statcom for renewable energy sources

8/18/2019 statcom for renewable energy sources

1/128


2/128

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


3/128

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,


4/128

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.


5/128

• 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


6/128


7/128

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


8/128

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


9/128

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


10/128

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.


11/128

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


12/128

• 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.


13/128

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


14/128

(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


15/128

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.


16/128

Rotor :

The aerodynamic forces acting on a wind turbine

theory. When the

(a)


17/128

(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.


18/128

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.


19/128

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


20/128

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:


21/128

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


22/128

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


23/128

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.


24/128

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


25/128

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.


26/128

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


27/128

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.


28/128


29/128

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


30/128

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


31/128

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.


32/128

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).


33/128

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:


34/128

‘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


35/128

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,


36/128

• 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


37/128

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


38/128

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


39/128

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


40/128

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


41/128

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.


42/128

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


43/128

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


44/128

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


45/128

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.


46/128

• 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


47/128

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-


48/128

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:


49/128

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


50/128

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:


51/128

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


52/128

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


53/128


54/128


55/128

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


56/128

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


57/128


58/128

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


59/128

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.


60/128

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


61/128

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


62/128


63/128

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


64/128

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.


65/128

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.


66/128

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.


67/128

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


68/128

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


69/128

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


70/128

.

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


71/128

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.


72/128

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.


73/128

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.


74/128

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.


75/128

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.


76/128

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;


77/128

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


78/128

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


79/128

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


80/128

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

statcom for renewable energy sources

Documents