ANDREA TORTELLA Wind Energy Tortella

Department of Industrial EngineeringDepartment of Industrial Engineering

OutlineOutline

• Introduction on wind energy

• Fundamentals of power conversion

• Wind turbine technology

• Operation of wind generators (fixed and variable speed)

• Power characteristic and energy production

2A. TortellaA. Tortella: : Recent progress in design and technology for wind power generationRecent progress in design and technology for wind power generation

• Main electrical components and control systems

Power converters for grid connection and generator control

Squirrel cage and doubly fed induction generators

Synchronous generators (mainly with PM excitation)

• Data of commercial WGs

Wind energyWind energy

• Clean and virtually endless source (generated from solar heating of the

atmosphere), available on a global scale

• Limited environmental impact with respect to fossil fuel or some other renewable

power plants (civil works and noise)

• Cubic relationship of wind speed to available power

• High time and geographic variability

• Influence of installation height and of local topography (natural or artificial


• Influence of installation height and of local topography (natural or artificial

obstacles, surface roughness, …) ⇒ turbulence, wind shear

• Limited power extraction efficiency (< 60%)

• Constraints on the operating speed range (cut-in and cut-out speed)

• Sophisticated control systems due to the wind speed variability (rated power

delivered on 10-15% of the whole operating period)

• Low density fluid ⇒ high weight (volume)/power (transport and installation issues)

• Turbine low speed operation to reduce noise ⇒ multi-stage gearbox ↔ low speed

generators plus grid converter

Wind Wind generatorsgenerators ((WGsWGs))

Micro-mini wind (<10

kW)

• Home loads (urban)

• Small farms

• Isolated

applications

(pumping, telecom

stations, mountain

refuges, heating, …)

Small wind (10 – 50 kW)

• Low medium industry services

• Parks or residential areas

• Large farms

• Small power stations

(distributed generation)

• Hybrid systems (micro-grid

integration with other


refuges, heating, …)integration with other

renewables)

Large wind (100 – 8000 kW)

• Single grid-connected on-

shore machines

• Wind farms with multiple

machines (on-shore or off-

shore) ⇒ 200 MW ÷ > 1 GW

Wind Wind energyenergy installationsinstallations

Highest capacity

among RE


500÷600 MW/year

Total ≈ 800 MW

Wind generator schemeWind generator scheme

Wind turbine (WT): wind to mechanical energy conversion Wv ⇒ Wm1 (lift ↔ drag principle,

horizontal ↔ vertical axis)

Electrical generator: mechanical to electrical energy conversion Wm2 ⇒ We at a fixed or

variable speed Ω2 (operating point defined by the shaft torque) by gearless (Ω1= Ω2) or

gearbox (Ω1< Ω2) transmission

Power converter: double stage electrical conversion interfacing the generator with loads

and/or the grid (voltage/frequency regulation, active/reactive power control according to the

grid dispatch strategy)


Power converter

Electric Electric generatorgenerator

wind

Wind turbineWind turbineV

ΩΩΩΩ2222 ~

Electric loads

Grid

Wv Wm1 We Wu

Low speed shaftElectrical Electrical terminalsterminals

Outputenergy

GearboxGearbox

ΩΩΩΩ1111

High speed shaft

Wm2

Single or three phase connection

grid dispatch strategy)

Extractable power (Betz theory)Extractable power (Betz theory)

A1

∆t

v1v2

P

A2

v

• Unfeasibility of the whole energy extraction from the air stream (it

should lead to the wind stopping)

• Extracted power P = Pv1-Pv2 calculated assuming constant mass flow

(mass conservation principle) and the law of conservation of momentum

Available power in a stream tube (section A, mass mv)

322

2

1

2

1

2

1vAv

dt

dmvm

dt

dP v

vv ρ==

=

Kinetic energy Mass flow: ρρρρ v A

Dependence

on v3

Uniform disk converter

put in free airstream

density


P (mass conservation principle) and the law of conservation of momentum

221 vv

v+=

Speed reduction equally distributed before and behind the converter

Speed value at the converter lower than undisturbed wind speed

Increase of the stream tube section behind the converter

ccpp = P/P= P/P11 extracted power fraction

(residual amount wasted behind)

ccp0,maxp0,max= 0.593 = 0.593 with v2/v1=1/3

In general, dependence on turbine

type, blade geometry and rotor

angular speed

( ) ( )pcvA

vvvvvAP 3

1

21

22123

1 2

1

2

11

2

1 ρ=−⋅+ρ=

undisturbed airstream power

Fraction dependent on v2/v1

Air density variationAir density variation

⋅−⋅=

⋅= 3048

297.0

0

mH

eTR

p ρρ

p: atmospheric pressure

R: gas constant

T: temperature

ρ0: air density at sea level = 1.225

kg/m3 (temp.15°C, 1 atm)

Hm: installation height2000


At Hm=2000 m, power

output can be reduced up

to ≈≈≈≈ 20% with respect to

the sea level value

0

500

1000

1500

80 85 90 95 100

Alti

tude

[m]

Variation with respect to sea level value [%]

Typical sizes of large wind generatorsTypical sizes of large wind generators

[m]160

120

80


40

Power [kW] 50 300 750 1000 2000 5000

Rotor diameter [m]

15 34 48 60 72 112

Tower height [m]

25 40 60 70 80 100

414 W/m2 510 W/m2283 W/m2 600 W/m2

Wind Wind shearshear

Class Z0 [m] Energy index [%] Terrain surface type

0 0.0002 100 Calm water

0.5 0.0024 73 Open and smooth terrain

( )( )

( )( )01

02

1

2

log

log

Zz

Zz

zv

zv =classification of the

surface structure

roughness length

ΔP=+ 60%ΔP=+ 50%


0.5 0.0024 73 Open and smooth terrain

1 0.03 52 Farm land, sparse buildings, short and bare hills

1.5 0.055 45 Farm land with sparse buildings, short hedges (<8 m); obstacle distance

about 1250 meters

2 0.1 39 Farm land with sparse buildings, short hedges (<8 m); obstacle distance

about 500 meters

2.5 0.2 31 farm land with several buildings, shrubs, hedges; obstacle distance

about 250 meters

3 0.4 24 Small town, farm land with woods and irregular terrain

3.5 0.8 18 Town with tall buildings

4 1.6 13 Large cities with tall buildings and skyscrapers

Drag driven WTsDrag driven WTs

Maximum drag in the wind direction ⇒ blades distributed on the whole swept area

Electrical

• Generally with vertical rotation axis (Savonius type)

• Low max power coefficient (≅19%) but high torque at low

wind speed

• Design to avoid stall positions (self-starting capability) and

high torque ripple (helical blades, double step rotors)

• Easy manufacturing and installation

• Reduced visual impact (building integration, flickering)


Electrical generator at

the shaft base

• Reduced visual impact (building integration, flickering)

• Basically adopted in mechanical drives (pumping systems) or

wind measurement equipment (cup anemometer)

Min drag Max drag

Sta

rtin

g t

orq

ue

0

Rotor 1Rotor 2

Total

Lift driven WTsLift driven WTs

• Blade section covering only a small

fraction of the swept area

• Horizontal axis (HA) type more

common because of the very high

conversion efficiency, proximal to

the theoretical maximum

• Vertical axis (VA) type adopted

Vertical and horizontal axis rotors operated by the lift (vertical) force

generated by the air stream pressure on the blade surface


• Vertical axis (VA) type adopted

mainly for small power applications33--bladed rotorbladed rotor22--bladed rotorbladed rotor11--bladed rotorbladed rotor

fT and fA dependent on α

(angle of attack), blade

geometry (lc, β) and on vr2

(or λ ‘tip speed ratio’)

fT/ fA maximization

by keeping low

values for α and β

lc

ΩΩ⋅⋅RRR

vv11

vvrrα β

Ω

Flow velocitycomposition

( ) ( ) ( )221

21

221

221

2 11 λ+=+=Ω+= vvvvRvv pr

lc

R

vr

α β Ω

Power coefficient (HAWT)Power coefficient (HAWT)

0.2

0.3

0.4

ββββββββ=0=0°°

β=4°

β=8°

β=12°

CCpMpM

1 2

0.5

Zone 1 (λ< λopt )

• When v1 increases or Ω decreases, the turbine

( )βλπρ=Ω= ∫ ,2

31

2

0 p

R

T cvRdrrfP• cp<cp,Betz because of the losses mainly

due to the turbine rotation/blade

effects (rotor wake,…)

• For a given β, maximum cp achieved at

a particular tip speed ratio λopt=ΩR/v1

(Ω≈v1 to keep λopt=const.)


Zone 2 (λ > λopt )

• Too high blade speed in relation to the air stream velocity ⇒ turbine comparable to a solid

disk overtaken by the air stream

• From the aerodynamic point of view, the angle of attack tends to decrease ⇒ relative

speed direction appears tangential to the blade, compromising the power extraction

0

0.1

0 3 6 9 12 15tip speed ratio

β=16°

λλλλλλλλoptopt

stall1

loses part of its capturing capability

• From the aerodynamic point of view, the angle of

attack tends to increase⇒ drag/lift increase, stall

condition at low λ (instability zone) where

however cp values are not technically significant

Vertical axis lift rotorsVertical axis lift rotors

Net force always in slanting direction

Ω

R

αUpwind blade

Downwind blade

Torque/angle characteristic (fixed λ)

• Lift force (torque) only if Ω ≠≠≠≠ 0 ⇒⇒⇒⇒ no self-

starting capability⇒ use of a hybrid

turbine (coupling with a Savonius one) or by

electrical spinning

• Variable angle of attack during rotation and

shadowing of the downwind blade ⇒reduced extracted power, oscillating

torque, fatigue stresses on blade


Ω R

Wind v1

αDownwind DownwindUpwind

0

Torque/angle characteristic (fixed λ)

• Use of multiple bladed rotor (e.g., 3 blades)

with twisted profile to have a smoother

torque profile

• Very high centrifugal stresses because of the

mass distribution at the periphery ⇒ strong

structure with brackets, twisted blades,

‘troposkien’ design

• Omni-directional operation (no yaw system)

WT comparisonWT comparison

• Only HAWTs

approach the

maximum

theoretical limit of

the conversion

efficiency

• VAWTs are

considered for

complex 0.3

0.4

0.5

0.6

Pow

er c

oeffi

cien

t

3-bladed

2-bladed

Single-bladed

Betz limit


complex

environments

(turbulence,

building

integration,…)

• Correlation

between λopt and

noisy operation

must be taken into

consideration

0 2 4 6 8 10 12 14 16 18

0.1

0.2

Pow

er c

oeffi

cien

t

0.0Savonius

Darrieus

Multi-bladed (american type)

λ = Ω·R / vTip speed ratio

Low speedLow speed

High torqueHigh torque

High speedHigh speed

Low torqueLow torque

Operation of wind generators (WGs)Operation of wind generators (WGs)

Nearly constant angular speed whatever

the wind speed value ⇒ optimal value for

only one operating speed

Angular speed adjustment according to wind speed

variation (inside allowable technical limitations) in

order to operate at the maximum power condition

Power Power

v4

v3

v C

C4A2=B2

A3

A4

B3

B4

A1,A2,A3,A4: maximum

power operating points

B1,B2,B3,B4: fixed speed

operating points

C1,C2,C3,C4: operating points


Wind speed

v2

v1

v1v2v3 v4

C1

C2

C3

A1 B1

C1,C2,C3,C4: operating points

with constant electrical load

(uncontrolled resistive

operation)Angular speed

60 rpm

Power

Wind speed [m/s]

55 rpm

50 rpm

45 rpm

Angular speed increase as wind increases to deliver more

power

Lower cut-in speed at low angular speed ⇒ increase of the

energy yield

WG WG typestypes

IG

Doubly fed induction generator

grid

IG

Squirrel cage induction generator


SG

Coil excited synchronous

generator

PM synchronous

generator

Output power characteristic (VS)Output power characteristic (VS)%

rat

edel

ectr

ical

pow

er

100

80

60

40

20

vcut-in

vrated

vcut-out

•• vvcutcut--inin = starting operating speed

determined by various

electromechanical issues (PM

cogging, too low efficiency at light

loads, turbine start mechanism…)

•• vvcutcut--outout = maximum admissible speed

after which the wind generator is

shut-off to prevent damage of

mechanical and electrical equipments

(electric or mechanical braking,

3

ccpp = = constconst

10 2

ΩΩ = = constconst

25°

50°

75°

100°

Pitch

angle


Wind speed v0

(electric or mechanical braking,

turbine furling)

0) Starting rotation without power production by a launching process and pitch regulation

1) Operation at a cp=cp,max=const. (Ω increases linearly with v by regulating the e.m. torque, P ≈ v3)

2) Operation at Ω=cost. up to the rated speed (cp < cp,max → power increases at a lower rate,

increased generator current and torque)

3) Operation at constant or decreasing output power to reduce electromechanical stresses →adoption of control systems of active type (pitch angle control→ see curve , active stall, yaw

control, generator braking torque regulation) or passive type (aerodynamic stall with flexible

blades, centrifugal mechanisms)

ControlControl algorithmalgorithm (v(v≤≤vvratedrated))

P-Ω characteristic with a well-

defined unique maximum point →dP/dΩ = 0 as the wind speed varies

• Application of a small increase and decrease to the speed Ω, measuring

continuously the output power ⇒⇒⇒⇒ calculation of ΔP / ΔΩ


continuously the output power ⇒⇒⇒⇒ calculation of ΔP / ΔΩ

Reduction of the generator torque with ΔP / ΔΩ > 0 (possibility to achieve

higher power by increasing the speed), alternatively the torque is increased to

keep ΔP / ΔΩ ≅≅≅≅ 0

Maximization of the electrical power could not coincide with the operation at

cp= cp,M

• Method insensitive to the errors in local wind speed measurement and also to

the wind turbine design but with accuracy problems in turbulent conditions

• Wind farms require separate control for each WT

Pitch angle variationPitch angle variationRotation of the blade to limit output power ⇒ active pitch active pitch (blade leading edge turned into the

wind, forward rotation) or active stallactive stall (blade leading edge turned out to the wind, backward

rotation → airflow no longer attached to the profile contour provoking stall)

Leading edge Leading edge

Trailing edge

Pitch angle [°]30

20

10

Active pitch control

P=const.=500 kW


• Active pitch control: wider variation range, possibility to implement control technique with

slow or fast pitch variation (slow suitable for VS turbines)

• Active stall control: very limited variations lead to remarkable power changes, small size but

precise actuators, possible stochastic variations depending also on the surface wear (delays

when applied, hysteresis effect when excluded)

Bearings

Actuator

0

-55 10 15 20 25

Wind speed [m/s]

0 30

Active stall control

EstimatedEstimated annualannual energyenergy production (AEP)production (AEP)

( ) ( ) ( )∫∫ ⋅⋅=⋅= outcut

incut

outcut

incut

v

v elech

v

v dhd dvvhvPTdvvwTW,

,

,

,

wd: energy density

Th: actual operating time (excepted

downtime for scheduled maintenance)

( )k

c

vk

ec

v

c

kvh

−−

⋅

=1

(hours/year for which v* ≤≤≤≤ v ≤≤≤≤ v* + Δv) / Δ v

Elaboration of wind speed measurements over a long

enough time interval (≈1 year for seasonal climate change)

8

10

12

k=2.0k=3.0k=1.0

% h

ours

/yea

r

c=10.0 m/s


k: shape factor shape factor (index of the time sharing between

low and high speed values)

⇒ k=1→3 tends to assume a ‘bell shape’ profile

⇒ k=2 more frequent (suggested in

regulations)

c: scale parameter scale parameter (shift the distribution

towards high speed values)

⇒ related proportionally to the average

measured speed ⟨v⟩Wind speed [m/s]

0

2

4

6

8

10

12

c=8.0 m/s

c=10.0 m/s

c=12.0 m/s

0 5 10 15 20

% h

ours

/yea

r

k=2.0⟨v⟩ =7.1 m/s

⟨v⟩ =8.9 m/s

⟨v⟩ =10.6 m/s

(hours/year for which v* ≤≤≤≤ v ≤≤≤≤ v* + Δv) / Δ v

0

2

4

6 k=1.5

% h

ours

/yea

r

Wind speed [m/s]0 5 10 15 20

Energy density Energy density distributiondistribution

k=2 – c=10 – c p=0.5


• Rated speed value must be set to obtain the maximum energy density

• Low energy contribution at the average speed

• High energy content at high speeds, even if they are less likely to occur

GearboxGearbox

• Adoption of planetary stages (typical gear ratio of up to 1:12)

⇒ lower mass and cost and higher efficiency than parallel

shaft gear (3-stages about 1/2 cost and 1/7 mass reduction)

• High speed machines ⇒ high gear multiplication (60-100)

requiring generally 3 stages ⇒ percentage on total weight can

reach 30% (over 20 tons for 2-3 MW rated machines)

• General issues

EfficiencyEfficiency: decrease with number of stages and load

2-stages: parallel

1-stage planetary

22--stage planetary gear stage planetary gear –– PPNN=1500 kW=1500 kW


(possible consideration of medium speed machines)

MaintenanceMaintenance: installation stiffness and quality of the

lubrication has been found to be a decisive factor for the

service life of the gearbox

Vibration/noiseVibration/noise: use of flexible joints and hybrid

configurations, e.g. 1 planetary stage + 2 helical parallel

stages, the latter less noisy

• Magnetic gear: physical isolation, higher efficiency, lubrication

free, very low acoustic noise and vibration but lower torque

density and very high costs

Sun gear (high speed)

Planet carrier

Ring gear (low speed)

PowerPower conversionconversion AC/DC/ACAC/DC/AC

• Back-to-back or active front-end inverter (fully

controlled converter) →→→→ decoupled and

independent control of the inverter pair

• DC link managed as a storage circuit for the

generated power to be exchanged with the grid

• Low harmonic distortion because of the high

commutation frequency


• Direct-drive generator with high pole

number ⇒⇒⇒⇒ modular windings with several

output terminals providing symmetrical

voltage groups (e.g., multiple stars)

• Multiple inverters supplied by the rectified

voltages and cascade connected (CHB) ⇒voltage increase and harmonic

compensation

• Number of voltage levels used to synthesize

the phase voltage dependent on the number

of cascaded cells (k cells → 2k+1 levels)

CHB11

CHB12

CHB21

CHB22

CHB31

CHB32

ExampleExample –– 5 5 cellscells CHB (11 CHB (11 levelslevels))


AC electrical machinesAC electrical machines: production of an e.m. braking torque Tem due to the interaction between

the main field (magnetic flux φ) and the reaction m.m.f (related to induced current I) ⇒Tem

∝∝∝∝ φ ⋅⋅⋅⋅I → regulation acting on field or current commands

: electrical power delivery because of the induced e.m.f. in the

stator windings (magnetic flux time variation due to current time variation ∝∝∝∝ φ⋅⋅⋅⋅ f or

conductor and main flux relative motion ∝∝∝∝ φ⋅⋅⋅⋅Ω)

: mechanical and electromagnetic power losses dependent amongst other

factors on winding currents, speed (frequency) and electromagnetic operation (harmonics,

saturation,…)


Field typeField type Induction machineInduction machine Synchronous machineSynchronous machine

Main

Line AC excitation or self-excitation (stator winding)

3-phase distributed winding

Semi-closed slots

Rotor DC excitation

Separate winding or permanent magnets

Anisotropic or isotropic magnetic circuit

Reaction

Frequency dependent on speed

Rotor 3-phase distributed or squirrel cage windings

Semi-closed slots

Frequency proportional to spee d

Stator 3-phase distributed winding or tooth-wound coils

Open or semi-closed slots

saturation,…)

Design approach focused on efficiencyDesign approach focused on efficiencyEXAMPLE:EXAMPLE: Wind turbineWind turbine connected to a electrical synchronous generator electrical synchronous generator operating at the

maximum power point (MPPT strategy)

Efficiency evaluation as a function of speed introducing the coefficient α= Ω/Ω* and assuming

a steady-state operation as for the electrical system (Ω*: reference speed)

• Turbine power PPmm ≈≈≈≈≈≈≈≈ ΩΩ33 (unchanged aerodynamic parameters)

• Generated e.m.f. E E ≈≈≈≈≈≈≈≈ Ω Ω (constant magnetic flux), II r.m.s. value of the supplied current

• Power balance (approximated) ⇒ PPmm≈≈≈≈≈≈≈≈ E∙IE∙I ⇒⇒⇒⇒⇒⇒⇒⇒ I I ≈≈≈≈≈≈≈≈ ΩΩ22

( ) ( ) ( )Ω++Ω ( ) α+α+α++⋅α

friction hysteresis eddy current windage conductors


( ) ( ) ( )( )Ω

Ω++Ω−==ηm

FecLmL

m

e

P

PIPP

P

P1

p

ααm

The condition α= αm identifies the more convenient operating point on a statistical base

( ) *4

*3

*2

2

*1 pp

ppp ⋅α++

α+

α=α

decrease as α ↑

pp**: per unit losses at

the reference wind

generator power

Increase as α ↑

( )*3

*4*3*2**

m

cLwFecpFehfr

P

PPPPP

αα+α+α++⋅α

Squirrel cage induction generator (SCIG)Squirrel cage induction generator (SCIG)

Different slot number

Bar welding to the end rings

Open bar ends

Rotors of high rated machines

Axial ventilation

duct

Thick stator yoke (low pole Thick stator yoke (low pole number number →→ 4 poles, 1500 rpm4 poles, 1500 rpm) )

Bar shape affecting rotor leakage

reactance and resistance ⇒ suitable

torque/speed characteristic


Stator and rotor laminations

Small rated machine High rated machineDifferent

winding types

poles 6 8

slip -0.7 % -0.8 %

synchr.

speed n0

1000

rpm

750

rpm

rated

torque Tn

2230

Nm

678

Nm

Grid connected SCIG applicationGrid connected SCIG application

Prime mover

ΩΩΩΩfixed

GridSCIG

Soft-starter

Limitation/control of the current requested for turbine starting (by-

passed at steady state to reduce losses)

Pole number selectorQc

• Low cost solution (standard equipments)

• Limited speed variation ⇒ power

fluctuations involve high torque ripple ⇒structural stress, high grid current variations

• Relevant capacitor size for power factor

correction, especially at light loads ⇒QC≈≈≈≈20-40% Prated depending on pole number

(4 poles → cosϕ ≅0.9, 12 poles → cosϕ

≅0.8)T4

i


SCIG

iaT1

a

b

cϕ1

ia1

ia

• Condition to obtain motor and discontinuous operation (low current) ⇒ 150150°°>> αα>>ϕϕ11

• High current harmonic content, however limited only to the starting phase

• Firing angle control ⇒ open loop (linear decrease with time with saturation on max current), closed current loop or

speed loop with maximum current limitation

firing angle α dynamically

adjusted to limit the

current

machine phase impedance

changes with speed

Pole changing SCIGPole changing SCIG

Low cost system to improve the machine operation in presence of large speed variation of the

prime mover ⇒ wind power ∝ v3 → by a pole changing (4/6) the obtained power reduction is

(4/6)3=8/27≅≅≅≅0.3, acceptable in practical fixed speed applications

• Two separate windings (higher cost but improved

design and performance) or a single winding

reconfigurable by suitable switches

• Pole combinations and winding characteristics

selected to keep the unvaried magnetic conditions

→ 1/2, 4/6, 6/8, 10/12


High pole number

Low pole number • Increase of the captured energy, reducing the

rotor losses and the gearbox noise (lower torque

fluctuation ⇒ better drivetrain exploitation)

• Optimal operating speed should be positioned on

the opposite side with respect to the expected

annual average wind speed

• Achievable also by adopting two different gear

ratio or two different generators (Danish concept)

• Check of transients and switch wear in presence of

wide and frequent speed changes

IsolatedIsolated SCIG application (SEIG)SCIG application (SEIG)

QLvariable

ΩΩΩΩvariable QCLext

Max (QL) ≈ Qc

SCIG

Ballast load

• Voltage regulation by controlling the

reactive power exchanged by a capacitor

bank and a variable external inductance

(step voltage regulator transformers or

AC/AC thyristors ⇒ TCITCI)

• Ballast load or energy storage to

compensate active power fluctuations

and stabilize the grid frequency


Load unaffected by

the variable

frequency

Load parameters adjusted to obtain a convenient

system operating point (e.g., maximum efficiency)

as function of the angular speed Ω

DoublyDoubly fedfed inductioninduction generatorgenerator (DFIG)(DFIG)Woundrotor

Brushes and slip ringsStator terminals Rectifier + chopper

•• DFIGDFIG (doubly fed induction generator): rotor

windings provided with sliding contacts

(brush and slip rings) for the connection to a

variable external resistance (A) or to an

AC/DC/AC or AC/AC power converter (B) ⇒variable speed operation

Low cost equipment


grid

ReqLow cost equipment

Possible use of optical commands instead

of slip rings (Req system on the rotor)

More promptness than pitch control

Limited speed variation (1÷1.1 Ω0)

High rotor losses at high speed

Wide speed range with sub- and super-

synchronous (Ωmax-Ωmin)/Ω0n=20-40%

Low converter ratings (20-30% Pn)

Separate regulation of active and reactive

power

Cost and control complexity

DFIG with AC/DC/AC converterDFIG with AC/DC/AC converterpublic grid

P'er

AC

DFIG

gearbox

P'm

s·fgrid

Pe=P'es+P'er

P'er

P'es

VDCDC

DC

AC

fgrid

frgrid

0.10.20.3

P/Pn

P'es

Pe

P'er

1

0.50

0.25

0.75

-0.25

s-0.3-0.2-0.1

Sub-synchronous Super-synchronous

n/n01.31.21.10.90.80.7

P’es: power delivered by the stator

P’er: power exchanged through the rotor

DFIG: doubly fedinduction generator

Input poweroutput power


DC ACtransformer

-0.25induction generator

Condition s=0 ⇒ operation as a synchronous

machine since s∙fgrid→ 0 ⇒ rotor DC supply

compensating the ohmic losses

Wind speed DFIG speed Slip P c/Pn Pturbine ∝∝∝∝v3

vmax n=1.25 n0= nmax s=-25%0.25/(1+0.25)=1/5

100 %

0.6 vmax n=0.75 n0= 0.6 nmax s=+25% 22 %

vmax n=1.33 n0= nmax s=-33%0.33/(1+0.33)=1/4

100 %

0.5 vmax n=0.66 n0= 0.5 nmax s=+33% 12.5 %

( )min

nminser,c s1

PsP'P

min −−==

CurvesCurves ofof a a DFIGDFIG--basedbased WGWG


(kW

)

DFIG drive for a WG systemDFIG drive for a WG system


SGs for variable speed WTsSGs for variable speed WTs

Classical SGClassical SG

MultiMulti--pole directpole direct--drive SGdrive SG Integration in the WT nacelleIntegration in the WT nacelle


Classical SGClassical SG

VAWT VAWT mastmastcouplingcoupling

MultiMulti--polepole directdirect--drivedrive SGSG

withwith outerouter rotorrotor and and surfacesurface PMsPMs

• Power converter for grid interfacing sized for the

maximum exchanged power

• Most of the commercial WGs without gearbox

(gearless or direct drive)

Larger machines, but better integration with the

turbine hub

Improved efficiency and reliability

Low maintenance and noise

• Issues: magnet cost for PMSG (large pole number!),

rotor DC supply for DC-excited SG (low speed!)

Efficiency in WGsEfficiency in WGs

High

benefit at

light loads


Variable speed turbines have the additional advantage of higher energy yield, but capital

costs are higher (increased generator sizes ⇒ nacelle sizes and mass, civil infrastructure,

transportation and installation)

DC excitation

Radial flux PMSGRadial flux PMSG

PM flux lines distributed in planes perpendicular to the rotating axis having a predominant radial direction (according to PM magnetization)


• Outer/inner rotor

• Surface/interior magnets (SPM/IPM)

• Conventional distributed windings with chorded coils

(integer n. slots/pole/phase q)/fractional pitch

windings (q fractional ⇒ tooth wound coils)

• Laminated stator with open/semi-closed slots

• Solid/laminated rotor (reduction of losses due to

stator harmonic and sub-harmonic fields)

StatorRotor

Magnets

SG for a small VAWTSG for a small VAWT

Inner rotor design

High manufacturing

automation (open slots,

rectangular teeth)

Design and construction of a radial flux SPM SG for a vertical axis

wind turbine (20 poles/24 slots 20 poles/24 slots - 1.5 1.5 kWkW)

Comparison of grid converter architectures

Set up of a sensorless control strategy

Very low ‘cogging’

Low weight

Natural air cooling (circumferential fins)


rectangular teeth)

SG for a small VAWTSG for a small VAWTPe decreasebecause of the Ω=const. strategy

0.00.0

0.50.5

1.01.0

1.51.5

2.02.0PPee [kW][kW]

202022 44 66 88 1010 1212 1414 1616 1818

[m/s][m/s]

back-to-back

rectif./boost/invert.

MPPTMPPT ΩΩ=const=const


[rpm]

0.00.0 202022 44 66 88 1010 1212 1414 1616 1818

SPM/IPMSPM/IPM--SG output curve limitSG output curve limit

1.0

1.2

1.4 Pem/Pn(Xq=Xd)

C/Cn(Xq=Xd)

Pem(Xq=Xd)

Pn(Xq=Xd)Pme(Xq=2Xd)

Pn(Xq=Xd)

( )

−⋅+Ψ= ααα dqqdqM

em LLIIIpT2

3reluctance term

PM term

• Electromagnetic torque ⇒ define design issues (pole pairs

pp, maximum flux/pole ΨΨMM, magnetizing and torque current

components IId,qd,q to be varied according to speed → α )

• Ld > Lq (inverse saliency) to have the additional reluctance

contribution

Torque and speed

range increased


1 2 3 4 5

0.2

0.4

0.6

0.8

α=f/fn

Cem(Xq=2Xd)

Cn(Xq=Xd)

Cem(Xq=Xd)

Cn(Xq=Xd)

range increased

by about 20%

adopting a

saliency ratio

Xq/Xd=2

Application of IPMApplication of IPM--SGSGbPM

hb

q d

NdFeB or Ferrite

(low cost) PMs

Laminated

poles

non magnetic

wedge

holes for

lamination

tie-rods

Air-gap g N S S

Higher mean torque/PM volume ⇒reluctance torque (LLqq > L> Ldd)

Negligible PM losses due to stator slotting

More complex manufacturing than SPM

machines

High torque ripple

Rotor losses (small air-gap, narrow d-q path

to increase reluctance effects)

Up-to-date commercial WGs

mainly focusing on SPM-SG


tie-rods

Non magnetic support, generally

aluminum to decrease mass and to

allow induced currents opposing

m.m.f. harmonics

DFIG + 3-stages gearbox + converter

Direct-drive IPM-SG + converter

Wind speed [m/s]

• Multi-pole modular configuration

⇒ each pole individually

assembled on the rotor support

(benefit for installation on large

diameter→ 3m)

• Very high efficiency with NdFeB

ExampleExample –– WG WG controlcontrol withwith PMPM--SGSG

PWMPWM

Voltage angle

calculator

PM SG

ΩΩΩΩΩΩΩΩ

ΩΩΩΩΩΩΩΩ, θθθθθθθθmm

Tem* iqs

* vqs*

vas*,vbs

*,vcs* vai

*,vbi*,vci

*

iag,ibg vag,vbg

VVdd

ii asas,i,i bsbs

GridWind turbine

θθθθθθθθ

Park transformationPark transformation PLL techniquePLL technique


abc→dq

dq→abc

dq→abc

MPPT

abc→dq

PI

PI

PI

decoupling PI

controller

ΩΩΩΩΩΩΩΩ Tem* iqs

ids*=0

vqs*

vds*

θθθθθθθθmm

θθθθθθθθee

θθθθθθθθee

idg,iqg vdg (vqg=0)

iqg*

idg* VVdd

**

VVdd

QQgg**

+

-

ii asas,i,i bsbs Reactive powerReactive power

Voltage feed-forward (calculation of the reference

inverter voltages)

iqs

ids

vdi* vqi

*

Gearless & geared comparisonGearless & geared comparison

• Multibrid technology ⇒ extensive comparable studies of

various drive train designs focusing top head masstop head mass, highest

possible systems reliabilitsystems reliability and annual energy production annual energy production

(AEP) per cost(AEP) per cost in the range 1.5 1.5 –– 5 MW5 MW (main reference off-

shore applications)

• Study (2009): cost-effective ranges of gearbox (GB) ratios

and power ratings of multibrid permanent-magnet (PM)

wind generator systems with single-stage gearbox

Higher GB ratios ⇒ lower generator size and cost,


Higher GB ratios ⇒ lower generator size and cost,

higher GB size and cost

Larger generator diameters ⇒ lower generator length

and active material cost, but higher structural cost for

the larger housing

Components cost in k€ of 1.5 MW WG (gear ratio 1:5.33↔direct drive)

Double star PMDouble star PM--SG with rectifier bridgesSG with rectifier bridges

I d

SG

v'

v"

vo

• Reduction of rectified voltage

harmonics because of the very

low speed

• Switches sized for ½ of the

rated output voltage

• Lower DC link losses than

parallel connection

• 50% available power in case of

one star fault

30° shifting

between the

2 stars


one star fault

2 coils/phase

Coil span 165°→ chording factor

= cos(7.5°) = 0.991

4 coils/phase

Voltage phasors 15° shifted → chording

factor = sin(30°)/(4*sin(7.5°)) = 0.958

Comparison single Comparison single –– double stardouble star

650

700

750

800[V]

double star winding

Main sizes ⇒ outer radius=182.5 mm, air-gap=1.5 mm, length=150 mm

Operational data ⇒ Ω=250 rpm – Id=5.4 A (Pout ≅≅≅≅ 4 kW)

145

150

155

160

165[Nm]

double star


500

550

600

650

0° 30° 60° 90°120°150°180°210°240°270°300°330°360°

single star winding

• Remarkable reduction of the rectified voltage ripple (-56%) and slight increase of the mean

value (+3%)

• Remarkable reduction of the torque ripple (1/3 than single star) and slight increase of the

mean value (+3.4%)

120

125

130

135

140

0° 30° 60° 90°120°150°180°210°240°270°300°330°360°

single star

High pole number AFHigh pole number AF--PM generatorsPM generators

• Direct-drive generators

• Large active surface enabling high

torque/volume

• Tooth wound coils placed in open

slots ⇒ easy manufacturing and

assembly, high filling factor, short

length end-windings

• Fractional pitch winding → nearly

Coil groups included in the same phase


• Fractional pitch winding → nearly

sinusoidal voltage waveform, very

low ‘cogging’ torque (decrease of

turbine cut-in speed)

• Adoption of more than one coil

group per phase →→→→ change of the

coil connection according to

current/voltage needs as function

of speed, multiple star windings

supplying separate converters

Example Example –– 4 stars winding4 stars winding

Ω

Tt

AFPMG AFPMG electricelectric modelmodel

+

−

Udc

E0,1

E0,2

E0,3

Rph Ls

Diode rectifier

b winding group

Idc

c winding group

d winding group

a winding grouprotor statorPMs

zr

ψ

Rm

la

31 2

Rm=120 mm, la=50 mm

Tem=150 Nm, Ω=120 rpm (Pem=1.9 kW)


d winding groupTem=150 Nm, Ω=120 rpm (Pem=1.9 kW)

• Desired speed Ω and turbine torque Tt (Idc

calculation) as input source signals

•• 44--stars conf. stars conf. ⇔⇔ 11--star conf. star conf.

o higher voltage (output power) and lower

current due to the lower voltage drop

(mainly at high speed)

o Lower rectified voltage ripple (6.2% ↔23.3%) because the phase voltage shifting

300

350

400

450

500

550

600

650

40 50 60 70 80 90 100 110 1200

0.5

1

1.5

2

2.5

3

3.5

UUdcdc

1-star4-stars

Ω [rpm]

[V] [A]

IIdcdc

Commercial Commercial productsproducts ofof largelarge WGsWGs




1. Rotor bearing/gearbox

2. Rotor: blade load-bearing elements in carbon fiber,

blade pitch system inside the rotor hub.

3. Air-treatment: draws in external air and filters salt

and moisture

4. Generator: rotor, mounted on the gearbox output

shaft, doesn't need its own bearings

WG WG costcost sharesshares


Source: Romax Technology (2011)

On On shoreshore WG WG costscosts


614 614 €€/kW/kW 530 530 €€/kW/kW

Commercial Commercial productsproducts ofof smallsmall WGsWGs

Airdolphin Z1000 Skystream 3.7 TVision QR5


Airdolphin Z1000 Skystream 3.7

Zephyr (Japan) Xzeres (USA)

Upwind Downwind

1 kW – 12.5 m/s 2.1 kW – 11 m/s

2.5 m/s (cut-in)50/60 m/s (cut-out/survival)

3.5 m/s (cut-in)63 m/s (survival)

D = 1.8 m D = 3.7 m

17.5 kg (57 W/kg) 77 kg (27.2 W/kg)

52 dBA (12.5 m/s,?) 60 dBA (10 m/s, 20 m)

Off- e on-grid On-grid (120 – 240 V)

TVision QR5

Ropatec (Italy) Quiet Revolution (UK)

3 kW – 13 m/s 6.2 kW – 14 m/s

4 m/s (cut-in)16 m/s (cut-out)

4.5 m/s (cut-in)16 m/s (cut-out)

620 kg (5 W/kg) 450 kg (13.8 W/kg)

7.26 m2 (410 W/m2) 13.6 m2 (456 W/m2)

42 dBA (10 m/s, 20 m) 58 dBA (10 m/s, ?)

Average cost: 4000 €/kW (HA), 6000 €/kW (VA)

SmallSmall WG WG componentscomponents


wind vane

Main bearings

Stator support

Generator support

Yaw bearings

Rotor hub

Pitch actuator

Nacelle frame

Example: small WG plantExample: small WG plant

Synchronous generator20 kVA

400 V, 50 Hz

3-phase rectifier

Booster (DC/DC)Braking resistances

Isolation transformer

meters

Line breaker

High speed

Line contactor

Inverter

Smoothing inductance

Interlocked with the line contactor and controlled by the booster

Power factor correction and filtering

capacitor bank

Galvanic separation with the public grid to block DC components, alternatively the inverter must include appropriate protections (IDC>0.5% Irms)

600 µH – 50÷80 A


Grounding disconnecting

switch

metersspeed circuit

breaker

Supply for protection devices warmer

24 V – 20 W

Booster and brake controlInverter control

ConclusionsConclusions

• Driving role of wind energy in RES

• Issues related to grid management with high penetration of wind generators (production variability and concentration)

• Reliability (basically adoption of mature technologies), but with continuous innovations (blade materials, drivetrainarrangement, power electronics)


arrangement, power electronics)

• Variable speed and PM generators assessed as the best combination as regards efficiency and reliability, though the high PM cost

• Possible introduction of new magnetic configuration for the electric generators in the low-mid power range

References and suggested readingsReferences and suggested readings• Books:o L. Freris, D. Infield, “Renewable Energy in Power Systems”, Wiley, 2008

o M.G. Simoes, F.A. Farret, “Integration of Alternative Sources of Energy”, Wiley, 2006

o J.F. Manwell, J.G. McGowan, A.L. Rogers, “Wind Energy Explained: Theory, Design and Application”, Wiley, 2009

o R. Gasch, J. Twele (editors), “Wind Power Plants: Fundamentals, Design, Construction and Operation”, Springer, 2012

o B. Wu, Y. Lang, N. Zargari, S. Kouro “Power conversion and control of wind energysystems”, Wiley – IEEE Press, 2011


o M. Mueller and H. Polinder (ed.), “Electrical drives for direct drive renewable energy systems”, Woodhead Publishing Limited, 2013

• Papers:o R. Datta and V. T. Ranganathan (2002). Variable-Speed Wind Power Generation Using Doubly Fed Wound

Rotor Induction Machine - A Comparison With Alternative Schemes. IEEE Transactions on Energy Conversion, Vol.17, No.3, September 2002, pp. 414- 421

o M. Andriollo, G. Martinelli, A. Morini, A. Tortella (2007). Performance Assessment of a Wind PM Generator-Rectifier System by an Integrated FEM-Circuit Model. IEEE International Electric Machines and Drives Conference (IEMDC 2007), 3-5 May, Antalya, Turkey.

o S. Soter, R. Wegener (2007). Development of Induction Machines in Wind Power Technology. IEEE International Electric Machines and Drives Conference (IEMDC 2007), 3-5 May, Antalya, Turkey, pp. 1490-1495.

ReferencesReferences and suggested readingsand suggested readings

o S. Kato, Y. Inui, M. Michihira, and Akira Tsuyoshi (2007). A Low-Cost Wind Generator System with a Permanent Magnet Synchronous Generator and Diode Rectifiers. Proc. Of ICREPQ 2007, paper 212, pp. 1-7

o M. Andriollo, M. De Bortoli, G. Martinelli, A. Morini, A. Tortella (2008). Control strategies for a VAWT driven PM synchronous generator. Symp. on Power Electronics, Electrical Drives, Automation & Motion (SPEEDAM2008), 11-13 June, Ischia, Italy.

o M. Andriollo, M. De Bortoli., G. Martinelli, A. Morini, A. Tortella (2008). Permanent Magnet Axial Flux Disc Generator for Small Wind Turbines. 18th Intl. Conf. on Electrical Machines (ICEM 2008), 6-9 Sept., Vilamoura, Portugal.

o H. Li, Z. Chen (2008). Overview of different wind generator systems and their comparisons. IET Renew. Power Gener., 2008, Vol. 2, No. 2, pp. 123–13.


Power Gener., 2008, Vol. 2, No. 2, pp. 123–13.

o D.J. Burnham,S. Santoso, E. Muljadi (2009). Variable Rotor-Resistance Control of Wind Turbine Generators. IEEE Power & Energy Society General Meeting 2009, 26-30 July, Calgary, pp.1-6

o M. Andriollo, M. De Bortoli, G. Martinelli, A. Morini, A. Tortella (2009). Control Strategy of a Wind Turbine Drive by an Integrated Model. Wind Energy, pp. 33- 49, vol.12

o M. Andriollo, M. De Bortoli, G. Martinelli, A. Morini, A. Tortella (2009). Analysis of the air-gap asymmetry in axial-flux permanent magnet generators. IEEE International Electric Machines and Drives Conference (IEMDC 2009), 3-6 May, Miami, USA

o M. Andriollo, G. Bettanini, G. Martinelli, A. Morini, A. Tortella (2009). Analysis of Double-Star Permanent-Magnet Synchronous Generators by a General Decoupled d-q Model. IEEE Transactions on Industry Applications, pp. 1416- 1424, vol. 45

o H. Li, Z. Chen, and H. Polinder (2009). Optimization of Multibrid Permanent-Magnet Wind Generator Systems. IEEE Transactions on Energy Conversion, Vol. 24, No. 1, March 2009, pp. 82-92

ReferencesReferences and suggested readingsand suggested readingso M. Andriollo, M. De Bortoli, A. Tortella (2010). Equivalent circuit for the dynamic analysis of a PM

synchronous generator/back-to-back converter drive for VAWT applications. Symp. on Power Electronics, Electrical Drives, Automation & Motion, 14-16 June, Pisa, Italy

o B. Backlund, S. Ebner (2011). The wind power converter for tomorrow is already here. EWEA Annual Conference Proceedings, 2011.

o F. Blaabjerg, M. Liserre, and K. Ma (2012). Power Electronics Converters for Wind Turbine Systems. IEEE Transactions on Industry Applications, Vol. 48, No. 2, March/April 2012.

o M. L. Henriksen, B. B. Jensen (2009). Comparison of Megawatt-Class Permanent Magnet Wind Turbine Generator Concepts. EWEA Annual Conference Proceedings, 2012

o S. Schmidt, A. Vath (2012). Comparison of Existing Medium-speed Drive Train Concepts with a Differential Gearbox Approach. EWEA Annual Conference Proceedings, 2012


o M. Andriollo, M. De Bortoli, A. Tortella (2013). Variable speed operation of a wind turbine driven self-excited induction generator connected to a low power DC-link. Intl. Conf. on Clean Electrical Power (ICCEP 2013), 11-13 June, Alghero, Italy.

o H. Polinder, J. A. Ferreira, B.B.Jensen, A.B. Abrahamsen, K. Atallah, R. A. McMahon (2013). Trends in Wind Turbine Generator Systems. IEEE Journal of Emerging and Selected Topics in Power Electronics, Vol. 1, No. 3, September 2013.

• Reports/Brochures:o REN21, “Renewables – Global Status Report 2013”, 2014

o EWEA, “Wind in Power – European statistics 2013”, 2014

o GWEC, “Global Wind Report – Annual Market Update 2013”, 2014

o WWEA, “Small Wind World Report 2014”, 2014

ANDREA TORTELLA Wind Energy Tortella

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