Lecture 4 Energy Conversion in Wind Turbine

Tuuli- ja aurinkovoimateknologia ja -liiketoiminta

Wind and Solar Energy Technology and Business

BL20A1200

Lecture 4

Energy Conversion in Wind Turbine

Equations for energy conversion

Newton’s second law for rotation

- angular acceleration [rad/s2_]

- angular speed [rad/s] - rotation angle [rad] T - torque [Nm] J - moment of inertia [kgm2_]

J

T

dt

d

dt

d

Kinetic energy in turbine

Turbine mechanical power

Generator electrical power

2

2

1

J

E

T

P

cos

3

cos

3

U

I

U

I

P

Equations for energy conversion

Effect of gearbox, gear ratio

•

Mechanical power is same on the both sides

•

In analysis parameters are reduced to the same speed

2

60

2

60

n

T

n

T

T

P

J

J

K

K

n=n

n=n

- angular speed [rad/s] K – shaft stiffness [Nm/rad]

J

k

J

K

k

_K

n=n

n

n

k

K

k

K

K

J

k

J

J

WIND TURBINE DRIVE TRAIN

• Wind turbine drive train includes all the components converting turbine mechanical power to electrical power

• Main components are gearbox, generator and power converter

• Traditional wind turbine drive train setup

• 3-stage gearbox

• high speed induction generator (double fed induction generator)

• Power converter for frequency control

• Sometimes called as ”Danish concept”

• PM generator technology and high gear box failure ratio in the past has brought alternative solutions to the market

• Modern turbines use also other drive train alternatives

• Direct Driven system without gearbox

• Low speed ratio gearbox

Drive train setup with high speed generator

WIND TURBINE DRIVE TRAIN

GEARBOX

• Typical gear topology consists of two planetary stages

and one helical stage or one planetary and two helical stages

• Gear ratio about 1:100 or more needed traditionally

• With one planetary stage about 1:6 gear ratio possible

• Planetary stages are used in low speed end

• Helical stages used at high speed end

• Efficiency approximately 98 %

• Oil cooling – oil to air heat exchanger

• Manufacturers Moventas, Hansen, Bosch Siemens, GE Transmission

www.moventas.com

WIND TURBINE DRIVE TRAIN

GEARBOX

• Example of compound planetary helical gearbox structure

http://www.nrel.gov/wind/pdfs/45325.pdf

WIND TURBINE DRIVE TRAIN

Example of WT speed and power [1]

• Figure shows 1 MW WT aerodynamic power

curves at different wind speeds (5 m/s…14 m/s)

• Additionally two possible generator power

curves have been presented

• If constant generator speed is applied, optimal tip

speed ration can be achieved only with a certain wind speed (here between 6 and 7 m/s)

• With variable speed generator the optimal

operational point can be achieved in the whole wind speed range

• In the past, fixed speed generators were applied,

• Today, all modern large scale (P > 1MW) wind

turbines have some method to vary the generator speed

WT power output at different wind

speeds and two different contol schemes [1]

Electric Drive in Wind Turbine

Traditional Electrical Drive solutions

• Fixed speed asynchronous generator

• Two speed asynchronous generator

• Wounded rotor asynchronous generator with variable rotor resistance

Modern Electrical Drive solutions

• Double fed induction generator (DFIG)

• Electrically excited synchronous generator (EESM)

• Permanent magnet synchronous generator (PMSM)

Drive Train Alternatives in WT

Fixed speed Generator

Doubly-fed induction Generator

Permanent Magnet Generator

Superconducting generator

1980 1990 2000 2010 2020 2030

Technology trends in WT Drive trains

Fixed speed asynchronous generator

• Asynchronous generator torque production is based on the slip frequency

• Slip s is relative speed difference between

synchronous speed and rotor mechanical speed

Example: f_grid = 50 Hz, number of pole pairs p = 2, nominal slip is -3% (negative slip in generator mode),

then nominal speed n_nom is

Drive Train Alternatives in WT

n

n

n

s

Torque curve of asynchronous

generator [1]. Speed range between n₀ And n_nom

p

f

n

60

1545

2

50

60

03

.

0

1

1

s

n

n

rpm

rpm

Two speed asynchronous generator

• As shown in previous example, asynchronous generator has a very narrow speed range, (1500-1545) rpm

• When generator speed can not be controlled, the blades must be designed to work properly on wider tip speed range

• If the generator has two sets of stator windings representing two different pole pair numbers, a two speed generator is obtained

• Then generator has two operational modes: low and high wind speed mode (this is referred as Danish concept in [2])

• If generator has pole pair numbers p=2 and p=3, (s= - 0.03, f = 50 Hz), two nominal speeds for low and high wind speeds are achieved

Drive Train Alternatives in WT

1030

3

50

60

03

.

0

1

n

1545

2

50

60

03

.

0

1

n

rpm

rpm

Wounded rotor induction generator

• This slip frequency can be adjusted by changing the rotor resistance

• If rotor resistance is to be changed, a wounded rotor and slip rings to access rotor winding system are needed

• If addditional resistance is connected to rotor winding via slip rings, a torque characteristics of the machine can be changed

• Fig. shows the torque curve with different rotor resistance values’

• Rotor resistance adjustment can be used to extend the torque control of asynchronous generator

• This feature has been used e.g. in Vestas Optislip turbine control method

Drive Train Alternatives in WT

Torque curves of asynchronous

Double fed induction generator

• In double fed induction generator stator is directly grid connected and wounded rotor is connected to the grid through a frequency converter

• This way the electrical frequency of the rotor can be tuned independently on the mechanical speed

• Example: 50 Hz DFIG, pole pair number p=2, should rotate 1350 rpm. What electrical frequency should be supplied to the rotor?

Stator has grid frequency 50 Hz and p=2 -> stator magnetic field has synchronous rotating speed n₀= 1500 rpm The electrical frequency in stator and rotor must be the same f_r,s= f_s=50 Hz in stator coordination system

Rotor rotates 1350 rpm, which is 90% of synchronous speed. Then electrical rotor frequency must add 10% to the mechanical speed (=5 Hz), to have the same electrical frequency on both stator and rotor.

Thus rotor inverter supplies + 5 Hz frequency to the rotor • Slip in the double fed drive is defined as

• In double fed system both stator and rotor windings participate to power generation

• Typically rotor winding creates maximum 1/3 of the whole generator power

• If rotor rotates slower then stator magnetic field, rotor consumes power

• If rotor rotates faster than stator magnetic field, rotor produces power

Drive Train Alternatives in WT

Layout of DFIG

Drive Train Alternatives in WT

Double fed induction generator layout, source The Switch

• Stator directly connected to the grid

• Rotor connected to the grid through power converter

• Additionally system includes crowbar to protect converter in grid voltage drop

Features of DFIG

Drive Train Alternatives in WT

• DFIG system design realizes typically 1:2 speed range

• Speed range limited by the rotor power

• Winding system

• Slip rings

• Converter

• At low rotational speed n < n_sync, rotor is consuming power (P₂ > 0)

• At high rotational speed, n>n_sync, rotor is producing power P₂ < 0)

• The total generator power (P_el) is a sum of rotor and stator power, as shown in figure.

• The rotor converter can also control reactive power of the system

• Reactive control capability is limited due to limited current ratings of the rotor converter

• In grid voltage transients the rotor circuit must be shortcircuited to protect rotor converter (crowbar)

• DFIG system has a limited capability to handle grid voltage transients

• Additional equipments might be ineccessary to fulfil new grid codes

3 4 3 2 sync n n

Stator (P1) and rotor power (P2) of DFIG generator as a function of speed [1]

Synchronous generators

• In synchronous generator rotor rotates in synchronism with the magnetic field

• Traditionally electrically excited synchronous generators (EESM) have been used in power production with direct grid connection

• Since the WT requires some torque and speed control, synchronous generator requires always frequency conversion, when applied to WT-application

• Slow speed EESM’s have been adapted also to wind power applications

• Permanent magnet synchronous machines (PMSM) have been developing rapidly

during the last decade offering an advantageous solution also for WTs

• Compared to EESM, PMSM is more simple due to lacking rotor windings and slip rings

• New turbine designs have more and more PM drives

• PMSM technology is available in different speed ranges

• high speed ( > 500 rpm)

• Medium speed (100…500 rpm)

• Direct driven (< 30 rpm)

Electrically exited synchronous generator

• ENERCON applies DD EESM in the range of 300 kW…7500 kW

• Diode rectifier is used to connect generator stator to the DC-link

• Torque control is implemented by controlling both excitation current and DC-link voltage

• Stator current results from the voltage difference between diode bridge output voltage and dc-link voltage

• Inverter controls both active and reactive power of the wind turbine

• Output current must be filtered to fulfill grid harmonic requirements

• IEC 6100-3-2 defines harmonics limits

Drive Train Alternatives in WT

Figure 9. Example of PWM waveforms [2] ENERCON drive train, [www.enercon.de]

Drive Train Alternatives in WT

Layout of PM drive

• Generator controlled by generator inverter

• Grid current controlled by grid inverter

• Good control dynamics on both generator and grid side

• Well controllable also during the grid transients

Permanent magnet synchronous generator

• Modern concept for wind power drive train is PM generator and full power converter

• Both generator and grid currents are fully controlled

• PM machine can be design with good efficiency both for high and low speeds

• Direct drive machines have higher losses due to more complicated winding structure, but on the other hand gearbox losses are elimiated

• Full power conversion allows speed range from zero to overspeed range

• PM-concepts enables thus wider design freedom to aerodymics of turbine

Drive Train Alternatives in WT

Examples of low, medium and high speed PM wind power generators

0 10 20 30 40 50 60 70 80 90 100 20 30 40 50 60 70 80 90 100 Dr ive train eff icienc y

Wind speed (% of rated)

DFIG PMHS PMMS PMDD Double-fed induction generator

Permanent magnet high speed

Permanent magnet medium speed

Permanent magnet direct drive

Drive Train Alternatives in WT

Comparison of typical efficiencies

EXAMPLE I, VESTAS V80 DRIVE TRAIN

http://www.vestas.com/en/media/brochures.aspx: V80

Rated power 2000 kW Wind class IEC1A

Rotor speed 10.8-19.1 rpm Generator 4 pole asynchonous Gearbox 3-stage planetary/helical Rotor diameter 80 m

Rated wind speed 16 m/s Tip speed ratio??

(rated point)

Gear ratio approximation

- Generator max. speed 1700..1800 rpm - Rotor maximum speed 19.1 rpm

0 . 5 16 60 40 2 1 . 19 94 ... 89 1 . 19 1800 .. 1700 k

DRIVE TRAIN EXAMPLES

DRIVE TRAIN EXAMPLES

BASE LINE

Alstom ECO 100 Platform

•

Double fed generator

•

Stator voltage 1000 V

•

Three stage gearbox

•

Tip speed ratio at

nominal point 6.57

www.alstom.com

DRIVE TRAIN EXAMPLES

BASE LINE

DRIVE TRAIN EXAMPLES

DIRECT DRIVEN

• Due to some failure cases related to gearboxes interest towards direct driven systems has increased

• Specially in offshore technology gearless systems are preferred

• In direct driven technology low speed synchronous generator is used

• Induction generator design not feasible for low speed due to complexity and heavy losses

• Electrically excited direct driven generator solution by Enercon has been on the market more than a decade

• Permanent magnet direct driven generators were introduced during the last decade

• Scanwind introduced 3.7 MW DDPM turbine 2005

• Goldwind has used PMDD outor rotor generator almost 10-years

DRIVE TRAIN EXAMPLES

DIRECT DRIVEN: CASE ENERCON

• Enercon has been a pioneer in direct driven technology

• Technology basis is electrically excited direct driven synchronous generator

• Relatively expensive machine structure due to wounded rotor

• Very good controllability-wide speed control area with high efficiency

• Own generator technology, not outsourced

DRIVE TRAIN EXAMPLES

DIRECT DRIVEN: CASE GE ENERGY

• Scanwind / GE Energy has a direct driven PM concept for offshre

• Generator technology has been developed by The Switch in co-operation with LUT

• Rotor and generator as counterweights

• Three parallel power threads

DRIVE TRAIN EXAMPLES

DIRECT DRIVEN: CASE SIEMENS

• Siemens has introduced new DDPM product SWT 3.0 101

• Outer rotor PMDD generator

• Nacell weight has been reduced

DRIVE TRAIN EXAMPLES

DIRECT DRIVEN: CASE GOLDWIND

• One of the main products is GW 1.5 MW PMDD

• Thousands of installed turbines in China

• Biggest PMDD market share in the world (OP’s estimate)

• Important customer to The Switch

DRIVE TRAIN EXAMPLES

MEDIUM SPEED DRIVE TRAIN

• Geared wind power drive train is lighter than direct driven

• When higher nacelle elevation is a target, nacelle weight should be reduced

• By reducing gear stages weight can be reduced and reliability increased

• Medium speed drive train system has been developed as a compromise

• Multibrid is a patented technology, where one stage gearbox has been integrated with generator

• Benefits from DD and geared system are combined

• WinWind uses multibrid technology in 1 MW and 3 MW turbines

DRIVE TRAIN EXAMPLES

MEDIUM SPEED DRIVE TRAIN: CASE WinWind

Source: WWD-3 datasheet

Note: Electric power is typically

NOT Electric power to the grid!!

DRIVE TRAIN EXAMPLES

SPECIAL TOPOLOGIES: CASE CLIPPER

Clipper Liberty uses multiple generators

TURBINE CONTROL BASICS

CONTROL MODES

•

At low wind speeds control goal is constant tip speed ratio

•

Pitch angle is not controlled

•

Torque is adjusted according to generator speed

•

Close to nominal wind speed control goal is constant rotational speed

•

Torque is adjusted by a speed controller

•

Above nominal wind speed control goal is constant power and speed

•

Generator torque and speed are kept constant

TURBINE CONTROL BASICS

CONTROL MODES

A – Constant tip speed ratio, torque adjusted by generator speed B – Constant speed – torque adjusted by speed controller

C – Constant power – turbine power adjusted by pitch control

Rotational speed vs. torque and power* Wind speed vs. torque and power

*)Calculated from Enercon E82 power curve with nominal speed assumption 17 rpm (uncertain)

40 60 80 100 120 140 160 180 0 500 1000 1500 2000 2500 0 5 10 15 20 25 0 500 1000 1500 2000 2500

A

B

C

A

B

C

TURBINE CONTROL BASICS

A.D. Wright and L.J. Fingersh , Advanced Control Design for Wind Turbines Part I: Control Design, Implementation, and Initial Tests, Technical Report , NREL/TP-500-42437, March 2008

TURBINE CONTROL BASICS

PITCH CONTROL

•

Above nominal wind speed turbine

power is kept constant by reducing the

power coefficient

•

In pitch controlled turbines this is done

by turning the blades

•

In commercial turbines pitch control uses

same reference for all the blades

•

The rate of change is typically less than

5 deg/sec in large turbines

•

Both hydraulic and electrical pitch

actuators are used

TURBINE CONTROL BASICS

YAW CONTROL

• Turbine should be directed to wind with good accuracy

• Yaw control is using wind direction measurement as feedback

• Yaw rates are typically less than 1 deg/s to avoid dangerous gyroscopic forces

• Simple PI-controller with a very slow bandwidht can be applied

• Additional logic is needed to avoid cable twisting in tower (max angle 360 deg)

Measured wind direction at Puumala, Finland. Masurements by LUT Energy & FMI

Note the wide fluctuation of wind direction at low hights (measurement points 90, 60 and 30 m)

WIND TURBINE STANDARDS

WIND TURBINE STANDARDS

LITERATURE

Interesting Web pages

Turbine manufacturer home pages

Vestas, Siemens, GE Energy, Gamesa, GoldWind, Sinovel, WinWind

Component manufacturers

LITERATURE

References

[1] Martin O.L. Hansen, Aerodynamics of Wind Turbines, 2nd edition, 2008

[2] Manfred Stiebler, Wind Energy Systems for Electric Power Generation, 2008

[3] National Renewable Energy Laboratory, WindPACT Drive Train Alternative

Design Study Report, 2005 USA

[4] Connection Code for Connection of Wind Power Plants to Finnish Power

System, Fingrid 31.3.2009