Final Seminar Report Sim 158
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Transcript of Final Seminar Report Sim 158
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A
SEMINAR REPORT
ON
HVDC (HIGH VOLTAGE DIRECT CURRENT)
&
FACTS (FLEXIBLE AC TRANSMISSION SYSTEM)
SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE AWARD OF THE
DEGREE OF
BACHELOR OF TECHNOLOGY
(ELECTRICAL ENGINEERING)
SESSION: 2012-2013
SUBMITTED BY:
SAURABH MEENA
ELECTRICAL ENGINEERING
ROLL NO. : 09EBNEE053
BANSAL SCHOOL OF ENGINEERING AND TECHNOLOGY,
JAIPUR
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ABSTRACT
HIGH VOLTAGE DIRECT CURRENT (HVDC)
High Voltage Direct Current (HVDC) technology has characteristics which make it especially attractive in
certain transmission applications. The number of HVDC projects committed or under consideration globally
has increased in recent years reflecting a renewed interest in this field proven technology. New HVDC
converter designs and improvements in conventional HVDC design have contributed to this trend. This paper
provides an overview of the rationale for selection of HVDC technology and describes some of the latest
technical developments.
FLEXIBLE AC TRANSMISSION SYSTEM (FACTS)
The philosophy of FACTS (Flexible AC Transmission Systems) is to use power electronic controlled devices to
control power flow in transmission network to utilize to its full capacity.
FACTS are one of the best ways to reduce the need for construction of new overhead transmission lines is to
increase power flow over existing lines.
Due to high capital cost of erection of new transmission line we have improved this technology called FACTS.
By using this technology we can improve the power transfer capability of the transmission line and these
devices have high switching capacity even at high frequencies as these devices have no moving parts we can
achieve high sensitivity and better control. There are many types of FACTS controller devices out of them
TCSC (Thyristor Control Series Capacitor) has more advantages than other types
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AN INTRODUCTION TO HVDC TRANSMISSION
One of the most exciting new technical development in electric power system in the last three decades has been
High Voltage Direct Current transmission. From the first of HVDC links to the recent, the voltage has
increased from 100 KV to 800 KV, the rated power from 20 MW to 6300 MW and the distance from 96 km to
1370 km.
Preceding and accompanying this rapid growth of Direct Current Transmission were developments in High
Voltage, High power valves, in control and protection system, in DC cables and in insulation for overhead DC
lines.
In India three HVDC projects are in operation.
(i) The Rihand-Delhi HVDC transmission project having 1500 MW capacity and 500 KV DCvoltage is the first commercial long distance DC transmission project in India.
(ii) Vindhyachal 2x250 MW Back to back DC converter station which asychronously connect the
Northern and Western regions for exchange of power.
(iii) The Nation HVDC experimental line project, which links Lower Sileru in A.P. to Barsoor in
M.P. Phase 1 of this project is capable of transmitting 100 MW at 100 KV DC.
The main advantages of High Voltage Direct Current transmission are
(1) Asynchronous operation
(2) Controllability
(3) Stability
(4) Reliability
(5) Low right of way requirement
(6) Economy on overall basis
(7) Greater power per conductor
(8) Simple line construction
(9) No skin effect, charging current and less corona loss and interference
(10) Ground return can be used
(11) Cables can be worked at a higher voltage gradient
(12) May inter connect AC systems of different frequencies
(13) Low short circuit current on DC line.
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APPLICATIONS OF DC TRANSMISSION
1. For cables crossing bodies of water wider than 32 km. [Ex Sweden-Got land link, a 20 MW, 100 KV
DC single conductor submarine link to supply power to the island of Got land.]
2. For inter connecting AC systems having different frequencies or where asynchronous operation is
desired.
3. For transmitting large amounts of power over long distances by overhead lines.
4. In congested urban areas where it is difficult to acquire right of way for overhead lines and where
lengths involved make AC cables impracticable.
Economic Factors
The cost per unit length of a DC is lower than that of an AC line of the same power capability with comparable
reliability, but the cost of the terminal equipment of a DC line is much more than that in an AC. A graph is
plotted between the cost of transmitting an amount of power by onemethod and the distance over which it is
transmitted, below:
The vertical intercept of each curve is the cost of the terminal equipment alone. The slope of each curve is the
cost per unit length of the line and of that accessory equipment which varies with length. The curve for AC
transmission intersects that for DC transmission at an X axis which is the break even distance, Transmission by
DC is cheaper than AC for distance above 500 km.
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WHY WE USE DC TRANSMISSION?
The question is often asked, Why we use DC transmission? One response is that losses are lower. But this is
not correct the level of losses is designed in to a transmission system and is regulated by the size of the
conductor selected. DC and AC conductors either as overhead transmission lines or submarine cables can have
lower losses but at higher expense since the larger cross-sectional are will generally results in lower but cost
more.
When converters are used for DC transmission, it is generally by economic choice driven by one of the
following reasons.
1. An overhead DC transmission line with its towers can be designed to be less costly per unit of
length than an equivalent AC line designed to transmit the same level of electric power. However the
DC converter stations at each end are more costly than the terminating station of an AC line and so
there is a breakeven distance above which the total costs of DC transmission is less than its AC
transmission alternative. The DC transmission has lower visual profile than an equivalent AC line and
so contributes to a lower environmental impact. There are other environmental advantages to a DC
transmission line through the electric magnetic fields being DC instead of AC.
2. If transmission is by submarine or underground cable, the breakeven distance is much less than
overhead transmission. It is not practical to consider AC cable systems exceeding 50km but DC cable
system are in service whose length is in the hundreds of kilometres and even distances of 600km or
greater have been considered feasible
3. Some AC power systems are not synchronized to the neighbouring networks even though their
physical distances between them os quite small. Thais occur in Japan where half the country is a 60hz
network and other is 50hz system. It is physically impossible to connect the two together by direct AC
methods in order to exchange electric power between them. However if a DC converter station is
located the required power flow even though the AC systems so connected remain asynchronous.
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TYPES OF DC LINKS
HVDC back to back link: This link is used to connect two AC grids, each AC grid can have its own load
frequency control. Direction of power flow can be controlled by adjusting the characteristics of convertor
valves. There is no increase in fault level and cascade trippings in the network are avoided. [Ex. Vindhyachal
Back to Back HVDC link].
Monopolar link: This links has one conductor, usually of negative polarity, and ground or sea return.
Bipolar link: This link has two conductors one positive, the other negative. Each terminal has two convertors
of equal rated voltages in series on the DC side. The neutral points (junction between convertors) are grounded
at one or both ends. If both neutrals are grounded, the two poles can operate independently. Normally operate
at equal currents: then there is no ground current. In the event of fault on one conductor, the other conductor
with ground return can carry up to half the rated load.
Homopolar link: This links has two or more conductors all having the same polarity, usually negative, and
always operate with ground return. In the event of a fault on one conductor, the entire convertor is available for
connection to the remaining conductor or conductors, which, having some over load capability, can carry more
than half of the rated power and perhaps whole rated power, at the expenses of increased line loss. In a Bipolar
scheme reconnection of the whole convertor to one pole of the line is more complicated and is usually not
feasible because of graded insulation. In this respect a Homopolar line is preferable to a bipolar line in cases
where continuous ground current is not objectionable. An additional advantage, through minor is less corona
loss and negative polarity is preferable to have less radio interference.
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Figure shows HVDC Bipolar system in which there are two poles one is negative and the other is positive.
Each pole consists of one 12 pulse covnertor at both ends in which sending end will act as rectifier and
receiving end will act as invertor. The 12 pulse convertor consists of two series connected 6 pulse bridges
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HVDC Bipolar System Layout
HVDC Rectifier and invertor station in HVDC Bipolar systems consists of following parts
1. AC switchyard
2. AC filter area
3. Convertor transformers
4. Valve hall and control room
5. DC switchyard and smoothing reactor
6. Electrical and mechanical ausiliaries
AC Switchyard
The AC switchyard is generally at 400 KV or 760 KV voltage level corresponding to the standard ofEHV/UHV transmission voltage. The AC yard is of one half breaker bus system. The advantage of one and half
a breaker system is it permits use of only three breakers for two circuits. In one and half a breaker system the
circuits one and two can take supply either from Bus I or Bus II, thus in the event of fault on any bus the supply
is maintained in the circuits by unfaulty bus. Hence, high security against loss of supply.
One and Half Breaker Scheme
The insulation coordination of the AC yard is correlated with that of DC yard and over voltages approaching
from DC side. Metal oxide arrestors are used in AC yard and DC yard. The AC yard is designed in similar
principles like usually EHV AC switchyards with following additional considerations:
A large area on AC yard is covered by AC harmonic filter bank.
More space is provided for movement of large convertor transformers and cranes.
No. of surge arrestors are provided at strategic locations in AC yard.
Protection and control of an eneterprise with valves and DC yard.
The circuit breakers used in HVDC substation have reinsertion resistors to reduce switchin over
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AC Harmonic Filter Area
A large portion of the area in AC yard is covered by AC filter bank. The filters are required to filter out
harmonics generated due to the operation of 3 phase AC/DC conversion which generates kp 1th harmonic on
AC side, p is the integer and K is the number of pulses of convertor valve. This is derived using fourier
analysis.
Harmonics in AC for 12 pulse system for which K = 12 are 1, 11, 13, 23, 25th
, 5th
and 7th
harmonics are of 10%to 25% which are generated due to the formation of 12 pulse by series connection of sixpulse connection.
Each filter bank has the following components.
1. AC filter capacitor bank
2. Reactor
3. Resistor bank
4. Current transformers
5. Circuit breakers
These AC harmonic filters are essential to reduce the harmonic contenet in the AC voltage within the limits.
AC filter capacitor also provide the leading reactive power consumed by the convertor (shunt compensation).
AC harmonic filters comprise RLC series circuit connected in shunt with the AC busbard. Separate branches
are provided for predominant 5th, 7th, 11th and 13th Harmonic and a high phase filter for higher than 23rd
harmonic and above.
A C Harmonic Filter Circuit
Reactive power demand and compensation: - The operation of the convertor requires a certain
amount of reactive power. This is due to
The manner of controlling HVDC convertors introduces a phase shift between the fundamentals of AC
current and voltage. The magnitude of this phase shift is strongly dependant on the firing angle and inrectifier and extinction angle y in invertor.
The commutation process, in which the DC current is connected from one valve to another, also
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Convertor consumes reactive power both when it operates as rectifier as well as invertor. Besides the reactive
power demand is also due to magnetizing current of convertor transformer. Considering normal valves of a
(rectifier) firing angle and extinction angel y (Invertor) the reactive power demand usually in the range of 50%
- 60% of the transmitted active power.
Convertor Reactive Power Demand
This reactive power in the range of 50% to 60% of the transmitted active power (each convertor station) is
compensated by several ways depending on the quality of the connecting AC network.
The different possibilities for suitable reactive power production are mentioned here.
I. AC filters
II. Shunt capacitors
III. Excessive reactive power from the AC network
IV. Static compensation
V. Synchronous condensers
Valve Hall and Control Room
The valve hall and control room are located between AC yard and DC yard. The valve hall houses quadruple
thyristor valves, air core reactors, terminal bushings associated bus bars and surge arrestor. The control room
building houses control panels for AC yard, DC yard and valves etc. in Bipolar HVDC substation there are two
valve halls, each valve hall houses three quadruple valves. The control room is in between two valve halls. The
valve hall is provided with uniform earthing mat in the flooring and uniform earthed screen in the wall and the
roof. The screen protects the control circuits from the electromagnetic interference produced by the operation
of thyristor valves. The valve hall is provided with air conditioning system. The temperature inside the valve
hall is high due to valve losses and the lowest temperature of valve hall maintained is 10C and the highest
temperature of valve hall maintained is 55C.
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The control room houses the following control panels:
Protection, metering and control panels for AC yard, DC yard and convertor transformers.
control panels for valves
PLC communication and tele control panels etc.
Monitoring panels.
The auxiliary switchgear, low voltage switchgear, DC supply systems is generally installed in a separate
floor of the control room building. The convertor valves are either supported on the valve hall floor on insulator
columns or are under hung from the roof by means of insulators.
Thyristor Valves
Since the individual thyristors has a limited voltage ratings nearly 7 KV, several thyristors are connected in
series to achieve desired rated voltage. The assembly formed is called a thyristor valve. A thyristor valve for an
HVDC convertor comprises of the following:
Several thyristors connected in series to achieve the required insulation level. Each thyristor has its
associated thyristor control unit.
Snubber (voltage grading) circuit for equal distribution of voltage across thyristors and protection of
thyristors in the string.
Cooling system to removal heat from the cathode silicon wager. In HVDC system pure deionised
water is circulated in a closed cycle to remove heat from heat sinks.
A valve is made up of stacking four valve modules in a vertical formation is called a quadruple valve. The
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A typical Bipolar twelve pulse convertor substation has two valve halls, one for each pole. Each valve hall
houses three quadruple valves.
The active part of thyristor is a semiconductor mono crystalline silicon wafer with a thickness of half a
millimetre and an area in the range 8 to 60 cm. The wafer has been treated to obtain P-N-P-N with desired
current and voltage properties. The junction temp. with stand capability is 100 to 125C. the water cooled
wafer has 45 cm area and a threshold voltage drop of 0.8 to 1.0 V. the thyristors are mounted on heat sinks.
The modules are cooled in parallel with two cooling circuits in each module giving equal coolings. As the
water should be insulating a special water processing unit is installed to deionise the water to limit the amount
of oxygen in the water. The valve losses are about 0.5 percent of DC power transfer.
Cooling System for Water Cooled Thyristor Valve
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DC Yard
The DC yard has the following essential equipment:-
DC smoothing reactor
DC filters
DC bus bars and isolators, earthing switches, current transducers, voltage dividers, surge arrestors.
Switchgear for switching from ground return to the metallic return.
Smoothing Reactor: HVDC smoothing reactors of a 0.4 henrys to 1 henry are generally used. There are oil
filled reactors. Smoothing reactor is connected in series with the convertor bridges in order to reduce the
current harmonics in the direct current and to reduce valve stresses due transients such as DC line faults and
commutation failures by limiting the fault current and the rate of rise current.
A DC smoothing reactor is located on the low voltage side and air core reactors on the line side of the
convertors. The later to limit any steep front surger entering the station from the DC side. Additional air core
reactors are installed in each phase on the AC side to reduce the rate of rise of current during thyristor turn on.
DC Harmonic Filters
Using Fourier analysis we can evaluate the harmonicas on DC side for 12 pulse connector which is Kp, p is
the integer and K is the pulse number. For 12 pulse system the harmonics generated on DC side are 0, 12, 24,
etc. a high pulse DC filter turned to 12
th
harmonic is usually provided on DC side.
Single Line Diagram of Single Pole Giving Details of DC Yard
1. Surge arrestor 6. Direct voltage measuring device
2. Converter transformer 7. DC filter
3. Air core reactor 8. Current measuring transducer
4. Thyristor valve 9. DC line
5. Smoothing reactor 10. Electrode line
The measuring equipment i.e. a voltage divider, current measuring transducer and current transformer, provide
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Earth Return
In monopolar configuration, the return path is usually through earth or sea. Earth return or sea return reduces
the cost of transmission system.
In Bipolar system the normal power flow is through pole conductors and only negligible out of balance current
flow through earth. The mid points of convertors at both ends are earthed. In monopolar the return is through
earth. The earth electrode station is usually built 10 to 25 km from main HVDC substation to avoid galvanic
corrosion of pipes, foundation structures, cable theatres, earthing material due to cathodic corrosion.
The connection between mid-point of convertor valve and a distant earth electrode is an electrode line.
Electrode line is insulated from earth and connected to the earth electrode.
1. Neutral Bus Switch
2. Switch for Metallic to Ground Transfer
3. HVDC Breaker of Ground to Metallic Return Transfer
DC circuit breaking is difficult due to non-availability of current zero in the DC. Hence links do not
have any provision of DC circuit breakers. HVDC links do not have parallel lines and T off lines due to lack of
HVDC circuit breaker. HVDC circuit breaker using artificial current zero is produced by discharging a
precharged capacitor bank through the breaker contacts has been developed but it is complex and not
economical.
Metallic Return: In the case of fault on a pole the power transfer taken place in Monopolar mode using
ground return in addition to this the line of the pole which is out of order can be used for return path. This type
of current return is called Metallic return.
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ENVIORMENTAL CONSIDERATIONS
The electrical environmental effects from HVDC transmission lines can be characterized by field and ion
effects as well as corona effects (4), (5). The electric field arises from both the electrical charge on the
conductor and for a HVDC overhead transmission line from charges on air ions and aerosols surrounding the
conductor. These gives rise to DC as well as due to ion current density flowing through the air. A DC magnetic
field is produced by DC current flowing through the conductors. Air ions produced by HVDC lines from
clouds which drift away from the line when blown by the wind and may come in contact with humans, animals
and plants outside the transmission lines right-of way or corridor. The corona effects may produce low levels
of radio interference, audible noise and ozone generation.
Field and corona effects
The field and corona effects of transmission lines largely favor DC transmission over AC transmission. The
significant considerations are a follows
1. For a given power transfer requiring extra high voltage transmission, the DC transmission line will
have a smaller tower profile than the equivalent AC transmission carrying the same level of power, this
can also lead to less width of right- of-way for DC transmission option.
2. The steady and direct magnetic field of DC transmission line near at the edge of transmission right-of-
way will be about the same value in magnitude as the earths naturally occurring magnetic field. For
this reason alone, it seems unlikely that this small contribution by HVDC transmission lines to the
background geometric field would be the basis of concern.
3. The static and steady electric field from DC transmission at the levels experienced beneath lines or
edges of the right-of way have no known adverse biological effects. There is no theory or mechanism
to explain how a static electric field at the levels produced by DC transmission lines could effect human
health. Electric fields from ac transmission lines have been under more intense scrutiny than fields
generated from dc transmission lines.
4. The ion and corona effects of dc transmission line lead to a small contribution of ozone production to
higher naturally occurring background concentrations. Exacting long term measurements are required
to detect such concentrations.
5. If ground return is used with monopolar operation, the resulting dc magnetic field can cause error in
magnetic compass readings taken in the vicinity of the DC line or cable. This impact is minimized by
providing a conductor or cable return path in close proximity to the main conductor or cable for
magnetic cancellation. Another concern with continuous ground current is that some of return current
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INHERENT PROBLEMS ASSOCIATED WITH HVDC
1. Expensive Convertors: Expensive convertor stations are required at each end of a DC transmission link,
whereas only transformer stations are required in an AC link.
2. Reactive Power Requirement: Convertors require much reactive power, both in rectification as well as in
inversion. At each convertor the reactive power consumed may be as much at 50% of the active power
rating of the DC link. The Reactive power requirement is partly supplied by the filter capacitance, and
partly by synchronous or static capacitors that need to be installed for the purpose.
3. Generation of Harmonics: Convertors generates a lot of harmonics both on the DC side and the AC side.
Filters are used on the AC side to reduce the amount of harmonics transferred to the AC systems. on the
DC system smoothing reactors are used. These components add to the cost of convertors.
4. Difficulty of Circuit Breaking: Due to the absence of a natural current zero with DC, circuit breaking is
difficult. This is not a major problem in single HVDC link systems, as circuit Breaking can be
accomplished by a very rapid absorbing of the energy back into the AC system.
5. Difficulty of Voltage Transformation: Power is generally used at low voltage, but for reasons of
efficiency must be transmitted at high voltage. Absence of the equivalent of DC transformers makes it
necessary for voltage transformation to carried out on the AC side of the system and prevents a purely DC
system being used.
6. Difficulty of High Power generation: Due to the problems of commutation with DC machines, voltage,
speed and size are limited.
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AN INTRODUCTION TO FACTS
Without electricity, modern society would cease to function. As the volume of power transmitted and
distributed increases, so do the requirements for a high quality and reliable supply. At the same time, rising
costs and growing environmental concerns make the process of building new power transmission and
distribution lines increasingly complicated and time-consuming. Making exciting lines as well as new ones
more efficient and economical then become a compelling alternative. The purpose of the transmission network
is to pool power plants and load canters in order to minimize the total power generation capacity and fuel cost.
The power systems of today, by and large, are mechanically controlled. There is a wide spread use of
microelectronics, computers and high-speed communications for control and protection of present transmission
systems; however, when operating signals are sent to the power circuits, where the final power control action is
taken, the switching devices are mechanical and there is little high-speed control.
Another problem with mechanical devices is that control cannot be initiated frequently, because these
mechanical devices tend to wear out very quickly compared to static devices. In India, for example there has
been a fourfold increase in the value of wholesale transactions during the last decade. At the same time,
however, addition of new transmission capacity to handle this growth has become increasingly difficult.
Fortunately, a new generation of technologies promises to solve the growth-capacity dilemma by offering
unconventional ways to increase transmission capacity with much less need for building new overhead lines.
Today, one such technological area that has the potential to revolutionize utility transmission system is
FLEXIBLE AC TRANSMISSION.FACTS devices are static equipment used for effective transmission. It
means to enhance controllability and increase power transfer capability. It was introduced by Dr N.G.Hingorani
in 1988 from the Electric Power Research Institute (EPRI) in the USA.
DEFINITION:
FACTS is defined by IEEE as power electronic based system and other static equipment that provide control
of one or more AC transmission system parameters to enhance controllability and power transfer capability.
Symbol for FACTS devices
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BASICS OF FACTS:
The main idea of FACTS can be explained by the basic equation of power transmission is shown in the below
figure. Power transmitted between two nodes of the system depends on voltages at both ends of the
interconnection. Different FAC TS devices can actively influence one or more of these parameters for power
flow control and improvement of voltage stability at node of interconnection. Depending on the system
configuration, the tasks of FACTS can be summarized as follows:
Meshed systems & bulk power transmission can cause power flow control.
Radial systems ¶llel lines can cause impedance control.
Weak systems can cause voltage control.
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FACTS CONTROLLERS:
GENARATIONS OF CONTROLLERS:
FIRST GENERATION: This uses a simple logic of capacitance of manually based upon a roughly pre-
manipulated data
SECOND GENERATION: The electronics makes its doubt here, forming a more smooth operation using relays
and semi-conductor switches. This even did not eradicate the demerit of manual presence.
THIRD GENERATION: This uses FACTS devices to control the flow of power in the transmission line, and
also increase the stability in transient conditions.
The power that can be transmitted over a line depends on three factors
1. Series reactance of the line
2. Bus voltages
3. Transmission angle d
v Voltage along the line can be controlled by reactance shunt compensation.
v Series line inductance can be controlled by series capacitive compensation.
v Transmission angle can be varied by phase shifting.
FACTS can be connected
1. In series with the power system.
2. In shunt with the power system.
3. Both in series and shunt with the power system.
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SERIES COMPENSATION
In series compensation FACTS are connected in series with power system. It works as a controllable
voltage source
Symbol for Series Controller
METHODS OF SERIES COMPENSATION:
Static synchronous series compensator (SSSC)
Thyristor controlled series capacitor (TCSC)
Thyristor switched series capacitor (TSSC)
Static synchronous series compensator (SSSC)
As mentioned, Static Synchronous Series Compensator (SSSC) is placed in the group of series connected
FACTS devices. SSSC consists of a voltage source inverter connected in series through a coupling transformer
to the transmission line.
A source of energy is required for providing and maintaining the DC voltage across the DC capacitor and
compensation of SSSC losses. Fig. 2 shows the model of SSSC which consists of a series connected voltage
source in series with an impedance. This impedance represents the impedance of coupling transformer.
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THYRISTOR CONTROLLED SERIES CAPACITOR (TCSC):
The basic thyristor controlled series capacitor scheme proposed in 1986 by Vithayathil
with others as a method of rapid adjustment of network impedance is shown in fig.It consists of the series
compensating capacitor shunted by a thyristor-controlled reactor. In a practical TCSC implementation several
such basic compensators may be connected in series to obtain the desired voltage rating and operating
characteristic
TCSC is a capacitive reactance compensator which consists of a series capacitive bank shunted by a thyristor
controlled reactor in order to provide smoothly variable capacitive reactance.
SINGLE LINE DIAGRAM OF TCSC
BENEFITS OF TCSC:
Study state control of power flow.
Transient stability improvement.
Balancing power flow in parallel lines.
To control line impedance.
Reduce transmission losses
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SHUNT COMPENSATION:
In shunt compensation power system is connected in shunt with the FACTS device .It works as a controllable
current source.
Symbol for shunt Controller
METHODS OF SHUNT COMPENSATION:
Static Synchronous compensator (STATCOM)
Static VAR Compensator (SVC)
Thyristor Controlled Reactor (TCR)
Thyristor Switched Reactor (TSR)
Thyristor Switched Capacitor (TSC)
Mechanically Switched Capacitor (MSC)
Static Synchronous compensator (STATCOM)
The Static Synchronous Compensator (STATCOM) is a shunt device of the Flexible AC Transmission Systems
(FACTS) family using power electronics to control power flow and improve transient stability on power grids
[1]. The STATCOM regulates voltage at its terminal by controlling the amount of reactive power injected into
or absorbed from the power system. When system voltage is low, the STATCOM generates reactive power
(STATCOM capacitive). When system voltage is high, it absorbs reactive power (STATCOM inductive).
The variation of reactive power is performed by means of a Voltage-Sourced Converter (VSC) connected on
the secondary side of a coupling transformer. The VSC uses forced-commutated power electronic devices
(GTOs, IGBTs or IGCTs) to synthesize a voltage V2 from a DC voltage source. The principle of operation of
the STATCOM is explained on the figure below showing the active and reactive power transfer between a
source V1 and a source V2. In this figure, V1 represents the system voltage to be controlled and V2 is the
voltage generated by the VSC.
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Operating Principle of the STATCOM
P = (V1V2) sin / X , Q = V1(V1 V2cos) / X
Symbol Meaning
V1 Line to line voltage of source 1
V2 Line to line voltage of source 2
X Reactance of interconnection transformer and filters
Phase angle of V1 with respect to V2
In steady state operation, the voltage V2 generated by the VSC is in phase with V1 (=0), so that only reactive
power is flowing (P=0). If V2 is lower than V1, Q is flowing from V1 to V2 (STATCOM is absorbing reactive
power). On the reverse, if V2 is higher than V1, Q is flowing from V2 to V1 (STATCOM is generating reactive
power). The amount of reactive power is given by
Q = (V1 (V1V2 )) /X.
A capacitor connected on the DC side of the VSC acts as a DC voltage source. In steady state the voltage V2
has to be phase shifted slightly behind V1 in order to compensate for transformer and VSC losses and to keep
the capacitor charged. Two VSC technologies can be used for the VSC:
VSC using GTO-based square-wave inverters and special interconnection transformers. Typically four
three-level inverters are used to build a 48-step voltage waveform. Special interconnection transformers are
used to neutralize harmonics contained in the square waves generated by individual inverters. In this type of
VSC, the fundamental component of voltage V2 is proportional to the voltage Vdc. Therefore Vdc has to be
varied for controlling the reactive power.
VSC using IGBT-based PWM inverters. This type of inverter uses Pulse-Width Modulation (PWM)
technique to synthesize a sinusoidal waveform from a DC voltage source with a typical chopping frequency of
a few kilohertz. Harmonic voltages are cancelled by connecting filters at the AC side of the VSC. This type of
VSC uses a fixed DC voltage Vdc. Voltage V2 is varied by changing the modulation index of the PWM
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The figure below shows a single-line diagram of the STATCOM and a simplified block diagram of its control
system.
Single-line Diagram of a STATCOM and Its Control System Block Diagram
The control system consists of:
A phase-locked loop (PLL) which synchronizes on the positive-sequence component of the three-phase
primary voltage V1. The output of the PLL (angle =t) is used to compute the direct-axis and quadrature-axis
components of the AC three-phase voltage and currents (labeled as Vd, Vq or Id, Iq on the diagram).
Measurement systems measuring the d and q components of AC positive-sequence voltage and currents
to be controlled as well as the DC voltage Vdc.
An outer regulation loop consisting of an AC voltage regulator and a DC voltage regulator. The output
of the AC voltage regulator is the reference current Iqref for the current regulator (Iq = current in quadrature
with voltage which controls reactive power flow). The output of the DC voltage regulator is the reference
current Idref for the current regulator (Id = current in phase with voltage which controls active power flow).
An inner current regulation loop consisting of a current regulator. The current regulator controls the
magnitude and phase of the voltage generated by the PWM converter (V2d V2q) from the Idref and Iqref
reference currents produced respectively by the DC voltage regulator and the AC voltage regulator (in voltage
control mode). The current regulator is assisted by a feed forward type regulator which predicts the V2 voltage
output (V2d V2q) from the V1 measurement (V1d V1q) and the transformer leakage reactance.
The STACOM block is a phasor model which does not include detailed representations of the power
electronics. You must use it with the phasor simulation method, activated with the Powergui block. It can be
used in three-phase power systems together with synchronous generators, motors, dynamic loads and other
FACTS and DR systems to perform transient stability studies and observe impact of the STATCOM on
electromechanical oscillations and transmission capacity at fundamental frequency.
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STATCOM V-I characteristic
The STATCOM can be operated in two different modes:
In voltage regulation mode (the voltage is regulated within limits as explained below)
In var control mode (the STATCOM reactive power output is kept constant)
When the STATCOM is operated in voltage regulation mode, it implements the following V-I characteristic.
The V-I characteristic is described by the following equation:
V= Vref+Xs I
Where
V Positive sequence voltage (pu)
I Reactive current (pu/Pnom) (I > 0 indicates an inductive current)
Xs Slope or droop reactance (pu/Pnom)
Static VAR Compensator (SVC):
The SVC regulates voltage at its terminals by controlling the amount of reactive power injected into or
absorbed from the power system.
When system voltage is low, the SVC generates reactive power (SVC capacitive). When system voltage is
high, it absorbs reactive power (SVC inductive).
Thyristor-controlled reactor (TCR): reactor is connected in series with a bidirectional thyristor valve. The
thyristor valve is phase-controlled. Equivalent reactance is varied continuously.
Thyristor-switched reactor (TSR): Same as TCR but thyristor is either in zero- or full- conduction.
Equivalent reactance is varied in stepwise manner.
Thyristor-switched capacitor (TSC): capacitor is connected in series with a bidirectional thyristor valve.
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FACTS CONCEPTS SIMILAR TO HVDC
While some of the relevant technology i.e., Static VAR Compensation is already in wide use, the FACTS
concept has brought to the table a tremendous potential for thyristor based
Controllers which will surely revolutionize the power system.
The technology offers the utilities the ability to:
1. Control power flows on their transmission routes;
2. Allow secure loading of transmission lines to their full thermal capacity.
FACTS technology, while allowing use of transmission to its thermal capacity, does not do away with the need
for additional transmission lines or the upgrading of existing lines where thermal limits have been reached or
when evaluation of losses added to the cost of FACTS technology shows that new lines or upgrading of
existing lines is the most optimum answer. Often, ac transmission systems are thought of as being "inflexible".
Power flow in ac networks simply follows Ohm's law and ordinarily cannot be made to flow along specific
desired paths. As a result, ac networks suffer from parallel-path, or "loop" flows. The power flows from source
to load in inverse proportion to the relative impedances of the transmission paths. Low impedance paths take
the largest fraction of flow, but all lines in the interconnection are a part of the flow path. Thus, utilities not
involved in an interchange power transaction can be affected.
A fundamental notion behind FACTS is that it is possible to continuously vary the apparent impedance of
specific transmission lines so as to force power to flow along a "contract path". This is a brand-new concept for
many system planners. As illustrated in Figure 1.3, with precise control of the impedance of transmission lines
using FACTS devices, it is possible to maintain constant power flow along a desired path in the presence of
continuous changes of load levels in the external ac network, and to react in a planned way to contingencies.
Just as in HVDC applications, FACTS controls could be designed to enhance the behaviour of the uncontrolled
systems. The flexible system owes its tighter transmission control to its ability to manage the interrelated
parameters that constrain today's systems, including series impedance, shunt impedance, phase angle, and the
occurrence of oscillations at various frequencies below the rated frequency. By adding to in this way, the
controllers enable a transmission line to function nearer its thermal rating. For example, a 500-kV line may
have a loading limit of 1000-2000MW for safe operation, but a thermal limit of 3000 MW. It is often not
possible both to overcome these constraints and maintain the required system reliability by conventional
mechanical means alone, such as tap changers, phase shifters, and switched capacitors and reactors
(inductors).Granted, mechanical controllers are on the whole less expensive, but they increasingly need to be
supplemented by rapidly responding power electronics controllers.
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Applying Flexibility to the Electric Power System:
The power industry term FACTS (Flexible AC Transmission Systems) covers a number of technologies that
enhance the security, capacity and flexibility of power transmission systems. FACTS solutions enable power
grid owners to increase existing transmission network capacity while maintaining or improving the operating
margins necessary for grid stability. As a result, more power can reach consumers with a minimum impact on
the environment, after substantially shorter project implementation times, and at lower investment costs - all
compared to the alternative of building new transmission lines or power generation facilities. The two main
reasons for incorporating FACTS devices in Electric power systems are:
Raising dynamic stability limits
Provide better power flow control
FACTS technologies deliver the following benefits
A Rapidly Implemented Installations: FACTS projects are installed at existing substations and avoid the
taking of public or private lands. They can be completed in less than 12 to 18 months a substantially shorter
timeframe than the process required for constructing new transmission lines.
Increased System Capacity: FACTS provide increased capacity on the existing electrical transmission system
infrastructure by allowing maximum operational efficiency of existing transmission lines and other equipment.
Enhanced System Reliability: FACTS strengthen the operational integrity of transmission networks, allowing
greater voltage stability and power flow control, which leads to enhanced system reliability and security.
Improved System Controllability: FACTS allow improved system controllability by building intelligenceinto the transmission network via the ability to instantaneously respond to system disturbances and gridlock
constraints and to enable redirection of power flows.
Seamless System Interconnections: FACTS, in the form of BTB dc-link configurations, can establish
seamless interconnections within and between regional and local Networks, allowing controlled power
transfer and an increase in grid stability
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CONCLUSION:
HVDC Transmission system:
HVDC transmission system is a very superior type of transmission system topology, which serves for power
transmission and thus contributes the advantage like Use of ground return possible, Skin effect, Tower size etc.
Although HVDC possess some disadvantages. The extent of advantages makes it a very suitable one for the
transmission. For long distance transmission of electricity HVDC transmission is the best one than Extra High
Voltage AC transmission.
FACTS :
It is envisaged that in future FACTS devices could be installed on wide scale by electrical utilities in an attempt
to control the power flows through their networks. Concern has been expressed that such wide scale application
of FACTS devices could cause conflict between the control systems of the different devices Using the
advanced solid state technology, FACTS controllers offer flexibility of system operation fast and reliable
control. They better utilization of existing power generation and transmission facilities without comprising
system availability and security .The planner has to select controller out of the set of FACTS controllers, for
improving the system operation based on cost benefit analysis.
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REFERENCES
HVDC
1. A Refined HVDC Control System- Ekstrom. A and Liss. G (IEEE)
2. Rapid City Tie-New Technology Tames The East, West Interconnection- M. Brahman, D. Dickson, P.
Fisher, M. Stolz.
3. Engineering book
FACTS
1. S.Nilsson, Special Application Considerations for Custom Power systems.
2. www.IEEE.com
3. www.siemens.org
http://www.ieee.com/http://www.siemens.org/http://www.siemens.org/http://www.ieee.com/