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ind Turbine Turbine Study vestigati vestigation on in to VT application in wind turbines A

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Martem

P.H.W.M.

Albers

Reportnumber: DCT 2003.90

A.H.J.A. Martens P.H.W.M. Albers Supe rvisor: Prof. Ir. N.J J. Liebrand, D r Hub van Doorn e Chair Coach: Dr P.A. Veenhuizen Eindhoven,

October 2 0 0 3

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technische universiteit universiteit eindhoven

W i n d Turbine S t u d v

Introduction

Wind turb ines th at o perate at a constant constant speed attain the greatest aerodynamic efficiency efficiency levels only when the wind speed is the same as the design wind speed. In variable variable speed wind power syste systems, ms, the turbine runs at a tip speed ratio which ensures its m aximum efficiency. efficiency. Variable speed systems have more advantages such as tha t the turbine is less sensitive sensitive to th e wind pattern of a given given location location and emits less noise at low speeds. Current varia bie speed systems generally iitilize electric generators, which are rigid ly fitted to th e turbine and are coupled coupled to an inverter, inverter, which ad justs the electric electric current generated generated t o th e frequency frequency required. required. Systems Systems equipped wit h inverters inverters present a number of drawbacks. drawbacks. They They are particularly expensi expensive ve and complex an d the inverter inverter and variabl speed ele ctrical ma chine are not very very efficient or reliable. The The goal o f this interns hip is to investigate the possibility of u sing a continuous variable trans mis sion CVT) CVT) betwe en a win d turbine and an electric generator in the design of a variable variable speed wind turbine. Such Such a solution will have the follow ing advantages: advantages: Maxim um efficiency efficiency of the wind device device even even with sudde n changes in win d speed. The generator p roduce s electric electric current at a constant frequency. Before Before a simulation can be made of the turbine first the specifications specifications of th e in go ing side and ou tgoin g side of the C T should be know. With With the ingo ing side the wind profile is meant and b y the outg oing side the kind of generator generator and it's it's properties. properties. Further it useful to kno w what the influences influences are are of these two aspects with respect respect to th e design o a wind turbine. The The goal of th is interns hip i s to investigat investigate e what th e advantages advantages and d isadvantages are of a C T in d turbin e app lication.

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technische universiteit universiteit eindhoven

W i n d Turbine S t u d v

Introduction

Wind turb ines th at o perate at a constant constant speed attain the greatest aerodynamic efficiency efficiency levels only when the wind speed is the same as the design wind speed. In variable variable speed wind power syste systems, ms, the turbine runs at a tip speed ratio which ensures its m aximum efficiency. efficiency. Variable speed systems have more advantages such as tha t the turbine is less sensitive sensitive to th e wind pattern of a given given location location and emits less noise at low speeds. Current varia bie speed systems generally iitilize electric generators, which are rigid ly fitted to th e turbine and are coupled coupled to an inverter, inverter, which ad justs the electric electric current generated generated t o th e frequency frequency required. required. Systems Systems equipped wit h inverters inverters present a number of drawbacks. drawbacks. They They are particularly expensi expensive ve and complex an d the inverter inverter and variabl speed ele ctrical ma chine are not very very efficient or reliable. The The goal o f this interns hip is to investigate the possibility of u sing a continuous variable trans mis sion CVT) CVT) betwe en a win d turbine and an electric generator in the design of a variable variable speed wind turbine. Such Such a solution will have the follow ing advantages: advantages: Maxim um efficiency efficiency of the wind device device even even with sudde n changes in win d speed. The generator p roduce s electric electric current at a constant frequency. Before Before a simulation can be made of the turbine first the specifications specifications of th e in go ing side and ou tgoin g side of the C T should be know. With With the ingo ing side the wind profile is meant and b y the outg oing side the kind of generator generator and it's it's properties. properties. Further it useful to kno w what the influences influences are are of these two aspects with respect respect to th e design o a wind turbine. The The goal of th is interns hip i s to investigat investigate e what th e advantages advantages and d isadvantages are of a C T in d turbin e app lication.

technische universiteit eindhoven

Wind

Turbine Study

Contents WHY INCORPORATE A CVT WIND CHARACTERISTICS

52.1

............................................................................

.................................................................................

WINDDISTRIBUTION ..............................................................................................

TURBULENCE ........................................................................................................ 5 2 . 3 WINDGUSTS ......................................................................................................... 52.2

52.4

WINDENERGY .......................................................................................................

...................................................................................

MOTORS1 MOTORS1 GENERATORS

GENERATO ........................................................................11 THESYNCH RONO US GENERATO .2 THEASYNCHRONOUS GENERATOR ..........................................................................12

.3 CHOICE OR KIND OFGRID CONNECTION ..................................................................13 3.3.1 .4

ND ADVANTAGE

SPEED, TWOSPEED,

DISADVA NTAGE S OF INDIRECT GRID CONNECTION ...........................14

.......................................................... ...................... ... 14 POLE CHA NGIN G GENERATORS .......................................

.5 STARTINGND STOPPING THE GENERATOR ...............................................................15 ~ I N N IPITCH NG CONTROLLED CONTROLLED TURBIN E AT VARIABLE

SPEE

......................................

3.7 COOLING YSTEM ............................................................................................... 16 3.

16 GEARBOX..........................................................................................................

THE CONTINUOUS VARIABLE TRANSMISSION

.1

TH

........................................17

DRY BELTVARIATOR .....................................................................................

.2 MODELING FTHE

........................ ........................ ........................ ......................... ......................... ........................ ............17 CVT ............ 17

.............................................

5 MATHEMATICAL MODEL OF THE TURBINE 6 SIMULATION

............................................................. ............................................................................................. ........................................... ...........20

SIMULATION URPOSE ......................................................................................... .2 SIMULATION ODELLING ...................................................................................... 20 .3 SIMULATIONESULTS .......................................................................................... 20 7 CONVERSATION AT WINDWAL

APPENDIX

...................................................................

.................................................................................................................29

IMULINKMOD EL OFTHE WIND TURBINE TURBINE EQUIPPED WITH A CVT ............................................29

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Wind Turbine Study

Wh incorporate a CVT? The first classic way of genera ting electricity with wind tur bin es shown in figure 1.1. The rotor of th e gene rator fed by the electrical grid or a permanent magnet. When the rotor rotate d from th e outsid e, inste ad of letting th e current from the grid m ove it, it will work like a gen erato r. The more torque you apply to th e turbine, t e more electricity you gen erate , but th e generator will still run a t the s am e speed dictated by the frequ ency of th e electrical grid also possibie to feed th e synchronou g e m r a t o r frcm ow separated grid. !n this ease yo will have to ciaiik th e generator/ rotor at a constzn speed In order to produ ce altern ating current with a c onsta nt frequency. C onsequently, with this type generator you will normally want to use an indirect grid connection of the generator. Now it pos sib le to run t he gene rator at different frequ encie s and th e A C inverter will match t he current to th e electrical grid it

rotor

Figure 1.1: constant speed wind turbine

grid

Figure 1.2: variablespeed wind turbine wi th nverter

The sec ond possibility (figure 1.2) is to use an a sync hrono us gene rator in comb ination with a n inverter. At lower wind spe ed, th e inverter sen ds a lower frequency to th asynchronous generator. Now it possible for th e gene rator to rotate wit slower speed and a s a consequenc e the rotor can rotate at lower speed s. Thi shown in figure 3.5 (s ee cha pte r 3). One of th e disadvantages f this system the wear and tear of the fixed gear, ca use d by the differences torque acting on t he rotor a s function of th e win spe ed. Especially when t he win turbulent there are huge fluctuations on t he rotor and thu s on th e fixed gear. But the main disadvantages are the costs for the inverter and t he gene rator. This will result in th e third option for a wind turb ine c onstruction.

The third sol ution for a variable speed wind turbin shown in figure 1.3. Now a continuous variabie transm ission ) ha s been placed between t he fixed gea r and th asy nch ron ous gen erato r. P urpose of this C to keep th e tip spee d ratio of th e rotor on a constant value (see chapter 2). also poss ible t ha t th e C T wiii ta ke over a cou pie of comparison with the CVT. The biggest problem here wear and tear.

also the need of gears, and th us

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Wind Turbine

aqunchronousgenerator

rotor

Figure 1.3: variable speed wind furbi ne wifh CVT

Study

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Wind T u r b i n e Study

Wind characteristics 32.1

Wind distribution

Wind is the movement o f air in comparison to th e earth surface. The geos trophic win ds are largely driven by temperature differences, and thus pressure differences, a nd a re no very much influenced by the surface of the earth. The geostrophic wind occurs at altitu des above 100 0 metres above ground level. Another force that causes air to move is the coriolis force which i s caused by the rotation of the earth. The wind speed at the usual measurement height of 1 0 meter is approximately twice as sma ll as the win d speed at the geostrophic height. This is because the friction of th earth surface slows the airflow down. The win d speed at a certain heig ht can be approximated by the following equation.

(eq. 2.1)

With

VTef

know n wind speed at a certain h eight zref an

is the roughness len gth in

the current win d direction. The roughness length may be found in certain reference manuals. For the design of a wind tu rbine wind is considered to consist of a constant part a nd a fluctuating part. T e constant part of the wind is of importance when de termining the place of a turbin e site. I t determines the q uantity of energy that can be extracted from the w ind over a lon g period of time. The fluctuating part of the wind (turbulen t effects) is considered when investigating the forces acting on a turbine. If a wind turbine is design ed for a certain site, detailed in forma tion is needed to calculate the energy present in the wind at that site at the turbine height. In general a wind turbine is design ed to deliver sufficient energy over a whole range of sites that sa tisfy a certain win d class. o show the information about the distributions of wind speeds, and the frequency o f the varying wind directions, one may draw a so-called win d rose on t he basis of m eteorological observations of win d speeds and wind directions. An example of a wind rose is given in figure 2.1

Figure 2.

Example

wind rose.

For th is particular win d rose de wind direction i s divided in twelve sec tions each covering thu s 30 degrees of wind direction. T e radius of the 1 2 outermost, wide wedges gives

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Wind Turbine Study

the relative frequency of each of the 12 wind directions, e. how m any percent of the tim e is the win d b low ing rom that direction. The second wedge gives the same information, bu t mu ltiplied by the average wind speed in each particular direction. The resu lt is then norma lised to add up to 10 0 per cent. This tells you h ow much each sector contributes t o th e average wind speed at our pa rticular location. The innermos t (red) wedge gives the same information as the first, but m ultiplied by the cube o f the wind speed in each particular location. The resu lt is then norma lised to ad up to 10 0 per cent. This tells you how much each sector contributes to the energy particular !omtion. content the wind When choos ing a suited site for your wind turbine meteorology data, idea lly in terms of a wind rose calculated over 30 years is probably your best guide, b ut these data are rarely collected directly at your site, and here are many reasons to be careful about t he use of me teorology data. If there are already wind turbines in the area, their prod uction resu lts are an excellent guide to local wind conditions. more accurate reproduction of the wind speed distribution in a certain direction an heig ht is the so-called Weibull distribution. The Weibull distribution is a statistica distribution of the wind speed. The wind speed pro bability function is given by: (eq. 2.2)

This is the prob abdity that the wind speed is equal or larger than wind speed n hours per year. In th is equ ation the parameters 'c ' and I<' are used for fitting the distributio n to a certain wind field. The wind speed distribu tion density is given by: 8760(k/c)(~/c)*-' ~ X ~ [ - ( V / C ) ' ] ~ V ,

feq. 2.3)

This is the probability that the wind spee actually between Vand I/+dVinhours per year. The relation w ith th e annu al average wind speed is given by: X [ l + ($)] with

hegamma function of he argument

The eib ull parame ters 'c' and I< have to b e corrected for the he ight at which the hub o the w ind tu rbine is located. With these formulas it is thus possible to calculate how many ho urs pe r year the wind blows with a certain speed in a certain direction. This information is needed for the calculation of the energy income of the turbin e pe r year A important detail is the number of hours per year that th e wind speed is between the 'cut i n wind speed' and the 'cut out of wind speed' of the turbine. Below the cut in win speed the wi nd speed is to low for the turbine to operate and above the cut ou t of wind speed of the turbin e the wind speed is to high for the turbine to operate safely. At that certain wi nd speed the turbine is shut down.

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Wind Turbine Stu dy

Figure 2.2: Example of a Weibull distribution wit h an average wind speed of approximately 7m/s.

32.2 Turbulence

All time variations in wind speed and direction w ith a period smaller than 0.1 hour ar considered as turbulence. Wind acting on a turbine can be divided in two c ategories. Th first one is caused by a constant wind speed and is also caiied quasi-static or tim averaged. The second one is caused by turbulence or win d gusts and is called dynamic. For the desc ription of turbule nt a irflow a number of different m odels are availab le varying in complexity. The true efficiency of a wind turbine depends on its behavior in a turbulent w ind flow field. r the description of a turbulent flow field in this thesis the spectral mo del of continues turbulence b y Kaimal [I9731 has been used. This model is based on a per ho ur average wind speed wi th a standard deviation o. The frequency behavior of the w ind speed fluctua tions i s described b y a mod el of the spectral power density S(n) wit h n th frequency.

(eq. 2.5)

With the heigh t above ground level and the coefficient qual to 0.06 for the longitud inal component of the win d and equal to 0.2 for the lateral component of the wind. Lateral (V and lon gitudina l (l$)components of the wind speed fluctuations can be calculated w ith: (eq. 2.6)

With i=x,y (lateral, l ong itud inal direction), m is the number of frequencies, An frequency spacin g and a random ph ase angle.


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Wind Turbine Stud

32.3 Windgusts

Wind patterns are considered to be wind gusts when their speed grows to on e and a half. time s their averag e spe ed n maximum time span of thre e second s. wind gus said to be heavy when th e wind s pee d during the gu st rises to 2 1-29 m/s. Very heavy wind gus ts have a minimum wind s pe ed o 29 m/s. discrete wind gus characterized by its amplitu de (the deviation from the average wind spe ed), its time spa n a nd a certain sh ap e function which gives th e variation of wind s pe ed s a function f time d uring the gust. commonly used sh ap e functio th e so-called 'one minus cosine' sh pe iiiiction: 0 . 5 ~ v [ l - cos(2m

v(t)

T)],

Wi h Au the am plitude of the wind gust, z th e period and f th e wind gu st.

(eq. 2.7)

t h e t im e s i n c e t h e b e g i nn i n g

discree t wind gust model a very idealized reprod uction f the reality. They ar e useful wind t u rb i ne a n d y s e s if gust is heavy enough comprehend t he en tire rotor of fo th e turbine. n suc h a c as e it can be said that th e wind s pe ed c han ges uniform over th tota l are a of th e turbine rotor. The minimum time that needed for g u s t t o a p p l y t o th e entire rotor given by: (eq. 2.8)

With coh,

th e coheren ce between two points that a re situated from each o the r in the

direction. It approaches or small fluctuations over a long period and app roac hes fo big fluctuation s over a shor t period. d, th e decay coefficient in th direction and /, the distance between two poin direction. Suitable values for th e deca coefficient bas ed on mea surem ents are dx=4.5, xFdz=7.5. For th e calcula tion of Av two empirical factors (F,,&) are needed. Av

F,

(eq. 2.9)

These factors are calculated on th e basis of years of measure ment of the horizontal wind p ea ks t the c oa st of Cape Kennedy Florida by Kaufman [1977]. These factors c an be use d for wind over a smoo th surface. In th e Netherlands wind gu sts with a wind sp ee d of 20 0 km/h seldom occur. Awind gus t that sufficiently big to co mpr ehe nd a rotor of m ha s th e following shape:

figure 2.3: W i n d g u s t 20 m/h (56m/s) w ifh of 5.5 sec. an d a constanf speed m/s.


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W i n d T u r b in e S t u d y

The maximum acceleration with such a wind gust is 27. m / s 2 .Due to th e very large inertia of th e turbine rotor this acceleration will no t be totally felt by the turbine. 52.4 Wind energy

The power in wind i s equal to the kinetic energy of the wind multiplied by the w ind speed. 0.5pv3,

Herein

s the power density

(eq. 2.10)

m 2 ] , is the air dens ity and

is the h orizontal

win d speed. The maximum power that can be extracted from the win d i s equal o the power density m ultiplied b y the turbine rotor area.

With the ro tor radius. wind power IJV] The power coefficient ( ) determ ines which part of the win d energy is extracted by the wind turbine. [eq 2.12)

The power coefficient depends on the design of the turbine and th e wind speed. According to Betz law the coefficient has a maximum of 16/2 7. Thus the maximum mechanical energy that can be extracted from the win d i s 16 /2 7 times the energy in wind. Because many design variables of a turbine influence the power coefficient it very useful to make a quick approximation of the power coefficient with a particular turbin e design. A empirical equation tha t gives an approximation o f the ma ximum power coefficient is the equation o f Wilson

With D /L the li ft to drag ratio, the rotor speed [rad/s]. Rw

A=--,

the tip speed ratio, B the number of rotor blade s and

[eq.2.14)

Calculation of th e ide al tip speed ratio demands detailed inform ation of th e turbine design. A rough estimation o f the optimum ti p speed ratio for the number o f rotor blades, the ratio at which m ost energy is extracted from the wind, is given i n table 2.1

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W i nd T u r b i n e S t u d

Table 2.1: Estimation o timum tip speed ratio

rc-TF-7

For every turbine for which the generated power is approximately linear with the win speed the de sign speed of the turbine is one and a ha lf times the cut in wind spee d o the turbine. Looking at fig 2.4 can best be seen what is meant b y a linear power system. This picture shows th e power output for different win d speeds (calculated with simulation, chapter ). With a linear model it is assumed that the power for win d speed /s is linear. After m/s it is assumed to be constant.

Figure 2.4: Power oufput for different w ind speeds. Originated from simulation Chapter 6, easured outpu t or diflerent w ind speeds and constant load.

For such a linear turbine (simplified case) the power coefficient can be calculated with:

th e cut in wind speed of the turbine. With With C,,max 0.47 and a cut in wind speed of

/s Cp as a function of the win d speed

Looks like:

5

10 wind

15

speed [ m l s ]

Figure 2.5: p as a function

f the win d speed.


Motors!

Wind T u r b i n e

Studv

Generators

The gen erator of the wind turbin e converts mechanical energy to electrical energy. The shaft that is d irectly coupled with the turbine rotor delivers the mechanical energy. Wind turbines can be b uild with synchronous or with asynchronous generators and can differ in way of elec trical grid connection. 53.1

synchronousgenerator

3-phase syneh ronoii i-noior uses rotating m a g ~ e t i cield. In the figure below there are three magnets, which are each bounded with there own phase of the three phase electrical grid. The rotor wi ll follow the magnetic field generated b y the stator exactly. That is why th is generator is called synchronous. The rotor makes one revo lution p er cycle. When the rotor is attached to a 50 Hz electrical grid the rotor w ill make 50x60 =3000 rounds per minute (rpm). When the 2-pole permanent magnet synchronous motor from figure 3.1 is enlarged to a 4-pole motor the rotor ill make a ha lf revolution per cycles. The result is that the motor speed is decreased to 1/ 2x 50 ~6 0= 15 00 prn.

figure 3.

synchronousmachine

In the next table there i s a view of m otor speeds at different number of po les and by different numbers o f frequencies of the electrical grid. Table3.1:rprn generator, dependentof requency and number ofpo les

The reason why the motor from figure s called a 2-pole motor is that it has one North and one South Pole. It looks like there are three poles, bu t the compass needle feels the pu ll from th e sum of the magnetic field around its own magnetic field. if he magne at the top is a strong South Pole, the two magnets at the bottom w ill ad up to a strong No rth Pole It is also p ossible to change the permanent magnet with an electromagnet which maintains its ma gnetism through a coil, which is fed with direct current. Th


Wind

Turbine Stud

electromagnet powered by using br ush es and slip rings on th e shaft of th e rotor. This preferred bec ause permanent m agnets will lose their force in a stron g magnetic field. Another reason tha t the permanent m agnet motors are expensive in buying. When the rotor rotated from the ou tsid e and not by th e electrical field gen era ted by th e sta tor, th e sync hronous motor w l work like a synchro nous generator. Via th e rotor and stator there sen d back a current into th e electrical grid. When th e rotor rotating with a con stant sp eed , the synchronous generator produces a voltage with a co nstan frequency. The more force is applied to th e gen erator th e m ore electricit generated, generator w6LI still rotated with the speed which defined by the electrical grid br?t frequency. 33.2 The asynchronousgenerator

r generati ng an alternating current an asynchrono us gene rator mostly used . The choice to u se an asynchron ous generator for wind turbine application t h a t t h e y a r e really reliable an d comparatively inexpensive. Other advanta ges of the asy nch rono us gen erato r are th e slip and a certain overload capacity. The difference between th e synchronous and asy nchrono us generator th e rotor. The rotor con sis ts of a number of aluminium or copper ba rs which are electrically con ne cte with th e electrical grid by aluminium end rings.

Figure 3.2: rotor o f he asynchronous machine

F i p u e 3.3: stator of he asynchronous machine

figure 3. you can s e the ou tside of the async hronous gen erator. It also co nsi sts like a syn chro nou s generator of a number of poles th at can produce a m agnetic field for th rotor. When t he cu rrent fed to the stato r the g enerator wi l act like a m otor and turns with a spe ed lower than th e synchronous speed. In

Figure 3.4: magnetic field in the rotor

In picture 3.4 the rotor shown with t he magnetic field coming f om th e stato r, which aro use s a current in the rotor bars. These ba rs will offer very little resistance , since the

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technische univ ersiteit eindhoven

Wind

Turbine Studv

ar e dragged alon g by the electromagn et force from the rotating m agnetic field in th stator. When th e rotor rotates with a speed e qual to the speed f th e asynchronous generator, forced by th e frequency from t he electrical grid and th e num ber of po les, th e gen erato will not have power outp ut. Thi because there no induction between th e rotor and stator. r gen eratin g a current the rotor w ll have to rotate at a higher level s the magne tic field. The hardery ou crank the rotor, the more power w l be transferred s an electromagn etic force to th e stator, and turn converted to electricity which fed into e'iectrica! grid. LL

So the s pee d of th e asynchron ous generator varies with th e torsion acting on the incoming shaft. The difference between the sp eed n peak power and on ideal pow er is in order of a number of percents. The difference percents of th e synchronous spe ed is called th e gene rators slip. 4-pole generator wi l rotate idea l with 15 0 rpm whe attached to the 5 Hz electrical grid. When th e generator prod uces its peak pow er, th gene rator rot ates abo ut 16 05 rpm. This wi l follow from figure 3.5 and equation (3.1).

Figure 3,5: lectromagnetic torqu as function of he speed

(eq. 3.1) The slip a function of the direct current resistan ce rotor windings of th e generator. n in cre ase in r esis tan ce will induce an increase in slip. By enlarging th e resis tan ce, th slip may increase to 0 percent A very useful mechanical p roperty of th e asyn chronou s generator th at it will not dec rea se of incr ease its spee d much when th e torsion varies. This me ans ther e is less wear an d te ar to th e gearbox, caused y lower peak forces. This one of the m ain reason s why people ch oose asynchronous generator rather than a syn chronous one. Another advantage th at th e rotor adap ts by itself th e number f po les the stator This why th e rotor can b e use d for a large variation o niimber poles. $3.3 Choice for ki nd o fg ri d connection

There are two po ssible fo rms for connection to the electrical grid: direct and indirect gri connection of th e genera tor. Direct connection means tha t th e gene rator will directly connected w ith a 3-p has e akerna ting current grid. By indirect grid connection t he current

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Wind Turbine Studv

passes troug h a num ber of electrical components in which the current is matched t o the grid. In case of an asynchronous generator th is i s done automatically. In the p ast wind turbines ran at almost constant speed with direct grid connection, bu nowadays m ore and m ore wind turbines run at variable speed. With variable speed it no t possible t o connect direct to the electricity grid. These wind turbines act on th eir own separated grids. An inverter controls this grid so the frequency of the a lternatin g current can be varied. In this way its possible to rotate the turbine with v arying speeds. The generator generates an alternating current with the same frequency, which is a pplied to stator. Altern ating current ( ) with a varying frequency can not be connected directly t o th grid. That i s why th e altern ating current is converged into a direct current (DC). For th conve rsion o f C to C you can use a thyristor or large power transistors. The fluc tua ting C wi ll be converged to an AC wit h th e same frequency as th e grid. This can be do ne wi th an inverter. The inverter has a low efficiency because there is not a smooth sinu s c om ing out of it. By filtering the signal you can make the sinus smooth, b ut it will not happen beautiful. 53.3.1Advantage and disadvantagesof hdirect grid

connectio

The main advantage of indirect grid connection i s that you can rotate the wind tu rbin with vary ing speeds. In case of a gust in wind th e rotor can turn faster, thus sto ring pa rt of th e excess energy as rotational energy un til the gust is over. r this case the system needs an inte lligent controller, since there m ust be made a difference between gusts and higher wind speeds. By storing the energy of a gust it is possible to reduce the peak torque, and so redu cing the wear and tear on the gearbox and generator. The second advantage is that th e electronics can contro l the reactive power. ith the reactive po wer is mended the phase sh ifting of current relative to voltage in the C grid. These sh ould be equa l to the power quality in the electrical grid. This may be useful particularly if a turbine is running on a weak electrical grid. The last advantage is that i t is possible to le t the generator work in its op timal point. The m ost basic disadvantage of indirect grid connection is the costs. As me ntioned before there sho uld be used a couple of electrical components to m atch the current from the generator to the grid. The costs for these components are nowadays hig h and n ot attractive. A nother disadvantage are th e ene rgy losses in de AC DC AC conversion. Also the power electronics may introduce harmonic d istortion of the alternating current in the electrical grid, thus reducing power quality. The problem of this harmonic d istortion arises because the filtering process mentioned above is not perfect

It's pos sible to e quip your wind turbine w ith two generators: A small generator for periods wit h low wind speeds and a large generator for periods with h igh wind speeds. The disadvantage of this con struction with two generators is the costs. A more common design o n n ewer machines is pole-changing generators. Depending on h ow the stator magnets are connected you may run the generator with a different number of poles, a nd thus w ith a different rotational speed.

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Wind Turbine Studv

Before you cho ose to u se two gen erators or a higher number of poles for low wind s pe ed you shou ld look wha t th e profit on this will be If the costs for such an application are higher then t e incomes, it's not recommend to build such a system. The energ y content for ow wind sp ee ds very small and s o thus t he electricity. n adv antag e to u se a twogenerator system tha t you can run your turbine at lower sp ee ds and thi s m ean s less noise from th e rotor blades.

3.5 Starting and stopping the generato When th generator is fe with current the generator acts Like motor. t low sp ee th e generator ha s no output. This not efficient bec aus e you wan current se nt back into the electrical grid. O nce the wind becom es powerful enoug h to turn th e rotor an generator at their rated speed its important that the generator becomes connected to t he electrical grid at th e right moment. When th e moment wrong there wi l b e mech anical resistan ce in the ge arbox and generator. The cons eque nce of this that t he rotor cannot acc ele rate or it will over spe ed . To prevent this the re ar e two safety devices: Aerodynamic braking system: tip brakes

The primary braking system for most modern wind turbin es th e aerodynamic braking system , which essentially consists turning the rotor blades about 9 0 deg rees along their longitudinal a xis ( n th e ca se of a pitch controlled turb ine or an active stall controlled turbin e or in turning th e rotor blade tips 9 0 deg ree s ( n th e ca se of a stall controlled turbine). These system s are usually spring operated, order to work even in ca se of electrical power failure, and the y are automatically activated if th e hydraulic system in th e turbine los es pres sure. The hydraulic system n th e turbine used turn th e blades or blade tips back in place on ce the dang erous situation over. The normal way of st op pin g a mo dern turbine (for any reason) therefore to u se the aerodynamic braking system Mechanicalbraking system

The mechanical brake used as a backup system for th e aerodynamic braking syste m, and s a parking brake, once th e turbine stopp ed in the ca se of a stall-controlled turbine. Pitch controlled turbines rarely need to activate the mechanical brake (except for mai ntena nce work), s the rotor cannot move very much once the rotor blad es a re pitched 90 degree s. o protect for a brownout it's not po ssible to use a normal switch for switching th t ~ r b i n e th e electrical grid. It's necessary to use a soft starting device with a thyristor. This thyristor gradually connec ts and disconn ects the ge nerator with th e grid. The loss es of these are per cent of the energy running through them . By using a by pas switch, which activated after th e turbine ha s been soft started, it is possible to minimize th e amo unt of energy wasted. y variat ions of th e wind sp ee d the re also a variation th e voltage applied to t he grid. This called flicker. When t he wind turbine connected to a weak grid you wi

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Wind Turbine

Studv

notice thi s flicker. There are various ways o f dealing with this issue in the des ign of the win d turbin e. This can be done mechanically, electrically and by us ing powe r electronics. In case tha t the wind turbine becomes disconnected from the grid, it's impo rtant that after the re-connection the current m atches with the grid. Otherwise it m ay cause hug current surges in the grid and the win d turbine generator. Like spoken before this w il cause a hug e blow of energy in the mechanical drive train (shafts, gearbox an d rotor). It's th e task for the electronic controller to m onitor constantly the voltage and frequency of the alterna ting current. In case the voltage or frequency of the local grid drifts outside certain h i t s , he tzrblne will automatica!ly disconnect from the grid and s top it se lf immec!Izte!y. $3.6 Running pitch controlled turbine

variable speed

As m entioned before it's possible to use pitch to slow down the speed of the turbine. The pu rpose of this is to control the torque and not to overload the gearbox and the generator. The advantage of variable speed is that you can rotate the gen erator on h al its s lip at his rated power. In case there is a gust in win d the speed o f the generator can be increased. In the m ean time it's possible to the pitch mechanism to pitch th e blades more out of the wind. Once the p itch mechanism has done its work, th e slip is decreased again. In case the win d suddenly drops, the process is applied in reverse order The m ain advantage o f this con trol strategy is that the fluctuations in power ou tput are liquida ted by varying the generators slip and s toring or releasing part of the energy as rotation al energy in the wind turbine rotor. A disadvantage is that if you run the generator with a high slip i t will produce more heat and thus runs less efficiently. $3.7 Coolhgsystem

During the o peration of producing electricity the generators need to be cooled. It's pos sible to use a large fan for air-cooling, b ut also water-cooled generators are used. The main advantage for using a water-cooled system is that it can be bu ilt more comp actly. This gives some electrical efficiency advantages. The disadv antage o f th is system is tha t they need a radiator to get rid of the heat. This radiator should be p laced in the nacelle. 3.8

Gearbox

When the w ind activates the rotor of the wind turbine, it wil l also rotate the shaft in direction o f the generator. The following pa rts of the power train wi ll follow the rotation of the rotor: the slow-speed shaft, the gearbox and the high-speed shaft To transport th rotational energy he th rotor to the generator ca a gearbox. The generator i s fed by the e lectrical grid with 50 Hz and makes 1500 rpm if here has been chosen a 4-pole generator. The turbine rotates much less and so the speed s hou ld be increased. This can be done with th e gearbox. The gear ratio is typica lly approximately 50. While usin g a gearbox you convert slow speed to high speed and high torque to low torque. In these situations the wind turbines does not change gears.

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W i n d T u r b in e S t u d

The 4.1

c o n t i n u o u s v a r i a b le t r a n s m i s s io n

e dry belt variator

By us ing a contin uou s variable transm ission it is possible to transform a win speed varying turbine speed i n an almost constant generator speed. This wi ll resu lt in a higher efficiency o f the generator. The ratio o f the C T wil l compensate the fluctuations in the wind. By usin g a dry b eit variator th e necessary pillley presslire is ieduced. This useful, because it is now possibie to actuate the piilley mechaiiicaUy. Th primary pu!ley driven by a sp indle m echanism i n combination with an electromotor. The force on th secondary pulle y is delivered by a spring. The efficiency of the T in a low ra tio i s highe than per cent when the inc oming torque is higher than 20 Nm. Modeling

For th n?de!Ing

he CVT

th CVT the next controller system can be used:

rItj

WI

r e f. r a t in r a te

ixntm ller

HI mo del

real ratio rate

Figure 4.1:controllersystem or the CM

The feedb ack c ontro ller c ons ists of a con troller C( ) a nd a mod el H s) o f th e C . In th mo del r(t) and y(t) are respectively the wanted ratio rate and the true ratio rate o f the . The tran sfer func tion of th describes the dynamic response of the ratio on a chan ging inco min g torque and voltage of the electro m otor. The transfer function for the CS PTO system de velope d at th e TU/e is by e quation 4.1: (eq. 4.1) The C T has a m aximu m com mute speed of -0.3 0. 11s. This commu te speed is imp lem ente d by u sin g a rate limiter. For th e C T the low and hig h ratio (0.505 respectively 2.17) are implemented by u sing a saturation. The total ratio range il l be 4.415. The M atlab / Simulink model is shown below: 14 &3~.2*17

Gain

Transfer Fcn

S a t u r a t ~ a n 5D5 2.171 R a t e l i r n ~ t t e r .D.3:!l.3]

n v e r h r e q v e r ha u d in g

Figure 3.2 Ma lab/Simulink model of the CM

In this thesis the simulation w ill not be calculated with th e dynamic response of the C This because the calculation o f the dynamic response ill cost a lo t of time and is no intere sting for this case. The value of H(s) and the gain i n this case il l be chosen 1. So the desired ratio times the rate limiter and the saturation is equ al to th e true ratio.

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Mathematical model

Wind Turb ine S t u d y

the turbine

The power in th e wind tha t is consum ed by th e turbine

equal to:

0 . 5 ~ ~ ~ 7 i ~ '

[see 32.4)

r a simplified situation under non -stead y st te conditions the simplified dynamic model for th e turbine connected to an over gear, CVT and a asynchronous generator can be derived:

rotor

Generator ilrn

It,(%

agen

Ig<
5.1)

(eq. 5.2)

Herein It

the inertia of the turbine rotor, wt

the rotor speed P,,,

th e electr

mech anical power of th e generator dep ende nt on its rotor spe ed (sli S). Furthermore qt, qcvt are t he efficiencies of th e over gear an d th e C T and t e me chan ical efficiency of th e ge nerato r. By integrating equatio 5.3 the turbine rotor speed can be calculated. The difference between the power tha consum ed from th e wind and t electro mechan ical power generated n the ge nerator used s kinetic energy for th turbine rotor. When th es e two powers are equa l the rotor spee d wi l be c onstan t. The inertia of th e turbine axel, the over gear, the CVT and the gene rator are neglected n this simplified situation . This possible beca use they are very small compared to th e rotor tur bin e inertia. Thinking f a turbine which co ns ist s of a over gear, a C T an d a generator th e Torque and the speed on going axel of th e C T can b e c alc ula ted by:

With th e ge ar ratio of the over gear. The use of this gea necessary to drive up the speed of the CVT to bring down its torque. The ingoing torque Tgen,,nd speed a,,,,, of th e gen era tor can then be calculated by respectively dividing and multiplying TCvt,,


o,,,,,

Wind Turbine Study

with the time varying gear ratio of th CVT. Furthermore the power has

multiplied by the efficiency of the CVT. [eq. 5.5)

be

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Wind T u r b i n e S t u d v

Simulation 6.1 Simulation purpose

The p urpose of the simulation is to see whether a C T in combination with a con troller can be used to let the rotor speed follow th e wind speed profile and thus kee ping he t ip speed ratio constant and op timising the maximum power coefficient (see eq. 2.13). o test the T to its lim its a turbulen t win d field and an extreme wind gust wi ll b e used as inpu ts. The C T ill thus take over the ro le of the inverter, bu t in compa rison to an inverter it ii i be a simpie and cheaper solution. In this simiilaiion th system w i t t i CVT speed c s i i t r o l iii b e compared to a constant speed system. This make it clear wh is a m ust. For a constant speed system the electric generator m ust either rotate at cons tant speed (synchronous generator) or at slightly varying speed (asynchronous generator). The CV on the other hand will co ntrol the turbine rotor speed to follow th win d speed exactly and keep the generator speed within its sma ll allowable range (asynchronous generator). Thus for a turbule nt win d field the efficiency o f a C T equipped system wi ll be com pared with an con ventiona l system at constant speed. Furthermore for a turbulen t wind field the efficiency of a CVT equipped system w ill be compared w ith a conventional constant speed system.

Simulation mo delling For the mod elling of the wind turbine the programme Ma tlab/ Simulink has been used. o get a win d turbine w ith an average outpu t of 1 0 KW a rotor radius of 5 m i s used. With an average win d speed of 1 0 /s lOKW of power is produced. For the calcu lation of the power coefficient a drag to lift ratio (D/L) of 0.01 is assu med (see eq. 2.13). Fu rthe rmo re a turbine rotor design with three blades and an approximated inertia of 1 00 0 kgm 2 has been used i n the simulation. For the Sirnulink models that w here used see the appendix. o generate th e set poin t for the CVT a very simple controller has been used. I t is no within the purpo se of this scope to develop a reliable and efficient controller. The set p oin t controller that has been used only looks at the difference b etween the turbin e roto r speed and the ideal rotor speed for optima l tip speed ratio that i depe nden t of the wind speed. This difference is then fed in to relay, which only loo ks at the sign of thi s difference. The output of the relay is high speed i f the difference is negative and low speed if it is positive. The p oint at which the relay changes outp ut so rt of ope rates as the c ontroller gain. The generator thus generates either a low or a h igh mechanical torque, which allows the turbine rotor speed to increase or decrease. The set po int co ntroller can be seen in the appendix.

First the dynamic response o f the turbine w ith CV Twill be investigated. As ing oin g win profile a wi nd gust that stays constant at peak speed will be used. The wind speed changes rap idly from 0 m/s to 17 m /s in approximately 2.5 seconds [see fig 6.11. This can be compa red to a step change in win d speed. As can be seen from figure 6.2 the rotor speed changes from to 13.6 radfs in 4.2 seconds this is due to the large inertia of the tu rbine rotor. In figure 6.3 can be seen tha t the C T changes its ratio slow ly in 4.

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tech nixh e universiteit eindhoven

Wind Turbine Study

seconds from 0.95 0.56. Thus even in this very extreme wind profile the CVT manages to control the rotor speed al l due to the large inertia of the turbine rotor. The dynamics tha t can be seen at constant r atio are due to the lack of a good controller. Furthermore the maximum torqu e and speed of the CVT ar 60 Nm and 62 rad/s this is within the allowa ble range of a dry belt CVT (max Nm and 838 rad/s).

Figure 6.1: Windgust which will be consfan at peak speed.

time [s

Figure 6.2: Rotor speed after a gust in wind speed.

Figore 6.3:

Mr at io after a gust in wind speed

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Wind T u r b i n e

Study

Now the behaviour o f the system with CVT ill be investigated with a turbulent win profile. The pr ofile used can be seen in figure 6. and is generated with equation 1 . 5 . As can be seen in figure 6. the rotor speed follows the win d speed very closely even wi th th e use of a very simple set point controller. That the m achine runs at optimum efficiency can be seen in figure 6.6. The tip speed ratio is alm ost constant a t its optimum value. Deviations of this value are caused b y the absence of a good controiler. As can be seen i n figure 6.7 the generator speed stays within its allowable asynchronou range so th e e lectric output of the generator has a con stant frequency. The C V i s thus usable as a replacemen t for an inverter. The ma ximum torqu e and speed on the CVT 60 Nm and rad/s, this allowable for a dry be lt CV

time [s

figure 6.4: Turbulentwindprofile

time

figure 6.5: Turbinerotor speed due to turbulent win profile,


Wind T u r b i n e

Study

time [s]

Figure 6.7: Generatorspeed.

Finally th e re spo nse f th e CW equipped system c o m pa re d t o a c o n s t a n t s p e e d syst em with th e sa e turbule nt wind profile. In figure 6.8 th e turbine rotor spee d of both s y s t e m s shown. Furthermore figure 6.9 sho ws th e difference in tip speed ratio, a s can be se en t he tip spe ed rati much more co nst an t for th e system with C . The system w ith CVTruns more efficient then t he con stant sp eed system.

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techn ische universiteit eindhoven

Wind T u r b in e S t u d

time

Figure 6.8: Turbinerotorspeed wi th and without C1/7:

time

Figure 6.9: np speed ratio for the CVTsystemand constant speed system.

The power delivered to th e generator of the sys tem with T and t e const ant s pe ed system a l m o s t t h e s a m e ( ab o u t 1.5% mo re for th e sy ste m with . This b e c a u s e t h e c o n s t a n t s p e e d s y st e m operated a t its ideal design wind sp ee d o ts tip spe ed rati then optimal around 4. Bu if we now u se a turbu lent wind profile wit ano ther m ean wind s pe ed you will se e that t he power output for th e C T system is th e sa me , th e tip s pe ed ratio of th e C T system will stay at its optimum 4, b u t t h e ~ t . i i i d pee d ali t h u s constan t spee d syste ms tip speed ratio wiii drop or rise w i t h influence t e pow er outp ut. We will now u se a different turbul ent wind profile with different mean wind sp eed ; lets choose it to be m/s [fig 6.101.

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W i nd T u r b i n e S t u d y

Figure 6.10: Turbulenf wind profile with mean wind speed of 1 m/s.

Now the tip speed ratio of b oth systems is compared (fig 6.11). The tip speed ratio for the constant speed system drops to aroun 3.2. Thus the power outpu t also significan tly drops. In figure 6.12 th e average power outp ut for both systems is compared. The power of the variable speed system is about 42 higher than th e constant speed system. This outlines th e reason why variable speed control is a must.

ZD

40

60

60

100

120

[s

Figure 6.12: Time averagepower oufput fo r fhe CVTsystem and the constant speedsysfem.

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Wind Turbine Studv

Conversation at Windw all

One of th e go als of this in ternsh ip is to exam ine the inte rest of th e wind turb ine ma nuf act ure rs for th e C . The biggest problem for th e C still th e low tor qu e it can handle on th e incoming shaft, namel 74 m. After a co upl e of calcu lations a v alu e fo th e maximum power of a wind turbine ha s been found, namely 30 kW. However, mos t o th e wind turbine m anufacturers built wind turb ine s over 0 . After a sear ch on th Internet there has been found a company who produces a new design of wind turbines. They built a H -type Darrieus turbine for placing inside th e build surroun ding on s lop ing an d fla t ro of s. Even piac ing th e turbine in verticai position possible. This design also convenient for turbuie nt wind conditions around buildings. The power of this H-type Darrieus . Big adv ant age s of this design a re the ow noise b ec aus e it rotating at low spee d a nd th e small radius of the D arrieus rotor.

Figure 7.1:H-type Darrieus wind turbine, used for the Windwall

After a visit to and conversation with on e of th e peo ple at Windwall the re can be ad som e conclusions: The C T has a gr eat adv antag e with re spec t to the gen erator c osts In the current application the generator is oversized, be caus e th e H-type Darrieus rotor sp ee ow and so the torqu ow. To get so e power out of th e gen era tor it chose n to be bigger than neede d. The C T will also give adv ant age s with re spe ct to th e co sts f th e inverter. If you use a CVT to hold the speed of the generator on a certain speed Level, it pos sible for th e C T to tak es over so e prope rties of the inverter. The inverter can b e replaced y a soft starter, which need ed to protect th e grid for a brownout. The bigge st problem for the m anufacturer of Windwall wa s the use f a fixed gea n th e design. development in the world of wind turbines is direct drive, in this application no ge ars are used and thus no wear and tear of transmissio ns occu rs. This with respe ct to th e gre at variations in torque that occur a wind turbine. Windwall also doesn't want to use fixed gea rs be caus e of wear a nd tea r. Without fixed g ea rs th e torque on t he incoming shaf t of th e C too high. o fix thi s probiem ther e shouid be investigated which other possibilities t he re are for transmission application between turbine and CVT. e impleme nted n the d esign of the Windwall ther e should first be taken ca re of the following points: hat will be th e lifecycle of th e C n which time intervals should m ainten ance be c onducted to th e C What th e profi in efficiency of the CVT in comparison with the inverter?

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Wind

Turbine Studv

Conclusion

The goal of th is internship is to investigate the possibility of u sing a CVT in a wind turbine application. After investigating the incoming side, the profile of the wind, and the out g oing side of th e wind turbine, the generator with its properties, it was possible to make a simulation. Because thi s is a first step in this particular research for th e use of a CVT in wind turb ine application, there are better results possible for the simula tion, such as a better set po int control for the CVT. After the simulations it was possible to iook at the idis) advantages of the CV application. One great advantage is the rem ovai of a great part of the inverter and th pro fits of a more efficient power output. With the use of CVT the tip speed ratio w ill be kep t constan t at varying speeds. A disadvantage i s the need of fixed gears, wh ich are nowadays m ore and m ore replaced by direct drive systems After a conversation with Windwall, a manufacturer of H-type Darrieus win d turbines, the general manager was interested, but he doesn't want to use fixed gears because of the wear an tear. Recommendations

When the CVT is used for wind turbine design i t is recommended to develop an easy and quick maintenance procedure. Further more, it's easier to b ui lt a complete CVT unit with bu ilt in gearbox, which generates low noise and little wear and tear. For better controlling th CVT and get b etter dynamic response it is recommended to design better controller.

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technische universiteit eindhove

Wind Turbine

Study

References

Mangialardi and G. Mantriota, Dynamic behaviour of wind pow er sys te equip ped with automatically regulated continuou sly variable transmis sion, Bari (1 5) 2. Daniel de C loe, The Constant Sp eed Power Take Off (CS-PTO), Eindhoven 3. Robert Gasch, Windkraftanlagen, Stuttg art (1996) Siegfried H eier, Windkraftan lagen im N etzbetrieb, Stu ttga rt (1996 5. David A. Spe ra, Fundamental conc epts of wind turbine en gineering (1982) E.H. Lysen, Introdu ction to wind energ 7. Eggleston and Stotta rd, Windturbine engineering design 1. L.

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Wind Turbine Stud

Simulinlc mod el o f the wind turbine equipped w ith CVT

omega

)omega

omega to cvt

mtor

h kappel ta turbine ma Eectm mech. knppel &etm mech. kappel Ulrlndspeed aver gear Rntar mindturbine

amq a cvt in

omega ovt nut

Bectm rneoh. koppel windspeed Setpoint

E&m mech. knppel desired speed cv oul

omega mtor raft

Setpoint Regelaar

omega cv nu

omega t n generato

Eectrn mech. koppel

Eeetm Mech kappel

---+ omega generatnr

&ch. koppel

Generator

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Wind Turbine Study

Rotor wind turbin

uindspesd

speed

Pradud

Pawerfrom rotor s p e e d

Windspeed

M e c h torque

Power from win

omega ruindspesd mnd weed

determmatlon

Cp determination

xu Fcnl

um3"0.B7 Fen2

Constant

PrnduEt

omega

cvt wind

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