statcom for renewable energy sources
TRANSCRIPT
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induction generator. From the obtained results, we have consolidated the
feasibility and practicability of the a pproach for t he a pplications c onsidered.
INTRODUCTION
To have sustainable growth and social progress,
the energy need by utilizing the renewable energy resources like wind,
biomass, hydro, co-generation, etc.
conservation and the use of renewable source are the key paradigm. The
need to integrate the re newable en ergy like wind energy into power s ystem
is to make it possible to minimize the environmental impact onconventional p lant. The integration of wind energy into existing power
system presents a technical challenges a nd that r equires co nsideration of
voltage regulation, stabil
an essential customer-focused measure and is greatly affected by the
operation of a distribution and transmission network. The issue of power
quality is o f great importance t o the wind turbine.
There has been an extensive growth and quick development in the
exploitation of wind energy in recent years. The individual units ca n be of
large ca pacity u p to 2 MW, feeding into distribution network, particularly
with customers connected in close proximity. Today, more
wind generating turbines are successful
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the xed-speed wind turbine operation, all the uctuation in the wind
speed are transmitted as uctuations in the mechanical torque, electrical
power on the grid and leads to large voltage uctuations. During the
normal operation, wind turbine produces a continuous variable output
power. These power variations are mainly caused by the effect of
turbulence, wind shear, and tower-shadow and of control system in the
power system.
Thus, the network needs to manage for such uctuations. The power
quality issues ca n be viewed with respect to the wind generation,
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transmission and distribution network, such as vol tage sa g, swells, ickers,
harmonics etc. However the wind generator introduces di sturbances into the
distribution network. One of the simple methods of running a wind
generating sy stem is t o use t he induction generator con nected directly to the
grid system. The induction generator h as inherent advantages o f cost
effectiveness a nd robustness. However; induction generators r equire reactive
power for magnetization. When the generated a ctive power of an induction
generator is vari ed due to wind, absorbed reactive power and terminal
voltage of an induction genera
control scheme in wind energy generation.
system is required under n ormal operating condition to allow the proper
control over t he active power p roduction. In the event of increasing grid
disturbance, a battery energy storage system for wind energy generating
system is generally required to compensate the uctuation generated by
wind turbine. A STATCOM based control technology has been proposed for
improving the power quality which can technically manages t he power level
associates with the commercial wind turbines. The proposed STATCOM
control scheme for grid connected wind energy generation for power
quality improvement has following objectives.
• Unity power factor at the sou rce side.
• Reactive power support only from STATCOM to wind Generator
and Load.
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• Simple bang-bang controller for STATCOM to achieve fast
dynamic response.
The paper is organized as fallows. The Section I
quality standards, issues and its consequences of wind turbine. The
Section III introduces the grid coordination rule for grid quality limits.
The Section IV describes the topology for e
Sections V, VI, VII describes t he con trol scheme, system performance an d
conclusion respectively.
1.2 Power quality
To have sustainable growth and social progress,
the energy need by utilizing the renewable energy resources like wind,
biomass, hydro, co-generation, etc.
conservation and the use of renewable source are the key paradigm. The
need to integrate the re newable en ergy like wind energy into power s ystemis to make it possible to minimize the environmental impact on
conventional p lant. The integration of wind energy into existing power
system presents a technical challenges a nd that r equires co nsideration of
voltage regulation, stabil
an essential customer-focused measure and is greatly affected by the
operation of a distribution and transmission network. The issue of powerquality is o f great importance t o the wind turbine.
There has been an extensive growth and quick development in the
exploitation of wind energy in recent years. The individual units ca n be of
large ca pacity u p to 2 MW, feeding into distribution network, particularly
with customers connected in close proximity. Today, more
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transmission and distribution network, such as vol tage sa g, swells, ickers,
harmonics etc. However the wind generator introduces di sturbances into the
distribution network. One of the simple methods of running a w ind generating
system is to u se t he induction generator con nected directly to the gri d system.
The induction generator has inherent
robustness. However; induction generators r equire reactive power for
magnetization. When the gen erated active power of an induction generator is
varied due to wind, absorbed react
voltage of an induction generat
scheme in wind energy generation.
system is required under normal operating condition to allow the proper
control over the active power p roduction. In the event of i ncreasing grid
disturbance, a battery energy storage system for wind energy generating
system is gen erally required to compensate the uctuation generated by wind
turbine. A STATCOM based control technology has been proposed for
improving the power quality which can technically manages the power level
associates with the commercial wind turbines. The proposed STATCOM
control scheme for grid connected wind energy generation for power quality
improvement has following objectives.
• Unity power factor at the sou rce side.
• Reactive power support only from STATCOM to wind Generator
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and Load.
• Simple bang-bang controller for STATCOM to ach ieve fast
dynamic response.
The paper is organized as fallows. The Section II int
quality standards, issues an d its con sequences of wind turbine. The Section
III introduces th e grid coordination rule for gri d quality limits. The Section
IV describes t he topology for p ower quality improvement. The Sections V, VI,
VII describes the control scheme, system performance and conclusion
respectively.
1.2 Power quality
The contemporary container crane industry,
segments, is often enamored by the bells an d whistles, colorful diagnostic
displays, high speed performance, and levels of automation that can be
achieved. Although these features an d
The contemporary container crane industry, like many other industry
segments, is often enamored by the bells a nd whistles, colorful diagnosticdisplays, high speed performance, and levels of automation that can be
achieved. Although these features an d
WIND ENERGY
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Wind power:
Wind is abundant almost in any part of the world. It
caused by uneven heating on the su rface of the earth as well as the eart h’s
rotation means that the wind resources will always be available. The
conventional ways o f generating electricity u sing n on renewable re sources su ch
as co al, natural gas, oil and so on, have great impacts on the en vironment as i t
contributes va st quantities o f carbon dioxide to the ea rth’s a tmosphere which
in turn will cause the temperature of the eart h’s su rface to increase, known
as t he green house effect. Hence, with the advances in science an d technology,
ways of generating electricit
wind are developed. Nowadays, the cost of wid wer
grid is a s ch eap as t he cost of generating electricity using coal and oil. Thus,
the increasing popularity of green electricity means the demand of electricity
produced by using non renewable en ergy is al so increased accordingly.
Fig: Formation of wind due to differential heating of land and sea
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Features of wind power systems:
There are some distinctive energy end use fea
i. Most wind power sites are in remote rural, island or marine areas.
Energy requirements in such places are d istinctive and do not require
the h igh electrical power.
ii. A power system with mixed quality supplies can be a good match with
total energy end use i .e. the su pply of cheap variable voltage p ower for
heating a nd expensive xed voltage el ectricity for l ights a nd motors.
iii. Rural grid systems are likely to be weak (low voltage 33 KV).
Interfacing a W ind Energy Conversion System (WECS) in weak grids is
difficult and detrimental to the workers’ safety.
iv. There are always periods without wind. Thus, WECS must be linked
energy storage or parallel generating system if supplies are to be
maintained.Power from the Wind:
Kinetic energy from the wind is used to turn the generator inside the
wind turbine to produced electri
the efficiency of the wind turbine in extracting the power from the wind. Firstly,
the wind speed is one of the important factors in determining how much power
can be extracted from the wind. This is because the power produced from the wind turbine is a function of the bed o
speed if doubled, the power produced will be increased by eight times the
original power. Then, location of the wind farm plays a n important role in order
for t he wind turbine to extract the most available power form the wind.
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The next important factor of the wind turbine is
blades length of the wind turbine
turbine since the power produced from the wind is also proportional to the
swept area of the rot or b lades i .e. the sq uare of the d iameter of the sw ept area.
Hence, by doubling the diameter of the swept area, the power produced
will be four fold incr
light an d durable . As the blade length increases, t hese qualities of the rotor
blades become more elusive. But with the recent advances
carbon-ber t echnology, the production of lightweight and strong rotor b lades
between 20 to 30 meters long is possib
rotor b lades a re ca pable to produce u p to 1 megawatt of power.The relationship
between the power produced by the wind source and the veloci
and the rotor blades sw ept diameter is sh own below.
The derivation to this fmula
some books d erived the formula in terms of the swept area of the rotor blades
(A) and the a ir density is d enoted as δ .
Thus, in selecting wind turbine ava
wind turbine is the one that can make the best
energy of the wind.
Wind power has the following advantages over
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• Improving price co mpetitiveness,
• Modular i nstallation,
• Rapid construction,
• Complementary generation,
• Improved system reliability, and
• Non-polluting.
Wind Turbines:
There are two types of wind turbi
• Horizontal-axis rotors, and
• Vertical-axis rotors.
In this rep ort, only th e h orizontal-axis w ind turbine will be d iscussed since th e
modeling of the wind driven electric generator is assumed to have the
horizontal-axis rotor.
The horizontal-axis wind turbine i
of the tower w ith respect t o the wind direction i.e. the axis of rotation are
parallel to the wind direction. These are ge nerally referred to as u pwind rotors.
Another type of horizontal axis wind
has blades r otating in back of the tower. Nowadays, only the u pwind rotors ar e
used in large-scale power gen eration and in this report, the term .horizontal-
axis wind turbine refers to the u pwind rotor arr angement.
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The main components of a wind turbine for elec
the transmission system, the generator, and the yaw and control system. The
following gures show the general layout of a typical h orizontal-axis wind
turbine, different parts of the typical grid-connected wind turbine, and cross-
section view of a nacelle of a wind turbine
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(c)
Figs: (a) Main Components of Horizontal-axis Wind Turbine
(b) Cross-section of a Typical Grid-connected Wind Turbine
(c) Cross-section of a Nacelle in A Grid-connected Wind Turbine
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The main components of a wind turbine can be classied as
system iii) Generator iv) Yaw v) Control system and vi) Braking and
transmission system
Tower:
It is the most expensive element of the wind turbine system. The lattice ortubular types of towers are constructed with steel or concrete. Cheaper and
smaller towers may be s upported by gu y wires. The major components such as
rotor b rake, gearbox, electrical switch boxes, controller, and generator are xed
on to or inside n acelle, which can rotate or yaw according to wind direction, are
mounted on the tower. The tower should be designed to withstand gravity and
wind loads. The tower has to be supported on a strong foundation i
ground. The design should consider the resonant frequencies of the tower do
not coincide with induced frequencies from the rotor and methods to damp out
if any. If the natural frequency of the tower lies above the blade passing
frequency, it is c alled stiff tower an d if below is ca lled soft tower.
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Rotor :
The aerodynamic forces acting on a wind turbine
theory. When the
(a)
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(b)
Fig (a) Zones of low and high-pressure (b) Forces act ing on the rotor bl ade
Aero foil moves in a ow, a pressure dist
symmetric aerofoil as sh own in the g.
A reference line from which measurements are made on an aerofoi
referred to as chord line and the length is known as ch ord. The angle, which
an aerofoil makes w ith the d irection of airow measured against the ch ord line
is called the angle of attack . The generation of lift force on an aerofoil
placed at an angle of attack to an oncoming ow is a consequence of the
distortion of the st reamlines o f the uid passing above and below the a erofoil.
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When a blade is subjected to unperturbed wind ow, the pre
towards the center of curvature of a streamline. The consequence is the
reduction of pressure (suction) on the u pper surface of the aerofoil compared to
ambient pressure, while on the lower si de th e pressure is positive or gr eater.
The pressure difference results in lif
blades. The drag force is the component that i
oncoming ow is sh own in Fig (b).
These forces are both proportional to the energy in
high efficiency of rotor in wind turbine design is for the blade to have a
relatively h igh lift-to-drag ratio. This ra tio can be va ried along th e length of the
blade to optimize the turbine’
force, drag force or both extract t he energy from wind. For aerofoil to be
aerodynamically efficient, the lift force ca n be 30 times greater t han the drag
force.
Cambered or asymmetrical aero foils have curved chord lines. The chordline is n ow dened as the st raight line joining the en ds of the ca mber line and
is measured from this chord line. Cambered aerofoil is preferred to
symmetrical aerofoil because th ey h ave higher l ift/drag ratio for p ositive a ngles
of attack. It is o bserved that the lift at zero angle of attack is no longer zero
and that the zer o lift occurs a t a small negative a ngle of attack of approximately
o. The center of pressure, which is at the ¼ chord position on symmetrical
aerofoil has a t the ¼ chord position on cambered aerofoil and moves towards
the t railing edge with increasing angle o f attack.
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Arching or cambering a at plate will cause
given angle of attack and blades with a ca mbered plate prole work well, under
the conditions experienced by high solidity, multi bladed wind turbines. For
low solidity tu rbines, the u se of aerofoil section is m ore eff ective.
The characteristics of an aerofoil
relative wind speed are the prime parameters respo nsible for t he lift and drag
forces. These forces acting on the blades of a wind turbine rotor are
transformed into a rot ational torque an d axial thrust force. The useful work i s
produced by the torque where as the thrust will overturn the turbine. This
axial thrust should be resisted by the tower and foundations.
Rotor speed:
Low speed and high-speed propeller are t he two types of rotors. A large
design tip speed ratio would require a long, slender blade h aving h igh aspect
ratio. A low design tip speed would require a short, at blade. The low
speed rotor runs with high torque and the high-speed rotor runs with low
torque. The wind energy converters of the same size have essen tially the
same power output, as the power output depends on rotor area. The low
speed rotor has curved metal plates. The number of blades, weight, and
difficulty of balancing th e b lades m akes t he rot ors t o be t ypically sm all.
They get self-started because of their aerodynamic characteri
propeller type rotor comprises of a few narrow blades with more
sophisticated airfoil section. When not working, the blades a re completely
stalled and the rot or ca nnot b e se lf-started. Therefore, propeller t ype rot ors
should be st arted either by changing the b lade p itch or by turning the rot or
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with the aid of an external power source (
to turn the rot or). Rotor is a llowed to ru n at variable sp eed or con strained
to operate at a constant speed. When operated at variable speed, the tip
speed ratio remains con stant and aerodynamic efficiency is increased.
Rotor alignment :
The alignment of turbine blades with the direction of wind is made by
upwind or downwind rotors. Upwind rotors face the wind in front of the
vertical tower and have the advantage of somewhat avoiding
effect from the presence of the tower. Upwind rotors need a yaw mechanism
to keep the rotor axis aligned with the direction of the wind. Downwind
rotors a re placed on the lee si de of the tower. A great disadvantage in this
design is the uctuations in the wind power due to the rotor passing
through the wind shade of the tower which gives r ise t o more fatigue loads.
Downwind rotors can be built without a yaw mechanism, if the rotor and
nacelle ca n be designed in such a way that t he n acelle will follow the wind
passively.
This may however include gyroscopic loads and hamper the possibility
of unwinding the cab les when the rotor ha s been yawing passively in the
same direction for a l ong time, thereby ca using the power cables t o twist.
Upwind rotors n eed to be rather inexible to keep the rotor b lades cl ear
of the tower, downwind rotors can be made more exible. The latter implies
possible savings with respect to weight an d may contribute to reducing the
loads on the tower. The vast majority of wind turbines in operation today have
upwind rotors.
Number of rotor blades:
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The three bladed rotors are the most common in modern aero generators.
Compared to three bladed concepts, the two and one bladed concepts h ave the
advantage of representing a possible sa ving in relation to cost and weight of the
rotor. However, the u se o f fewer r otor b lades implies t hat a higher r otational
speed or a larger chord is needed to yield the sam e energy output as a three
bladed turbine of a similar
more u ctuating loads b ecause of the vari ation of the inertia, depending on the
blades being in horizontal or vert
speed when the blade is pointing upward or downward.
Therefore, the two and one bladed concepts usually have so-called
teetering hubs, implying that t hey have the rotor h inged to the main shaft.
This design allows the rotor to teeter
unbalanced loads. One bladed wind turbines are less widespread than two–
bladed turbines. This is because
more noise a nd visual intrusion problems, need a counter weight to balance
the rot or b lade.
Generator:
Electricity is an excellent en ergy vector to transmit t he high quality
mechanical power of a wind turbine. G enerator is usually 95% efficient and
transmission losses should be less than 10%. The frequency and voltage of
transmission need not be standardized, since the end use requirements
vary. There are already many designs of wind/ elect
a wide range of generators. The distinctive features of wind/electricity
generating syst ems a re:
(i) Wind turbine efficiency is greatest if rotational frequency varies to
maintain constant tip
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speed ratio, yet electricity generation is m ost effi cient at con stant or
near con stant frequency.
(ii) Mechanical control of turbine to maintain constant frequency
increases complexity and expense. An alternative method, usually
cheaper and more efficient is to vary t he electrical load on the turbine
to control the rot ational frequency.
(iii) The optimum rotational frequency of a turbine in a particular wind
speed decreases w ith increase i n radius in order to m aintain constant
tip speed ratio. Thus, only sm all turbines of less t han 2 m radius can
be coupled directly to generators. Larger machines require a gearbox
to increase t he gen erator d rive frequency.
(iv) Gearboxes are relatively expensive and heavy. They require
maintenance and can be noisy. To overcome this problem, generators
with a large number of poles are being manufactured to operate at
lower f requency.
(v) The turbine can be coupled with the generator to provide an indirect
drive through a mechanical accumulator (weight lifted by hydraulic
pressure) or ch emical s torage (battery). Thus, generator control is
independent of turbine operation.
The generators used with wind machines are i) Synchronous AC generator
Induction AC generator a nd iii) Variable sp eed generator
Synchronous AC generator :
The Synchronous speed will be in the range of 1500 rpm – 4
– 6 pole or 750 rpm, - 8 pol
moisture is to be avoided by providing suitable protection of the gen erator. Air
borne noise is reduced by using liquid cool
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increase of the damping in the wind turbine d rive train at the exp ense of losses
in the rotor can be obtained by high slip at rated power output. Synchronous
generators run at a xed or synchronous speed, . We have ,
where is the number of poles, is the electrical fncy d is the
speed in rpm.
Induction AC generator:
They are identical to conventional industr
on constant speed wind turbines. The torque is applied to or removed from theshaft if the rotor speed is above or below synchronous. The power ow
direction in wires is the factor to be considered to differentiate between a
synchronous generator and induction motor. Some design modications are to
be incorporated for induction generators
regime of wind turbines an d the n eed for h igh efficiency at part load, etc.
Variable speed generator :
Electrical variable sp eed operation can be a pproached as:
All the output power of the wind turbine may be passed through the
frequency co nverters t o give a broad range o f variable sp eed operation.
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A restricted speed range may be achieved by converting only a fraction
of the output power.
Yaw system:
It turns t he n acelle a ccording to the a ctuator en gaging on a gear r ing a t the
top of the tower. Yaw control is the arran gement in which the en tire rotor is
rotated horizontally or y awed out of the wind. During normal operation of
the syst em, the wind direction should be perpendicular t o the swept area of
the rotor. The yaw drive is con trolled by a slow closed- loop control system.
The yaw drive is operated by a wind vane, which is usualtop of the nacelle sensing the rel ative wind direction, and the wind turbine
controller. In some designs, the nacelle is yawed to attain reduction in
power during high winds.
In extremity, the turbine can be stopped with nacelle turned such that the
rotor ax is i s a t right angles t o the w ind direction.
One of the more difficult parts of a wind turbine designs is the yaw system,
though it is ap parently simple. Especially in turbulent wind conditions, the
prediction of yaw loads is u ncertain.
Control systems :
A wind turbine power plant operates in a range of two charact
speed values referred to as Cut in wind speed and Cut out wind speed
. The turbine starts to produce power at Cut in wind speed usually
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between 4 and 5 m/s. Below this speed, the turbine does not generate
power. The turbine is stopped at Cut out wind speed usually at 25 m/s t o
reduce load and prevent damage to blades. They are designed to yield
maximum power at wind speeds that lies usually between 12 and 15 m/s.
It would not be econ omical to design turbines at s trong winds, as they are
too rare. However, in case of stronger w inds, it is n ecessary t o waste part of
the excess energy to avoid damage on the wind turbine. Thus, the wind
turbine needs some sort of au tomatic control for the protection and
operation of wind turbine. The functional capabilities o f the control system
are required for:
i Controlling the automatic startup
ii Altering the blade pitch mechanism
iii Shutting down when needed in the normal and abnormal condition
iv Obtaining information on the status of operation, wind speed,
direction and power production for m onitoring purpose
As can be seen in gure 1 (c),
are t he generator, yaw motor, gearbox, tower, yaw ring, main bearings, main
shaft, hub, blade, clutch, brake, blade and spinner. Other equ ipment that is
not shown in the gure m ight include the an emometer, the con troller inside the
nacelle, the sen sors a nd so on. The gen erator i s respo nsible for t he conversion
of mechanical to electrical energy.
Yaw motor is used power the yaw drive to turn te
the wind. The gearbox is u sed to connect the low-speed shaft (main shaft in the
gure) to th e h igh-speed shaft which drives t he gen erator r otor.
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The brake is used to stop the main shaft from over speeding. The
are u sed to extract the kinetic power from the wind to mechanical power i.e.
lifting and rotating the blades. The tower is made from tubular st eel or st eel
lattice and it is u sually very h igh in order t o expose th e rot or b lades to higher
wind speed.
Induction generator :
An induction generator is a type of electrical generator that is
mechanically and electrically similar to a polyphase i nduction motor. Induction
generators p roduce electrical power w hen their sh aft i s rotated faster t han the
synchronous frequency of the eq uivalent induction motor. Induction generators
are of ten used in wind turbines an d some m icro hydro installations d ue to their
ability to produce useful power at varyi ng rotor sp eeds. Induction generators
are m echanically and electrically simpler t han other gen erator t ypes. They are
also m ore rugged, requiring no brushes or commutators.
Induction generators are not self-exciting, meaning they require anexternal supply to produce a r otating magnetic ux. The external supply can be
supplied from the electrical grid or from the generator itself, once it st arts
producing power. The rotating magnetic ux from the stator induces cu rrents
in the rotor, which also p roduces a magnetic eld. If the rotor t urns sl ower t han
the rat e of the rotating ux, the machine acts like an induction motor. If the
rotor is turned faster, it acts like a generator, producing power at the
synchronous frequency.
In induction generators t he m agnetizing ux is est ablished by a capacitor
bank connected to the machine in case of standalone syd
grid connection it draws magnetizing current from the grid. It is mostly
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suitable for w ind generating st ations a s in this ca se sp eed is a lways a variable
factor.
Why Induction Generator :
Induction generator is commonly used in the wind turbine electric
generation due to its reduced unit cost, brushless rotor construction,
ruggedness, and ease of Maintenance. Moreover, induction generators have
several characteristics over the synchronous generator. The speed of the
asynchronous generator will vary a ccording to the turning force (moment, or
torque) applied to it. In real life, the d ifference b etween the rot ational speed atpeak power and at idle is very sm all approximately 1 percent. This is com monly
referred as the generator’s slip which is the difference between the
synchronous speed of the induction generator and the actual speed of the
rotor.
This speed difference is a very important variable for the induction
machine. The term slip is u sed because it describes what an observer riding
with the stator eld sees looking a
backward [35]. A more useful form of the slip quantity
expressed on a per unit basis using synchronous speed as the reference. The
expression of the sl ip in per u nit is sh own below.
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Fig: Torque vs. Speed Characteristics of Squirrel-cage Induction Generator
In the gure, it can be seen that when the induction machine is running at
Synchronous sp eed at t he p oint where t he sl ip is zer o i.e. the rot or is sp inning
at the sa me sp eed as t he rotating magnetic eld of the st ator, the torque of the
machine is zero. If the induction machine is to be operated as a motor, the
machine is t o operated just below its syn chronous sp eed.
On the other ha nd, if the induction machine is to be operated as a generator,
its st ator t erminals sh ould be con nected to a constant-frequency voltage sou rce
and its r otor is d riven above synchronous speed (s
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Fig. : Per-Phase E quivalent Circuit of An Induction Machine
In this project, star-connected induction machine is evaluated. All the
calculations a re in per-phase va lues. Hence, for a star-connected stator:
In order t o analyze t he behavior of an induction generator, the operation of anInduction motor must be fully understood. Once, the equivalent circuit
parameters h ave been obtained, the performance of an induction motor is easy
to determine. As sh own in Fig, the total power Pg transferred across t he a ir gap
from the st ator i s
And it is evident from gure
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Therefore, the internal mechanical e
From the power point of v iew, the equivalent circuit of gure 3 can be
rearranged to the following gure, where the mechanical power per stator
phase i s equ al to the p ower absorbed by the resistance R2(1-s)/s.
Fig: Alternative Form for P er-Phase E quivalent Circuit
The analysis of an induction motor is
diagram as shown in the following gure in conjunction with the equivalent
circu it.
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Fig: Power Flow Diagram
Where,
The parameters of an induction generator cn
load test and block rotor t est (The st eps in calculating th e parameters a nd the
test results ob tained from a 440V, 4.6A, 2.2kW induction motor).
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POWER QUALITY
The contemporary container crane industry, like many other industry
segments, is often enamored by the bells and whistles, colorful diagnosticdisplays, high speed performance, and levels of automation that can be
achieved. Although these features a nd their indirectly related computer b ased
enhancements are key issues to an efficient terminal operation, we must not
forget the foundation upon which we are b uilding. Power quality is the mortar
which bonds the foundation blocks. Power quality also affects terminal
operating economics, crane reliability, our en vironment, and initial investment
in power distribution systems to su pport new crane installations. To quote the
utility company newsletter which accompanied the last monthly issue of my
home utility billing: ‘Using electricity wisely is a good environmental an d
business practice which saves you money, reduces emissins
plants, and conserves our n atural resources.’ As we are a ll aware, container
crane performance requirements continue to increase a t an astounding rate.
Next gen eration container cr anes, already in the bidding process, w ill require
average power demands of 1500 to 2000 kW – almost double the total average
demand three years ago. The rapid increase in power demand levels, an
increase in container crane population, SCR converter c rane drive retrots a nd
the large AC and DC drives needed to power and control these cranes will
increase a wareness of the power quality issue in the very n ear f uture.
POWER QUALITY PROBLEMS
For t he purpose of this a rticle, we sh all dene power quality problems a s:
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‘Any power problem that results in failure or misoperation of customer
equipment, manifests itself as an economic burden to the user, or produ ces
negative impacts on the environment.’
When applied to the container crane indust
power qu ality include:
• Power Factor
• Harmonic Distortion
• Voltage Transients
• Voltage S ags or Dips• Voltage S wells
The AC and DC variable speed drives utilized on board container cranes
are signicant con tributors t o total harmonic current an d voltage distortion.
Whereas SCR phase control creates the desirabl
drives op erate a t less t han this. In addition, line n otching occurs when SCR’s
commutate, creating transient peak recovery voltages t hat can be 3 to 4 times
the nominal line voltage depending upon the system impedance and the size of
the drives. The frequency and severity of these power system disturbances
varies with the speed of the dr
drives will be highest when the drives are operating at slow speeds. Power
factor will be lowest when DC drives are operating at slow speeds or during
initial acceleration and deceleration periods, increasing to its maximum value
when the SCR’s are phased on to produce rated or base speed. Above base
speed, the power f actor essen tially remains constant. Unfortunately, container
cranes can spend considerable time at low speeds a s the operator at tempts to
spot and land containers. Poor power factor places a greater kVA demand
burden on the utility or
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can also affect the voltage stability which can ultimately result in detrimental
effects on the
life of sensitive el ectronic equipment or even intermittent malfunction. Voltage
transients created by DC drive SCR line notching, AC drive voltage chopping,
and high frequency harmonic voltages a nd currents are al l signicant sources
of noise a nd disturbance t o sensitive electronic eq uipment
It has been our experience that end users often do not associate power
quality problems with Container cranes, either because they are totally
unaware of such issues or there was n o economic Consequence if power quality
was not addressed. Before the advent of solid-s
factor was reasonable, and harmonic cu rrent injection was minimal. Not until
the crane Population multiplied, power demands per crane increased, and
static power con version became the way of life, did power qu ality issues b egin
to emerge. Even as harmonic distortion and power Factor issues su rfaced, no
one was really prepared.
Even today, crane builders a nd electrical drive System vendors avoi d the
issue during competitive bidding for new cranes. Rather than focus on
Awareness and understanding of the potential
is intentionally or U nintentionally ignored. Power qu ality problem solutions are
available. Although the solutions a re n ot free, in most cases, they do represent
a good return on investment. However, if power qu ality is n ot specied, it most
likely will not be d elivered.
Power quality can be improved through:
• Po wer factor co rrection,
• Harmonic ltering,
• S pecial line n otch ltering,
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• Tr ansient voltage su rge su ppression,
• Proper earthing syst ems.
In most cases, the person specifying and/or buying a container crane may not
be fully aware of the potent
nothing else, we would hope to
provide that awareness.
In many cases, those involved with specication and procurement of
container cranes m ay not be cognizant of such issues, do not pay the utility
billings, or consider ie
specications may not include denitive power qu ality criteria such as power
factor corr ection and/or h armonic ltering. Also, many of those specications
which do
require p ower qu ality equipment do not properly dene t he cri teria. Early in the
process o f preparing the cr ane sp ecication:
• Consult with the utility company to determine regulatory or contract
requirements that must be
satised, if any.
• Consult with the electrical drive su ppliers and determine the power qu ality
proles t hat can be
expected based on the drive sizes and technologies proposed for t he specic
project.
• Evaluate the economics of power qu ality correction not only on the presentsituation, but consider t he impact of future u tility deregulation and the future
development plans for the terminal
THE BENEFITS OF POWER QUALITY
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Power quality in the container terminal environment impacts the
economics of the terminal operation, affects reliability of the terminal
equipment, and affects other consu mers served by the same utility service.
Each of these con cerns is exp lored i n the following paragraphs.
1. Economic Impact
The economic impact of power quality is the foremost incentive to
container terminal operators. Economic impact can be signicant and manifest
itself in several ways:
a. Power Factor Penalties
Many utility companies invoke penalties for low power factor on monthly billings. There is no
of metering and calculating power factor penalties vary from one utility
company to the next. Some utility companies actually meter kVAR usage and
establish a xed rate times the number of kVAR-hours con sumed. Other utility
companies monitor kVAR demands and calculate power factor. If the power
factor falls b elow a xed limit value o ver a demand period, a penalty is b illed in
the form of an adjustment to the peak demand charges.
A number of utility companies servicing container terminal equiment do
not yet invoke power factor p enalties. However, their servi ce contract with the
Port may still require that a minimum power factor over a dened demand
period be met. The utility company may not continuously monitor power factor
or kVAR usage a nd reect them in the monthly utility billings; however, they do
reserve t he right to monitor t he Port service at an y time. If the power factor
criteria set forth in the servi ce co ntract are n ot met, the u ser m ay be p enalized,
or required to take corrective actions at the user’s expense. One utility
company, which supplies power service to several east coas t container
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terminals in the USA, does not reect power factor p enalties in their m onthly
billings, however, thei
‘The average power factor u nder op erating conditions of customer’s load
at the point where ser vice is m etered shall be not less t han 85%. If below 85%,
the customer may be required to furnish, install and maintain at its expense
corrective a pparatus which will increase t he
Power factor of the en tire installation to not less t han 85%. The cu stomer sh all
ensure that n o excessive harmonics or transients are introduced on to the
[utility] system. This may require special power con ditioning equipment or
lters.
The Port or terminal operations personnel, who are responsible for
maintaining container cranes, or specifying new container crane equipment,
should be aware of these requirements. Utility deregulation will most l ikely
force u tilities t o en force r equirements su ch as t he exa mple ab ove.
Terminal operators who do not deal with penalty issues today may be
faced with some rather severe penalties in the future. A sound, future terminal
growth plan should include con tingencies for ad dressing the possible econ omic
impact of utility d eregu lation.
b. System Losses
Harmonic currents and low power factor created by nonlinear loads, not
only result in possible power factor penalties, but also increase the power
losses in the distribution system. These losses are not visible as a separate
item on your monthly utility billing, but you pay for them each month.
Container cranes are signicant contributors to harmonic currents and low
power factor. Based on the typical demands of today’s high speed container
cranes, correct ion of power factor a lone on a typical state of the a rt quay crane
can result in a red uction of system losses t hat converts to a 6 to 10% reduction
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in the monthly utility billing. For most o f the larger terminals, t his is a
signicant annual saving in the co st of operation.
C. Power Service Initial Capital Investments
The power distribution system design and installation for w
as well as modication of systems for t erminal capacity u pgrades, involves h igh
cost, specialized, high and medium voltage equipment. Transformers,
switchgear, feeder ca bles, cable ree l trailing cables, collector b ars, etc. must be
sized based on the kVA demand. Thus cost of the equ ipment is directly relatedto the total kVA demand. As the relationship above indicates, kVA demand is
inversely proportional to the overall power factor, i.e. a lower power factor
demands higher kVA for the same kW load. Container cranes are one of the
most signicant users of power in the terminal. Since con tainer cranes with
DC, 6 pulse, SCR drives operate at r elatively low power factor, the total kVA
demand is si gnicantly larger than would be t he ca se i f power factor corr ection
equipment were supplied on board each crane or at some common bus location
in the terminal. In the absence of power quality corrective equipment,
transformers are l arger, switchgear current ratings m ust be h igher, feeder cab le
copper si zes a re larger, collector sy stem and cable reel cables must be larger,
etc.
Consequently, the cost of the initial power distribution system
equipment for a system which does n ot address p ower qu ality will most likely
be higher than the same system which includes wer
2. Equipment Reliability
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Poor power quality can affect machine or equipment reliability and
reduce the life of components. Harmonics, voltage transients, and voltage
system sags and swells are all power quality problems and are all
interdependent.
Harmonics affect power factor, voltage transients can induce harmonics,
the same phenomena which create harmonic current injection in DC SCR
variable speed drives are responsibl
varying power factor of the me dr
effects of harmonic distortion, harmonic cu rrents, and line notch ringing can
be mitigated using specially
3. Power System Adequacy
When considering the installation of additional cranes to an existig
power distribution system, a power system analysis should be completed to
determine the adequacy of the system to su pport additional crane loads. Power
quality corrective actions m ay be dictated due to inadequacy of existing power
distribution systems to which new or relocated cranes a re to be connected. In
other words, addition of power quality equipment may render a workable
scenario on an existing power distribution system, which would otherwise b e
inadequate to su pport additional cranes without high risk of problems.
4. Environment
No issue might be as important as the effect of power quality on our
environment. Reduction in system losses and lower demands equate to a
reduction in the consumption of our natural nm resources and reduction in
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power p lant em issions. It is our r esponsibility as occupants of this planet t o
encourage conservation of our natural resources and support measures which
improve our ai r qu ality.
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FLEXIBLE AC TRANSMISSION SYSTEMS (FACTS)
Flexible AC Transmission Systems, called FACTS, got in the recent years
a well known term for higher controllability in power systems by means of
power electronic devices. Several FACTS-devices have been introduced for
various applications worldwide. A number of new types of
stage o f being introduced in practice.
In most o f the applications the controllability is used to avoid cost
intensive or landscape requ iring extensions of power syst ems, for instance likeupgrades or ad ditions of substations a nd power lines. FACTS-devices p rovide a
better adaptation to varying opera
existing installations. The b asic a pplications o f FACTS-devices a re:
• Power ow control,
• Increase of transmission capability,
• Voltage control,
• Reactive power compensation,
• S tability improvement,
• Power quality improvement,
• Power conditioning,
• F licker m itigation,
• Interconnection of renewable a nd distributed generation and storages.
Figure shows the basic idea of FACTS for transmission systems. The
usage of lines for act ive power transmission should be ideally up to the thermal
limits. Voltage a nd stability limits sh all be sh ifted with the m eans o f the se veral
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different FACTS devices. It can be seen that with growing line length, the
opportunity for FACTS devices gets more and more important.
The inuence of FACTS-devices is achieved through switched or
controlled shunt com pensation, series compensation or phase shift con trol.
The devices work electrically
The power electronic allows very short
second.
The development of FACTS-devices has started with the growing
capabilities o f power el ectronic components. Devices for h igh power levels h ave
been made available in converters f
overall starting points are n etwork elements inuencing the reactive power or
the impedance of a part of the power system. Figure 1.2 shows a number of
basic devices separated into
For t he FACTS side the taxonomy in terms of 'dynamic' and 'static' needs
some explanation. The term 'dynamic' is u sed to express t he fast controllability
of FACTS-devices p rovided by the power electronics. This is one of the main
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differentiation factors from the conventional devices. The term 'static' means
that the devices h ave no moving parts like mechanical switches to perform the
dynamic controllability. Therefore most of the FACTS-devices can equally be
static an d dynamic.
The left column in Figure 1.2 contains the conventional dev
of xed or m echanically switch able com ponents like resistance, inductance or
capacitance together with transformers. The FACTS-devices contain theseelements as well but u se additional pow er electronic valves or converters to
switch the elements in smaller s teps or with switching patterns within a cycle
of the alternating current. The left column of FACTS-devices uses Thyristor
valves or converters. These valves
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years. They have low losses because of
cycle in the converters or the usage of the Thyristors to simply bridge
impedances in the valves.
The right column of FACTS-devices contains more advanced technology
of voltage source converters based today mainly on Insulated Gate Bipolar
Transistors (IGBT) or Insulated Gate Commutated Thyristors (CT).
Source Converters provide a free con trollable voltage in magnitude and phase
due to a pulse width modulation of the IGBTs or IGCTs. High modulation
frequencies allow to get low harmonics in the output signal and even to
compensate disturbances coming from the network.
The disadvantage is that with an increasing switching frequency, the
losses are increasing as well. Therefore s pecial designs of the converters a re
required to compensate t his.
CONFIGURATIONS OF FACTS-DEVICES:
SHUNT DEVICES:
The most used FACTS-device is the SVC or the version with Voltage
Source Converter called STATCOM. These shunt devices are operating as
reactive power compensators. The main applications in transmission,
distribution and industrial networks a re:
• Reduction of unwanted reactive power ows and therefore reduced network
losses.
• Keeping of contractual power exchanges with balanced reactive power.
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• Compensation of consumers and improvement of power quality especially
with huge demand uctuations like industrial achie
railway or underground train systems.
• Compensation of Thyristor converters e.g. in conventional HVDC lines.
• Improvement of static or transient stability.
Almost half of the SVC and more than half of the STATCOMs are used for
industrial applications. Industry as w ell as co mmercial and domestic groups of
users requ ire power qu ality. Flickering lamps a re no longer accep ted, nor a re
interruptions of industrial processes due to insufficient power qu ality. Railway
or underground systems with huge load variations require SVCs or STATCOMs.
SVC:
Electrical loads both generate and absorb reactive power. Since the
transmitted load varies considerably from one hour to another, the reactive
power ba lance in a grid varies a s well. The res ult can be u nacceptable voltage
amplitude variations or even a voltage depression, at t he extreme a voltage
collapse.
A rapidly operating Static Var Compensator (SVC) can continuously
provide the reactive power requ ired to control dynamic voltage oscillations
under various system conditions and thereby improve the power system
transmission and distribution stability.
Applications of the SVC systems in i
a. To increase active power transfer capacity and transient st ability
margin
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b. To damp power oscillations
c. To achieve eff ective vo ltage c ontrol
In addition, SVCs are also u sed
1. in transmission systems
a. To redu ce temporary over voltages
b. To damp sub synchronous resonances
c. To damp power oscillations in interconnected power systems
2. in traction systems
a. To balance loads
b. To improve power factor
c. To improve vo ltage regu lation
3. In HVDC systems
a. To provide reactive p ower t o a c–dc co nverters
4. In arc furnaces
a. To reduce vo ltage va riations a nd associated light icker
Installing an SVC at one or more suitable points in the network can
increase transfer cap ability and reduce losses while maintaining a smooth voltage prole under different network conditions.
mitigate act ive power oscillations through voltage am plitude modulation.
SVC installations consist of a number of building blocks. The most
important is t he Thyristor val ve, i.e. stack assemblies o f series co nnected anti-
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parallel Thyristors t o provide controllability. Air co re rea ctors a nd high voltage
AC capacitors are the reactive wer
valves. The step up connection of
achieved through a p ower transformer.
SVC building b locks a nd voltage / current characteristic
In principle the SVC consists of Thyristor Switched Capacitors (TSC) and
Thyristor Switched or Controlled React
of a combination of these branches varies the reactive power as shown in
Figure. The rst commercial SVC was installed in 1972 for an electric arc
furnace. On transmission level the rst SVC was u sed in 1979. Since then it is
widely used and the most accepted FACTS-device.
SVC
SVC USING A TCR AND AN FC:
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In this arrangement, two or more FC (xed capacitor) banks are
connected to a TCR (thyristor controlled reactor) through a step-down
transformer. The rating of the reactor is chosen larger t han the rating of the
capacitor by an amount to provide the maximum lagging vars that have to be
absorbed from the system.
By changing the ring angle of the t hyristor co ntrolling th e react or f rom
90° to 180°, the reactive power can be varied over the entire range from
maximum lagging vars to leading vars that can be absorbed from the system by
this com pensator.
SVC of the FC/TCR type:
The main disadvantage of this conguration is the signicant harmonics
that will be generated because of the partial conduction of the large react or
under normal sinusoidal steady-state operating condition when the SVC is
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absorbing zero MVAr. These harmonics are ltered in the following manner.
Triplex harmonics are canceled by arranging the TCR and the secondary
windings of the step-down transformer in delta connection. The capac
banks with the help of series react
other higher-order h armonics as a high-pass lter. Further losses a re high due
to the circulating cu rrent between the reactor an d capacitor b anks.
Comparison of the loss characteristics of TSC–TCR, TCR–FC
compensators and synchronous condenser
These SVCs do not have a short-time overload capability because the
reactors are usually of the air-core type. In applications requiring overload
capability, TCR must be designed for short-time overloading, or separate
thyristor-switched overload reactors m ust be em ployed.
SVC USING A TCR AND TSC:
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This compensator overcomes two major shortcomings of the earlier
compensators by reducing losses under operating conditions and better
performance u nder large syst em disturbances. In view of the sm aller rating of
each capacitor bank, the rating of the reactor bank will be 1/n times the
maximum output of the SVC, thus reducing the harmonics generated by the
reactor. In those situations where harmonics have to be reduced further, a
small amount of FCs tuned as lters may be connected in parallel with the
TCR.
SVC of combined TSC and TCR type
When large disturbances occur in a power system due to load rejection,
there is a possibility for large voltage transients because of oscillatory
interaction between system and the SVC capacitor ba nk or the parallel. The LC
circuit of the SVC in the FC compensator. In the TSC–TCR scheme, due to the
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exibility of rapid switching of capacitor b anks w ithout appreciable d isturbance
to the power sys tem, oscillations can be avoided, and hence t he transients in
the syst em can also b e avoided. The cap ital cost of this SVC is h igher than that
of the earlier one due to the increased number of capacitor switches and
increased control complexity.
STATCOM:
In 1999 the rst SVC with Voltage Source Converter called STATCOM
(STATic COMpensator) went into operation. The STATCOM has a characteristicsimilar to the synchronous condenser, but as an electronic device it h as no
inertia and is su perior to the syn chronous con denser in several ways, such as
better dynamics, a lower investment co
costs.
A STATCOM is build with Thyristors with turn-off capability like GTO or
today IGCT or with more and more IGBTs. The static line between the current
limitations h as a certain steepness determining the control characteristic for
the vo ltage.
The advantage of a STATCOM is that the reactive power provision is
independent from the act ual voltage on the con nection point. This can be seen
in the diagram for the maximum currents being independent of the voltage in
comparison to the SVC. This means, that even during most severe
contingencies, the S TATCOM keeps i ts full capability.
In the distributed energy sector t he usage of Voltage Source Converters
for grid interconnection is com mon practice today. The next step in STATCOM
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STATCOM Equivalent Circuit
Several different control techniques ca n be used for t he ring control of
the STATCOM. Fundamental switching of the GTO/diode once per cycle can beused. This approach will minimize sw itching losses, but will generally utilize
more com plex transformer topologies. As an alternative, Pulse Width Modulated
(PWM) techniques, which turn on and off the GTO or IGBT switch more than
once per cycle, can be used. This approach allows for simpler transformer
topologies at the ex pense o f higher sw itching losses.
The 6 Pulse STATCOM using fundamental switching will of course
produce the 6 N 1 harmonics. There are a variety of methods to decrease the
harmonics. These methods include the basic 12 pulse conguration with
parallel star / d elta transformer connections, a complete elimination of 5th and
7th harmonic current using series connection of star/star and star/delta
transformers an d a quasi 12 pulse m ethod with a single star-star t ransformer,
and two secondary windings, using control of ring angle to produce a
30 phase sh ift between the two 6 pulse b ridges.
This method can be extended to produce a 24 pulse and a 48 pulse
STATCOM, thus eliminating harmonics even further. Another possible
approach for h armonic cancellation is a multi-level conguration which allows
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for more than one switching element per level and therefore more than one
switching in each bridge arm. The ac voltage derived has a staircase effect,
dependent on the number of levels. This st aircase voltage ca n be controlled to
eliminate h armonics.
SERIES DEVICES:
Series devices have been further developed from xed or mechanically
switched compensations to the Thyristor Controlled Series Compensation(TCSC) or even Voltage S ource Converter based devices.
The main applications are:
• Reduction of series voltage decline in magnitude and angle over a
power l ine,
• Reduction of voltage uctuations within dened limits d uring changing
power transmissions,
• Improvement of system damping resp. damping of oscillations,
• Limitation of short circuit currents i n networks o r su bstations,
• Avoidance of loop ows resp. power ow adjustments.
TCSC:
Thyristor Controlled Series Capacitors (TCSC) address specic dynamical
problems in transmission systems. Firstly it increases damping when large
electrical systems are i nterconnected. Secondly it can overcome the problem of
Sub Synchronous Resonance (SSR), a phenomenon that involves an interaction
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ADVANTAGES
• Continuous con trol of desired compensation level
• Direct smooth control of power ow within the n etwork
• Improved capacitor bank protection
• Local mitigation of sub synchronous resonance (SSR). This permits
higher levels of compensation in networks where interactions with
turbine-generator t orsional vibrations or with other con trol or m easuring
systems are of concern.
• Damping of electromechanical (0.5-2 Hz) power osci llations which often
arise between areas in a large interconnected power network. Theseoscillations are d ue to the dynamics of inter are a power transfer an d
often exhibit poor d amping when the aggregate power tranfer over a
corridor i s h igh relative t o the t ransmission strength.
SHUNT AND SERIES DEVICES
DYNAMIC POWER FLOW CONTROLLER
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A new device in the area of power ow control is the Dynamic Power Flow
Controller (DFC). The DFC is a hybrid device between a Phase Shifting
Transformer (PST) and switched series compensation.
A functional single line diagram of the Dynamic Flow Controller
in Figure 1.19. The Dynamic Flow Controller consists of the following
components:
• a st andard p hase sh ifting transformer with tap-changer (PST)
• ser ies-connected Thyristor Switched Capacitors and Reactors (TSC /
TSR)
• A mechanically switched shunt capacitor (MSC). (This is optional
depending on the system reactive power requirements)
Based on the system requirements, a DFC might consist of a number of
series TSC or TSR. The mechanically switched shunt capacitor (MSC) will
provide voltage su pport in case of overload and other conditions.
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Normally the reactance of reactors an d the ca pacitors are sel ected based
on a binary b asis t o result in a desired stepped reactance va riation. If a higher
power ow resolution is needed, a reactance equivalent to the half of the
smallest one can be added.
The switching of series reactors occurs at zero current to avoid any
harmonics. However, in general, the principle of phase-angle control used in
TCSC can be applied for a continuous control
is b ased on the following ru les:
• TSC / T SR are switched when a fast response is required.
• The relieve of overload and work in stressed situations is handled by
the TSC / TSR.
• The switching of the PST tap-changer should be minimized particularly
for t he cu rrents h igher than normal loading.
• Th e total reactive power consumption of the device can be optimized by
the operation of the MSC, tap changer and the switched capacities an d
reactors.
In order to visualize the steady state operating range of the DFC, we
assume an inductance in parallel representing parallel transmission paths. The
overall control objective in steady state would be to control the d istribution of
power ow between the branch with the DFC and the parallel path. Thiscontrol is a ccomplished by control of the injected series vo ltage.
The PST (assuming a quadrature booster) will inject a voltage in
quadrature with the node voltage. The controllable reactance will inject a
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voltage in quadrature with the thrughput
ow has a load factor cl ose to one, the two parts of the seri es vo ltage will be
close to collinear. However, in terms of speed of control, inuence on reactive
power balance an d effectiveness at high/low loading the two parts of the ser ies
voltage has quite different character
loadings up to rated current is illustrated in Figure 1.20, where the x-axis
corresponds to the throughput current and the y-axis corresponds to the
injected series v oltage.
Fig. Operational diagram of a DFC
Operation in the rst and third quadrants corresponds to reduction of
power through the DFC, whereas operation in the second and fourth quadrants
corresponds to increasing the power ow through the DFC. The slope of the
line passing through the origin (at which the tap is at zero and TSC / TSR are
bypassed) depends on the short circui
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Fig. Principle con guration of an UPFC
The UPFC consists of a shunt and a series transformer, which are
connected via two voltage sou rce converters with a common DC-capacitor. The
DC-circuit allows the active power exchange between shunt and series
transformer to control the phase shift of the series voltage. This setup, as
shown in Figure 1.21, provides the full controllability for voltage and power
ow. The seri es con verter needs to be protected with a Thyristor b ridge. Due to
the high efforts for the Voltage Source Converters an d the protection, an UPFC
is getting quite expensive, which limits the practical ap plications where the
voltage and power ow control is r
OPERATING PRINCIPLE OF UPFC
The basic components of the UPFC are two voltage source inverters (VSIs)
sharing a common dc storage capacitor, and connected to the power system
through coupling transformers. One VSI is connected to in shunt to the
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transmission system via a shunt transformer, while the other one is con nected
in series t hrough a series t ransformer.
A basic UPFC functional scheme is shown in g.1
The series inverter is controlled to inject a symmetrical t
voltage system (Vse), of contr
the line to control active and reactive power o ws o n the t ransmission line. So,
this inverter w ill exchange active a nd reactive p ower w ith the line. The reactive
power i s electronically provided by the series inverter, and the active power i s
transmitted to the dc terminals. The sh unt inverter is operated in such a way
as to demand this d c t erminal power (positive or n egative) from the line keeping
the voltage across the storage capacitor Vdc constant. So, the net real power
absorbed from the line by the UPFC is equ al only to the losses of the inverters
and th eir transformers.
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The remaining capacity of the shunt inverter can be used to exchange
reactive p ower w ith the line so to provide a voltage regu lation at the co nnection
point.
The two VSI’s can work independently of each other by separating the dc
side. So in that case, the shunt inverter is operating as a STATCOM that
generates or ab sorbs reactive power to regulate the voltage magnitude at t he
connection point. Instead, the series inverter is operating as SSSC that
generates or ab sorbs r eactive power to regulate the cu rrent ow, and hence the
power low on the transmission line.
The UPFC has many possible operating modes. In particular, the shunt
inverter i s o perating in such a way to inject a controllable cu rrent, ish into the
transmission line. The sh unt inverter can be con trolled in two different modes:
VAR Control Mode: The reference input is an inductive or capacitive VAR
request. The shunt inverter control translates the var reference into a
corresponding shunt current request an d adjusts gating of the inverter to
establish the desired current. For this mode of control a feedback signal
representing th e d c b us vo ltage, Vdc, is a lso required.
Automatic Voltage Control Mode: The shunt inverter reactive current is
automatically regulated to maintain the transmission line voltage at the point
of connection to a reference value. For t his mode of control, voltage feedback
signals are obtained from the sending end bus feeding the shunt coupling
transformer.
The series inverter controls the magnitude and angle of the voltageinjected in series with the line to inuence the power ow on the line. The
actual value of the injected voltage ca n be ob tained in several ways.
Direct Voltage Injection Mode: The reference inputs are directly the
magnitude an d phase a ngle of the ser ies voltage.
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Phase Angle Shifter Emulation mode: The reference input is phase
displacement between the sending end voltage and the receiving end voltage.
Line Impedance Emulation mode: The reference input is an impedance value to
insert in series w ith the line impedance
Automatic Power Flow Control Mode: The reference inputs are values of P
and Q to maintain on the transmission line despite system changes.
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STATIC SYNCHRONOUS COMPENSATOR (STATCOM)
Introduction
The STATCOM is a solid-state-based power converter version of the
SVC. Operating as a shunt-connected SVC, its capacitive or i nductive output
currents can be controlled independently from its terminal AC bus voltage.
Because of the fast-switching characteristic of power converters, STATCOM
provides much faster response as compared to the SVC. In addition, in the
event of a rapid change in system voltage, the ca pacitor vol tage d oes n ot change
instantaneously; therefore, STATCOM effectively reacts for the desired
responses. For exa mple, if the syst em voltage drops for a ny reaso n, there is a
tendency for STATCOM to inject capacitive power to support the dipped
voltages.
STATCOM is capable of high dynamic performance and its
compensation does not depend on the common coupling voltage. Therefore,
STATCOM is very effective during the power system disturbances.
Moreover, much research conrms several advantages of
STATCOM. These advantages compared to other shunt compensators include:
• S ize, weight, and cost reduction
• E quality of lagging a nd leading output
• Preci se a nd continuous r eactive power control with fast response
• P ossible a ctive h armonic lter ca pability
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This chapter describes the structure, basic operating principle and
characteristics of STATCOM. In addition, the concept of voltage source
converters an d the co rresponding control techniques a re illustrated.
STRUCTURE OF STATCOM
Basically, STATCOM is comprised of three main parts (as seen
from Figure below): a voltage source converter (VSC), a step-up coupling
transformer, an d a controller. In a very-high-voltage system, the leakage
inductances of the step-up power transformers can function as coupling
reactors. The m ain purpose of the cou pling inductors i s t o lter out the cu rrent
harmonic components that are generated mainly by the pulsating output
voltage of the power converters
Reactive power generation by a S TATCOM
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CONTROL OF STATCOM
Introduction
The controller of a STATCOM operates the converter in a
particular way that the phase angle between the converter voltage and the
transmission line voltage is d ynamically adjusted and synchronized so that the
STATCOM generates or absorbs desired VAR at the point of coupling
connection. Figure 3.4 shows a simplied diagram of the STATCOM with a
converter vol tage so urce __1 E and a tie reactance, connected to a system with a
voltage source, and a Thevenin reac TIEX_THV TH .
Two Modes of Operation
There are two modes of operation for a STATCOM, inductive mode
and the capacitive mode. The STATCOM regards an inductive reactance
connected at its terminal when the converter voltage is higher than the
transmission line voltage. Hence, from the sy stem’s p oint of view, it regards t he
STATCOM as a capacitive reactance and the STATCOM is considered to be
operating in a capacitive mode. Similarly, when the system voltage is higher
than the converter voltage, the system regards an inductive reactance
connected at its terminal. Hence, the STATCOM regards the system as a
capacitive reactance and the STATCOM is considered to be operating in an
inductive mode
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.
STATCOM operating in inductive or capacitive modes
In other words, looking at t he phasor d iagrams on the right of Figure 3.4,
when1 I, the reactive current component of the STATCOM, leads ( THVE −1) by
90º, it is in inductive m ode a nd when it lags b y 9 0º, it is in capacitive m ode.
This dual mode capability enables the STATCOM to provide
inductive compensation as well as capacitive compensation to a system.
Inductive compensation of the STATCOM makes it unique. This inductive
compensation is to provide inductive reactance when overcompensation due to
capacitors banks occurs. This happens during the night, when a typical
inductive load is a bout 20% of the full load, and the ca pacitor b anks along the
transmission line provide with excessive ca pacitive react ance due to the lower
load. Basically the con trol system for a S TATCOM consists of a current control
and a voltage co ntrol.
Current Controlled STATCOM
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Current controlled block diagram of STATCOM
Figure above shows the reactive current control block diagram of
the STATCOM. An instantaneous t hree-phase set of line voltages, v l, at BUS 1
is u sed to ca lculate the reference a ngle, θ, which is p hase-locked to the phase a
of the l ine voltage, v la . An instantaneous three-phase set of measured converter
currents, i l, is d ecomposed into its real or d irect component, I 1d , and reactive o r
quadrature component, I 1q , respectively.The qu adrature com ponent is com pared
with the desired reference vale 1q * and the error is passed through an error
amplier which produces a relative angle, α, of the converter voltage with
respect to the transmission line voltage. The phase a ngle, θ 1, of the converter
voltage is calculated by adding
and the phase – l ock-loop angle, θ. The reference qu adrature component, I 1q *, of
the converter current is dened to be either positive if the STATCOM is
emulating an inductive reactance or negative if it is emulating a capacitive
reactance. The DC capacitor voltage, v DC , is dynamically adjusted in relation
with the converter voltage. The control me descr
implementation of the inner cu rrent control loop which regulates t he react ive
current ow through the STATCOM regardless of the line voltage.
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Voltage Controlled STATCOM
In regulating th e line voltage, an outer vol tage control loop must
be implemented. The outer voltage control
the ref erence rea ctive cu rrent for t he inner cu rrent control loop which, in turn,
will regulate the l
Voltage controlled block diagram of STATCOM
Figure 3.6 shows a voltage control block diagram of the STATCOM. An
instantaneous three-phase set of m easured line voltages, v 1, at BUS 1 is
decomposed into its real or d irect component, V 1d , and reactive or qu adrature
component, V 1q , is co mpared with the d esired reference va lue, V 1 *, (adjusted by
the droop factor, K droop ) and the error is passed through an error amplier
which produces the reference current, I 1q *, for th e inner cu rrent con trol loop.
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The droop factor, K droop , is dened as the allowable voltage error a t t he rated
reactive current ow through the STATCOM.
BASIC OPERATING PRINCIPLES OF STATCOM
The STATCOM is connected to the power system at a PCC (point of
common coupling), through a step-up coupling transformer, where t he voltage-
quality problem is a concern. The PCC is also known as t he terminal for which
the t erminal voltage is U T . All required voltages a nd currents a re m easured and
are fed into the controller to be compared with the commands. The controller
then performs feedback control and outputs a set of switching signals (ring
angle) to drive the main semiconductor switches of the power converter
accordingly to either increase the voltage or to decrease it a ccordingly. A
STATCOM is a controlled reactive-power sou rce. It provides voltage support by
generating or absorbing reactive power at the point of co mmon coupling
without the need of large external react
controller, the VSC and the coupling transformer, the STATCOM operation is
illustrated in Figure b elow.
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STATCOM operation in a power system
The charged capacitor C dc provides a DC voltage, U dc to the
converter, which produces a set of controllable three-phase output voltages, U
in synchronism with the AC system. The synchronism of the three-phase
output voltage with the transmission line voltage has to be performed by an
external controller. The amount of desired voltage across STATCOM, which is
the vo ltage reference, Uref, is s et manually to the c ontroller. The vo ltage control
is thereby to match U T with Uref which has been elaborated. This matching of
voltages is done by varying the amplitude of
done by the ring angle set by the controller. The controller thus sets U T
equivalent to the Uref. The reactive power exchange between the con verter an d
the AC system can also be controlled. This reactive power exchange is the
reactive current injected by the STATCOM, which is the current from the
capacitor produced by absorbing real power from the AC system.
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where I q is the reactive current injected by the STATCOM
U T is the STATCOM terminal voltage
Ueq is the equivalent Thevenin voltage seen by the STATCOM
X eq is t he equ ivalent Thevenin reactance of the power system seen by the
STATCOM
If the a mplitude of the ou tput voltage U is increased above that of
the AC system voltage, U T , a leading current is produced, i.e. the STATCOM is
seen as a conductor by the AC system and reactive power is generated.
Decreasing the amplitude of the ou tput voltage below that of the AC system, a
lagging current results an d the STATCOM is seen as a n inductor. In this case
reactive power is absorbed. If the amplitudes are equal no power exchange
takes p lace.
A practical converter is not lossless. In the case of the DC
capacitor, the energy stored in this capacitor would be consumed by the
internal losses of the co nverter. By making the output voltages o f the co nverter
lag the AC system voltages b y a small angle, δ, the converter absorbs a small
amount of active power from the AC system to balance the losses in the
converter. The diagram in Figure below illustrates t he phasor d iagrams of the
voltage at the terminal, the
quadrants of the PQ plane.
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Phasor diagrams for STATCOM applications
The mechanism of phase angle adjustment, angle δ, can also be
used to control the reactive power gen eration or a bsorption by increasing or
decreasing the ca pacitor vol tage U dc , with reference w ith the ou tput voltage U.
Instead of a capacitor a b attery can also b e u sed as DC energy. In
this case the converter can control both reactive and active power exch ange
with the AC system. The capability of control
power exchange is a signicant feature which can be used effectively in
applications requ iring power osci llation damping, to level peak power dem and,
and to provide u ninterrupted power for cri tical load.
CHARACTERISTICS OF STATCOM
The derivation of the formula for the transmitted active power
employs con siderable cal culations. Using the vari ables d ened in Figure below
and applying Kirchoffs laws t he following equ ations can be written;
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Two machine system with STATCOM
By equaling right-hand terms of the above formulas, a formula for the cu rrent
I1 is ob tained as
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Where U R is the STATCOM terminal voltage if the STATCOM is ou t of operation,
i.e. when I q = 0. The fact that I q is sh ifted by 90◦ with regard to U R can be used
to express I q as
Applying the sine law to the diagram in Figure below the following two
equations result
from which the formula for sin α is derived as
The formula for the transmitted
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To dispose of the term U R the cosine law is applied to the diagram in Figure
above Therefore,
Transmitted power versus transmission angle char
With these concepts of STATCOM, it is thus important to utilize
these principles in accommodating shunt compensation to any system. Since
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this thesis only reects on the voltage control and power increase, the
requirements of the STATCOM would be further elaborated.
FUNCTIONAL REQUIREMENTS OF STATCOM
The main functional requirements of the STATCOM in this thesis
are t o provide sh unt compensation, operating in capacitive m ode only, in terms
of the following;
• Voltage st ability control in a power syst em, as t o compensate the loss vol tage
along transmission. This compensation of voltage has to be in synchronism
with the AC system regardless of di
• Transient stability during disturbances i n a sy stem or a ch ange of load.
• Direct vol tage sup