308666.5_very good tech dfig ig etc

7/29/2019 308666.5_very Good Tech DFIG IG Etc

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!Drago BanDamir arkoFaculty of Electrical Engineering and Computing, Zagreb, Croatia

Miroslav MaeriKonar- KET d.d, Zagreb, Croatia

Zvonko uligMarijan PetriniInstitute for Electrical Engineering Inc., Zagreb, Croatia

Branko TomiiJosip tudir

Konar-GIM d.d, Zagreb, Croatia

GENERATOR TECHNOLOGY FOR WIND TURBINES, TRENDS IN APPLICATION

AND PRODUCTION IN CROATIA

ABSTRACT

In the paper an overview of achievements known in the world in the area of construction andapplication of various electric generator systems for wind turbines is given. Using the available literature andother technical data a comparison is made between the following commonly used types of generator:

induction, synchronous with classical excitation, synchronous permanent-magnet and doubly-fed inductiongenerator. A basic concept and some selected details from the design and construction of the first windgenerator developed and built in Croatia are given. The generator is three-phase, low speed, 60 poles,designated for direct-drive, power rating 1000 kVA. At rotating speeds between 10 rpm and 30 rpm, thegenerator voltage and frequency change in the range of 250-690 V and 5-14,5 Hz. Based on experienceacquired by carrying out the first project involving construction of the wind turbine and generator system, theother types of generator are also considered and the optimal solutions are sought for future constructions.Some theoretical and practical significance is given to the possibility and justification for construction of adirect-drive permanent magnet generator, which is estimated to be the most promising technical solution.

Keywords: Wind power plant, synchronous generator, doubly-fed induction generator, permanent-magnetgenerator, optimization.

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60, direct-drive 1000 kVA. 10 - 30 r/min, 250-690 V, 5 - 14,5 Hz. . , .

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1 INTRODUCTION

The development, construction and exploitation of wind turbines on the global scale have achievedan unexpected growth in the last decade. The total capacity for production of electric energy from windinstalled worldwide by the end of 2006 reached 74223 MW, while in 2006 alone the capacity was expanded

by 15197 MW. The expected increase in construction of new capacities is around 32 % annually [15].Although in 2005 only 1 % of the total world production of electric energy was generated by wind turbines[20], that sector becomes an important segment in the world energy market.

By the end of 2006 the total power rating of wind turbines installed in Europe was 48000 MW, andthe annual growth was around 19 % [16]. The number of active manufacturers of wind turbines in the worldalready exceeds 30, and the leaders are European manufacturers. The leading manufacturers of wind turbinesare Germany, Spain, USA, India and Denmark. Those countries also have the highest number ofcommercially exploited wind turbines. In 13 countries with the highest number of installed wind turbineunits the total power rating exceeds 1000 MW [20].

There is currently a large conjuncture on the market and a shortage of production capacities. A greatnumber of specialists around the world are conducting scientific research. The possibilities of more efficientutilization of wind potential on acceptable locations with reduced environmental impact are being explored.

The larger and more efficient turbines and generators are being built. The new and improved systems forguidance and control and for connecting the wind turbine with the power grid are also developed. The resultof progress made in development and construction of wind turbines is evident from the fact that in 1980 the

power rating of a single unit was 30 kW, while in 2006 it reached 5000 kW. The course of developmentthrough increase of power rating and generated electrical energy per unit in the last 25 years is illustrated inFig. 1. In 25 years the energy production from wind turbines has increased around 500 times [1, 20].

According to trends in Europe and the obligations of European countries to produce a part of electricenergy from renewable sources, Croatia has also involved in utilization of its wind potential by purchasingwind turbines on the world market and installing them on the following locations: island of Pag, 7 units,850 kW each, and near ibenik, 14 units, 800 kW each. According to a study made by Institute Hrvoje Poarand ordered by Croatian utility company (HEP), the estimated wind capacity in Croatia is around 400 MW.The development of the first Croatian prototype of a wind turbine is conducted by a group of Croatiancompanies led by KONAR Electrical Industry Inc. The aim of this paper is to give a brief overview of thestate-of-the-art and conditions on the world market and to present the basic features of the development of adomestic prototype. As a reminder some basic equations and terminology from the area of wind powerengineering are given in the paper.

1.1 Basic equations related to wind power

The energy and power of wind can be illustrated using the familiar expression for kinetic energy ofmoving air

( )2 21 1

2 2E mv P S v v= = (1)


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Year 1980. 1985. 1990. 1995. 2000. 2005.

Rated power [kW] 30 80 250 600 1.500 5.000Rotor diameter [ m] 15 20 30 46 70 115

Hub height [m] 30 40 50 78 100 120

Annual production[kWh]

35.000 95.000 400.000 1.250.000 3.500.000 17.000.000

Fig. 1 Growth of power and size of wind turbines in the period between 1980 and 2005where E is the energy, P is the power, m (kg/m3) is the mass of moving air, (kg/m3) is the density of air,v (m/s) is the wind speed and S (m2) is the effective area circumscribed by the turbine blades. The windturbine can utilize only a part of the total wind energy which is taken into account by power coefficient Cpwhich is a function of aerodynamic properties of the turbine and the orientation of the turbine with respect tothe wind direction. The theoretical maximum for Cp is 0,593 (Betzs law). Taking into consideration (1) the

power captured by the wind turbine can be written as

31

2 pP C S v= (2)

The wind power is proportional to its speed raised to the power of three according to which theturbine should be controlled. In an ideally designed turbine, according to the theory by German scientist

Betz, only 59,3 % of the wind energy can be captured [5]. In real life the power coefficient is achieved in therange 0,25 - 0,45. For the optimal Cp the speed of rotation of a turbine should be adjusted according toconstantly changing wind speed. In technical considerations Cp is calculated for a concrete turbine as afunction of coefficient , which is the ratio of tip speed [m/s] over wind speed [m/s], and the blade pitchangle which depends on wind direction. The expression for the power of wind turbine can now be writtenin the form

( ) ( )3 2 31 1

, ,2 2p p

P C S v C r v = = (3)

For turbines whose speed is not controlled the coefficient is constant, and for uncontrolled it varies.Practically all installed wind turbines are based on three basic types:

1. fixed-speed,2. semi-variable speed,

3. regulated variable-speed.

The principle characteristic of a wind turbine showing the dependence of power on wind speed isshown in Fig. 2. The generator and the power control system with varying wind speed are designedaccording to that characteristic. The rated power of the turbine is achieved in the wind speed range from vratedto vmax. For wind speeds from cut-in speed vc to rated speed vrated the power of the unit is optimized byadjusting the aerodynamic profile of the blades in order to maximize the power output. From rated speedvrated to cut-out speed vmax the turbine must be controlled to limit the mechanical and electrical power outputto their rated values. Various techniques of optimization and power limitation are applied (stall control,

active stall control, pitch control). The details are given in [5], [13].


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Fig. 2 Typical static power characteristic of a wind turbine with active stall control

2 GENERATOR TYPES FOR WIND TURBINES

Since rotational speeds of wind turbines are naturally low, approximately in the range 5 to 30 rpm,the selection of the type of generator, regardless of the applied type of control, depends on the type of

connection to the power grid and whether or not a gearbox is used. In modern constructions induction andsynchronous AC generators are used in several variants defined later in the paper. DC generators are used for

power levels of several kW.

For fixed-speed wind turbines squirrel cage or wound rotor induction generators are commonly usedwhich are directly connected to the power grid (without a power converter). A gearbox is necessary whosegear ratio is high enough so that a 4-pole or a 6-pole induction generator can be used. The inductiongenerator with high number of poles (2p>20) is unacceptable for practical applications due to low powerfactor cos (0,6 or less without compensation) and consequently poor power conversion capability. Forturbines with variable or semi-variable speed there is a variety of applied power conversion systems. Thedevelopment of these systems involves detailed technical and economic studies dealing with selection of thegearbox, electric generator, power converter and connection to the power grid. Further in the paper

descriptions of various drive-trains, significant for practical implementation in newly built wind turbines, aregiven.

2.1 Fixed-speed wind turbine with squirrel cage induction generator

Between the rotor of a turbine and the induction generator a gearbox is located (usually three-stagewith high gear ratio up to 1:100) which is selected so that a 4-pole or a 6-pole three-phase squirrel cageinduction generator can be used. Such a generator has a simple construction and can be picked from acatalogue of standard induction machines. It is connected directly to the 50 Hz or 60 Hz power grid via soft-start device which reduces the starting current. The load of the generator is limited by aerodynamicconstruction of the turbine blades using the stall principle, and the speed of the series turbine+ gearbox+generator varies very little around the rated speed of the generator. The slip of the generator is around 1-2 %

so that losses in the rotor are tolerable. The wind turbine cannot operate without connection to the power gridfrom which it draws reactive power for magnetization. For low power and island operation capacitors can beused for excitation of the generator. For compensation of reactive power the capacitors are used. Very often asquirrel cage induction motor with selectable number of poles, usually for two different speeds as shown inFig. 3, is used. The company Mitsubishi uses such technology in their units rated 1000 kW [17].

The turbine is constructed for two fixed speeds which are optimized to obtain maximum power fromthe wind and to reduce noise in changing conditions. There are constructions using wound-rotor inductionmotor in which rotor resistance is changed electronically to regulate the speed and reduce mechanical stresson the power generating unit.


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Fig. 3 Fixed-speed wind turbine with induction generator

2.2 Variable-speed turbine with synchronous or doubly-fed induction generator

Classical synchronous generators constructed for fixed speed set by a primary mover have a DCcurrent excitation on the rotor. In general, such units (generator + primary mover) are main producers of

electric energy in power plants, for 50 or 60 Hz systems. If it is required to regulate the speed of the turbineand the generator to improve the overall efficiency or for some other reasons, and at the same time thegenerator should be connected to the power system with fixed frequency, it can be done in two ways:

1. The power converter located between the armature winding of the generator and the power gridof fixed frequency can be used, as illustrated in Fig. 4. The excitation current in the rotor windingwith voltage regulation system or permanent magnets are required.

2. The AC current in the rotor winding of a doubly-fed induction generator can be used which isgenerated by a cycloconverter or a two-way static power converter connected to the same powergrid as the armature winding (see fig. 5). The stator winding of the generator is connecteddirectly to the 50 Hz power grid. The rotor winding is connected via frequency converter and theaccompanying power transformer. Due to great significance of this type of power generating unit

for wind turbines and hydroelectric power plants, it will be described in more detail further in thepaper.

Fig. 4 Variable-speed wind turbine with synchronous machine

For wind turbine applications there is always a gearbox between the turbine and the generator, whichincreases the speed of the generator to 1000 rpm or 1500 rpm (6-pole or 4-pole machine at 50 Hz). Inhydroelectric power plants there are no gearboxes because natural speeds of hydro turbines are higher thanthe speeds of wind turbines. From the aspect of construction, the doubly-fed induction machine is a wellknown wound-rotor induction machine in which the stator and rotor windings are connected to the voltagesources of different frequencies. The classical synchronous machine is also a doubly-fed machine, only inthis case the rotor current frequency is equal to zero (DC excitation). According to basic classification ofelectric machines with rotating filed, the doubly-fed machine can be classified as a synchronous machine.


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Fig. 5 Variable-speed wind turbine with doubly-fed induction generator

The construction shown in Fig. 5 is more favorable than the one in Fig. 4 due to smaller size and costof the power converter, lower harmonic content in the power grid generated by the converter, bettercontrollability, smaller required space, better stability of the power system and possibility to generatereactive power. The further advantage is the possibility to increase the efficiency of primary movers invariable operating conditions, e.g. pump-turbine in reversible hydroelectric power plants of power rating up

to several hundred MW [11] and wind turbines up to 5 MW.The method which involves AC current in the rotor winding, i.e. the additional voltage of variable

frequency, is based on the knowledge that for electromechanical power conversion magnetic fields generatedby stator and rotor currents must be mutually static, i.e. they have to rotate with the same speed. The statorand rotor windings are three-phase, rotor has slip rings and brushes and the rotor winding is constructed inthe same way as the winding of a classical three-phase wound rotor induction motor. Hence, this machine isusually called doubly-fed induction machine. The brushes and slip rings are its main disadvantage becausethey wear out and require constant maintenance.

In the following expressions the basic relations in the case of AC excitation of the rotor winding aregiven. In the rotor winding which has 2p2 poles and is connected to the three-phase voltage of frequency f2 arotating magnetic field is created which rotates at speed of n2 = f2 /p2 with respect to the rotor. The turbine

rotates the rotor at speed of n. In these conditions the rotating field of the rotor winding will rotate withrespect to the stator winding with p1 pole pairs at the speed of

2 2 2on n f p= + (4)

The frequency of the voltage induced in the stator winding due to rotating field of the rotor will be

1 2 1 2( )f n n p np f= + = + (5)

because for power conversion we must always have p1 = p2= p. If direction of the rotor field changes, thenwe will have

1 2( )n f f p= (6)

Since in such applications the stator frequency is always fixed, the rotor frequency and the turbinespeed are regulated. For example, if the rotor of a 4-pole three-phase generator rotates at n = 27,5 s-1 and thefrequency of the grid is 50 Hz, then rotor must be excited with current of frequency f2 = 5 Hz in order toconvert mechanical power into electrical. The relations between the rotor speed n, rotor frequency f2 andstator frequency f1 are illustrated in Fig. 6.

Fig. 6 Relations between frequency and speed of a doubly-fed induction machine


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It follows from Fig. 6 that different relations between f1, n i f2 are possible. Since f1 is set by thepower grid, the frequency of the rotor current is adjusted according to the varying speed of the turbine. Byregulating the speed of a turbine or a pump the best energy efficiency can be achieved in concrete conditionsof wind speed or water level in the accumulation lake of a reversible hydroelectric power plant. The abovementioned system of speed control can be used to generate both inductive and capacitive reactive power offrequency equal to the frequency of the power system. The doubly-fed induction generator has a wideapplication in wind turbines [5,11,13] because of the unpredictable and constantly changing wind speedwhich is the reason why it is necessary to regulate the turbine speed. The power generating units areconstructed for rotor frequencies which are roughly 4-10 times lower than the stator frequency which allowsthe rated power of the converter to be around 35% of the rated power of the generator.

Let us consider the power conversion relations in a wind turbine (for simplicity the losses in thegenerator are neglected) according to notation in Fig. 5. The mechanical power of the turbine at the exit froma gearbox Pmech is by means of rotating magnetic field converted into electric power Pg, which is delivered tothe power grid, and the slip power in the rotor Pr= - sPg. This can be shown using expression

g meh gP P sP= + (7)

The rotor slip s is defined as the difference between the speed of the rotating stator field and the rotorspeed s = (ns n)/ ns and in the generating mode it is always negative. In generating mode the sign of powerPr will be positive (into the grid), and since the slip s is positive for motoring mode, the power Pr in themotoring mode will be negative (from the grid). For the usual range of turbine speed regulation, from 70 %to 110 % of the rated speed, and the synchronous speed of the generator of around 90 % of the rated speed ofthe turbine, the slip is in the range 23%, and the required power of the converter in the rotor is around 30 %of the power which is electromechanically converted in the machine.

The variable-speed wind turbine technologies are becoming dominant, especially for high power (1-5MW) wind turbines. In 2002, according to [3], 47 % of all wind power units were made with doubly-fed

induction machine. The power converters were rated around 35 % of the rated power of the generator.In electrical engineering there are many known solutions for large controlled electric drives (power

up to 50 MW) with subsynchronous cascades where the only difference is that they are used insubsynchronous motoring mode where the rotor slip is always positive. In large reversible power plants ratedup to several hundred MW doubly-fed induction machines are used for higher energy efficiency at transitionfrom pump (motoring) to turbine (generating) mode of the hydroelectric power unit [13]. The efficiency oflarge hydraulic machines can differ by several percent in turbine and pump mode at different hydraulicconditions. The speed variation of such units is in the range 10-15 %.

2.3 Variable-speed turbine and synchronous generator

Besides induction generator a synchronous generator for small rotating speeds (5-30 rpm) can be

used. Synchronous generator with high number of poles (60 or more) with classical excitation or permanentmagnets can be connected directly to the turbine without a gearbox, and the connection to the power grid iscarried out via power converter. The power rating of the converter is equal to the power rating of thegenerator. Due to large number of poles and low rotating speed the generator must develop large torque.Consequently it has relatively large weight and dimensions, which inevitably affects the constructions anddimensions of the nacelle. An example of a system with this type of generator is shown in Fig. 7. These arecompletely new, not classical constructions of the generator for which separate research and developmentwere required. Several leading manufacturers have successfully implemented their own technical solutionswhich are still not fully optimized and are continuously being developed.


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Fig. 7 Synchronous machine with classical excitation for direct-drive wind turbine

Both permanent magnet technology and classical excitation on the rotor have been developed. Details

can be found in numerous scientific papers. This type of power conversion unit is closely related to theconstruction of the power converter which must deliver full power of the generator. The development andapplication of direct-drive generators are inseparable from the development, application and cost of the

power converter. The reduction of cost of permanent magnets and their availability on the market havesignificantly influenced the development and application of permanent-magnet generators for direct-drivesystems and for systems with single or three-stage gearboxes. Compared to generators with excitation currenton the rotor, permanent-magnet generators for the same power rating have higher efficiency due to lowerrotor losses, smaller dimensions of the rotor, simpler cooling circuit because rotor does not require cooling,simpler maintenance, and no separate excitation system. Whether or not permanent-magnet generators will

prevail will depend on price and availability of the magnets on the market, solution of technical problemsrelated to attachment of the magnets to the rotor, and cost and reliability of the power converter. In that areathere is a noticeable research and development [14,17] going on and an increasing number of companies is

manufacturing or developing prototypes of permanent-magnet generators (Siemens, ABB, WinWinD,Mitsubishi, KD Blansko,...)

2.4 Single gearbox for multiple generators

Fig. 8 illustrates solutions by some manufacturers of wind turbines in which several induction orsynchronous generators are connected to a single gearbox. With this type of construction the currentlyavailable wind energy can be utilized in the best manner, because the number of active generators is adjustedaccording to immediate power of the turbine.

a) b)

Fig. 8 Wind turbine with multiple generators : a) induction generators, b) synchronous generators


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2.5 Comparison of different constructions

Table 1 contains comparison of basic properties, relative advantages and disadvantages for six mostcommonly used power generating units in wind turbines.

Table 1 Comparison of various generator technologies for wind turbines

Generator Advantages Disadvantages

Squirrel-cage single-speed ortwo-speed induction generator (2p=4 ili 6)

- simple construction

- simple maintenance- attenuated pulsations of turbine torque

- low cost

- direct connection to the power grid

- requires reactive power

- requires soft start device forinitial connection to the grid- applicable only for fixed turbinespeed- requires a gearbox

- cannot be used for large numberof poles (>20)

Doubly-fed induction generator

- significantly reduced power rating andcost of the converter

- possible speed regulation for optimalutilization of energy (typically 20-25%)

- reactive power for magnetization of themachine is provided by the powerconverter

- subsynchronous and supersynchronousoperation is possible

- slip rings and brushes, wear and

tear, maintenance- complex control of the entireunit

- direct connection to the grid issomewhat difficult

Synchronous generator with rotor excitationwinding

- simple control of reactive power- wide range of speeds

- simple control

- requires power converter of thesame power rating as the machine- requires an excitation system

- slip rings and brushes, wear andtear, maintenance

DIRECT-DRIVE

- no gearbox

- higher efficiency- large dimensions and weight,problems with construction,transportation and installation

WITH GEARBOX

- small dimensions and weight- standard construction can be used

- high cost, losses ( 2-3) %

- problematic maintenance of thegearbox

Synchronous permanent magnet generator- simple rotor with no parts prone to wear

and tear- very low rotor losses

- high cost of permanent magnets

- possibility of demagnetization

- insufficient experience inconstruction and installation

DIRECT-DRIVE

- no gearbox- higher efficiency

- simple maintenance

- large dimensions and weight,problems with construction,transportation and installation

WITH GEARBOX

- small dimensions and weight- standard construction can be used

- high cost, losses ( 2-3) %

- problematic maintenance of thegearbox


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2.6 Generator systems of leading manufacturers of wind turbines

In Table 2 a brief overview of generator systems of worlds leading manufacturers is given. Onlysystems with rated power higher than 100 kW have been selected.

Table 2 Worlds leading manufacturers of wind turbines and the types of systems they use

ManufacturerConcept/design*

Power range [kW] Generator

ABB (SE)** VS/PC100 500500 5000

600 4000

squirrel-cage single-speed and two-speed induction generatorsynchronous, classical and permanent-magnet generator

doubly-fed induction generator

Bonus/(DK)

Siemens Wind Power

CS/CS

CS/AS

600

1000 2300

squirrel-cage induction generator


KD Blansko (CZ) VS/PC 1000 3000 synchronous permanent magnet generator - single-stage gearbox

DeWind (GB/DE) VS/PC 600 2000 doubly-fed induction generator

Ecotechnia (ES)CS/CS

VS/PC750 3000 doubly-fed induction generator

Enercon (DE) VS/PC 300 4500 direct drive synchronous generator

GE Wind Energy(US/DE)

CS/CS

VS/PC

600

1500 36002500 (new)


doubly-fed induction generatorsynchronous permanent magnet generator + gearbox

Jeumont (FR) VS/PC 750 1500 direct drive synchronous generator

Made (ES)

CS/CS

VS/PCVS/PC

660-1320

8002000

squirrel-cage induction generator, two-speed (2p=4 , 2p=6)

synchronous generator (2p = 4) + gearboxbrushless synchronous generator (2p=4)

Mitshubishi (JP) CS/CS250 10002000

two-speed squirrel-cage induction generatordirect drive synchronous permanent-magnet generator

NEG micon/Vestas

(DK)

CS/CSCS/AS

VS/PC

600 15001500 2000

2750 - 4200

squirrel-cage induction generatorsquirrel-cage induction generator

doubly-fed induction generatorNordex (DK) VS/PC 1300 2500 doubly-fed induction generator

Repower Systems (DE)

CS/CS

VS/AS+PC

VS/PC

600 750

1050

1500 5000




Siemens (DE)VS/PCVS/PC

3600400 2300

3000


squirrel-cage induction generator (gearbox 1:119) 2p=4two-speed squirrel-cage induction generator (2p=4, 2p=6)direct drive synchronous permanent-magnet generator and gearbox

Turbowinds (BE) CS/AS 400 600 two-speed squirrel-cage induction generator (2p=4, 2p = 6)

Vestas (DK)RB/PC

PRS/OS

660 2750

850 3000


two-speed squirrel-cage induction generator (opti-slip)

Winwind (FI) RB/PC 1000 3000 synchronous permanent-magnet generator

Zephyros (NL) VS/PC 1500 - 2000 synchronous permanent-magnet generator

Konar (HR)(Prototype)

VS/PC 1000 direct drive classical synchronous generator (testing stage)

* Concept / design of a turbine

CS/CS constant speed/classic stall - fixed pitch angle

of the blades

CS/GP constant speed/combination with pitch adjustment

CS/AS constant speed/active stall negative pitchangle of the blades (3-5)

PRS/OS partly regulated speed, pitch control + OptiSlip, regulation 10 %

VS/PC variable speed/pitch control combined withclassic stall

VS/AS+PC constant speed + pitch adjustment

**only electrical equipment for wind turbinesSOURCE:www.ArchiExpo.com- Wind Turbines All the Manufacturers


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3. WIND TURBINE MANUFACTURED IN CROATIA

After detailed analysis of worldwide known technical solutions in construction of wind turbines andtrends in their development it was decided in 2004 in Konar Inc. to begin with development andconstruction of the first wind turbine in Croatia. The wind potential at locations in Croatia was tested. Theresults of these tests and the ability of Croatian industry to manufacture as many components of the wind

turbine as possible were also considered. The wind turbine with pitch control, power rating 750 kW anddirect-drive synchronous generator was selected. During research and development of the project and afteranalyzing the wind potential it was decided to increase the rated power of the turbine to 1000 kW and toadjust the design of the accompanying electrical equipment to that power rating.

With the engagement of numerous Croatian companies and scientific institutions (Konar Inc., uroakovi Inc., Institute Hrvoje Poar, Croatian shipyards, civil engineering companies, transportationcompanies etc.) the prototype was finished early in 2007. During writing of this paper the tests had beenconducted in the company Konar-GIM. The initial field test is planned for June and July 2007 at locationPometeno Brdo near Split. The basic technical data of the turbine is given in Table 3.

Table 3 Technical data of the wind turbine prototype KO VA 57/1

Type KO VA 57/1Diameter of the blades 57,4 m

Power regulation pitch controlRated power 1000 kW

Speed 10 30 rpm

Hub height 60 mTower height 59 mNacelle weight 62 t

Tower weight 82 t

Total weight 144 t

A three-phase synchronous generator entirely developed and manufactured by Konar - GIM isinstalled in the wind turbine. It is a completely new construction of the generator since this is the first time agenerator was built for such a specific purpose. It is rated 1000 kVA, 60 poles, frequency 5 14,5 Hz,connection to the grid via power converter. The detailed technical data of the generator is given in Table 4,and two photos from the final stage of construction are shown in Figs. 9 and 10.

The outer diameter of the generator is 4200 mm. This dimension is crucial for the selection of thewind turbine drive and the type of generator that would be used. Due to transportation problem in certaincountries it is required that this diameter should be limited to 4 m. Since this is a prototype of a generatormanufactured in Croatia, it is expected that during testing the valuable data will be collected to be used forthe construction of similar machines in the future.

Since there is a growing trend in the world of construction of high power (over 1000 kW) generators

for wind turbines, we will consider briefly advantages and disadvantages of two main constructions of suchgenerators, the permanent-magnet generator and the generator with classical excitation on the rotor.

Due to high cost of permanent magnets and the lack of experience in their application, until a decadeago no permanent-magnet generators for wind turbines were constructed, except in rare cases. Wind turbineswere built with several versions of induction generators and synchronous generators with classical excitation.Konar has obviously chosen a direct-drive synchronous generator with DC current excitation in the rotorwinding. In the case of permanent-magnet generators an experience in design and construction is required,especially for problems related to installment and magnetization of the magnets on the rotor.

For the purpose of research we designed a permanent-magnet version of Konars generator andcompared these two designs from the aspects of weight and key parameters.


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Fig. 9 Rotor of the generator SV 4205 60 Fig. 10 Hub mounted on the rotor of thegenerator SV 420560

Table 4 Technical data of the three-phase synchronous generator

Type SV 4205 60Rated power 1000 kVA

Rated voltage and frequency 690 V , 14,5 Hz

Rated current 836,7 ARated speed 29 rpm

Range of speed regulation 10 to 30 rpm

Range of frequency variation 5 do 14,5 HzVoltage regulation 150 760 VRated power factor , cos 0,95

Critical speed 36 rpm

Number of poles 60Number of phases 3

Insulation class of stator and rotor HCooling air

Excitation system separate/self-excitation

Rated excitation voltage 170 VRated excitation current 119 A

0,25 0,5 0,75 1 load P/Prated

Efficiency 93,9 94,9 95,3 95,3 efficiency %Total weight 27 t

4. PERMANENT-MAGNET GENERATOR

In the case of wind turbines it is very important to have a simple construction and maintenance of thegenerator. This is why permanent-magnet generator appears to be a very good solution because it hardlycontains any parts that require maintenance. For these reasons many manufacturers are constructing suchgenerators (Siemens, ABB, Mitsubishi, ...). The connection of this type of generator to the power grid isillustrated in Fig. 11.


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Fig. 11 Direct-drive permanent-magnet synchronous generator and its connection to the power grid

Until recently AC permanent-magnet generators were used only for specific applications and for lowpower. The appearance of modern ferrite and neodymium permanent-magnet materials with magneticproperties far better than the properties of previously used permanent-magnet materials, it became possible tobuild electrical machines of power ratings similar to the machines with classical excitation. One of mostfrequently used materials of the rare earth type is neodymium-iron-boron (NdFeB). The cost of suchmaterials has become acceptable and is continuously dropping (10 EUR/kg according to [14]).

Hence it considered that in Croatian industry the development and construction of such generatorsshould be initiated as well. There are various problems to be solved and the lack of experience inconstruction of high power permanent-magnet synchronous machines with high number of poles (up to 100).These problems include magnetization and fixation of the magnets on the rotor, demagnetization of themagnets at high temperatures or in some undesirable regimes like short-circuits. Since there is almost no

experience in domestic industry, the only data from which one can start the design process are similaritieswith already manufactured machine with classical excitation and limitations with respect to dimensionsinside the nacelle. The known data are power rating, voltage, frequency, characteristics of the turbine andsimilar dimensions.

4.1 An example of optimal design of a permanent-magnet generator

The analyses of various types of permanent-magnet generators described in [14] have shown thatmagnets with arc shape mounted on the rotor surface are optimal from the aspect of machine cost for acertain power level. Generator design is a complex procedure of determining its geometric dimensions and

parameters. Due to high number of variables which must be varied during design and imposed limitations,the optimization is used as a systematic, mathematically based system of decision making in the selection of

the machine dimensions. The general definition of the generator design optimization problem is: find avector of variables [ ]1 2, ,..., ,

D

Dx x x x R= G G

where ( ) ( ) , 1,...,G Di i ix x i D = which must satisfy m

inequality constraints ( ) 0, 1,...,jg x j m =G

and minimize the goal function ( )f xG

. The optimization method

is Differential Evolution (DE) [24]. The DE performs calculations on a population of vectors which mutatein every iteration and converge towards solution space where inequality constraints are satisfied and wherethe goal function is minimized. Table 5 contains the definition of the optimization problem for the design ofa permanent-magnet generator.

The rated data 1 MVA, 690 V, 29 rpm is the same as for the generator described in chapter 3. Themodel of the generator is analytical based on field calculations inside the air-gap by means of conformalmapping [25]. The generator is connected to the active rectifier by which the armature current is kept in

phase with the induced voltage to achieve required power with minimum current. The design data of theoptimized generator are given in Table 6, and the partial cross-section of the generator is shown in Fig. 12.


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Table 5 Definition of optimization problem of a permanent-magnet generator

Generatorvariables

(x)

1. Do/Do0 : Ratio of stator outer diameter and maximum allowed outer diameter (Dv0 = 4 m)

2. D/Do : ratio of stator inner and outer diameter

3. la/la0 : ratio of core length and maximum allowed core length (la0 = 0.6 m)

4. hs/[(Do-D)/2]: ratio of slot height and the difference between stator outer and inner radius

5. Br : permanent-magnet data

6. Qs : Number of slots

7. p : number of pole pairs

8. bt/s : ratio of tooth width and the slot pitch on the stator inner diameter

9. lm/ : ratio of magnet thickness and the air-gap length

10. p : part of the pole pitch covered by magnet

Inequality

constraints

g(x)

1. Bt 1,7 T : maximum flux density in the stator tooth

2. By 1,2 T : maximum flux density in the stator yoke

3. P 1 MW : minimum electrical output power

4. K 60000 A/m : maximum linear current density

5. J 4 A/mm2 : maximum current density

7. 1 0,71max : limitation of the fundamental component of the armature winding MMF for protection of

the magnet from demagnetization (1max - armature winding MMF on the knee of the magnetization curve)

8. 0.95 : minimum efficiency

9. T/T0.025 : maximum pulsating torque (2.5% of the average torque)

Goal function

f(x)V/V0 : ratio of active volume and maximum allowed active volume (V0=D

2o0/4lao)

Table 6 Results of the optimized design of a permanent-magnet generator

Rated power 1 MVA Magnet data at 120 0CVACODYM 669AP

Br=1.05 T, Hc=801 kA/m

Rated voltage 690 V Number of turns per coil 1

Rated frequency 15,95 Hz Number of parallel paths 1

Rated current 908 A Coil pitch 5

Rated speed 29 min-1 Slot fill factor 0,4

Rated power factor 0,92 Current density 3,89 A/mm2

Number of poles 66 Linear current density 60000 A/m

Number of phases 3 Flux density in stator tooth 1,70 T

number of slots 333 Flux density in stator yoke 1,20 T

Stator outer diameter 3463 mm Flux density in rotor yoke 1,22 T

Stator inner diameter 3210 mm Torque pulsations 1.2%

Rotor outer diameter 3203 mm Efficiency 95,0 %

Rotor inner diameter 3026 mm Magnet mass 1162 kg

Core length 459 mm Copper mass 639 kg

Air gap length 3,5 mm Iron mass 7947 kg

Magnet thickness 40,5 mm Total mass (active parts) 9748 kg


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Fig. 12 Partial cross-section of the optimized permanent-magnet generator

5. CONCLUSION

A research of current situation and trends in further development and application of wind turbineshas been conducted. The real boom in construction of wind turbines all over the world is continued. Manyscientific and expert papers have been published and the research is continuing.

In wind turbine systems induction and synchronous generators are used, depending on the turbinesize, the type of control and the connection to the power grid. Both direct-drive systems and systems with agearbox are used.

Induction squirrel-cage generators for power levels up to 1000 kW have advantage at fixed turbinespeeds, while doubly-fed induction generators have advantage at higher power levels and in the case oflimited regulation of turbine speed, 20-25% around rated speed. Induction generators always require agearbox.

Synchronous generators are used for turbines with regulated speed and for both direct-drive systemsand systems with a gearbox. Permanent-magnet generators are becoming more significant and it is realisticthat in new applications they will push out generators with classical excitation and the need for a gearbox.Significant research and development is conducted in that area. Various patents have been applied for.

The wind power plant manufactured in Croatia is an important achievement. Although this is aprototype, its parameters are comparable with those on the world market. For wind potentials found in

Croatia the selected power level and the dimensions of the generator are appropriate.Using optimization for the design of a permanent-magnet generator it has been shown that it is

possible to design a generator whose dimensions and parameters make it a serious competitor to thegenerator with classical excitation. The development and construction of a permanent-magnet generatorwould bring Croatian trends of development closer to the trends in the rest of the world.

6. REFERENCES

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308666.5_very good tech dfig ig etc

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