RGPV EX7102 UNIT II

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Prof MD Dutt HOD Ex Department SRCT ThuakhedaBhopal MP India
READING MATERIAL FOR B.E. STUDENTS
OF RGPV AFFILIATED ENGINEERING COLLEGES
BRANCH VII SEM ELECTRICALAND ELECTRONICS
SUBJECT EHV AC AND DC TRANSMISSION
Professor MD Dutt
Addl GeneralManager(Retd)
BHARAT HEAVY ELECTRICALS LIMITED
Professor(Ex) in EX Department
Bansal Institute of Science and Technology
Kokta Anand Nagar BHOPAL
Presently Head of The Department ( EX)
Shri Ram College OfTechnology
Thuakheda BHOPAL
Sub Code EX 7102 Subject EHV AC AND DC TRANSMISSION
UNIT II

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EX 7102
RG PV Syllabus
UNIT II EHV AC AND DC TRANSMISSION
FACTS devices, basic type of controllers, series controller. Static Synchronous Series
CompensatorSSSC. Thyristor controlled series capacitor TCSC. Thyristor controlled
series reactor TCSR . Shunt controller STATCOM, Static VAR (SVC). Series – Series
controllers. Combined series shunt controller. UPFC and TCPST.
INDEX
S No Topic UNIT II Page
1 FACTS devices, basic type of controllers, series controller 3- 4
2 Static Synchronous Series CompensatorSSSC 5
3 Thyristor controlled series capacitor TCSC 6-9
4 Thyristor controlled series reactor TCSR 9-10
5 Shunt controller STATCOM, Static VAR (SVC). 10-13
6 Series – Series controllers. Combined series shunt controller
UPFC and TCPST
13-18

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FACTS DEVICES, BASIC TYPE OF CONTROLLER
SERIES CONTROLLER
A.C transmission system that employ power electronic based and other static controllers
to enhance controllability and to increase power transfer capability are defined as
FACTS i.e Flexible Alternating Current Transmission systems.
Benefits of using FACTS controllers are:-
a) They help in obtaining optimal operation by reducing power losses and
improving voltage profile.
b) Due to controllability of FACTS, the power carrying capacity of lines can be
increased upto thermal limits.
c) The transient stability limit is increased thereby improving the dynamic security
of the system.
d) Some FACTS controllers suchas TCSC can damp the Sub Synchronous
Resonance SSR.
e) The problem of dynamic over voltage can be overcome by use of FACTS
controllers.
TYPES OF FACT DEVICES
There are two group of FACTS devices that follow two distinctly different technical
approach.
The first group of FACT controllers are known as variable impedance type FACT
controllers. They are
i) Static VAr compensator SVC
ii) Thyristor controlled Series Capacitor TCSC
iii) Thyristor controlled Phase Shifting Transformer. TCPST
The second group uses self commutating static convertor operating as controlled
voltage sources, The direct current in a voltage sourced convertor flows in both
direction therefore the convertor valves are to be bi directional. These FACTS
controllers are known as Voltage SourceConvertor VSC based controllers. They are
i) Static Synchronous Compensator STATCOM
ii) Interline Power Flow Controllers IPFC
iii) Unified Power Flow Controllers UPFC

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a) Series controller b) shunt controller c) combined series series controller d) combined
series shunt controller
SERIES CONTROLLER
The series controller may be a variable capacitor, inductor or a variable frequency
source. A series controller injects a variable series voltage (productof current and
variable reactance in the line. A voltage in series with the transmission line can control

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the current flow and there by the power transfer from the sending end to the receiving
end. When the injected series voltage is in phase Quadrature with the line current, the
controller generates or consumes only reactive power. If the injected series voltage is
not in phase Quadrature, the controller handle both reactive and active power. Series
capacitive impedance can be decrease the overall effective series transmission
impedance from sending end to the receiving end and thereby increasing the
transmittable power. The inductive series compensation may be used when it is
necessary to decrease the power flow in the line. However, the capacitive compensation
is more commonly used some of series controllers are:-
i) Thyristor controlled Series Capacitor TCSC
ii) Thyristor controlled Series Reactor TCSR
iii) Static Synchronous Series Compensator SSSC
Thyristor switched series controller
2 STATIC SYNCHRONOUS SERIES COMPENSATOR
A static synchronous series compensatoris a device whose output voltage (injected
voltage) is in Quadrature with the line current for the purposeof changing the overall
reactive drop in the line. The output voltage of a device is controlled independently and
is normally quite small as compare to the line voltage. The SSSC may be with storage
or without storage facility. The storage system may be a battery storage or a
superconducting magnetic storage device which injects a voltage vector of variable
angle in series with the line.
The static synchronous series compensatormay be used for current control, stability
improvement and fro damping oscillations during disturbances.

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SSSC without energy storage system
3. Thyristor controlled Series CapacitorTCSC
The use of thyristor control to provide variable series compensation makes it attractive
to employ series capacitor in long lines. Controlled series compensation can be
achieved in two ways
1. Discrete controlusing thyristor switched series capacitor TCSC
2. GTO Thyristor controlled series capacitor GCSC
TCSC consists ofa number of capacitors in series. Each shunted by a switch
composed oftwo anti parallel thyristor as shown below
Thyristor switched series capacitor

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A capacitor is inserted by turning ‘off’ the thyristor switch. Similarly it is by passed by
turning ‘on’ the thyristor. If all the switches are ‘off’, the effective capacitance becomes
Ceq = C/m where ‘m’ is the total number of capacitors. Similarly if all the switches are
simultaneously turned ‘on’, Ceq is zero. Therefore the effective capacitance and hence
the degree of series compensation are controlled in a stepped manner by changing the
number of capacitors inserted in the circuit.
TCSC is a mature technology available for application in EHV AC lines. Parallel
combination of switched capacitors and controlled reactors provide a smooth current
control range from capacitive to inductive values by switching the capacitors and
controlling the current in reactor. The figure below shows linear reactor ‘L’ connected
to AC sourceVs through two thyristor connected in anti parallel. During positive half
cycle of source voltage, T1 is turned on and during the negative half cycle T2 is turned
on. For firing angle α =90˚, the sourcecurrent is continuous as shown below. The
circuit behaves as if the inductance L is directly connected to the source without
thyristor. Forα=90˚, Is a sine wave, its fundamental componentIf1 is same as Is and is
therefore maximum. As a result, inductive reactance offered by reactor Xl=Vs/If1 is
minimum. Here Vs is the rms value of sourcevoltage and If1 is the rms value of the
fundamental component of sourcecurrent, which for α=90˚ is equal to Is( rms value of
sourcecurrent).
Thyristor controlled reactor

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α>90° α>90°
forT1 for T2
Circuit diagram and its voltage and current waveform
For firing angle α>90˚, current is discontinuous, as shown in figure b) . Its fundamental
component If1 again lags Vs, by 90˚ Its fundamental component If1 decreased,
therefore the inductive reactance offered by reactor (=Vs/If1) has become more. If α is
further increased, fundamental component of is would be further reduced and therefore
reactance offered by the reactor would be more pronounced. Forfiring angle α=180˚,
is=0, if1=0 and theoretically, the inductive reactance offered by the reactor would be
infinite. This shows that with firing angle control from α=90˚ to180˚: the effective
reactance of the reactor, as seen by the source, can be regulated from its actual value
Xl=2πfl when α=90˚, to an infinite value when α=180˚.
As the fundamental component of sourcecurrent lags the sourcevoltage by 90˚, the
reactor consumes no power. It draws only reactive power.
Actually for 0˚≤ α ≤ 90˚, there is no control over inductor L therefore
Xl =Vs/Is = Vs/If1 for 0˚≤ α ≤ 90˚,
Or
L =Vs/ωIs = Vs/ωIf1 for 0˚≤ α ≤ 90˚,
For α > 90˚, the Fourier analysis of inductor current wave form gives the fundamental
component If1 as under
If1= Vs/πωL [ 2π-2α+sin2α]
The reactive power drawn at α=90˚ or for 0˚≤ α ≤ 90˚ is
Q =VsIf1 = Vs Is = Vs²/ωL
For 90˚ ≤ α ≤ 180˚ ,
Q = Vs.If1 = Vs²/πωL [ 2π - 2α +sin 2α]

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At α= 90˚, reactive power drawn is maximum, when α=180˚, reactive power is zero
A simple understanding of working of TCSC can be obtained by analyzing a variable
inductor connected in parallel with affixed capacitor as shown below
Working of TCSC b)capacitive operation c)Inductive operation
ZTCSC = -XC jXTCR = -j XC
j(XTCR - ¡ XC) 1 - XC
XTCR
The current through the TCR (ITCR)is given by
ÎTCR = -j XC Î L = Î L
j(XTCR - XC) 1 - XTCR
XC
Since the losses are neglected, the impedance of the TCSC is purely reactive.
The capacitive reactance of TCSC
XTCSC = XC
1 - XC
XTCR
Just the opposite of the convention used in circuit analysis and load flow studies.
The reactance of TCSC is capacitive as long as the reactance of the capacitor Xc
is less than the reactance of the TCR’XTCR’. When the thyristor are blocked, the
reactance of TCR is infinite and current through it is zero. For Xc < XTCR, The
current through the TCR 180˚ out of phase with the line current ‘Il’ for Xc >
XTCR,, The effective reactance of TCSC is negative and it behaves as an inductor.
In this case the line current and the current through the TCR ‘ITCR’ are in phase.
When the triggering delay angle of TCR is 180˚, the reactor becomes non-
conducting and the series capacitor has its normal value. There is no difference
in the performance of TCSC in this mode with that of a fixed capacitor. This
operation mode is also known as ‘Waiting mode’ and is normally avoided. As

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the delay angle is reduced to less than conducting and the net impedance of the
controller becomes inductive. The variation of TCSC reactance with firing angle
α is shown below.
The feasibility of fast control of thyristor enables the improvement of stability
and damping of oscillations using appropriate control strategies. TCC may be
used for current control, stability improvement, damping oscillation, and for
limiting fault current
Variation of TCSC reactance with firing angle α
4 Thyristor controlled series reactorTCSR
A thyristor controlled series reactor comprises of a series reactor shunted by a thyristor
controlled reactor (TCR). When the triggering delay angle of TCR is 180˚, the reactor is
non conducting and the uncontrolled reactor works as a fault current limiting reactor.
As the delay angle of 90˚, the reactor becomes fully conducting and the net inductance
value is because of the parallel combination of two inductances. Thus a smoothvariable
reactance control is obtained.
Thyristor – controlled series reactor are used for current control, stability
improvement, damping oscillation, and for limiting fault current

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5 Shunt controller STATCOM, Static VAR (SVC).
SHUNT CONTROLLER
A shunt controller is a variable reactance connected in parallel to a transmission line .
The controller injects current into the system at the point of connection. If the injected
current is in phase Quadrature with the line voltage, the controller handles only reactive
power. For any other phase angle between the current and the line voltage it handles
both active and reactive power.
Reactive shunt compensation can significantly increase the maximum transmittable
power. The transient stability at a given power transmission level and the fault clearing
time, is determined by P- δ characteristics of the postfault system. Since appropriately
controlled shunt compensation can provide effective voltage support, it can increase the
transmission capability of the postfault system thereby enhance transient stability.
Thus with suitable and fast controls, shunt compensation will be able to change the
power flow in the system during and following dynamic disturbances so as to increase
the transient stability limit and provide effective power oscillation damping.
Some basic type of shunt controllers are
a) Static VAR compensatorSVC
b) Static Synchronous Compensator STATCOM
c) Static Synchronous Generator SSG
d) Thyristor controlled Dynamic Brake TCDB
STATIC SYNCHRONOUS COMPENSATOR
The static synchronous compensatoror simple static compensatorSTATCOM is a
shunt connected device developed as an advanced static VAr compensatorwhere a
voltage sourceconvertor VSC is used instead of controllable reactors and switched
capacitors. The use of VSC requires self commutating devices such as GTO ,IGBT ,
IGCT,MCT etc, which make them costlier.
A STATCOMbased on voltage source convertor is shown in figure below. From a
given input of DC voltage, voltage sourceconvertor produces a set of 3 phase ACout
put voltages, each in phase with and coupled to the corresponding AC system voltage
through a relatively small reactance. The reactance is provided by either an interface
reactor or the leakage inductance of a coupling transformer. By suitable control, the
phase and magnitude of the AC voltage injected by VSC can be controlled. The output
control is independent of the system voltage.

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A STATCOMis comparable to a synchronous condenser which can supply variable
reactive power and regulate the voltage of the bus to which it is connected. Te Ac
voltage is directly proportional to the DC voltage Vdc across the capacitor. If any
energy source, a battery or a rectifier, is present on the DC side, the voltage Vdc can be
held constant. The self commutated switches GTO are switched on and off once in a
cycle. The conduction period of each switch is 180˚. The switches are synchronized to
the supply voltage ‘V’. If the line voltage ‘V’ is in phase with the convertor output
voltage ‘E’ and has the same magnitude, no current flows into or out of the
compensator. Thus there is no exchange of reactive power with the line. If the convertor
voltage is increases, the voltage difference between ‘V’ and ‘E’ appears across the
leakage reactance of the step down transformer. As a result, a leading current( leading
by V) is drawn and the compensatorbehaves as a capacitor, generating reactive power..
On the other hand if V> E, then the STATCOMdraws a lagging current, behaves as a
reactor and absorbs reactive power. Thus compensator operates like a synchronous
machine.
A STATCOMhas many technical advantages over SVC they are
i) Faster response
ii) Requires less spaceas bulky components such as reactors are not required.
iii) Modular and reloadable
iv) Can be interfaced with real energy sources such as battery , SMES
v) The reactive current can be maintained therefore superior performance is
achieved during low voltage conditions. Even if the reactive current can be
increased under transient condition if the devices are rated for transient
overloads.
STATCOM
A STATCOMis controlled reactive power source. It provides voltage supportby
generating or absorbing reactive power at the point of coupling without the need of
large external reactors or capacitors banks. Thus STATCOMmay be used for voltage
control, reactive power compensation and damping oscillations.

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STATIC VAR COMPENSATOR SVC
The static Var compensator SVC is first generation FACTS controllers. It is a variable
impedance device in which the current through a reactor is controlled by backto back
connected thyristor . These thyristor valves are rated for lower voltages as the SVC is
connected to the transmission line through a step down transformer or through the
tertiary winding of a power transformer. The location of SVC is important in
determining its effectiveness. They should be located at load centre or midpoint of
transmission line.
There are two type of SVC
1. Fixed Capacitor- Thyristor Controlled Reactor (FC-TCR)
2. Thyristor Switched Capacitor – Thyristor Controlled Reactor (TSC-TCR)
The second type of SVC is more flexible, requires smaller reactor and hence generates
less harmonics.
Figure below shows a static VAR compensator. It is a shunt connected combination
which includes a separate thyristor controlled or thyristor switched reactor for
absorbing reactive power and thyristor switched capacitor for supplying the reactive
power.
Static VAR compensator
The TCR and TSC are connected on the secondaryside of a step down transformer. The
TSC is switched in using two thyristor connected back to backat the instant in a cycle
when the voltage across valve is minimum and positive. This results in minimum
switching transient. The current in a TCR can be continuously varied from zero to
maximum by phase control in which the firing angle α is varied from 180˚ to 90˚. The
harmonics in SVC are generated by the TCR. Neither TSC or TSR generates
harmonics. The TCR current contains odd harmonics. Tuned and high pass filters are
also used in parallel which provide capacitive reactive power at fundamental frequency.
To limit the harmonics entering the system, some of the fixed capacitors are connected
as series tuned filters. To reduce the harmonics further, a twelve pulse configuration of
TCR should be used.
The use of SVC improves transmission capacity and steady state limit. SVC can be
used for stability improvements both during small and large disturbances. Its use can
also damp the sub synchronous oscillations. The costof a SVC is lesser as compared to
a STATCOM.

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6. Series – Series controllers. Combined series shunt controller UPFC and TCPST
It is a combination of two or more separate series controllers with each series controller
connected in a transmission line or in a multi-line transmission system. All the
controllers connected in series are controlled in a coordinated manner.
Another variation of a series series controller is the inter – line power flow controller
IPFC. This is recently introduced 1998 controller having a combination of two or more
static synchronous compensatoras shown in figure. The SSSCs are coupled through
common DC link.
With this arrangement, in addition to providing series reactive compensation, any
convertor can be controlled to supply real power to the common DC link from its own
transmission line .Thus real power can be made available from the under utilized lines
and can be used by other lines.
Consider an IPFC scheme consisting of two backto back DC to AC, each compensating
a transmission line by injecting a series voltage as shown in figure.
INTERLINE POWER FLOW CONTROLLER
AN IPFC ARRANGEMENT
The two synchronous voltages V1c and V2c represent the two convertors. X1 and X2
are he reactance’s of Line 1 and line 2 respectively. For clarity both the sending end and
receiving end of two lines are assumed to be constantwith fixed magnitudes and fixed

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angles; resulting in identical transmission angles ( δ1 and δ2) for the two systems. The
two line reactance’s and the compensating voltages are also assumed to be identical.
Although in actual the two system could be different with different voltages,
impedances and angle. System 1 is selected as the prime system for which free
controllability of both real and reactive power is stipulated.
PHASOR DIAGRAM OF A SYSTEM WITH IPFC
Figure above shows the Phasordiagram showing V1s , V1c, I1, V1x (Voltage across
reactance X1) and the injected compensating voltage V1c with controllable magnitude
( 0 ≤ V1c ≤ V1cmax) and angle ( 0 ≤ P1 ≤360˚). The rotation of Phasor V1c with angle
P1 varies both the magnitude and angle of V1x and results in the change in real power
and reactive power.
Thus it manages the overall real and reactive power management of a multi line
transmission system and therefore optimizing the capability of the transmission system.
The IPFC arrangement essentially requires the rigorous maintenance of the overall
power balance at the DC terminals by appropriate control action and real power
transfer. The IPFC together with independent controllable series reactive compensation
of each transmission line helps as follows.
1. Reduce the burden of overloaded line by real power transfer.
2. Compensate for resistive line drops and the corresponding reactive power
demands.
3. Increase the effectiveness of the overall compensating system against dynamic
disturbances.
COMBINED SERIESSHUNT CONTROLLER
A combined series shunt controller has separate series and shunt controllers whose
operation is coordinated. The series controller injects voltage in series with the line
voltage and the shunt controller injects current into the location of the controller.

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UNIFIED POWER FLOW CONTROLLER UPFC
In the UPFC, the shunt and series controllers are unified. A unified power flow
controller is a combination of static synchronous compensator(STATCOM)and a
static synchronous series compensator(SSSC). The STATCOMand SSSC are coupled
by a common DC link as shown in figure. The DC link allows bi- directional flow of
real power between the series output terminals of the SSSC and the shunt output
terminals of STATCOM. UPFC is controlled to provide concurrently active and
reactive series line compensation. It is able to control, simultaneously or selectively .
all the parameters affecting power flow in the transmission line that is transmission line
voltage , impedance and angle, therefore the active power and reactive power flow in
line. The UPFC may also provide independently controllable shunt reactive
compensation.
Conceptually, the UPFC is generalized synchronous voltage source represented by
voltage PhasorVc with controllable magnitude Vc ( 0 ≤ Vc ≤ Vcmax) and angle
( 0 ≤ P1 ≤360˚) in series with the transmission line. Convertor 2 (SSSC)provides the
main function of the UPFC by injecting the voltage Vc with controllable magnitude and
phase in series with the line through a transformer. The transmission line current
flowing through this voltage results in reactive and real power exchange between it and
the AC system.
The real power can freely flow in either direction between AC terminals of the two
convertors. Since a synchronous voltage sourceis able to generate only the reactive
power, the real power exchanged is supplied by one of the buses. The real power
exchanged at the AC terminals is converted into DC power which appears at the DC
link.
The basic role of convertor 1 (STATCOM)is to supply or absorb the real power
demanded by convertor 2 at the DC link to supportthe real power exchange resulting
from the series voltage injection. Whereas there is a direct path for the real power
through convertor 1 and 2 back to the line, the corresponding reactive power exchanged
is supplied or absorbed locally by convertor 2.

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UNIFIED POWER FLOW CONTROLLER
In addition, convertor 1 can also generate or absorb controllable reactive power, if
needed and hence provide independent shunt reactive compensation for the line. The
reactive power exchange of convertor 1 is independent of the reactive power exchanged
by convertor 2. Thus each convertor can independently generate or absorb reactive
power at its own Ac output terminal. There can be no reactive power flow through the
DC link. UPFC are employed for controlling active and reactive power, voltage control,
damping oscillations and limiting faults current.
THYRISTOR CONTROLLED PHASE SHIFTING TRANSFORMER
A special form of 3 Phase regulating transformer is realized by combing a transformer
that is connected in series with a line to a voltage transformer equipped with a tap
changer. The windings of the transformer are so connected that on its secondary side,
phase Quadrature voltages are generated. The secondaryvoltages of the voltage
transformer are fed into the secondarywindings of the series transformer. Thus the
addition of small ,phase Quadrature voltage components to the phase voltages of the
line creates phase shifted output voltages without any appreciable change in magnitude.
A phase shifted transformer is therefore able to introduce a phase shift in line.

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The Phasordiagram is shown in figure shows the phase shift realized without
appreciable change in magnitude by the injection of phase Quadrature voltage
components in a 3 phase system .When a phase shifting transformer with on load tap
changer is used, controllable phase shift is obtained. Phase shifting transformers have
been in use since 1930’s for controlof power flow in transmission lines in steady state.
In spite of their low MVA capacity, these phase shifting transformers can exercise a
significant real power control. A promising use of these devices is in creating active
power regulation on selected lines and securing active power damping.
Controlled shunt compensation increases transient stability by increasing or maintaining
the transmission line voltage during the accelerated swing of the disturbed machine.
Controlled series reactive compensation improves transient stability by increasing the
power transmission during the first swing by reducing the effective line impedance. The
ability of the phase shifting transformer or the phase angle regulator to maintain the
maximum effective transmission angle during the first swing can also be used
effectively to increase the transient stability limit. The phase shifting transformer can
provide a substantial increase in the transient stability margin. The increase in stability
margin is proportional to the angular range and which in turn depends on the VA rating
of the phase shifting transformer.
The modification of voltage magnitudes and or their phase by adding a control voltage
is an important conceptand forms the basis of some of the new FACTS devices.
By using electronic controllers, the operation of phase shifting transformer can be made
fast which enables dynamic regulation of power flow and improvement of power flow
system stability and dynamic security.
Both the conventional thyristor based and the GTO based phase angle regulator inject a
voltage between the given bus and the controlled line. The major difference is that
whereas the thyristor based regulator obtains the voltage to be injected from appropriate
taps of the regulating transformer, the GTP based regulator generates this voltage from
DC supply. Therefore, the function of thyristor based regulator is that of an on load tap
changer, selecting the propertap and injecting the thus obtained voltage to the line. The
function of GTO based regulator is to generate the required voltage and to inject it in
series with the line just as the thyristor based regulator. Thus the injected voltage need
not be realized through electromagnetic winding arrangements; instead by using high
speed semiconductor switches such as GTO thyristors, voltage sourceinvertors (VSI)
phase shifted components are produced.

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PHASE SHIFTING TRANSFORMER AND ITS PHASOR DIAGRAM

RGPV EX7102 UNIT II

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