elektrane 08 ve

Elektrane

VjetroelektraneElektraneFER 2009.

Energijske tehnologije2Energijske tehnologije: Energija vjetra 22008. 2

Energija vjetra – kinetička energija

Vjetar je masa zraka u pokretu:• uzrokuje ga razlika tlakova

(rezultat razlike temperatura)• posljedica sunčeve energije (1 do 2 %)• značajan utjecaj rotacije Zemlje i konfiguracije tla•globalni i lokalni uvjeti

Energijske tehnologije3Energijske tehnologije: Energija vjetra

Prvi vjetroagregati - Europa i SAD (kraj 19. st.)

Danska - La Cour’s testni vjetroagregat 1897

SAD -Charles

Bush 1890

Novi početak –California 1985.

Altamonte Pass 4000 x 40 kW

Boone, NC 2 MW Mod-1

Energijske tehnologije4

Danas – energija vjetra dio rješenja

41x1 MW Mitsubishi MWT-1000A, 55 m stup,29.5 m lopatice, Combine Hills projekt u NE Oregon

10 x 2,3 MW Bonus (Siemens) VAvisina stupa 61 m, dubina vode 15 m

http://www.samsohavvind.dk/windfarm/

http://www.samsohavvind.dk/windfarm/


Porast instalirane snage u vjetroelektranama u svijetu


Instalirana godišnja snaga u vjetroelektranama po regijama


Instalirana snaga vjetroelektrana u 10 vodećih zemalja (kraj 2008)

Country MW Capacity % of Global Capacity

US 25,170 MW 20.8%

Germany 23,903 MW 19.8%

Spain 16,754 MW 13.9%

China 12,210 MW 10.1%

India 9,645 MW 8.0%

Italy 3,736 MW 3.1%

France 3,404 MW 2.8%

UK 3,241 MW 2.7%

Denmark 3,180 MW 2.6%

Portugal 2,862 MW 2.4%

Total top 10 104,104 MW 86.2%


Vjetroelektrane• Najznačajniji obnovljivi izvor• Značajan udjel u instaliranim kapacitetima• Vjetroparkovi/vjetrofarme imaju neka od

obilježja klasičnih elektrana• Jedini obnovljivi izvor koji ćemo razmatrati • Zbog načina proizvodnje električne energije u

vjetroturbinama ove elektrane imaju i neke specifičnosti

• Zanima nas i način rada na mreži i uvjeti za priključivanje vjetroelektrana

• Dodatni tehnički uvjeti za priključak i pogon VE na prijenosnoj mreži, HEP 2008

Energijske tehnologije9Energija vjetra 9

Energija i snaga vjetra• energija mase zraka je kinetička:

– masu zraka određuje gustoća, površina kroz koju struji, brzina i vrijeme: m=ρAvt

• snaga vjetra:– derivacija energije po vremenu, dE/dt:

• gustoća zraka:– ovisi o temperaturi, tlaku i vlažnosti– za standardne uvjete na moru

ρ=1,225 kg/m3 (101,3 kPa i 15 °C)– moguće je koristiti standardnu gustoću uz

korekciju faktorom odstupanja (prosjeka) stvarnog tlaka i temperature:

– unutar uobičajenih promjena temperature i tlaka gustoća varira do 10%

• brzina zraka:– raste s visinom i vrlo promjenjiva

2

2vmEk⋅

=

tvAEk ⋅⋅⋅

=2

3ρ

vA

d=vt

2

3vAPvjetra⋅⋅

=ρ

2008.

3,101pcH =

TcT

1,288=


Continuity

Energijske tehnologije11Energija vjetra 11

Teorijski iskoristiva snaga vjetraIskorištena snaga ovisi o brzini kojom vjetar

dolazi (v) i brzini kojom odlazi (w):– posebno odvajamo masu jer ovisi o prosjeku brzina:– neka omjer brzina w/v bude x

v w

( )22

2)(

2wvwvAP −⋅

+⋅

⋅=ρ ( )23 1

2)1(

2xxvAP −⋅

+⋅⋅

⋅=ρ

pcvAP ⋅⋅⋅

= 3

2ρ

( )212

)1( xxcp −⋅+

=

x

cp

– za x= w/v=1/3 , cp ima maksimalnu vrijednost cp.max :cp.Betz=16/27=59,3%

- stvarni stupanj djelovanja je uvijek manji od maksimalnog

2008.


Brzina vjetra u ovisnosti o visini

• Brzina se vjetra povećava s visinom– to ovisi o konfiguraciji

tla, temperaturi i tlaku– važno za procjenu

brzine na raznim visinama jer određuje snagu i naprezanje VA

• Jednostavan model preko koeficijenta terena α:– mirna voda i glatko i

tvrdo tlo: α=0,10– visoka trava α=0,15– šumovito α=0,25– grad sa velikim

zgradama α=0,40

Energijske tehnologije: Energija vjetra 122008.

α

⎟⎟⎠

⎞⎜⎜⎝

⎛=

00 H

HvvH

Varijacija brzine vjetra [m/s] s visinom [m] ovisno o dobu dana i godišnjem dobu (siječanj – crno i srpanj– crveno). (B. Sørensen: Renewable Energy).

α=0,4

α=0,28

α=0,16

450

300

150

30

4,4 m/s (16 km/h)

4,4 m/s

4,4 m/s

3,12,21,4

Visina

[m]


Tipovi vjetroturbina

• Klasične vjetrenjače (nisu od interesa za proizvodnju električne energije)

• Vjetroturbine s horizontalnom i vjetroturbine s vertikalnom osovinom (horizontal axis wind turbines – HAWT, vertical axis wind turbines -VAWT)

• Jednu vjetroturbinu zovemo i vjetroagregat (VA)• Grupe vjetroturbina na jednoj lokaciji

označavamo pojmom vjetrofarma ili vjetropark• Ovisno o priključku jedan ili više vjetroparkova

označavamo kao vjetroelektranu (VE)


Vertical Axis Wind Turbines

• Darrieus rotor - the only vertical axis machine with any commercial success

• Wind hitting the vertical blades, called aerofoils, generates lift to create rotation

• No yaw (rotation about vertical axis) control needed to keep them facing into the wind

• Heavy machinery in the nacelle is located on the ground

• Blades are closer to ground where windspeeds are lower


Horizontal Axis Wind Turbines

• “Downwind” HAWT – a turbine with the blades behind (downwind from) the tower

• No yaw control needed- they naturally orient themselves in line with the wind

• Shadowing effect – when a blade swings behind the tower, the wind it encounters is briefly reduced and the blade flexes


Horizontal Axis Wind Turbines

• “Upwind” HAWT – blades are in front of (upwind of) the tower

• Most modern wind turbines are this type• Blades are “upwind” of the tower• Require somewhat complex yaw control to keep

them facing into the wind• Operate more smoothly and deliver more power


Number of Rotating Blades

• Windmills have multiple blades– need to provide high starting torque to overcome weight of the

pumping rod– must be able to operate at low windspeeds to provide nearly

continuous water pumping– a larger area of the rotor faces the wind

• Turbines with many blades operate at much lower rotational speeds - as the speed increases, the turbulence caused by one blade impacts the other blades

• Most modern wind turbines have two or three blades


Zadatak 1. Snaga vjetroagregataOdrediti specifičnu i ukupnu

električnu snagu vjetroagregata (VA) uz:gust. zraka ρ = 1,225 kg/m3;

brzinu vjetra v = 12 m/s;

promjer lopatica D = 50 m;

efikasnost cp = 0,4

el. meh. stup. djelovanja η = 0,8

P, p

P = η ⋅ cp⋅ 0,5⋅ρ⋅A⋅v3 [W]

p = P/A = η ⋅ cp⋅0,5⋅ρ⋅v3 [W/m2]

Rješenje:

p = 0,8 ⋅ 0,4 ⋅ 0,5 ⋅ 1,225 ⋅ 123

= 0,8⋅0,4 ⋅ 1058 = 339 [W/m2]

A = r2 ⋅ π = D2⋅π/4 [m2]

P = p ⋅ A = 339 ⋅ 502 ⋅ 3,14/4= 339 ⋅ 1963 = 665 [kW]

Za vježbu: 1. odrediti P i p uz v= 9 m/s.2. odrediti maksimalnu snagu uz iste

brzine (η=1 i cp=cpBetz)

Rj.:

P9m/s= 280 kW; p9m/s= 143 W/m2

P9m/s.max= 520 kW; p9m/s.max= 265 W/m2

P12m/s.max= 1232 kW; p12m/s.max= 628 W/m2

Energijske tehnologije19Energijske tehnologije: Energija vjetra 19

Snaga vjetroagregata za razne brzineUtjecaj brzine vjetra na snagu

odražava cp:

– cp =ηvjetrenjače⋅cp.Betz) ovisi o aerodinamici lopatica: brzina i položaj

– često se uzima ukupna vrijednost koja u sebi sadrži i stupnjeve djelovanja mehaničke i električne pretvorbe: cpe =ηe⋅cp

P = cpe⋅ 0,5⋅ρ⋅A⋅v3 [W]

– uobičajeno je prikazivati cp u ovisnosti o tzv. omjeru brzine vrha lopatice prema brzini vjetra (λ)

– promjena cpe u ovisnosti o brzini okretanja rotora za različite brzine vjetra pokazuje pomicanje optimuma

2008.


Efficiency of power production


• Not all of the power in the wind is retained - the rotor spills high-speed winds and low-speed winds are too slow to overcome losses

• Depends on rotor, gearbox, generator, tower, controls, terrain, and the wind

• Overall conversion efficiency (Cp·ηg) is around 30%

Estimates of Wind Turbine Energy

WPBP EP

Power in the Wind

Power Extracted by Blades

Power to ElectricityPC

Rotor Gearbox & Generator

gη


Tip-Speed Ratio (TSR)

• Efficiency is a function of how fast the rotor turns

• Tip-Speed Ratio (TSR) is the speed of the outer tip of the blade divided by windspeed

Rotor tip speed rpm DTip-Speed-Ratio (TSR) = Wind speed 60v

π×=

• D = rotor diameter (m) • v = upwind undisturbed windspeed (m/s) • rpm = rotor speed, (revolutions/min)


Koeficijent cp ovisi o izvedbi vjetroagregata

5/29/20092008. Energijske tehnologije: Energija vjetra 23

Moderni VA sa tri lopatice


Električna snaga vjetroagregataSnaga vjetra proporcionalna je :

• gustoći zraka• trećoj potenciji brzine vjetra

Dobivena snaga iz vjetra određena je brzinom vjetra i karakteristikom vjetroagregata: el. snaga

brzina vjetra

1. startna brzina2. efikasnost se mijenja3. maksimalna snaga

generatora4. maksimalna brzina

1

2

3

4

Pa

va vb

Pb

Energijske tehnologije26Energijske tehnologije: Energija vjetra 26

Ovisnost snage VA o brzini vjetra• Svaki VA ima

karakteristiku snage u ovisnosti o brzini vjetra

• Karakteristika snage ovisi o tehničkoj izvedbi – pasivna samoregulacija (stall)– aktivna regulacija (pitch)

2008.


Wind Turbine as Electricity Generator


Zadatak 2. Snaga vjetroagregataOdrediti električnu snagu

vjetroagregata (VA) za šest točaka iz krivulje snage:nazivna snaga Pn = 2 MW

brzina vjetra: 3, 10, 12, 15, 20 i 30 m/spostotak nazivne snage: 0, 50, 80, 100, 100 i 0 %

Pi

Pi = ci.pns⋅ Pn

Rješenje:P3 = 0 ⋅ 2 = 0

P10 = 0,5 ⋅ 2 = 1,0 MW

P12 = 0,8 ⋅ 2 = 1,6 MW

P15 = 1 ⋅ 2 = 2,0 MW

P20 = 1 ⋅ 2 = 2,0 MW

P30 = 0 ⋅ 2 = 0

Energijske tehnologije29Energijske tehnologije: Energija vjetra

Spektar varijabilnost vjetra u vremenu

2008. 29

god. mjesec dan sat minuta sekunda

Dnevno (često uzrokovano temp.):• more• padine planina

Turbulencije, od:• tla, prepreka• olujnih fronti

Veliki vremenski sistemi• visoki/niski tlačni sistemi• sezonske varijacije

Vremenska skala

Doprinos varijabilnosti

“spektralni prekid”


prepreke

Energijske tehnologije: Energija vjetra

Varijabilnost vjetra u prostoru

2008. 30

vrsta tlatopografija

meteorologija

<< 1 km

~ 1 km~ 10 km

~ 100 km

vrijeme [s]

brzi

na[m

/s]

Kratkotrajne varijacije brzine vjetra na dvije lokacije mogu biti korisne za smanjivanje ukupne varijabilnosti proizvedene energije.Primjer za dvije lokacije udaljene 90 m:


0 5 10 15 20 25 30

Power Out put

Windspeed Dist r ibut ion

Energy Out put

m/s

How it Works – Pitch Control


Kako dobiti statistiku vjetra za lokaciju?

dva pristupa

lokalna mjerenja

podaci s ostalih lokacija

kratko vrijeme (brzina, smjer i temperatura)

mjerenja

ostale lokacije ne odgovaraju

Problem: Rješenje:

koreliranje s drugim lokacijama gdje je duže mjereno

koreliranje(tlo, prepreke)

“Predviđanje koreliranjem

mjerenja”

Korelacioni atlas vjetra


Utjecaj VE u EE sistemu

velika varijabilnost

mala predvidljivost

upravljivost

može se dijelom smanjiti uključivanjem VE na širokom području

korištenje poboljšanih metoda predviđanja vremena (= vjetra)

korištenje modernih VE s kontrolom nagiba lopatica i varijabilnom brzinom


Tehnologija: veličinemale

1 ~ 100 kWsrednje i velike

100 ~ 1500 kW(pučina )

> 1500 kW

Daleka izolirana mjestaRaznolikost rješenja

Na mrežiSamostalne i u grupi1000 kW posve komercijalne (velike serije)

Na pučini (stotine MW)Razvija se

Mikro Vrlo male Male Srednje Velike1 10 100 750 [kW]

Energijske tehnologije35Energijske tehnologije: Energija vjetra 352008. Energija vjetra 35

Tehnologija: osnovne komponente

Lopatice rotora

kućište

toranj

temelji

pozicioniranje(prijenos)generatorkontrola

HAWT


Different Wind Turbine Concepts


Current situation


Current situation• at present, no clear winner in the competition of

concepts• 3-bladed turbines are the favourite solution• both fixed and variable speed machines, but

more new large designs chose variable speed• both stall and pitch controlled, but more larger

machines with pitch or active stall control• both gear box and direct drive• both glass fibre and wooden (composite) blades,

but majority uses glass fibre and epoxy resin


Fixed speed stall-controlled turbine


Variable speed pitch-controlled turbine


Blades

Hub

Gearbox

Yaw system

Generator

Control system,

converter & transformer

Tower

Tower16%

Nacelle & yaw system

13%

Hub9%

Gearbox20%

Blades23%

Electric & control system

19%

Anatomy of a Wind Turbine

€


Rotor40T

Tower200T

Nacelle70T

Rotor

Nacelle

Tower

Anatomy of a Wind Turbine

Mass

Vestas V90-3MW 110T

Siemens SWT3.6-107 220T

Multibrid M5000 310T

REpower 5M + Bard VM 440T


Lift and Drag Forces


How Rotor Blades Extract Energy from the Wind• Air is moving towards

the wind turbine blade from the wind but also from the relative blade motion

• The blade is much faster at the tip than at the hub, so the blade is twisted to keep the angles correct


Angle of Attack, Lift, and Drag

• Increasing angle of attack increases lift, but it also increases drag

• If the angle of attack is too great, “stall” occurs where turbulence destroys the lift


Aerodynamic Control of Wind Turbine• Stall:

– fixed blade pitch– passive power control by stall effect– control parameter: wind speed

• Pitch:– blade pitch activated by WT control– active power control– control parameter: power output, wind speed and

rotor speed


Stall control – stand still


Stall control – start, slow speed


Stall control – power generation


Stall control – power limitation


Pitch control – stand still


Pitch control – start of operation


Pitch control – power generation


Pitch control – power limitation


Main parts of wind turbine


1. Hub controller 11. Blade bearing2. Pitch cylinder 12. Blade3. Main shaft 13. Rotor lock system4. Oil cooler 14. Hydraulic unit5. Gearbox 15. Machine foundation6. Top Controller 16. Yaw gears7. Parking Break 17. Generator8. Service crane 18. Ultra-sonic sensors9. Transformer 19. Meteorological gauges10. Blade Hub

10

1617

12

5

12

Nacelle Components


Lopatice, prijenos, generator i kontrola

Direktni pogon:sinkroni generator

Indirektni pogon: asinkroni generator


Trendovi

• Jedinice od 2 MW i više sve važnije od 2002.• Poboljšana efikasnost

– Odabir lokacije– Bolja oprema– Porast efikasnosti 2-3% godišnje

• Stalni pad cijene investicija

φ - promjer


1.5 MW wind turbine in comparisonto a Boeing 747, Jumbo-Jet


SPEED n 20 17 13 5 – 15 3 – 10 rpm

HE

IGH

T [M

]

Development of HAWT


Increasing rotor dia - advantages


Increasing rotor dia - disadvantages


Physical development limits of wind energy


Proizvođači vjetroturbina i veličine• Vergnet: 5, 10, 15, 20, 60, 250kW• AOC: 10, 50kW• Enercon: 330, 800, 2000, 4500kW• Vestas: 850, 1650, 2000, 3000, 4500kW• Bonus: 1000, 1300, 2000kW• GE-Wind: 1500, 2000, 3600kW• Nordex: 1300, 1500, 2500, 2300kW


Wind Turbine Generators

• A key difference between wind turbines and convention generators is maximum power can be extracted from the wind when the turbines spin at varying speeds– Conventional generators are fixed speed, which is ideal for the

use of synchronous generators

• Several different technologies are used for wind turbine generators– Variable frequency synchronous generators– Squirrel cage induction generators– Doubly fed induction generators

• Mechanical power control of the turbine blades is a key design consideration (either stall or pitch control)

• Another important point is possibility of generator related speed control


Wind energy conversion


Proposed Standard Models

• Based on characteristics of grid interface– Type A – conventional induction generator (Constant speed, (active) stall

controlled)– Type B – wound rotor induction generator with variable rotor resistance– Type C – doubly-fed induction generator (Variable speed, pitch controlled)– Type D – full converter interface (Variable speed, pitch controlled)

Type A Type B Type C Type D


Comparison of Wind Turbine Types


Grid Interaction of Wind Turbine Types


Synchronous Machines

• Spin at a rotational speed determined by the number of poles and by the frequency

• The magnetic field is created on their rotors• Create the magnetic field by running DC through

windings around the core• A gear box is needed between the blades and

the generator• 2 complications – need to provide DC, need to

have slip rings on the rotor shaft and brushes


Asynchronous Induction Machines

• Do not turn at a fixed speed• Acts as a motor during start up as well as a

generator• Do not require exciter, brushes, and slip rings• The magnetic field is created on the stator

instead of the rotor• Less expensive, require less maintanence• Most wind turbines are induction machines


The Inductance Machine as a Motor

• The rotating magnetic field in the stator causes the rotor to spin in the same direction

• As rotor approaches synchronous speed of the rotating magnetic field, the relative motion becomes less and less

• If the rotor could move at synchronous speed, there would be no relative motion, no current, and no force to keep the rotor going

• Thus, an induction machine as a motor always spins somewhat slower than synchronous speed


Slip

• The difference in speed between the stator and the rotor

1 (6.28)S R R

S S

N N NsN N−

= = −

• s = rotor slip – positive for a motor, negative for a generator

• NS = no-load synchronous speed (rpm) • f = frequency (Hz) • p = number of poles• NR = rotor speed (rpm)

120S

fNp

=


The Induction Machine as a Motor

• As load on motor increases, rotor slows down• When rotor slows down, slip increases• “Breakdown torque” increasing slip no longer

satisfies the load and rotor stops• Braking- rotor is forced to operate in the

opposite direction to the stator field

Torque- slip curve for an induction motor, Figure 6.17


The Induction Machine as a Generator

• The stator requires excitation current– from the grid if it is grid-connected or– by incorporating external capacitors

• Windspeed forces generator shaft to exceed synchronous speed

Figure 6.18. Single-phase, self-excited, induction generator


The Induction Machine as a Generator

• Slip is negative because the rotor spins faster than synchronous speed

• Slip is normally less than 1% for grid-connected• Typical rotor speed

(1 ) [1 ( 0.01)] 3600 3636 rpmR SN s N= − = − − ⋅ =


Wind turbine with Induction Generator, direct coupled to the grid


Wind turbine with asynchronous generator


Squirrel Cage Induction Generators

• Squirrel cage induction motors are the most widely used type of motor. Because there is no external connection between the rotor and stator they can be extremely reliable

• Losses in a traditional induction motor vary according to the slip

• Torque-speed curves can be determined by solving the circuit for different assumed slip values

Induction Machine Circuit Model


Induction generator for wind turbines


Torque diagram – grid connected


Squirrel Cage Induction Generators

• Synchronous speed can be changed discretely by changing the number of poles (2=3600 rpm, 4=1800 rpm, 6=1200 rpm; 8=900 rpm; pole switching can be used to change the synchronous speed (similar to what is done with low, medium, high for an induction motor fan)

• Squirrel cage induction generators have the advantage of no slip rings, but have the disadvantage that once they are constructed the rotor resistance can not be changed– They are cheap to make, but are usually only used for low power

applications– They also can place high torques on the mechanical systems if the

rotor torque-speed curve has a steep slope


Wound Rotor Induction Generators

• Wound rotor machines avoid the need for fixed Rrotor by replacing the squirrel cage rotor with a wound rotor that includes external resistors. The value of the rotor resistance can then be changed dynamically– Usually the external resistors are located off of the rotor,

requiring slip rings and brushes. – Opti Slip (Vestas) solves the problem of needing slip rings and

brushes by mounting everything on the rotor, including the control. Control communication is via an optical fiber attached to the stator, which communicates with the rotor whenever it rotates past.

– Suzlon uses their Flexi-slip control system


OptiSlip Example: Vestas V47

By increasingthe slipthe generatorcan run slightly fasterduring a gustallowing timefor the pitchmechanismto respond

Source: www.creswindfarm.gr/site1/Articles/V47_US.pdf


Double fed induction generator variable speed with pulse width modulated inverter


Control of wind turbine with DFIG


Double fed induction generator


Wind turbine with inverter system in the main power circuit (variable speed)


Variable Frequency (Pulse-modulated) Synchronous Generators• Traditional, grid-connected synchronous

machines do not work because they are fixed speed, but variable frequency synchronous machines can be used. However, they require a power converter (ac-dc-ac) rated for the full generator output, which is expensive.

Source: S. Muller, M. Deicke, R.W. DeDoncker, “Doubly Fed Induction Generator Systems,” IEEEIndustry Applications Magazine, May/June 2002; Figure 4


Variable Frequency Synchronous Generators• Usually permanent magnet (multi-pole)

synchronous generators are used since they avoid the need for an excitation system– Power converter provides control

• Either low speed (gearless) or high speed systems can be used– Mitsubishi MWT-S2000 (2MW)

is an example of a low speed, gearless system (8-24 rpm)


Variable Frequency Synchronous Generators, cont.• Advantages include

– wide range variable speed operation, allowing higher annual production

– The ability to store wind gusts as rotational energy (changing rotor speed) reducing mechanical stresses

– Power converter provides reactive power control

• Disadvantages include– Costs associated with the power converter since full generator

output must go through the converter– Losses and harmonic distortion caused by the converter– Potential loss of rotating inertia as seen by the grid


Advantages of Variable-Speed Turbines


Variable-Speed Turbines


Doubly-Fed Induction Generator (DFIG)


Full-Power AC-AC Converter Drive


Complete variable-speed turbine


Vjetroelektrana(VE)


Efekt sjene za jednu vjetroturbinu


Lokacije VE: efekt više VA u blizini

pov. turbulencije

reduciranje brzine

Slabljenje vjetra turbine niz vjetar:• manje brzine: manje snage• veće turbulencije: više opterećenja• veći broj VA u VE povećava gubitke

• optimiranje pozicioniranja za snagu i trošenje (računalni programi za simulacije i mjerenje)• razmak u dominantnom smjeru od 4 do 9 promjera

- gubitci od 5 do > 60%, za manji (2x2) ili veći (10x10) broj VA u VE


Wind Farms

Figure 6.28

For closely spaced towers, efficiency of the entire array

becomes worse as more wind turbines are added


Shading effects in wind farms


Wind Farms – Optimum Spacing

Figure 6.29Optimum spacing is estimated to be 3-5 rotor diameters between towers and 5-9 between rows

5 D to 9D

3 D to 5D


Shading intensity depends on:• Geometry• Wind Direction• Wind Speed• Power Curve, Thrust Coefficient Curve• Turbulence Intensity (depends on site and

atmospheric stratification

• Definition of Park Efficiency:


Primjer razmještaja VA unutar vjetroparka


Power Plant Functions• Power generation

– Prime mover of conventional generation is controllable as opposed to the wind

– Conventional generators are synchronous generators– Wind turbines use squirrel cage generators or variable speed

generators with power electronic converters

• Short term balancing• Long term balancing• Voltage control• Fault current supply• Ride-through capability: ability to operate

through grid faults (e.g. Short circuit) and help restore the grid operation after the fault


Short term Balancing• Characteristics:

– No storage capabilities– Power balance continuously required to stabilize

frequency– Power plants maintain balance/frequency

• Effects on short term balance normally limited:– With few wind turbines, short term fluctuations are

too small to effect system balance– With many wind turbines, short term fluctuations

even out


Long term Balancing• Effected by generation and load• Long term balancing complicated because:

– Long term wind speed fluctuations can effect large regions

– Remaining conventional generators face complicated demand pattern

– Wind turbine availability depends on wind availability


Voltage Control• Goal of grid voltage control• Keeping node voltages close to their nominal values in

order to:– assure correct working of customer equipment– prevent equipment, both of the grid company and the customer, from

being damaged

• Voltage control is necessary due to line impedances• Depends strongly on wind turbine type

– Variable speed turbines can generate reactive power and contribute to voltage control if the converters are overdimensioned

– Doubly fed induction generator: reactive power dependent on rotor current

– Direct drive: reactive power dependent on converter grid current– In case of constant speed turbines extra equipment needed


Voltage Control


Fault Current Supply• Power system protection protects equipment from

damage caused by fault currents:• Current is monitored continuously • If large current is sensed, faulted element is

disconnected by switchgear• Switchgear must be able to interrupt fault current

• Constant speed wind turbines supply fault current• Wind turbines with doubly fed induction generator are

disconnected to protect the converter: no fault current• Wind turbines with direct drive generator are

disconnected, but could feed nominal current into fault


Conclusions• Generators play important role in power system

operation• Wind turbines differ much from conventional

generators• This effects their capability to fulfill the functions

of power plants• Exact capabilities vary between wind turbine

types


Power Grid Integration of Wind Power

• Currently wind power represents a minority of the generation in power system interconnects, so its impact of grid operations is small

• But as wind power grows, in the not too distant future it will have a much larger, and perhaps dominant impact of grid operations

• Wind power has impacts on power system operations ranging from that of transient stability (seconds) out to steady-state (power flow)

• Local and global impact


Wind Power, Reserves and Regulation

• A key constraint associated with power system operations is pretty much instantaneously the total power system generation must match the total load plus losses– Excessive generation increases the system frequency, while

excessive load decreases the system frequency

• Generation shortfalls can suddenly occur because of the loss of a generator; utilities plan for this occurrence by maintaining sufficient reserves (generation that is on-line but not fully used) to account for the loss of the largest single generator in a region (e.g., a state)


Wind Power, Reserves and Regulation, cont.

• A fundamental issue associated with “free fuel” systems like wind is that operating with a reserve margin requires leaving free energy “on the table.”

• Because wind turbine output can vary with the cube of the wind speed, under certain conditions a modest drop in the wind speed over a region could result in a major loss of generation– Lack of other fossil-fuel reserves could exacerbate the situation


Wind Power and the Power Flow

• The most common power system analysis tool is the power flow (also known sometimes as the load flow)– power flow determines how the power flows in a network– also used to determine all bus voltages and all currents– because of constant power models, power flow is a nonlinear

analysis technique– power flow is a steady-state analysis tool– it can be used as a tool for planning the location of new

generation, including wind


Grid codes and integration of wind energy

• Grid Code: Technical document containing the rules governing the operation, maintenance, & development of the system

• Grid Codes description and purpose in this context

• Operate a wind farm/wind turbine like a power station/plant


Grid codes and integration of wind energy

• Steady state– Frequency /Power control– Low/high frequency support– Voltage support/reactive power compensation– Power Quality, flicker, harmonics

• Transient /dynamic state– Fault ride through, to stay connected during low voltage

on the grid– Ramp rate

• Communication /power dispatch– Reliable communication– Wind forecasting– Participate power market


Grid Integration Levels• Large network• Micro grid• Stand alone system• The main questions for system integration

– Penetration of wind energy in the grid (Share of energy produced by WTGs to the energy supply in a grid)

– Diversity of the total power generation– Structure and pattern of load


Integration of wind energy into power system• Integration into:• large networks

– national or continental interconnected systems– grid connection is only a question of available grid capacity or

grid reinforcement

• small grids– separate isolated distribution systems not connected to large

networks– grid integration is a matter of adapting the wind farm size to the

capacity of the existing power generation and consumption

• stand-alone systems – no connection to a grid– most difficult case: production has to meet consumption at any

time on a small scale


Large network integration• In the large networks up to now the penetration

is below 5-6%.• In regions or local areas in Europe the

penetration is above 50 % in average• Grid integration at this stage is of no problem

– Due to the network extension power generation of wind energy is more steady

– Large diversity of power generation– Power quality at point of common coupling is the main concern– In future control energy will be a matter of discussion


Structure of large network


Connection to different voltage levelsof the electrical network


Grid connection of a single WT


Grid connection of a wind farm in ringconfiguration


Example of grid connection of a windfarm in radial configuration


Priključak na mrežu i višak vjetra

0

500

1000

1500

2000

2500

3000

0 2000 4000 6000 8000 10000

sati

MW

Krivulja trajanja opterećenja

Elektrane koje moraju raditi cijelo

Konstantno opterećenje

Višakvjetra

Iskoristiva energija vjetra

1000 MW nazivni kapacitet vjetra36% faktor opterećenja

• Odbacivanje energije iz vjetroelektrana za snagu koja prelazi opterećenje minus bazna proizvodnja

• Provodi se na nivou regionalne interkonekcije• Promjenjivo za sve periode


Power Quality and Wind Turbines• power factor (reactive power)• power variations (10min,1min, instantaneous)• switching operations (current spike factor)• flicker and voltage fluctuations

– Voltage flicker describes dynamic variations in the network voltage which may be caused by wind turbines or by varying load.

– The origin of the term is the effect of the voltage fluctuations on the brightness of incandescent lights and the subsequent annoyance to people.

• harmonics (Integer harmonics and interharmonics)


Basic control of active and reactive power in a wind turbine (DFIG and PMG)


Micro grid and stand-alone• In principle Micro grid and stand-alone are quite

similar• The main question is the amount of renewable

energy in a system, the penetration• An often economic viable solution for existing

diesel supply is the extension to a wind-diesel-system

• The most simple systems for stand alone operation is the fuel saver


Micro grid and stand-alone• Due to the limited grid extension and low

diversity of power generation wind energy integration is more difficult

• An integration of 10 to 15% wind energy share to installed capacity is unproblematic

• Further wind energy share to installed capacity up to 30 or 40% is possible, if controllable turbines with grid supporting characteristics are installed and a common control for the diesel generators is possible

• For higher penetration energy storage systems and load management strategies have to be implemented


Wind-Diesel-System with short and mediumterm storage and control

elektrane 08 ve

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