MAT Lab Implementation of Sen Transformer as a FACTS Device

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International Journal of Engineering Technology, Management and Applied Sciences
www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476
MAT Lab Implementation of Sen Transformer as a FACTS
Device
P Yogananda Reddy
Assistant Professor
VR Siddhartha Engineering College,
Vijayawada, A.P.,India
R Giridhar Balakrishna
Assistant Professor
VR Siddhartha Engineering College,
Vijayawada, A.P.,India
ABSTRACT
In conventional AC transmission
system, the ability to transfer AC power is
limited by several factors like thermal limits,
voltage limit, short circuit current limit etc.
These limits define the maximum electric
power which can be efficiently transmitted
through the transmission line without causing
any damage to the electrical equipments and the
transmission lines. This is normally achieved
by bringing changes in the power system
layout.
Flexible ac transmission systems
(FACTS) have been developed for better
control of electric power flow through the
efficient utilization of existing transmission
lines. , In its most exotic form we are using
Unified Power Flow Controller (UPFC), static
synchronous compensator techniques. But these
are employed with power electronic
components which are complex in nature.
A method is proposed by using the traditional
technology of transformer and tap changer.
This novel technique includes simplicity of
control and by avoiding unnecessary
complexity of power electronics. A MAT LAB
model is designed for the proposed method of
Sen Transformer.
increased voltage variation, and ―loop flow‖ of
power. The construction of new transmission
lines is becoming increasingly difficult because
of various reasons, such as unfavorable
regulatory, environmental, and public policies
and the escalating cost. The power industry is
in constant search for the most economic way
to transfer bulk power along a desired path. The
significant increase in the utilization of the
existing transmission systems should be limited
by the thermal and not the stability limit.
Electric power flow through an ac transmission
line is a function of the line impedance (R, XL),
the magnitude of the sending-end voltage VS ,
and the receiving –end voltage Vr, and the phase
angle between these voltages as shown in
Figure 1. The expressions for power flow at the
receiving-end of the line are shown,
considering the line is represented in its
simplest form with a reactance XL.
1. INTRODUCTION
The demand for electrical energy around the
world is continuously increasing. The locations
for electric generation are based on energy
availability and environmental acceptability.
The transmission lines are becoming
overloaded and experiencing reduced stability,
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P Yogananda Reddy, R Giridhar Balakrishna
An uncompensated active and reactive power
flow in a transmission line is typically not
optional. If the reactive power flow in the line
is reduced, the freed up capacity of the line can
be effectively utilized to carry an increased
amount of active power. As a consequence, the
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generator is no longer required to supply the
reactive power. The efficiencies of the
generator and its coupling transformer also
increase. Therefore, the independent control of
active and reactive power flow in a
transmission line delivers the most revenue
from an ac transmission system.
Each of the power flow control
parameters (voltage, angle, and reactance) can
be changed with the use of the following
existing solutions:

Shunt inductor/capacitor for voltage
regulations.

Phase-shifting transformer for phase
angle regulation.

Series inductor/capacitor for series
reactance regulation.
By changing any one of the parameters using a
power flow controller, both the active and
reactive power flow in a transmission line can
be affected. Consider that the point of
compensation in the transmission line is at its
sending-end. Assuming that there are no
charges in the transmission line‘s impedance
and the voltage at the receiving-end, a power
flow controller can control the flow of active
and reactive power ( P and Q) to be a particular
pair of values by modifying the transmission
line‘s sending –end voltage to be of one
particular magnitude and at a particular angle.
A series-connected compensating voltage can
modify the transmission line voltage. For a
desired amount of active and reactive power
flow in the line, the compensating voltage has
to be of one particular magnitude and at a
particular angle with respect to the line voltage.
The compensating voltage is also at any angle
with the prevailing line current, and therefore,
emulates in series with the transmission line a
capacitor that increases the power flow of the
line or an inductor that decreases the power
flow of the line and a positive resistor that
absorbs active power from the line or a
negative resistor that delivers active power to
the line. Therefore, the desired compensating
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P Yogananda Reddy, R Giridhar Balakrishna
voltage is actually an impedance emulator.
Through the use of either the voltage-sourced
converter (VSC)-based unified power flow
controller (UPFC) or the traditional technology
of transformer and tap changer-based ―Sen‖
Transformer (ST), a variable series-impedance
is emulated.
A 160 MVA-rated UPFC was commissioned in
1998. This UPFC demonstrated for the first
time that active and reactive power flow in a
transmission line could independently be
regulated while maintaining a fixed line voltage
at the point of compensation. The VSC-based
technology has the capability of providing a
fast dynamic response, but this capability is not
required in most utility applications where the
need is to regulate the line voltage and the
power flow in the line(s) in a ―slow‖ manner.
Although the UPFC is the most versatile power
flow controller that has been ever built, its high
installation and operating costs must be reduced
before it can be successful commercially in
utility applications. The ST is a promising, lowcost power flow controller that provides voltage
regulation at a point in a transmission line.
Additionally, the ST provides the same
independent active and reactive power flow
control as the UPFC, albeit at a reduced
dynamic rate. The ST uses reliable, costeffective, and proven transformer and tap
changer-based technology. Hence, the ST is
adequate and economically attractive to meet
today‘s utility‘s need for independent control of
active and reactive power flow in a
transmission line.
The objective in this document is to compare
the merits and demerits of the traditional
technology of transformer and tap changer with
the emerging technology of VSC and reveal the
need for the new ST as a cost-effective power
flow controller. Within the scope of this paper,
an ST and a UPFC are studied with both power
flow controllers connected to a simple two-bus
network.
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2.
TRANSFORMER
AND
TAP
CHANGING TECHNOLOGY
The main use of the traditional transformer and
tap changer-based technology is as follows.
2.1. Voltage Regulation
A voltage-regulating transformer (VRT)
connects a voltage that is either in-phase or outof-phase with the phase-to-neutral voltage of
the transmission line and in series with the
transmission line as shown in Figure2. The
result is that the voltage at any point in a
transmission line is regulated. The bipolar
compensating voltage in any phase is induced,
through autotransformer action, in two
windings placed on the same phase of the
transformer core.
In this configuration as shown in Figure 2, a
VRT is a single-core transformer. The exciter
unit consists of a three-phase (A, B and C). Yconnected primary winding and is connected in
shunt with the line. The three-phase primary
winding is excited from the three-phase line
voltage (VsA,VsB and VsC). A three-phase
bipolar compensating voltage (Vs’sA,Vs’sB and
Vs’sC) that is either in-phase or out-of-phase
with the corresponding phase-to-neutral voltage
is generated from the induced secondary
voltages. The voltage-regulating unit consists
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P Yogananda Reddy, R Giridhar Balakrishna
of a total of six secondary windings (two
windings in each phase). With the use of taps,
the magnitude of the compensating voltage Vs‘s
is varied. The line is regulated at a voltage Vs‘
from the uncompensated voltage Vs. The
corresponding phasor diagram is shown in
Figure. The controller, as shown in Figure, is
fed with two input signals-one is the exciting
voltage Vs and the other is the reference voltage
Vs ‗*. The tap control unit, in the controller,
monitors the magnitude of the exciting voltage
Vs and the reference voltage Vs ‗*, and turns on
the appropriate tap, in the voltage-regulating
unit, in order to regulate the line voltage at Vs
*
‗ .Figure shows the schematic diagram of a
thyristor-controlled tap changer. A transformer
winding is tapped at various places. Each of the
tapped points is connected to one side of a
back-to-back thyristor (triac) switch. The other
side of each thyristor switch is connected
together at point A. Depending on which
thyristor is on, the voltage between points A
and B can be varied between zero and the fullwinding voltage with desired steps in between.
In the mechanical version of this arrangement,
a load tap changer (LTC) connects with one of
various taps to give a variable number of turns
between the connected tap and one end of the
winding.
2.2
Phase Angle Regulation
A phase angle regulator (PAR), also known as a
phase-shifting transformer, connects a voltage
that is in quadrature with the phase-to-neutral
voltage of the transmission line in series with
the transmission line as shown in Figure. The
series-connected
compensating
voltage
introduces a phase shift [Figure] whose
magnitude (for small change) in radians varies
with the magnitude of the compensating
voltage in per unit where the phase-to-neutral
voltage of the transmission line is the base
voltage.
In this configuration as shown in Figure, a PAR
is a single-core transformer. The exciter unit
consists of a three-phase (A, B and C), Y-
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www.ijetmas.com May 2015, Volume 3, Issue 5, ISSN 2349-4476
connected primary winding and is connected in
shunt with the line. The three-phase primary
winding is excited from the three-phase line
voltage (VsA, VsB and VsC). A three-phase
bipolar compensating voltage (Vs’sA,Vs’sB and
Vs’sC) that is in quadrature with the
corresponding phase-to-neutral voltage is
generated from the phase-to-phase of the
induced secondary voltages. The phase angleregulating unit consists of a total of 12
secondary windings (four windings in each
phase). With the use of taps, the magnitude of
the compensating voltage Vs‘s is varied. The
three-phase compensating voltage is electrically
isolated and connected in series with the
transmission line. The line is regulated at a
voltage Vs‘ from the uncompensated voltage Vs.
The corresponding phasor diagram is shown in
Figure.
together, it requires the use of a single-core
transformer with a three-phase, Y-connected
primary winding in the exciter unit and a total
of 18 windings (six for voltage regulation and
12 for phase angle regulation) and nine LTCs
(three for voltage regulation and six for phase
angle regulation) in the voltage and phase
angle-regulating unit. It would be advantageous
to use a scheme that is based on a single-core,
three-phase transformer and tap changers in
order to generate the required compensating
voltage Vs‘s that modifies the effective sendingend voltage Vs‘. The new ST requires the use of
only nine secondary windings as compared to
18 windings that are needed when the
compensating voltage is segregated into its
direct and quadrature components and
controlled separately using a VRT and a PAR,
respectively.
3.
The magnitude and the angle of the effective
sending-end voltage Vs‘ can be regulated with
the use of a VRT and a PAR, respectively. In
order to implement both of these functions
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P Yogananda Reddy, R Giridhar Balakrishna
SEN TRANSFORMER
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Figure.3 shows an ST, which is a single-core,
three-phase transformer with a Y-connected
primary winding and nine secondary windings.
The ST provides two functions
 Voltage regulation;
 Impedance regulation for independent
control of bidirectional active and reactive
power flow
As shown in Figure 3, the voltage Vs at any
point in the electrical system is applied to a
shunt-connected
single-core,
three-phase
transformer‘s primary windings. A total of nine
secondary windings (a1, c2, and b3 on the core
of A-phase, b1, a2, and c3 on the core of Bphase, and c1, b2, and a3 on the core of Cphase) constitute the voltage and impedanceregulating unit. By choosing the number of
turns of each of the three windings, and
therefore, the magnitudes of the components of
the three 120degree phase-shifted induced
voltages, the compensating voltage Vs’s in any
phase is derived from the phasor sum of the
voltages induced in a three-phase winding set
(a1, a2, and a3 for injection in A-phase, b1, b2,
and b3 for injection in B-phase, and c1, c2, and
c3 for injection in C-phase). The compensating
voltage is of line frequency and is connected in
series with the line through autotransformer
action.
When an ST is used as a voltage regulator
[Fig.3(b)], the in-phase component of the
compensating voltage for any phase is induced
in a winding that is placed on the corresponding
phase of the transformer core. The out-of-phase
component of the compensating voltage for that
phase is derived from the phasor sum of the
voltages induced in two equal-turn windings
that are placed on the remaining two phases of
the transformer core. For example, the in-phase
component of the compensating voltage for the
A-phase is induced in a winding that is placed
on the core with the exciting primary winding
of the A-phase. The out-of-phase component of
the compensating voltage for the A-phase is
derived from the phasor sum of the voltages
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P Yogananda Reddy, R Giridhar Balakrishna
induced in two equal-turn windings that are
placed on the core with the exciting primary
windings of the B-phase and the C-phase,
respectively. The effect is such that the
transmission line voltage at a point is regulated.
When an ST is used as an impedance regulator
[Fig.3(c)], the series-connected compensating
voltage Vs’s modifies the effective sending-end
voltage Vs’ in order to selectively control the
active and the reactive power flow of the line.
The compensating voltage is at any angle with
the prevailing line current. The active or direct
component of the compensating voltage
provides the series resistance emulation,
whereas the reactive or quadrature component
provides the series reactance emulation.
Both functions of voltage regulation and
impedance regulation can be implemented in
just one unit by proper programming of the tap
control unit. Notably, each of a1, b1, and c1 is
tapped at the same number of turns; each of a2,
b2, and c2 is tapped at the same number of
turns; each of a3, b3, and c3 is tapped at the
same number of turns. However, the number of
turns in the a1-b1-c1 set, a2-b2-c2 set, and a3b3-c3 set can be different from each other.
The series-connected compensating voltage is
derived from the line voltage through
transformer action with the shunt-connected
primary windings. Therefore, the exchanged
active and reactive power with the line must
flow through the primary windings to the line.
A series-connected compensating voltage that
is X% of the line voltage provides a shunt
current that is the same X% of the line current.
The shunt current through the exciter unit has
both active and reactive components. The
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loading effects of these two currents on the
power system network are independent of each
other as shown in Fig.4. Therefore, if it is
desirable to compensate the combined loading
effects of the active and the reactive current
through the exciter unit into the power system
network, a separate shunt-connected reactance
compensator, such as a static var compensator
(SVC) may be considered.
There are instances when only any one of the
three secondary windings in each phase is
required to carry the rated current at rated
voltage. Therefore, each of the nine secondary
windings must be designed to carry the rated
current at rated voltage. This require the
magnetic rating of the ST to be 2p.u. Note that
the efficiency of an ST is in the range of 99.7%
when mechanical LTCs are used and in the
range of 99% when thyristor-controlled LTCs
are used. The ST, with 360degree of voltageinjection capability, uses only three primary
windings and at the most six secondary
windings at any given operating point. The
remaining three secondary windings stay
inactive. Therefore, it is possible to achieve a
360degree operating range just by using six
secondary windings instead of nine secondary
windings with hardware configuration in every
120degree of operation. In this case, the
magnetic rating of the ST is only 1.5 p.u.,
instead of 2 p.u. Note that by using six
secondary windings, one of the six operating
regions (0 to 120°, 120 to 240°, 240 to 360°, 60 to 60°, 60 to 180°, and 180 to 300°) can be
selected.
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P Yogananda Reddy, R Giridhar Balakrishna
A comment on the ST‘s operation during line
faults is as follows. Generally, to limit a phaseto-neutral fault current, additional inductive
impedance in the path of the fault is needed. If,
for example, the additional emulated
impedance is equal to the equivalent line
impedance, the steady-state fault current is
reduced to half of the natural fault current. The
steady-state voltage across the emulated
impedance is half of the phase-to-neutral
voltage. In the worst case, the emulated
impedance must withstand twice the voltsecond that is needed to support half of the
steady-state phase-to-neutral voltage in order to
avoid saturation. The relatively insignificant
(typically 0.05 to 0.15 p.u.) voltage rating of
the series core of an ST would saturate early in
the fault cycle. The saturated windings do not
provide significant current-limiting effect in the
ST. Therefore, the ST rides through a fault.
Figure.4.3. shows a basic ST model. The
voltage and impedance-regulating unit injects a
voltage Vs’s whose active and reactive
components with load convention Vd and Vq,
respectively, in series with the transmission
line. This, in turn, changes the voltage VX
across the transmission line, and hence, the
current and the power flow through the
transmission line change. The compensating
voltage Vs’s is at any angle with the prevailing
line current I. The component Vd of the
compensating voltage that is either in-phase or
out-of-phase with the line current emulates a
positive or a negative resistor in series with the
transmission line. The remaining component Vq
that is in quadrature with the line current
emulates either an inductor a capacitor in series
with the transmission line. The compensating
voltage Vs’s delivers and absorbs both active
and reactive power (Peach and Qeach), which are
defined as
Peach = - Vs’s.I=VdI=VsId
Qeach= -Vs’s*I=VqI=VsIq
Note that when the series-connected
compensating unit emulates a capacitor, the
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shunt-connected exciter unit emulates an
inductor and vice versa. Also, when the seriesconnected compensating unit emulates a ―+R‖,
the shunt-connected exciter unit emulates a ―R‖ and vice versa.
4.
ALGORITHM
The series compensating voltage Vs‘s in any
phase is derived from the contributions of the
compensating windings of the ST from three
different phases. If the phase angle of the series
compensating voltage is exactly at 0degree,
120degree or 240degree, it can be constructed
from only one of the three phases a, c or b,
respectively. For any other angle, the series
compensating voltage is constructed from two
adjacent voltages.
Consider an ST, as shown in Fig.5, which has
four tap position in each of the nine
compensating secondary windings. Each tap
position provides a voltage of 0.1 p.u. and
therefore, a maximum of 0.4 p.u. is obtained
from each phase. The possible combinations of
voltage tap-setting positions are shown by the
dotted grid in Fig.4.2. Let Vs‘s be the required
compensating voltage, at an angle with
reference to the corresponding phase angle.
Then, one of the four combinations enclosed by
the dashed circle must be selected. In addition,
the selected combination must be the nearest to
the voltage vector, Vs‘s. In Fig.6.1, the circle is
shown in an enlarged view, where the four
combinations marked as 1, 2, 3 and 4 are vector
distances T1, T2, T3 and T4, respectively, apart
from the desired series voltage Vs‘s. These four
vector distances indicate the error introduced
due to the selection of any particular
combination. Of the four distances, the one
with the least magnitude will introduce the least
error and the corresponding tap-setting
combination would be selected to construct
Vs‘s. Let Vs‘sx and Vs‘sy donate the two
rectangular components of Vs‘s in the Cartesian
coordinate system. Similarly, let Vkx and Vky be
the components of Vk (k=1, 2, 3, 4), formed by
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P Yogananda Reddy, R Giridhar Balakrishna
the four possible tap-setting combinations.
Then, the error with respect to Vs‘s is defined as
The tap-setting combination corresponding to
Vk with the smallest is selected as the best tap
setting to implement Vs‘s. Thus, the algorithm to
determine the best tap setting for Vs‘s consists
of the following steps.
Step 1) Get the input (magnitude Vs‘s, and
leading phase angle, β) about required series
voltage injection in phase a.
Step 2) Based on β, identify the zone into
which the series voltage phasor falls: Zone 1 (0
°< β ≤ 120°), Zone2: (120°< β ≤240°) and
Zone3: (240°< β≤ 360°). Next, identify the
contributing (one or two) phase(s) and set the
contribution of the zero other zero phase(s).
When β is exactly equal to 0°, 60°, 120°, 180°,
240° and 300° and the magnitude is exactly
midpoint in between two consecutive grid
positions, select the higher position.
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Step 3) Based on the magnitude of Vs‘s, identify
the four nearest tap-setting positions [dot
positions on the grid in Fig.6.1].
Step 4) Calculate the normalized vector
distances between voltages produced by these
four tap positions and the compensating voltage
Vs‘s. This will give the magnitude of errors ϵk
(k= 1, 2, 3, 4) that would be introduced due to
the selection of corresponding tap settings.
Step 5) Compare the errors and identify the tapsetting combination that yields the minimum
error.
Step 6) Implement the tap setting in
corresponding phase(s) in the ST through the
use of load tap changers.
Step 7) Implement similar tap settings for
voltages to be added in series with phases b and
c.
5.
MODELLING
OF
“SEN”
TRANSFORMER
A digital computer simulation model of the ST
has been developed using MAT Lab. The
model consists of two subsystems: the electrical
subsystem and the tap-selection algorithm
subsystem. Figure illustrates the interface
between the two subsystems.
5.1. ELECTRICAL SYSTEM
Figure 7 show the electrical system and the ST.
The electrical system is comprised of two ac
systems connected by a three phase
transmission line. The ST is connected at the
sending end of the transmission line. Table1
gives the parameters for both the ST and the
network.
5.1.1. Electrical Network Model: The ac
sources at both sending and receiving ends are
modeled as infinite sources with the same
magnitude but at a phase difference of 20° (the
receiving end voltage lags the sending end
voltage). The transmission line is modeled as
lumped series impedance.
5.1.2. ST Model: The ST is a specially
designed transformer with multiple windings
having multiple tap positions in the secondary.
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P Yogananda Reddy, R Giridhar Balakrishna
The model for such a transformer is not
available in MATLAB. Therefore, nine singlephase transformers, each having on-load tap
changing capability have been used to model
the ST. By using single-phase transformers,
inter-phase mutual flux linkage and thus mutual
inductance has not been considered, which may
cause some discrepancies in the results. These
nine single-phase transformers are modeled
with a small resistance and leakage reactance as
shown in Figure. Output voltages of three
transformers (contributing from phase a, b and
c) are added in series and then fed to one phase
of the transmission line. The nine outputs (aa,
ab, ac, ba, bb, bc, ca, cb and cc) from the tapselection algorithm supply the value of tap
setting to all nine transformer Tap terminals.
Should these outputs undergo any changes; the
transformers readjust their tap positions and
produce the required compensating voltages.
5.1.3. Tap Changer Model: In a practical
transformer, tap changing is performed through
a tap selector; where a resistor or inductor is
used in parallel with the tap positions to limit
the current through a shorting winding segment
between two consecutive taps. In Figure, an
example of tap-changing operation has been
shown along with the equivalent MATLAB
model for each position of the tap selector.
Although, practical transformers on load tap
changers (OLTC), such as the ones form
Reinhausen use taps with voltage difference in
the range of 0.02 p.u. to 0.067 p.u., in this
model a voltage difference of 0.1 p.u. between
taps has been assumed for the clarity of
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simulation. The time required to move the tap
selector between adjacent tap positions is 2 s. In
order to move the tap selector from its initial
position, terminal E (0.2 p.u.) to its final
position, terminal C (0.1 p.u.), the following
four steps of approximately 0.5 s each are
performed.
Step 1: In position 1, the tap selector is
connected to terminal E; therefore, in the model
the circuit breaker (CB) connecting terminal E
to line is closed and the rest of the CBs are
open. Now, the selector is moved to a position
where it connects both terminal D and E;
however, the current flows through terminal E
alone. To model this situation, the CB
connecting terminal D is closed.
Step 2: The tap selector moves further down
and is connected to terminal D. The line current
now flows through the resistor, and depending
on the value of the resistance, a slight dip in the
voltage may occur. In the model, CB
connecting terminal E is opened.
Step 3: In this step, the tap selector is moved to
a position, where it connects both terminals C
and D. This is the situation where a circulating
current will flow within the loop formed by the
terminals C, D and the resistor. The higher the
value of the resistor, the lower will be the
circulating current, however, too high a value
of resistance would a voltage sag. In the model,
the CB connecting terminal C is now closed.
Step 4: In the final step, the tap selector is
moved to allow contact with
terminal C
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P Yogananda Reddy, R Giridhar Balakrishna
alone, which is the required final position. In
the model, the CB connecting terminal D is
opened.
5.1.4. Measurements: Measurement blocks are
used to measure electrical signs and hence, to
calculate powers at the receiving end using the
following equations:
Pr= (vaia+ vbib+ vcic)
Qr= √3 (vaic – vcia)
Where va, vb, and vc voltages of phase a, b and
c, respectively, and ia, ib and ic are the currents
in the respective phases.
5.2. TAP-SELECTION ALGORITHM:
The tap-selection algorithm has been
implemented as a FORTRAN program linked
to the electrical system using an interfacing
block created in MATLAB. Although
MATLAB has the capability of interfacing
other scripts such as MATLAB, C or C++,
FORTRAN has been chosen for its simplicity
and the speed of implementation. The inputs to
the interfacing block are the magnitude (Vs‘s)
and the phase angle (β) of the compensating
voltage and the outputs are the tap positions for
the nine compensating windings of the ST. Any
change in the demand of series voltage is
passed to the FORTRAN program which
implements the necessary tap positions which
are sent to the electrical system and
implemented through the on-load tap changer
of the transformer.
A Time block has been used to synchronize the
instant of inputs (Vs‘s and β) with the simulation
time. Two table blocks have been introduced to
serve the purpose of a lookup table. The inputs
which vary with respect to time, can be
predefined through these blocks for both Vs‘s
and β. Based on these inputs, the tap-selection
program produces the value of tap ratio through
the outputs aa, ab, ac, ba, bb, bc, ca, cb and cc.
The first letter of the output variable (for
example, aa) denotes the contributing phase and
the second letter indicates the phase in which
the voltage is added in series. These outputs are
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then passed to the compensating transformers
for the readjustment of tap setting.
6.
RESULTS
All the off-line and real-time simulations are
performed using a time step of 100. In these
simulations, it is assumed that each secondary
winding is capable of injecting up to 0.4 p.u.
series voltage in the line (9 tap positions for
each secondary winding), although in Fig.4.2.
only 5 tap positions are shown. Before injecting
any series voltage, the system was simulated in
an uncompensated mode with the network
parameters listed
Steady State Results:
In this case the simulation of the ST is carried
out for a compensated series voltage of 0.1 p.u
0.4 p.u. The angle β is varied at a discrete step
of 1° in the range of 0° to 360° for a tap
resolution of 5%. The variation of both Pr and
Qr are shown in Fig.10 respectively, for four
magnitudes of injected voltage as well as the
uncompensated mode (zero voltage injection).
It is evident that both of the active and reactive
power follows an almost first-order-hold
sinusoidal pattern by varying the phase angle.
Nevertheless looking at the active power
profile, it can be seen that the difference
between the peak magnitude of the
compensated and uncompensated active power
in the first half cycle is smaller than that in the
second half cycle. In other words the ST has a
higher capacity when regulating active power
below the natural power rather than above it.
The reason for this asymmetry in the behavior
of the ST can be described as follows. As we
reached the peak magnitude of active power, a
large amount of in phase currents flow through
325
P Yogananda Reddy, R Giridhar Balakrishna
the transmission line and ST impedances. Also,
the ST is modeled as a set of 12 coupled
windings with nonzero coupling coefficients
between the secondary windings. This means
that if more than one secondary winding has a
non-zero tap value, then there would be a
secondary effect voltage drop or voltage boost
in the ST injected voltage due to the currents
flowing through the other secondary windings.
Table II shows the tap positions of the
ST secondary windings for four extreme values
of Pr and Qr, As can be seen, at the peak
magnitude of active power ( Prmax) secondary
windings of S1 and S2 are in series with the
line and both of them are in their full tap
positions ( maximum resistance and inductance
). The effect of interaction of these two sets of
secondary winding on the ST injected voltage
at high levels of active power can be explained
by the Phasor diagram shown in fig 11, which
is drawn based on the following assumptions.
Resistances of secondary windings are
neglected. All the inter phase coupling
coefficients are assumed to be zero, in practice
inter phase couplings are very small compared
to same-phase couplings.

Quadrature component of line current is
neglected in the peak value of active power, in
the simulation results it was about 7% of the inphase component.
For the case of reactive power, the situation is
reversed. Flow of high quadrature current in the
line will increase the ST injected voltage rather
than decreasing it. This can be seen from fig
9.1, where the distance between the negative
peak value of Qr and Qn is larger than the
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distance between positive peak value of Qr and
Qn.
It is to be noted that if separate units (zero
mutual couplings) are used for different sets of
secondary windings, the mutual effect of
secondary‘s on each other will be eliminated
and the above asymmetry in active and reactive
powers will disappear.
The relationship between Pr and Qr for Vs1s of
0.1 p.u, 0.2 p.u, 0.3 p.u and 0.4 p.u is shown fig
. It was found that the ST produces a nearly
hexagon profile compared to the circular profile
of UPFC(2).
7.
CONCLUSION
The ST is a unique device for controlling
voltage. Phase angle and power flow in a
transmission network. Despite its relatively
slower response in comparison to the UPFC.
The advantages of the ST lie in the simplicity
of its control. the absence of unnecessary
complexity of power electronics. and the
overall lower cost and higher efficiency of the
power-flow controller. Up to now a detailed
transient model of the ST was not available.
Which prompted users to employ approximate
models of simplified transformers available in
EMTP—type programs.
This precluded transparency in the simulation
and control design stages. This paper has
developed an accurate ST transient model from
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P Yogananda Reddy, R Giridhar Balakrishna
re-alistic components thus providing greater
insight into the ST functionality. The two
salient features of this model are the effect of
mutual coupling in the ST secondary windings.
and the nonlinear excitation characteristic
heretofore ignored in the previous works. The
inclusion of these effects made the ST model
developed more realistic. This also opens the
way for incorporating the model into
commercial EMTP—type programs.
Real-time implementation of the entire system
model and the controller was carried out on a
state-of-the art simulator. The real-time
simulation is fully interactive as though the user
is communicating with the real system. Realtime oscilloscope results are given to
demonstrate the performance of the model and
the controller.
The execution time of the simulation is quite
low: on a 100µs time-step the ST model
execution is only 75 µs. These numbers show
that the developed ST model is computationally
efficient and that the control algorithm is
suitable for online applications.
REFERENCES

K. K. Sen and M. L. Sen, ―Introducing
the family of "sen" transformers : A set of
power flow controlling transformers,‖ IEEE
Trans. Power Del.,vol. 18, no. 1, pp. 149–157,
Jan. 2003.

M. O. Faruque and V. Dinavahi, ―A
tap-changing algorithm for the implementation
of ―sen‖ transformer,‖ IEEE Trans. Power Del.,
vol.22, no. 3, pp. 1750–1757, Jul. 2007.

Babak aschari, M. Omar faruque and
Venkata Dinavahi‖, Detailed real time transient
model of the ―sen‖ transformer‖, IEEE Trans.
Power Del. Vol.23,no.3,July 2008.

K. K. Sen and M. L. Sen, ―Comparison
of the sen transformer with the unified power
flow controller,‖ IEEE Trans. Power Del., vol.
18, no. 4,pp. 1523–1533, Oct. 2003.
 Understanding FACTS by NARAIN G.
HINGORANI, LASZLO GYUGYI.
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