temperature superconducting materials are utilized for
this purpose. A number of manufacturers are now marketing second-generation
superconducting bands on a commercial basis. These bands, which are cooled by
liquid nitrogen, completely lose their direct-current resistance in the superconducting
state. In alternating current applications, an extremely small residual resistance
remains. This resistance normally has virtually no impact on current flows. If a certain
“current value” is exceeded, the superconductor reacts immediately and generates
resistance in milliseconds. This resistance limits the increase in current generated by
short-circuits extremely quickly and very effectively, thereby protecting the power
grid. The superconductor is heated in the process. After a short regeneration phase,
the current limiter can return to normal operation. Regeneration like current limitation
is fully automatic and takes place without external intervention.
In the particular case of the superconducting fault current limiter being developed for
Stadtwerke Augsburg, a short-circuit limiting reactor is bypassed by a
superconducting fault current limiter and a downstream circuit breaker. In normal
operation, current flows through the no-loss superconductor. When a short-circuit
current occurs, the superconductor limits it very quickly and shunts it into the limiting
reactor. Current flow through the superconductor is then interrupted to allow the
superconductor to regenerate. As a result, the losses to the power grid from the
limiting reactor occur only during breakdowns. During normal grid operation, the
limiting reactor is “invisible” and current flows through the superconducting fault
current limiter.
3.2. INTERESTING FACTS ABOUT THE SUPERCONDUCTING FAULT
CURRENT LIMITER
 Unlike conventional limiting reactors, resistive superconducting fault current
limiters (SFCLs) have no resistance and no negative impact on a power grid’s
stability. As a result, SFCLs protect grids when short-circuits occur but are
“invisible” for the grid during normal operation.
 Short-circuit limiting reactors are a conventional alternative to

superconducting fault current limiters. They are used, if at all, in distribution
grids (up to 30 kV) primarily in the industry sector. Only in exceptional cases

are short-circuit limiting reactors used in distribution grids (110 kV and
higher).
 As designed by Siemens, superconducting fault current limiters consist of an
SFCL element that bypasses a parallel-connected short-circuit limiting reactor
in normal grid operation. As a result, the limiting reactor is current less and
causes no losses during normal operation. However, the SFCL itself needs
energy for cooling purposes. On a rough estimate, the power loss of an SFCL
can be assumed to total about 50% of the energy consumed by a comparable
short-circuit limiting reactor.
 A large number of short-circuit limiting reactors are currently installed in

power grids. According to some estimates, there are as many as 44,000
installed worldwide. In normal operation, a short-circuit limiting reactor
always has resistance, which typically causes a power loss of 25 kilowatts. As
a result, up to 1,100 megawatts of electricity – or the total output of a large-
scale power plant – are lost worldwide due to the use of short-circuit limiting
reactors.
 Siemens researchers are attempting to reduce these losses by 50% or more

through the use of superconducting fault current limiters – SFCLs themselves
require energy for cooling purposes.
 A superconducting fault current limiter with a rated current of 817 amperes

will secure the connection between Stadtwerke Augsburg’s grid and an
industrial company. In operation, an energy load with a maximum feed-in
power of 15 megawatts will be fed from the company’s grid into the
Stadwerke’s grid. Without appropriate measures, this load would exceed the
permissible short-circuit level.
 During the field test, a short-circuit limiting reactor, which will be used as a
backup solution, will be bypassed by a superconducting fault current limiter.
As a result, the negative impact and the losses due to the limiting reactor in
normal operation will be avoided. The SFCL will only limit the current and
the limiting reactor will only be used if there is a short circuit. This special
arrangement will also make it possible to directly compare the operation of a

conventional solution (short-circuit limiting reactor) with that of a
superconducting fault current limiter (SFCL).
 What is a fault current?

A fault current is the current which flows during a short circuit. At home
we have fuses or magnetic circuit breakers to switch off in case of a short circuit. In a
substation the situation is the same – only the current is much higher – thousands of
ampere. Such a fault current is dangerous for two reasons: Magnetic forces: during
short circuit enormous magnetic forces are created which try to move electric
conductors away from each other. Thermal energy: The high current during a short
circuit heats up all electric conductors in fractions of a second and can lead to a fire.
3.3. SUPERCONDUCTING FAULT CURRENT LIMITER (SFCL)
3.3.1. Fault Current Limiter and SFCL
There are several types of faults in electrical network; they are lightning, short
circuits, grounding etc., so a large fault current is produced. The power system
security is not properly controlled by large currents; due to this unexpected condition
happened like fire, equipment damage, and even blackout [10]. Therefore, by
installing Circuit Breakers cut off fault current, however, to cut it takes minimum
breaking time, and sometimes fail to break. At fault condition to limit very high
current in high speed a Fault Current Limiter (FCL) is applied .A fault current is
limited within 1/4 cycle with high fault limiting speed. Fast and automatically this
task will be recovered. Several types of FCLs are developed and few of them used in
power system. The main function of FCL is to change the circuit from low impedance
circuit to high impedance circuit. Power electronics device and/or Circuit breakers are
used to organize FCL circuits. For the protection of high recovery voltages a fuse or
snubber circuits are used. For the implementation of normal conductor these FCLs are
attractive, however, few weak points are there such as speed and big size in
distribution and transmission level as well current limiting is slow.
For prevention of the fault current in the power grid .The Superconducting fault
current limiter (SFCL) is done a promising duty. It has some characteristics, they are
at critical temperature (Tc) and critical current (Ic) the resistance is zero .A
superconductor lose its superconductivity when fault current exceeds Ic and to limit
circuit current the resistance increase radically (called quench).

3.3.2. Classification of SFCL
Various types of SFCLs have been built and showed desired current limitation up to
medium voltages. Some of them were actually field-tested in the electrical power grid.
However, the SFCLs seem to be not near to commercial operation in the grid. This
means that the SFCL is not ready to satisfy the utilities in various conditions. The
conditions are dependent upon the application conditions, general purpose
applications and special purpose ones. We can classify these SFCLs as three types,
which are resistance type(R-type), Inductance type (L-type) and saturable core type.
R-type makes use of quench resistance of superconductor directly [11]. L-type makes
use of superconductor as trigger element for circuit inductance which limits fault
current. Saturable core type makes use of superconductor magnet to saturate reactor
iron core. In normal operation, this reactor has a little reactance in saturation state.
However in fault state, fault current releases saturation state and increases impedance,
therefore limits fault current.
3.3.3. Superconducting Technologies
For the carrying of electric power and for the limitation of peak currents the
superconductors are used. It has been around since the realization and the discovery of
superconductors that they have extremely non-linear properties. Other distinctively,
based on nonlinear response to temperature, the current and magnetic field variations
current limiting behaviour depending. Any of these three parameters can cause a
transition between the superconducting and the normal conducting system. The curve
in the lower half of is a normalized plot showing the non-linear relation between
current flow in a superconductor and its resistance.
The data for the curve was measured while the superconductor was in a constant
magnetic field and a constant temperature. Similar curves can be produced for
changes in temperature and magnetic field. Sections of superconductor caused by
current increase and happen to resistive that the generated heat cannot be isolated
locally. This surplus heat is transferred through the conductor, so the temperature of
nearby sections is increases. The mutual current and temperature can cause these
regions to become usual and generate heat. For the description of the propagation of
the normal zone through a superconductor the word “quench” used. The quench
process is frequently speedy and uncontrolled once initiated.

The quench process is uncontrolled after initiation, the amount of the normal region
and the rise of temperature in the materials to be calculated. Therefore in
superconducting components designing the quench process can be used .Several
hundred patents exist showing theoretical ways in which this phenomenon might be
used to control fault currents in the electric power grid. However, efforts to develop
the concepts into commercially viable product have culminated in only a few practical
designs and even fewer working prototypes. Many of these designs have
shortcomings (e.g. size, performance, reliability, recovery under load, or cost) that
hinder them from reaching full commercial potential. Most SFCL designs use the
aforementioned quench behaviour to limit fault currents within the first cycle.
However, that is often where the similarities end as each SFCL design has its own
methods of sustaining the limiting action once the superconductor becomes resistive.
To help establish a fundamental understanding of SFCL technologies, this section
provides a simplified and hopefully effective overview by condensing various design
concepts into three SFCL types. The first two types depend on the quenching action
of superconductors and the third type uses DC HTS magnet windings to saturate an
iron core.
3.3.4. Resistive SFCL
Resistive SFCLs utilize the superconducting material as the main current carrying
conductor under normal grid operation. The principle of their operation is shown in
the one-line diagram at the top of Figure 3.1. As mentioned above, the lower figure is
a normalized plot of voltage across Rsc as a function of the ratio of current through
the device, Iline, to the “critical current”, Ic, of the superconducting element.
At present, for HTS materials, the convention is to define “critical current” as the
current at which a voltage drop of 1.0 μV/cm is observed along the conductor. When a
fault occurs, the current increases and causes the superconductor to quench thereby
increasing its resistance exponentially. The current level at which the quench occurs is
determined by the operating temperature, and the amount and type of superconductor.
The rapid increase in resistance produces a voltage across the superconductor and
causes the current to transfer to a shunt, which are a combined inductor and resistor.
The shunt limits the voltage increase across the superconductor during a quench. In
essence, the superconductor acts like a switch with millisecond response that initiates

the transition of the load current to the shunt impedance. Ideally, the incipient fault
current is limited in less than one cycle.
Iline/Ic
Fig. 3.1.Plot of voltage and current for SFCL
Early resistive SFCL designs experienced issues with “hot spots”, or non-uniform
heating of the superconductor during the quench. This is a potential failure mode that
occurs when excessive heat damages the HTS material. Recent advances in
procedures for manufacturing HTS materials coupled with some creative equipment
designs have reduced the hot-spot issue. The grid characteristic of the resistive SFCL
after a quench is determined by the shunt element.
Thus, because the shunt is typically quite reactive, a resistive SFCL typically
introduces significant inductance into the power system during a fault. During the
transition period when current is being transferred from the superconductor to the
shunt, the voltage across the combined element shown in Fig 3.1 is typically higher
than it is after the current has transitioned into the shunt. The dynamics of this process
depend on the two elements and their mutual inductance.
The quench process in resistive SFCLs results in heat that must be carried away from
the superconducting element by the cryogenic cooling system. Typically, there is a
momentary temperature rise in the superconducting element that causes a loss of
superconductivity until the cryogenic system can restore the operating temperature.
This period of time, known as the recovery time, is a critical parameter for utility
systems (which may see multiple fault events occurring close together in time) and is
a key distinguishing characteristic among various SFCL designs.

Some resistive SFCLs include a fast switching component in series with the
superconducting element. This switch quickly isolates the superconductor after most
of the current has transitioned to the shunt element, allowing the superconducting
element to begin the recovery cycle while the limiting action is sustained by the shunt.
The fast-acting switch reduces the peak temperature within the superconductive
material and allows for faster recovery times than for purely resistive SFCLs. This
type of SCFL is sometimes referred to as a hybrid SFCL.
3.3.5. Shielded-Core SFCL
One of the first SFCL designs developed for grid deployment was the shielded-core
design, a variation of the resistive type of limiter that allows the HTS cryogenic
environment to remain mechanically isolated from the rest of the circuit. By using
mutual coupling of AC coils through a magnetic field, an electrical connection is
made between the line and the HTS element.
Essentially, by using an HTS element a transformer with the secondary side is
shunted. At fault condition, the current is increased on the secondary causes the
quenching of the HTS element, so across L1 the voltage increases, that opposes the
fault current.
3.3.6. Operation of The Saturable-Core SFCL
Under nominal grid circumstances (when the AC current does not exceed the
maximum rating for the local system), the iron is fully saturated by HTS coil. So the
relative permeability is one. The iron acts like air in the AC coils.
So the AC impedance (inductive reactance) is related to that of an air-core reactor.
Under fault conditions, due to negative and positive current peaks the core comes out
from saturation, causing the line impedance is increasing in part of each half cycle. as
a result peak fault current is reduced considerably. During a limiting action, the
dynamic action of the core moving instantaneously in and out of saturation produces
current harmonics in the waveform. Nevertheless, at normal operating conditions, the
voltage and current waveforms are mostly unaffected by the saturable-core SFCL.
Basically, the saturable-core SFCL is an iron-core reactor with variable-inductance
under normal grid conditions that has the impedance of an air-core reactor and at fault
conditions has very high impedance. Distinct with resistive SFCLs, they need time
among limiting actions to cool the superconducting elements, the saturable-core move
towards can handle quite a few actions in sequence since the superconductor does not

quench. In detail, here not use a superconducting coil in the saturable-core FCL;
nevertheless, operating losses are reduced and make the more compact winding with
the use of an HTS DC field winding.
A most important disadvantage of saturable-core SFCL technology is the size and
weight related with the heavy iron core; on the other hand, manufacturers hope to
improve this subject in future prototypes. a prototype saturable-core SFCL’s energy
has newly tested based on an completely new design model that is four times smaller
than its predecessor. Grid ON, an Israeli-based start-up company, is in the process of
developing a saturable-core concept intent on reducing size and weight to more
accommodating levels for commercial use.
3.3.7. Major Benefits from SFCL Applications
 Effective Fault Current Limitation
 Transparency
 Passivity / self-triggered
 Fast reaction (sub-cycle reaction)
 Resettable
 No environmental impact
3.3.8. Modelling of Resistor-Type SFCL:
In order to limit a fault current, many models for the SFCL have been developed:
resistor-type, reactor-type, transformer type, etc. In this study, we modelled a resistor-
type SFCL that is mostly basic and used widely which represents the experimental
studies for superconducting elements of SFCL. Quench characteristics and recovery
characteristics of a resistor-type SFCL are modelled based on.
And an impedance of the SFCL according to time t is given as follows:
0 (t<t0)
zn [1- exp(- ( − 0)/ )] (t0≤t<t1) (3.1)
Z(t) = a1(t-t1)+b1 (t1≤t<t2)
a2(t-t2)+b2 (t≥t2)

3.4. FAULT MODELLING OF THE TRANSMISSION LINE IN THE
ELECTRIC RAILWAY:
An electric railway is connected to a 154 kV transmission line through a Scott
transformer in Korea. Fig.3.2 shows a transmission system connected with railway
equivalent model. A TCSC facility is installed to control power flow in transmission
line, and an electric railway includes a single phase load that causes a voltage
unbalance. Table 3.1shows the transmission and electric railway system parameters of
simulation. In addition, the self- and mutual impedance of transmission and rail of the
Korean electric railway system were considered. Here, in order to analyze the
influence of an electric railway connection on the transmission line fault.
TABLE 3.1
Fig.3.2. Transmission line three-phase fault simulation equivalent circuit and fault
location

We simulated a fault situation. Fig. 3.2 shows fault location on the transmission line
with TCSC. Fault starting time is 0.4 s and fault duration time is 0.1 s. show
simulation results for a 3-phase fault for an electric railway connection, respectively.
In case of fault on the transmission line without electric railway, 3-phase voltage
decreased owing to fault and after fault removal, there is a transient phenomenon that
has a small offset voltage but returned to a steady state. In addition, this simulation
showed a small magnitude difference between offset voltages. Current is generated a
large transient current during fault and reach to steady state, load current, after fault
removal. In contrast, large offset voltages are represented in simulation result about
fault on transmission line with electric railway. Especially, voltage of phase B is
increased up to 350 kV in a moment. In addition, voltage unbalances of transmission
line became more serious compared with above case. Line currents are also increased
and caused unbalance owing to transient voltage. We think that closing phase angle
control of TCSC system is influenced by generated transient voltage and current as
the cause of these results. This phenomenon will cause problem about voltage
stability and malfunction of protection scheme in the transmission grid. These results
show that an electric railway connection aggravates voltage unbalance in the
transmission line.
Fig 3.3 Voltage and current results from the simulation without railway

4. SIMULATION THEORY
4.1GENERAL
MATLAB (matrix laboratory) is a numerical computing environment and
fourth-generation programming language. Developed by MathWorks, MATLAB
allows matrix manipulations, plotting of functions and data, implementation of
algorithms, creation of user interfaces, and interfacing with programs written in other
languages, including C, C++, Java, and Fortran. Although MATLAB is intended
primarily for numerical computing, an optional toolbox uses the MUPAD symbolic
engine, allowing access to symbolic computing capabilities. An additional package,
Simulink, adds graphical multi-domain simulation and Model-Based Design for
dynamic and embedded systems.
In 2004, MATLAB had around one million users across industry and
academia. MATLAB users come from various backgrounds of engineering, science,
and economics. MATLAB is widely used in academic and research institutions as
well as industrial enterprises.
4.2MATLAB HISTORY
Cleve Moler, the chairman of the computer-science department at the
University of New Mexico, started developing MATLAB in the late 1970s. He
designed it to give his students access to LINPACK and EISPACK without them
having to learn Fortran. It soon spread to other universities and found a strong
audience within the applied mathematics community. Jack Little, an engineer, was
exposed to it during a visit Moler made to Stanford University in 1983. Recognizing
its commercial potential, he joined with Moler and Steve Bangert. They rewrote
MATLAB in C and founded MathWorks in 1984 to continue its development. These
rewritten libraries were known as JACKPAC. In 2000, MATLAB was rewritten to
use a newer set of libraries for matrix manipulation, LAPACK.
MATLAB was first adopted by researchers and practitioners in control
engineering, Little's specialty, but quickly spread to many other domains. It is now
also used in education, in particular the teaching of linear algebra and numerical
analysis, and is popular amongst scientists involved in image processing.

4.3SIMULINK
Simulink, developed by MathWorks, is a commercial tool for modeling,
simulating and analyzing multi-domain dynamic systems. Its primary interface is a
graphical block diagramming tool and a customizable set of block libraries. It offers
tight integration with the rest of the MATLAB environment and can either drive
MATLAB or be scripted from it. Simulink is widely used in control theory and digital
signal processing for multi-domain simulation and Model-Based Design
Simulink is a block diagram environment for multi-domain simulation and
Model-Based Design. It supports system-level design, simulation, automatic code
generation, and continuous test and verification of embedded systems. Simulink
provides a graphical editor, customizable block libraries, and solvers for modeling
and simulating dynamic systems. It is integrated with MATLAB, enabling you to
incorporate MATLAB algorithms into models and export simulation results to
MATLAB for further analysis.
4.4BUILDING THE MODEL
Simulink provides a set of predefined blocks that you can combine to create a
detailed block diagram of your system. Tools for hierarchical modeling, data
management, and subsystem customization enable you to represent even the most
complex system concisely and accurately.
4.4.1SELECTING BLOCKS
The Simulink Library Browser contains a library of blocks commonly used to
model a system. As shown in Fig.4.2, these include:
Fig.4.1: Building a new model

 Continuous and discrete dynamics blocks, such as Integration and Unit Delay
 Algorithmic blocks, such as Sum, Product, and Lookup Table
 Structural blocks, such as Mux, Switch, and Bus Selector
We can build customized functions by using these blocks or by incorporating
hand-written MATLAB, C, Fortran, or Ada code into the model. Thecustom blocks
can be stored in their own libraries within the Simulink Library Browser.
Fig.4.2: Commonly used blocks

Simulink add-on products let you incorporate specialized components for
aerospace, communications, PID control, control logic, signal processing, video and
image processing, and other applications. Add-on products are also available for
modeling physical systems with mechanical, electrical, and hydraulic components.
To build a model as shown in Fig.4.1 by dragging blocks from the Simulink
Library Browser into the Simulink Editor, we then connect these blocks with signal
lines to establish mathematical relationships between system components. Graphical
formatting tools, such as smart guides and smart signal routing, help we control the
appearance of the model as we build it. We can add hierarchy by encapsulating a
group of blocks and signals as a subsystem in a single block.
The Simulink Editor gives a complete control over what we see and use within
the model. For example, we can add commands and submenus to the editor and
context menus. We can also add a custom interface to a subsystem or model by using
a mask that hides the subsystem's contents and provides the subsystem with its own
icon and parameter dialog box.

4.4.2NAVIGATING THROUGH THE MODEL HIERARCHY
The Explorer bar and Model Browser in Simulink helps to navigate the model.
The Explorer bar indicates the level of hierarchy that we are currently viewing and
lets we can move up and down the hierarchy. The Model Browser provides a
complete hierarchical tree view of your model, and like the Explorer bar, can be used
to move through the levels of hierarchy.
4.4.3MANAGING SIGNALS AND PARAMETERS
Simulink models contain both signals and parameters. Signals are time-
varying data represented by the lines connecting blocks. Parameters are coefficients
that define system dynamics and behavior. Simulink helps to determine the following
signal and parameter attributes as shown in Fig.4.3:
 Data type—single, double, signed, or unsigned 8-, 16- or 32-bit integers;Boolean;
enumeration; or fixed point
 Dimensions—scalar, vector, matrix, N-D, or variable-sized arrays
 Complexity—real or complex values
 Minimum and maximum range, initial value, and engineering units
If we choose not to specify data attributes, Simulink determines them
automatically via propagation algorithms, and conducts consistency checking to
ensure data integrity. These signal and parameter attributes can be specified either
within the model or in a separate data dictionary. We can then use the Model Explorer
to organize, view, modify, and add data without navigating through the entire model
as shown in Fig.4.4.
Fig.4.3: Signal Attributes tab

Fig.4.4: Model Explorer Window

4.4.4SIMULATING THE MODEL
We can simulate the dynamic behavior of the system and view the results as
the simulation runs. To ensure simulation speed and accuracy, Simulink provides
fixed-step and variable-step ODE solvers, a graphical debugger, and a model profiler.
4.4.5CHOOSING A SOLVER
Solvers as shown in Fig.4.5 are numerical integration algorithms that compute
the system dynamics over time using information contained in the model. Simulink
provides solvers to support the simulation of a broad range of systems, including
continuous-time (analog), discrete-time (digital), hybrid (mixed-signal), and multirate
systems of any size.
Fig.4.5: Configuration Parameters dialog box showing the Solver pane

These solvers can simulate stiff systems and systems with
discontinuities. We can specify simulation options, including the type and properties
of the solver, simulation start and stop times, and whether to load or save simulation
data. We can also set optimization and diagnostic information. Different
combinations of options can be saved with the model.

4.4.6RUNNING THE SIMULATION
We can run your simulation interactively from the Simulink Editor or
systematically from the MATLAB command line. The following simulation modes
are available:
 Normal (the default), which interpretively simulates the model
 Accelerator, which increases simulation performance by creating and
executing compiled target code but still provides the flexibility to change
model parameters during simulation
 Rapid Accelerator, which can simulate models faster than Accelerator mode
by creating an executable that can run outside Simulink on a second
processing core
To reduce the time required to run multiple simulations, we can run those
simulations in parallel on a multi-core computer or computer cluster.
4.4.7ANALYZING SIMULATION RESULTS
After running a simulation, we can analyze the simulation results in MATLAB
and Simulink. Simulink includes debugging tools to help to understand the simulation
behavior.
4.4.8VIEWING SIMULATION RESULTS
We can visualize the simulation behavior by viewing signals with the displays
and scopes provided in Simulink. We can also view simulation data within the
Simulation Data Inspector, where we can compare multiple signals from different
simulation runs. Scope is the block in Simulink by which we can measure and view
the voltage, current, and power in electrical domain. Fig.4.6 shows the output of a
multilevel converter through scope.
Alternatively, we can build custom HMI displays using MATLAB, or log
signals to the MATLAB workspace to view and analyze the data using MATLAB
algorithms and visualization tools.
Fig.4.6: Multi-step waveform

4.4.9DEBUGGING THE SIMULATION
Simulink supports debugging with the Simulation Stepper, which lets we step
back and forth through your simulation viewing data on scopes or inspecting how and
when the system changes states. With the Simulink debugger we can step through a
simulation one method at a time and examine the results of executing that method. As
the model simulates, you can display information on block states, block inputs and
outputs, and block method execution within the Simulink Editor.
4.5SIM POWER SYSTEMS
SimPowerSystems™ provides component libraries and analysis tools for
modeling and simulating electrical power systems. The libraries include models of
electrical power components, including three-phase machines, electric drives, and
components for applications such as flexible AC transmission systems (FACTS) and
renewable energy systems. Harmonic analysis, calculation of total harmonic distortion
(THD), load flow, and
other key electrical power system analyses are automated. SimPowerSystems was
developed by Hydro-Québec of Montreal.
SimPowerSystems models as shown in Fig.4.7 can be used to develop control
systems and test system-level performance. We can parameterize the models using
MATLAB® variables and expressions, and design control systems for the electrical
power system in Simulink®. We can add mechanical, hydraulic, pneumatic, and other
components to the model using Simscape™ and test them all in a single simulation
environment. To deploy models to other simulation environments, including
hardware-in-the-loop (HIL) systems, SimPowerSystems supports C-code generation.
Starting with MathWorks Release 13, SimPowerSystems and SimMechanics
of the Physical Modeling product family work together with Simulink® to model
electrical, mechanical, and control systems. Electrical power systems are
combinations of electrical circuits and electromechanical devices like motors and
generators. Engineers working in this discipline are constantly improving the
performance of the systems. Requirements for drastically increased efficiency have
forced power system designers to use power electronic devices and sophisticated
control system concepts.

That tax traditional analysis tools and techniques. Further complicating the
analyst’s role is the fact that the system is often so nonlinear that the only way to
understand it is through simulation.
Land-based power generation from hydroelectric, steam, or other devices is
not the only use of power systems. A common attribute of these systems is their use
of power electronics and control systems to achieve their performance objectives.
SimPowerSystems was designed to provide a modern design tool that allows
scientists and engineers to rapidly and easily build models that simulate power
systems.
SimPowerSystems uses the Simulink environment, allowing you to build a
model using simple click and dragprocedures. Not only can you draw the circuit
topology rapidly, but your analysis of the circuit can include its interactions with
mechanical, thermal, control, and other disciplines. This is possible because all the
electrical parts of the simulation interact with the extensive Simulink modeling
library. Since Simulink uses MATLAB® as its computational engine, designers can
also use MATLAB toolboxes and Simulink block sets. SimPowerSystems and Sim
Mechanics share a special Physical Modeling block and connection line interface.
Users can rapidly put SimPowerSystems to work. The libraries contain models
of typical power equipment such as transformers, lines, machines, and power
electronics. These models are proven ones coming from textbooks, and their validity
is based on the experience of the Power Systems Testing and Simulation Laboratory
of Hydro-Québec, a large North American utility located in Canada. The capabilities
of SimPowerSystems for modeling a typical electrical grid are illustrated in
demonstration files. And for users who want to refresh their knowledge of power
system theory, there are also self-learning case studies.
Fig.4.7:SimPowerSystems pane

4.6MODELING ELECTRICAL POWER SYSTEMS
With SimPowerSystems, we build a model of a system just as we would
assemble a physical system. The components in the model are connected by physical
connections that represent ideal conduction paths.
This approach describes the physical structure of the system rather than
deriving and implementing the equations for the system. From the model, which
closely resembles a schematic, SimPowerSystems automatically constructs the
differential algebraic equations (DAEs) that characterize the behavior of the system.
These equations are integrated with the rest of the Simulink model.
We can use the sensor blocks in SimPowerSystems to measure current and
voltage in your power network, and then pass these signals into standard Simulink
blocks. Source blocks enable Simulink signals to assign values to the electrical
variables current and voltage. Sensor and source blocks connect a control algorithm
developed in Simulink to a SimPowerSystems network.
4.7MODELING CUSTOM COMPONENTS
SimPowerSystems enables to model custom components by using the
fundamental elements included in its libraries and by combining these elements with
Simulink blocks.
Fig.4.8:Simpower system Libraries

Components provided in SimPowerSystems as shown in Fig.4.8 include:
Electrical elements: Linear and saturable transformers; arrestors and breakers;
and transmission line models.

Electric machinery: Models of synchronous, permanent magnet synchronous,
and DC machines; excitation systems; and models of hydraulic and steam turbine-
governor systems.
Power electronics: Diodes, simplified and complex thyristors, GTOs,
switches, IGBT models, and universal bridges that allow selection of standard bridge
topologies.
Control and measurement: Voltage, current, and impedance measurements;
RMS measurements; active and reactive power calculations; timers, multimeters, and
Fourier analysis; HVDC control; total harmonic distortion; and abc-to-dq0 and dq0-
to-abc transformations.
Electrical sources: To implement sinusoidal current source, sinusoidal
voltage source, generic battery model, Controlled AC Current and Voltage sources,
DC Voltage Source. To implement three-phase voltage source with programmable
time variation of amplitude, phase, frequency, and harmonics, and to implement
three-phase source with internal R-L impedance. The entire blocksets is shown in
Fig.4.9.
Fig.4.9: Blocksets of electrical sources used in SimPowerSystems

Three-phase components: RLC loads and branches; breakers and faults; pi-
section lines; voltage sources; transformers; synchronous and asynchronous
generators; and motors, analyzers, and measurements.
Electric Drives and Other Application Libraries
SimPowerSystems provides the following specialized application libraries:

Flexible AC Transmission Systems (FACTS): Phasor models of flexible AC
transmission systems.
Distributed Resources: Phasor models of wind turbines.
Electric Drives: Editable models of electric drives that include detailed
descriptions of the motor, converter, and controller for each drive. The Electric Drives
library includes permanent magnet, synchronous, and asynchronous (induction)
motors. The converters and controllers implement the most common strategies for
controlling the speed and torque for these motors, such as direct-torque control and
field-oriented control.
SimPowerSystems supports the development of complex, self-contained
power systems, such as those in automobiles, aircraft, manufacturing plants, and
power utility applications. You can combine SimPowerSystems with other
MathWorks physical modeling products to model complex interactions in multi-
domain physical systems. The block libraries and simulation methods in
SimPowerSystems were developed by Hydro-Québec of Montreal.
Fig.4.10: Circuit of a transmission line
Fig.4.11: Same circuit designed in Simulink window

Thus users can rapidly put SimPowerSystems to work. The libraries that
containing models of typical power equipment such as transformers, lines, machines,
and power electronics are used to construct a electrical circuit shown in Fig.4.10 and
the completely designed circuit of the same in Simulink window as shown in Fig.4.11.
4.8CONNECTING TO HARDWARE
We can connect the Simulink model to hardware for rapid prototyping,
hardware-in the-loop (HIL) simulation, and deployment on an embedded system.
4.8.1RUNNING SIMULATIONS ON HARDWARE
Simulink provides built-in support for prototyping, testing, and running
models on low-cost target hardware, including Arduino®, LEGO® MINDSTORMS®
NXT, PandaBoard, and BeagleBoard. We can design algorithms in Simulink for
control systems, robotics, audio processing, and computer vision applications and see
them perform in real time.
Fig.4.12: Hardware Interface to simulink
Simulink provides built-in support for prototyping, testing, and running

models on low-cost target hardware, including Arduino®, LEGO® MINDSTORMS®
NXT, and BeagleBoard as shown in Fig.4.13.

Fig.4.13:Low-cost target hardware
With Real-Time Windows Target™, we can run Simulink models in real time
on Microsoft® Windows® PCs and connect to a range of I/O boards to create and
control a real-time system as shown in Fig.4.12. To run the model in real time on a
target computer, we can use xPC Target™ for HIL simulation, rapid control
prototyping, and other real-time testing applications. See xPC Target Turnkey for
available target computer hardware. Simulink models can be configured and made
ready for code generation. By using Simulink with add-on code generation products,
you can generate C and C++, HDL, or PLC code directly from your model.
4.9APPLICATIONS
A number of MathWorks and third-party hardware and software products are
available for use with Simulink. For example, Stateflow extends Simulink with a
design environment for developing state machines and flow charts. Coupled with
Simulink Coder, another product from MathWorks, Simulink can automatically
generate C source code for real-time implementation of systems. As the efficiency
and flexibility of the code improves, this is becoming more widely adopted for
production systems, in addition to being a popular tool for embedded system design
work because of its flexibility and capacity for quick iteration. Embedded
Coder creates code efficient enough for use in embedded systems.
Target together with x86-based real-time systems provides an environment to
simulate and test Simulink and Stateflow models in real-time on the physical system.
Embedded Coder also supports specific embedded targets, including Infineon
C166, Motorola68HC12, Motorola MPC 555, TI C2000, TI C6000, RenesasV850 and
RenesasSuperH. With HDL Coder, also from MathWorks, Simulink and Stateflow
can automatically generate synthesizableVHDL and Verilog.
Simulink Verification and Validation enables systematic verification and
validation of models through modeling style checking, requirements traceability and
model coverage analysis. Simulink Design Verifier uses formal methods to identify
design errors like integer overflow, division by zero and dead logic, and generates test
case scenarios for model checking within the Simulink environment. The systematic
testing tool TPT offers one way to perform formal test- verification and validation
process to stimulate Simulink models but also during the development phase where
the developer generates inputs to test the system. By the substitution of the Constant
and Signal generator blocks of Simulink the stimulation becomes reproducible.
Sim Events adds a library of graphical building blocks for modeling queuing
systems to the Simulink environment. It also adds an event-based simulation engine to
the time-based simulation engine in Simulink.

5. Simulation Results
Fig 5.1Typical Primary and Secondary Voltages of a Scott t/f under T-Phase

Fig.5.2. Results for typical primary and secondary voltages of a Scott transformer
under a T-phase load.

Fig.5.3. Simulation Circuit for voltage and current with TCSC

Fig.5.4. voltage and current results from the simulation with TCSC method

Fig.5.5. Fault current limiting, improvement in voltage unbalance, TCSC and SFCL
quenching characteristics of the proposed method




An electric railway characteristic that most utilities are concerned with is current un
balance produced by large single-phase loads. These unbalanced currents cause 3-
phase voltage unbalance. To minimize voltage unbalances in 3-phase power feed
networks, Scott transformers are widely used. Nevertheless, an unbalance can be
generated owing to large rapidly changing single-phase loads. Fig. 5.2 shows an
unbalance in a 3-phase transmission line voltage produced by a single-phase load in
T-phase of the railway.
In case of fault on the transmission line without electric railway, 3-phase voltage
decreased owing to fault and after fault removal, there is a transient phenomenon that
has a small offset voltage but returned to a steady state. In addition, this simulation
showed a small magnitude difference between offset voltages. Current is generated a
large transient current during fault and reach to steady state, load current, after fault
removal. In contrast, large offset voltages are represented in simulation result about
fault on transmission line with electric railway. Especially, voltage of phase B is
increased up to 350 kV in a moment. In addition, voltage unbalances of transmission
line became more serious compared with above case. Line currents are also increased
and caused unbalance owing to transient voltage. We think that closing phase angle
control of TCSC system is influenced by generated transient voltage and current as
the cause of these results. These phenomenon will cause problem about voltage
stability and malfunction of protection scheme in the transmission grid. These results
show that an electric railway connection aggravates voltage unbalance in the
transmission line..
If a fault occurs, the proposed method clears voltage unbalance and protects TCSC.
Thus, transmission system can quickly return to operating in a conventional state. As
a result, we will expect improvement effect for the problems about voltage stability.
We propose a method to improve voltage unbalance using an SFCL. If a transmission
line fault occurs, fault current is decreased by the SFCL. After fault is cleared, voltage
unbalance produced by railway’s single-phase loads can quickly be reduced. As a
result, a transmission and electric railway system return to the steady state. In
particular, SFCL has advantage of fast operation within 1/4 cycle. The operation
characteristics of conventional relay and breaker are greater than 5–15 cycles.

6. CONCLUSION
6.1 CONCLUSION
This project proposed a method to reduce voltage unbalance for a TCSC-
compensated transmission line using an SFCL. First, the configuration and operation
of a compensated transmission line and connected electric railway system were
modelled and detailed. Next, voltage unbalance in transmission line was studied when
line fault occurs. Finally, the method for alleviating this problem with SFCL was
considered. The proposed method showed the following improvements for
transmission line faults:
1) The fault current was decreased as compared to the existing system fault current.
2) Voltage unbalance in the transmission system was quickly improved after the fault
was removed.
In future, we will study a protection scheme using an SFCL for a compensated
transmission system to improve system stability.
6.2. FUTURE SCOPE
A conventional superconducting fault current limiter (SFCL) is usually only
connected to a power system for fault current limitation. However, attempts to use the
hybrid SFCL application to reduce the transformer inrush current. To accomplish this,
first suggests the concepts to expand the scope of the SFCL application in the power
system. The power system operator should first determine the proper amount of
current-limiting resistance (CLR) of the hybrid SFCL. Therefore a decision scheme of
the optimal insertion resistance in an SFCL application to reduce the transformer
inrush current. This scheme and the SFCL model are implemented using the
electromagnetic transient program (EMTP).

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Voltage unbalance enhancement using SFCL in a Power feed

Network with Electric railway system

Fig 1.1 Series compensation concept for a power system

Department of EEE Y.S.R Engineering college of Y.V.U Page 1

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Fig 1.2.Operational limits of transmission lines for different voltage levels

Constraints preventing the use of full thermal capability of conventional AC

 Transient, dynamic and voltage instability.

 Voltage control and associated reactive power flow problems.

 Fault level constraints.

Advantages of FACTS technology over other solutions to network reinforcement

 Less environmental impact than most alternative techniques of transmission

Fig 1.3 conventional& FACTS devices

 Combined series-series Controllers

 Combined series-shunt controllers

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Fig 1.4: Series Controllers Fig 1.5: Shunt Controllers

Fig 1.6: Combined series-series controller Fig 1.7: Combine series-shunt

1.2 STATIC VAR COMPENSATOR:

Fig 1.8: Static VAR Compensator

SVC is a general phrase for a thyristor-controlled or thyristor switch reactor And/or

Fig 1.9: V-I Characteristics of SVC Fig1.10: V-Q Characteristics of SVC

Fig 1.11: Susceptance model of SVC

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Fig1.12.firing angle model of SVC

1.3. STATIC SYNCHRONOUS COMPENSATOR (STATCOM)

The STATCOM is a synchronous voltage source which generates a balanced 3-ph

Fig.1.14.Schematic Diagram of a Typical DVR

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Fig.1.15.Schematic Diagram of UPQC

Fig 1.16: Thyristor Controlled Series Compensator model

Thyristor Controlled Series Capacitors (TCSC) is used to solve a specific problem in

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 In the network the power flow is controlled by direct smooth control.

 Capacitor bank protection is also improved.

 Sub-synchronous resonance (SSR) is improved locally. The higher levels of

 In a large interconnected power network areas electromechanical (0.5-2 Hz)

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Fig1.18: TCSC equivalent circuit Fig1.19: TCSC equivalent circuit

1.6.2. Static Model of TCSC:

1.7. POSSIBLE BENEFITS FACTS CONTROLLERS:

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 Especially at industrial machines, metal melting plants, railway or

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2.1. TCSC-COMPENSATED TRANSMISSION:

Power transmitted between a sending-end bus and a receiving-end bus in an AC

If a series capacitor is inserted into transmission line, the inductive reactance of

Fig2.1. TCSC Closed-loop constant current control methodology

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2.2 OPERATION AND CONTROL OF A TCSC SYSTEM:

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Fig 2.3. Transmission system connected with a railway

TABLE 2.1 Load parameters and values

Parameter Zn(Ω) TF(s) a1 a2 b1 b2

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 Short-circuit limiting reactors are a conventional alternative to

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