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TRANSCRIPT
Determination of Best Drive Train Technology for Future
Onshore Wind Turbines as a Function of the Output Power
Emre Aydin
Master of Science ThesesBarcelona, Spain
2013
!The!Department!of!Electrical!Engineering!of!the!Eindhoven!University!of!Technology!accepts!no!responsibility!for!the!contents!of!MSc.!theses!or!practical!training!reports!
Determination of Best Drive Train Technology for Future
Onshore Wind Turbines as a Function of the Output Power
Emre Aydin
Master Theses in Sustainable Energy Technologyat the Department of Electrical Engineering
of the Eindhoven Technical University Eindhoven, Netherlands
July 2013
Supervisors
Prof. Elena Lomonova, TU/eDr. Sergi Ratés, Alstom Power
Abstract
Till today several unique drivetrain designs have been developed to reduce the weight and cost however it resulted in differing problems such as high cost, heavy weight, low reliability, dependence on supplier, transportability etc. Additional to that, the fact that there is quite few onshore wind turbines in the 3-5 MW range constituted the scope of this study, which conveys the comparison work of different drivetrains and as a result shows the best drivetrain decision for future onshore topology.
Chapter 2, 3 and 4 describe the cost and weight calculation of wind turbine generators, gearboxes and converters, respectively, by developing new formulas.
Chapter 5 summarizes the requirements for accesses and the challenges faced during transportation process of the drivetrain components.
Chapter 6 outlines the O&M frequencies and costs, bringing up preventive acts in order to avoid acute reactive maintenances.
Chapter 7 informs about grid integration and transmission topics along with advantages and disadvantages of different voltage drivetrain applications.
Chapter 8 discusses the one of the most important subject, reliability of drivetrain components.
Chapter 9 provides a futuristic look on drivetrain developments and abridges the new trends in the market.
Ultimately, the comparison of different drivetrain topologies is presented and the conclusion for the best drivetrain configuration is attained.
!
Acknowledgment
Since the beginning of my research traineeship along with my master thesis on September 2012, I have had the great eleven months at Alstom Power in Barcelona. For that reason, I would like offer my sincerest gratitude to Marc Sala and Maurits Ornstein who provided me this opportunity.
I would like to deeply thank you to my supervisors Prof. Elena Lomonova and Dr. Sergi Ratés for their support throughout my thesis with their patience and knowledge.
I would like to present my highest appreciation to Héctor Ortiz de Landázuri Díaz for his friendly and cheerful attitude in my daily work.
I am very grateful to Jessica Svensson, the special person in my life, who did not deny anything but gave me her love and continuous support.
I would like to express my thankfulness to my beloved mother, who highly believed in the importance of education and went to great lengths to be able to fulfill my academic desires.
Lastly, I give my special thanks to my dear friend, Stefan Kamp, for his extreme generosity, support and loyalty.
Barcelona / 2013
Emre Aydin
i
Contents!
CONTENTS' I!
LIST'OF'FIGURES' III!
LIST'OF'TABLES' IV!
GLOSSARY' V!
1.! INTRODUCTION' 1!
2.! COST'AND'WEIGHT'CALCULATION'OF'WIND'TURBINE'GENERATORS' 6!2.1.! DESIGN'AND'ECONOMIC'MODEL'OF'HIGH:SPEED'WIND'GENERATORS' 6!2.1.1.! DESCRIPTION!OF!DFIG_3G! 6!2.1.1.1.! Weight!and!cost!estimations!of!DFIG_3G! 7!2.1.2.! DESCRIPTION!OF!SCIG_3G! 9!2.1.2.1.! Weight!and!cost!estimations!of!SCIG_3G! 10!2.1.3.! WEIGHT!AND!COST!ESTIMATIONS!OF!PMSG_3G! 13!2.1.4.! WEIGHT!AND!COST!ESTIMATIONS!OF!PMSG_4G! 15!2.2.! DESIGN'AND'ECONOMIC'MODEL'OF'MEDIUM:SPEED'WIND'GENERATORS' 17!2.2.1.! WEIGHT!AND!COST!ESTIMATIONS!OF!PMSG_1G! 18!2.2.2.! WEIGHT!AND!COST!ESTIMATIONS!OF!PMSG_2G! 21!2.3.! DESIGN'AND'ECONOMIC'MODEL'OF'LOW:SPEED'WIND'GENERATORS' 24!2.3.1.! WEIGHT!AND!COST!ESTIMATIONS!OF!PMSG!_DD! 25!
3.! COST'AND'WEIGHT'CALCULATION'OF'WIND'TURBINE'GEARBOXES' 29!3.1.! DESIGN'AND'ECONOMIC'MODEL'OF'SINGLE:STAGE'GEARBOX' 29!3.2.! DESIGN'AND'ECONOMIC'MODEL'OF'TWO:STAGE'GEARBOX' 32!3.3.! DESIGN'AND'ECONOMIC'MODEL'OF'THREE:STAGE'GEARBOX' 35!3.4.! DESIGN'AND'ECONOMIC'MODEL'OF'FOUR:STAGE'GEARBOX' 38!
4.! COST'AND'WEIGHT'CALCULATION'OF'WIND'TURBINE'CONVERTERS' 40!4.1.! DESIGN'AND'ECONOMIC'MODEL'OF'PARTIAL:SCALE'POWER'CONVERTER' 41!4.2.! DESIGN'AND'ECONOMIC'MODEL'OF'FULL:SCALE'POWER'CONVERTER' 44!
5.! TRANSPORTABILITY'OF'DRIVETRAIN'COMPONENTS' 47!
6.! O&M'FREQUENCIES'AND'COSTS' 52!
7.! GRID'INTEGRATION'&'TRANSMISSION' 56!
8.! RELIABILITY' 61!
ii
9.! NEW'TRENDS' 65!
CONCLUSION'&'RECOMMENDATIONS' 67!
BIBLIOGRAPHY' 72!
iii
List!of!Figures!
Figure'I'Sketch'of'wind'turbine'drivetrain'system'and'data'flow'within'[1]' 1!Figure'II'Typical'configuration'of'DFIG_3G'wind'energy'system'[50]' 6!Figure'III'DFIG_3G'weight'as'a'function'of'rated'power' 8!Figure'IV'DFIG_3G'cost'as'a'function'of'rated'power' 9!Figure'V'Variable'speed'squirrel'cage'induction'generator'[51]' 10!Figure'VI'SCIG_3G'weight'as'a'function'of'output'power' 11!Figure'VII'SCIG_3G'cost'as'a'function'of'output'power'compared'to'DFIG_3G'cost' 13!Figure'VIII'PMSG_3G'generator'speed'vs.'generator'torque'as'a'function'of'rated'power' 14!Figure'IX'PMSG_3G'generator'weight'and'cost'estimation' 15!Figure'X'PMSG_4G'generator'speed'vs.'generator'torque'as'a'function'of'rated'power' 16!Figure'XI'PMSG_4G'generator'weight'and'cost'estimation' 17!Figure'XII'MediumQspeed'PMSG'system'[60]' 18!Figure'XIII'Lamination'stack'mass'in'percentage'in'PMGs'as'a'function'of'generator'speed' 19!Figure'XIV'Material'shares'in'the'generator'of'PMSG_1G' 20!Figure'XV'PMSG_1G'generator'weight'and'cost'estimation' 21!Figure'XVI'Copper'winding'usage'in'percentage'in'PMGs'as'a'function'of'generator'speed' 22!Figure'XVII'Material'shares'in'the'generator'of'PMSG_2G' 23!Figure'XVIII'PMSG_2G'generator'weight'and'cost'estimation' 24!Figure'XIX'LowQspeed'PMSG'system'outline'incl.'fullQscale'power'converter'[59]' 25!Figure'XX'Structural'mass'in'percentage'in'PMGs'as'a'function'of'generator'speed' 25!Figure'XXI'PMSG_DD'weight'as'a'function'of'rated'power' 26!Figure'XXII'Estimated'generator'weight'distribution'for'directlyQdriven'PMG' 27!Figure'XXIII'PMSG_DD'total'cost'as'a'function'of'rated'power'and'materials'costQbreakdown' 29!Figure'XXIV'Proposed'gear'design'for'singleQstage'gearbox'(a)'in'reference'to'M5000'(b)'[16]' 30!Figure'XXV'PMSG_1G'weight'estimation'as'a'function'of'rated'power'and'gear'ratio' 31!Figure'XXVI'PMSG_1G'(gearbox'+'generator)'cost'estimation'as'a'function'of'rated'power' 32!Figure'XXVII'2Qstage'gear'technology'from'Winergy'[16]' 33!Figure'XXVIII'2Qstage'gearbox'weight'and'cost'estimation' 34!Figure'XXIX'Conventional'drivetrain'vs.'PMSG_2G' 35!Figure'XXX'Several'threeQstage'gearbox'concepts'[19]' 36!Figure'XXXI'3Qstage'gearbox'weight'estimation'as'a'function'of'rated'turbine'torque' 36!Figure'XXXII'3Qstage'gearbox'cost'estimation'as'a'function'of'rated'turbine'torque' 37!Figure'XXXIII'4Qstage'differential'gearbox'design'from'Bosch'Rexroth'[23]' 38!Figure'XXXIV'4Qstage'gearbox'weight'and'cost'estimation' 40!Figure'XXXV'Outline'of'DFIG'system'together'with'partialQconverter' 42!Figure'XXXVI'Commercial'LV'partialQscale'wind'converters’'weight'as'a'function'of' 43!Figure'XXXVII'PartialQscale'power'converter'weight'and'cost'estimation' 44!Figure'XXXVIII'Main'HW'diagram'of'B2B'multilevel'IGBT'full'power'converter' 45!Figure'XXXIX'FullQscale'power'converter'weight'and'cost'estimation' 46!Figure'XL'Sketch'of'xDFM'topology'[29]' 47!Figure'XLI'a)'Modular'heavy'haul'trailer'with'special'lowQdeck'[30],' 48!Figure'XLII'Horizontal'swept'path'analysis'of'nacelle'transportation'[31]' 49!Figure'XLIII'Nacelle'and'tower'deployment'of'DFIG_3G'wind'turbine'on'a'sea'vessel'in'an' 50!Figure'XLIV'Scope'of'wind'turbine'O&M'[40]' 53!Figure'XLV'Schema'of'power'network'interconnection' 56!Figure'XLVI'Reactive'power'variation'capability'at'different'regions' 57!Figure'XLVII'Onshore'HVDC'configurations'[47],' 58!Figure'XLVIII'Relative'costs'for'connecting'wind'parks'across'EUQ27'[49]' 60!Figure'XLIX'Failure'distribution'within'gearbox'[40]' 62!
iv
Figure'L'Failure'distribution'within'generator'[42]' 63!Figure'LI'Annual'failure'rate'and'down'time'statistics'[33]' 65!Figure'LII'Comparison'of'seven'drivetrain'topologies'for'3'MW'category' 68!Figure'LIII'Comparison'of'seven'drivetrain'topologies'for'4'MW'category' 70!Figure'LIV'Comparison'of'seven'drivetrain'topologies'for'5'MW'category' 70!
!
!
List!of!Tables!
Table'1'Wind'turbine'categorization'as'a'function'of'generator'speed' 4!Table'2'Wind'turbine'characteristics'used'for'the'drivetrain'comparison' 5!Table'3'Advantages'and'disadvantages'of'DFIG'system' 7!Table'4'Material'properties'of'stator'laminations'[7]' 18!Table'5'Material'specific'costs'in'April'2013' 28!Table'6'Comparison'of'partialQscale'and'fullQscale'power'converter'[33][37][38][39]' 41!Table'7'Transportation'requirements'for'nacelle'of'DFIG_3G' 51!Table'8'Transportation'cost'model'for'a'recent'wind'farm'project'in'Europe'(2013)' 51!Table'9'Preventative'O&M'frequencies'of'DFIG'system’s'drivetrain' 55!Table'10'Key'features,'investment'costs'and'surface'occupation'of'FACTS'technologies'[48]' 59!Table'11'Comparison'of'LV'and'MV'drivetrains'[34][35][36]' 61!Table'12'Top'failures'in'drivetrain'[28][43][44][45]' 62!
v
Glossary!
AEP' Annual!Energy!Production'
BDFIG' Brushless!Doubly!Fed!Induction!Generator!
B2B' Back!to!Back'
CAPEX' Capital!Expenditure'
CC' Chain!Chassis'
CMS' Conditional!Monitoring!System'
CRB' Cylindrical!Roller!Bearing'
CVT' Continuously!Variable!Transmission!
DFIG_3G' Doubly!Fed!Induction!Generator!with!3Vstage!Gearbox'
DSO' Distribution!System!Operator'
EEG' Electrically!Excited!Generator'
FACTS' Flexible!Alternating!Current!Transmission!System!
FOC' Field!Oriented!Control'
FRT' Fault!Ride!Through'
GCC' Grid!Connected!Converter'
GR' Gear!Ratio!
GSC' Generator!Side!Converter'
HS' High!Speed'
HSS' High!Speed!Shaft'
HTS' High!Temperature!Superconductivity!
HVAC' High!Voltage!Alternative!Current'
HVDC' High!Voltage!Direct!Current!
IGBT' Insulated!Gate!Bipolar!Transistor'
IGCT' Insulated!Gate!Commutated!Transistor'
IMS' Intermediate!Shaft'
LCOE' Levelized!Cost!of!Energy'
LIDAR' Light!Detection!and!Ranging'
LS' Low!Speed'
LSC' Line!Side!Converter'
LSS' Low!Speed!Shaft'
LTS' Low!Temperature!Superconductivity'
LV' Low!Voltage!
LVRT' Low!Voltage!Ride!Through'
vi
MS' Medium!Speed'
MSC' Machine!Side!Converter'
MTBF' Mean!Time!Between!Failures'
MV' Medium!Voltage!
OHS' Occupational!Health!and!Safety'
OPEX' Operating!Expenditure'
O&M' Operation!&!Maintenance'
PCC' Point!of!Common!Coupling'
PM' Permanent!Magnet!
PMG' Permanent!Magnet!Generator'
PMSG_DD! Direct!Drive!Permanent!Magnet!Synchronous!Generator'
PMSG_1G' Permanent!Magnet!Synchronous!Generator!with!singleVstage!Gearbox!
PMSG_2G' Permanent!Magnet!Synchronous!Generator!with!2Vstage!Gearbox'
PMSG_3G' Permanent!Magnet!Synchronous!Generator!with!3Vstage!Gearbox'
PMSG_4G' Permanent!Magnet!Synchronous!Generator!with!4Vstage!Gearbox!
PWM' Pulse!Width!Modulation'
SCADA' Supervisory!Control!and!Data!Acquisition'
SCIG_3G' Squirrel!Cage!Induction!Generator!with!3Vstage!Gearbox'
SRB' Spherical!Roller!Bearing'
STATCOM' Static!Synchronous!Compensator'
SVC' Static!VAR!Compensator'
TC' Truck!Chassis'
TRB' Tapered!Roller!Bearing'
TSO' Transmission!System!Operator'
WRSG' Wound!Rotor!Synchronous!Generator'
Aydin, 7/21/13 1
1. Introduction!
Figure I Sketch of wind turbine drivetrain system and data flow within [1]
As the advancements in wind power technologies speed up the focus is
concentrated largely on the methods of designing the most cost-competitive,
efficient and reliable wind turbines. In order to realize this aim various
drivetrain topologies – one of the highest priced key component in a standard
wind turbine (see Figure I)– are designed and presented to the wind energy
market. Till today several unique drivetrain designs have been developed to
reduce the weight and cost however it resulted in differing problems such as
low reliability, dependence on supplier, transportability etc. Some
Aydin, 7/21/13 2
manufacturers stay conservative and continue their turbine assembly with the
conventional group of Doubly-Fed-Induction-Generator (DFIG), partial-scale
power converter and different-staged gearboxes whereas some try to catch up
with the new trends and shift to the group of Permanent-Magnet-Generator
(PMG) and full-scale power converter eliminating the gearbox from the
nacelle. The recent tendency shows that hybrid design by combining PMG
and lower-stage gearbox unit attracts the market’s attention in great numbers.
Provided with this general picture the first thing that springs to mind is that:
Which one of those three is the best drivetrain technology? Following this
question the further inquiries could be: Which parameters should be taken into
account by determining the best technology? What are the strengths and
weaknesses of those existing systems? What can be the new trends in the
future? Could hydraulic-drive variable transmission systems and direct-drive
superconducting generator systems play a role in the futuristic trend? In this
graduation project the answers to those questions are provided and further
studies are carried out.
The aim of the project is to determine the best drivetrain technology for the
future wind turbines by decreasing the weight, CAPEX/OPEX and losses of
the drivetrain, and further by improving its reliability, which will result in
higher annual energy yield. To detail more specifically, it is to develop a new
onshore wind turbine drivetrain, which will have a rated power range between
3 MW and 5 MW, for Alstom Power, Barcelona.
The methodology of realizing the project begins with comparison of 38
existing onshore wind turbines with the rated power of 3 MW and more
among each other. Following the comparison, 14-selected wind turbines,
based on their various applied drivetrain technologies, segmented generator,
generator voltage, rated power, accessibility to the product documentation, are
sub-categorized into 7 categories (see Table 1). The categorization is
Aydin, 7/21/13 3
performed according to generator types and number of gearbox stages as a
function of generator speed.
Aydin, 7/21/13 4
Table 1 Wind turbine categorization as a function of generator speed
Aydin, 7/21/13 5
For the next step, alternative drivetrain concepts are further looked into
together with Alstom’s ECO 100 – 3 MW wind turbine and new drivetrain
concept is determined by taking into account those two major parameters:
weight and cost. The secondary, also highly important parameters are:
reliability, grid integration, maintainability, transportability, different
operating voltages and possible risks. During the study, trade-off is set among
various criteria so that the optimum wind drivetrain is determined objectively.
In Table 2 wind turbine characteristics used in the study for drivetrain
comparison are depicted.
Table 2 Wind turbine characteristics used for the drivetrain comparison
Rated&power&(kW) 3000 4000 5000
Rotor&diameter&(m) 100 110 120
Rated&speed&(rpm) 14 12.7 11.6
Angular&speed&(rad/s) 1.47 1.33 1.22
Tip&speed&(m/s)
Rated&torque&(Nm) 2046277.8 3013584.7 4109699.0
Air&density&(kg/m3)
Maximum&power&coefficient
Optimum&tip&speed&ratio
SingleMstage&gearbox&service&factor&FsSingleMstage&gearbox&weight&factor&FwPlanet&wheels&number&in&a&singleMstage&gearbox&ZWheel&ratio&RwSingleMstage&gearbox&ratio&RratioSingleMstage&gearbox&output&torque&Tm-(kNm) 227.36 334.84 456.63
Lamination&stack&specific&cost&(€/kg)
Structure,&bearings,&cooling&specific&cost&(€/kg)
Copper&wire&specific&cost&(€/kg)
Permanent&magnet&specific&cost&(€/kg)
Wind&turbine&characteristics
Cost&modeling
3
15
15
167
10
73.00
27.40
1.225
0.48
7
Gearbox¶meters
1.25
3.5
9
Aydin, 7/21/13 6
2. Cost'and'Weight'Calculation'of'Wind'Turbine'Generators'
2.1. Design'and'economic'model'of'high;speed'wind'generators'
2.1.1. Description'of'DFIG_3G'
DFIG_3G system is the most preferred drivetrain configuration comprising of
variable speed doubly-fed induction generator (see Figure II), three-stage
gearbox and a partial-scale power converter. The system connects the stator
directly to the grid to where the rotor circuit follows a connection, too,
through the power converter. In most standard systems of DFIG, the variable
speed range is around ±30% of the synchronous speed and the partial-scale
power converter rated between 25-30% is sufficient for controlling the rotor
within required speed range. Advantages and disadvantages of DFIG system
are presented in Table 3.
'Figure II Typical configuration of DFIG_3G wind energy system [50]
DFIG Advantages
Disadvantages
independent active/reactive power control limited reactive power supply
mature and economic design
high torques under faulty conditions
voltage support to grid
complex converter control
magnetization from rotor circuit
high maintenance of brush/slip ring
power factor control
limitation required on start-up
Aydin, 7/21/13 7
current
constant voltage frequency and amplitude reduced torsional stiffness
annual replacement of brush intensive gearbox maintenance
Table 3 Advantages and disadvantages of DFIG system
2.1.1.1. Weight*and*cost*estimations*of*DFIG_3G*
In this study, Alstom Power’s ECO 100–3 MW, 1800 rpm/50 Hz and ECO
80-1.7 MW, 1800 rpm/50 Hz onshore wind turbines are taken as reference.
For further support referencing and proof-checking Fingersh et al.’s technical
report “wind turbine design cost and scaling model” is used, which suggests
the following formula [2]:
!"#"$%&'$!!"#$%&! = !!.!"! ∗ !!"#$%&'!!"#$%&!.!""# ( 2.1 )
Turbine rating: in kW
Generator weight: in kg
Applying eq. ( 2.1 ) on this paper’s focus turbine ratings of 3 MW, 4 MW and
5 MW, the results return as it is presented in Figure III.
Aydin, 7/21/13 8
Figure III DFIG_3G weight as a function of rated power
Using the results from Figure III and comparing with internal data provided
from two other generator suppliers the estimated results prove themselves to
be correct. However, since the eq. ( 2.1 ) doesn’t hold correctly for doubly-fed
generators with lower rating than 3 MW, a calculation model for DFIG_3G as
in eq. ( 2.2 ) is suggested in order to decrease the slight deviation created by
eq. ( 2.1 ).
!"#"$%&'$!!"#$%&!"#$_!" !
= !!.!"## ∗ !"#$%&'!!"#$%&! + !.!!"# ∗ !"#$%&'!!"#$%&!+ !.!"!#!
( 2.2 )
Turbine rating: in MW
Generator weight: in tons
According to DFIG producers DFIGs featuring three-stage gear and 1800
rpm/50 Hz can be priced between 7.1 €/kg and 7.4 €/kg. Pricing could be
quite volatile due to negotiating conditions (e.g. serial production or
prototype, cover of maintenance costs etc.). In reference to those conditions
0"2"4"6"8"10"12"14"16"18"
0" 1" 2" 3" 4" 5" 6"
Wei
ght o
f DFI
G (t
ons)
Rated Power (MW)
DFIG_3G generator weight calculation
Aydin, 7/21/13 9
and in order to provide a standardized cost calculation model for the generator
of DFIG_3G system, specific cost of 7.1 €/kg is recommended as in eq. ( 2.3 )
and the costs for 3 MW, 4 MW and 5 MW generators are projected in Figure
IV.
Figure IV DFIG_3G cost as a function of rated power
!"#"$%&'$!!"#$!"#$_!" != !!.! ∗ !"#"$%&'$!!"#$%&! ( 2.3 )
Generator cost: in €
Generator weight: in kg
2.1.2. Description'of'SCIG_3G'
SCIG_3G concept (see Figure V) utilizes variable speed squirrel cage
induction generator, three-stage gearbox and a full-scale power converter. The
SCIG is known as rugged and economical machine with its mechanical
simplicity, whereas the system has some drawbacks e.g. increased converter
0"
20000"
40000"
60000"
80000"
100000"
120000"
140000"
0" 1" 2" 3" 4" 5" 6"
Cos
t of D
FIG
(€)
Rated Power (MW)
DFIG_3G generator cost estimation
Aydin, 7/21/13 10
losses, excitation current obtainment etc. together with intensive gearbox
maintenance.
Figure V Variable speed squirrel cage induction generator [51]
2.1.2.1. Weight*and*cost*estimations*of*SCIG_3G*
In order to estimate the weight of SCIG featuring 3-stage gear, 1500 rpm/50
Hz, 3 kV, ABB’s IC611/IP55 air-to-air cooled high-voltage induction
generators [3] are taken as reference. Firstly, to be able to see how the IG
voltage change acts on the weight, generators with 3 kV, 6 kV and 10 kV are
compared and the 3 kV type is preferred to continue with, following the
conclusion that voltage increase results in escalation of the weight.
Aydin, 7/21/13 11
Figure VI SCIG_3G weight as a function of output power
As seen in Figure VI, the weight values deviate between 8.1-9.7 tons for 3 kV
SCIG with shaft center height 560 mm, whereas it fluctuates between 11.4-
12.2 tons for 3 kV SCIG with shaft center height 630 mm. This is because the
enlargement of the generator volume is required to meet the desired generator
output power.
Based on the figures from ABB and proof-check with the datasheet of
Siemens SWT 3.6/107, further weight estimations are realized:
Shaft center height 560 mm
!"#"$%&'$!!"#$%&!"#$_!" != !!.!" ∗ !"#"$%&'$!!"#$"#!!"#$% + !"#$.!! ( 2.4 )
Shaft center height 630 mm
!"#"$%&'$!!"#$%&!"#$_!" != !!.!"# ∗ !"#"$%&'$!!"#$"#!!"#$% + !"#$.!! ( 2.5 )
Generator output power: in kW
Generator weight: in kg
0"
2000"
4000"
6000"
8000"
10000"
12000"
14000"
3000" 3500" 4000" 4500" 5000" 5500" 6000"
Wei
ght (
kg)
Output power (kW)
SCIG_3G generator weight calculation
Shaft"center"height"560"mm" Shaft"center"height"630"mm"
Aydin, 7/21/13 12
Concerning the study, eq. ( 2.4 ) is applied for 3150 kW and 4200 kW SCIGs,
whereas eq. ( 2.5 ) is applied for 5250 kW SCIG due to its bigger volume and
related shaft center height.
As it is observed from the weight comparison of DFIG_3G and SCIG_3G,
squirrel-cage induction generator is lighter than doubly-fed induction
generator owing to lacking of slip rings and rotor windings.
The induction generator cost of SCIG_3G system should be scaled with 0.89
of the induction generator cost of DFIG_3G system as shown in eq. ( 2.6 ).
The estimation is based on Tavner et al.’s comparative assessment of
medium-speed brushless DFIG with alternative drivetrain designs [4] and
Alstom Power’s experience which conveys SCIG cost is ~10% less than
DFIG cost having both the same output power. Figure VII displays the
detailed comparison.
!"#"$%&'$!!"#$!"#$_!" != !!.!" ∗ !"#"$%&'$!!"#$!"#$_!"! ( 2.6 )
Generator costs: in €
Aydin, 7/21/13 13
Figure VII SCIG_3G cost as a function of output power compared to DFIG_3G cost
2.1.3. Weight'and'cost'estimations'of'PMSG_3G'
In reference to technical specifications and drawings of The Switch (3.3 MW-
1500 rpm and 2.7 MW-1500 rpm) [11] high-speed PMGs, which have 560
mm shaft center height, and by the help of internal experience from The
Switch and Alstom Power, it is concluded that the specific generator weight of
PMSG_3G system equals to 363.5 kg/kNm, as it is shown in eq. ( 2.7 ).
!"#"$%&'$!!"#$%&!"#$_!" != !!"!.! ∗ !"#$%!!"#$%&!"#"$%&'$! ( 2.7 )
Generator weight: in kg
Rated torque: in kNm
The cost estimation for the generator of PMSG_3G system is realized by the
following eq. ( 2.8 ), which means that the current high-speed PM generator
price is tariffed with 5.73 €/Nm of its rated torque. This rate reflects the
50000"
60000"
70000"
80000"
90000"
100000"
110000"
120000"
130000"
2500" 3500" 4500" 5500"
Cos
t of S
CIG
(€)
Output Power (kW)
SCIG_3G generator cost estimation
DFIG"cost" SCIG"cost"
Aydin, 7/21/13 14
related market average values in April 2013 and it is liable to the dynamic
magnet prices.
!"#"$%&'$!!"#$!"#$_!" != !!.!" ∗ !"#$%!!"#$%&!"#"$%&'$! ( 2.8 )
Generator cost: in €
Rated torque: in Nm
In Figure VIII generator characteristics of studied PMSG_3G system are
projected. The gear ratio applied on the system is equivalent to 107 and the
generator speed values are 1498 rpm, 1356 rpm and 1243 rpm for 3 MW, 4
MW and 5 MW generators respectively.
By performing eq. ( 2.7 ) and eq. ( 2.8 ), generator weight and cost values are
produced for PMSG_3G system as they are given in Figure IX.
Figure VIII PMSG_3G generator speed vs. generator torque as a function of rated power
0"5000"10000"15000"20000"25000"30000"35000"40000"45000"
0"
200"
400"
600"
800"
1000"
1200"
1400"
1600"
3"MW" 4"MW" 5"MW"
Gen
erat
or T
orqu
e (N
m)
Gen
erat
or S
peed
(rpm
)
Rated Power (MW)
PMSG_3G generator characteristics
Generator"Torque" Generator"Speed"
Aydin, 7/21/13 15
Figure IX PMSG_3G generator weight and cost estimation
2.1.4. Weight'and'cost'estimations'of'PMSG_4G'
Adding the fourth gear stage to the drivetrain results in escalating generator
speed which climbs up to 1792 rpm, 1622 rpm and 1487 rpm for 3 MW, 4
MW and 5 MW PM generators, respectively, with a total gear ratio 128.
In this study to realize the estimations, Vestas V112-3.0 MW wind turbine’s
in-house design generator [12], Winergy AQWA-560LS-08A, manufactured
by Winergy [22], is taken as reference. The generator consists of 68 kg
neodymium and 7 kg dysprosium [13].
According to the weight estimation provided by eq. ( 2.9 ), which is the same
formulation as eq. ( 2.7 ), it can be stated that weight of permanent magnets
encompassed by V112’s PM generator stands for %1.23 of total generator
weight. In this point, it is observed that weight ratio of permanent magnets in
medium-speed PM generators decreases from ~3% to ~1% in high-speed PM
80,000"
110,000"
140,000"
170,000"
200,000"
230,000"
260,000"
0.0"
2000.0"
4000.0"
6000.0"
8000.0"
10000.0"
12000.0"
14000.0"
16000.0"
3 MW-1498 rpm
4 MW-1356 rpm
5 MW-1243 rpm
Gen
erat
or C
ost (€)
Wei
ght (
kg)
PMSG_3G generator weight and cost estimation
Weight" Cost"
Aydin, 7/21/13 16
generators. Expanding on eq. ( 2.9 ) and eq. ( 2.7 ), both of them are the same
formulas and hold for the weight calculation of high-speed PM generators.
!"#"$%&'$!!"#$%&!"#$_!" != !!"!.! ∗ !"#$%!!"#$%&!"#"$%&'$! ( 2.9 )
Generator weight: in kg
Rated torque: in kNm
Figure X PMSG_4G generator speed vs. generator torque as a function of rated power
Previously mentioned generator speed values are displayed in Figure X with
the generator torque values.
Cost estimation follows the same method used in PMSG_3G’s generator cost
estimation and specific generator cost holds the same value, 5.73 €/Nm, as
given by eq. ( 2.10 ).
0"
5000"
10000"
15000"
20000"
25000"
30000"
35000"
40000"
600"
800"
1000"
1200"
1400"
1600"
1800"
2000"
3"MW" 4"MW" 5"MW"G
ener
ator
Tor
que
(Nm
)
Gen
erat
or S
peed
(rpm
)
Rated Power (MW)
PMSG_4G generator characteristics
Generator"Torque" Generator"Speed"
Aydin, 7/21/13 17
!"#"$%&'$!!"#$!"#$_!" != !!.!" ∗ !"#$%!!"#$%&!"#"$%&'$! ( 2.10 )
Generator cost: in €
Rated torque: in Nm
When the generator prices of PMSG_3G and PMSG_4G systems are
compared (see Figure IX and Figure XI) it is noted that PMSG_4G’s
generator price covers 84% of the PMSG_3G’s generator price, which is
mainly caused by less PM usage.
Figure XI PMSG_4G generator weight and cost estimation
2.2. Design'and'economic'model'of'medium;speed'wind'generators'
With the aim of decreasing dependency on rare earth materials and providing
lower cost energy production, some turbine manufacturers shifted slightly
from their induction generator based productions to developing medium-speed
hybrid drive systems. This system, as presented in Figure XII, constitutes of
80,000"
110,000"
140,000"
170,000"
200,000"
0.0"
2000.0"
4000.0"
6000.0"
8000.0"
10000.0"
12000.0"
14000.0"
3 MW-1792 rpm
4 MW-1622 rpm
5 MW-1487 rpm
Gen
erat
or C
ost (€)
Wei
ght (
kg)
PMSG_4G generator weight and cost estimation
Weight" Cost"
Aydin, 7/21/13 18
different number of gears, permanent magnet generator and a full-scale power
converter.
Figure XII Medium-speed PMSG system [60]
2.2.1. Weight'and'cost'estimations'of'PMSG_1G'
While the generator speed increases from low-speed to medium-speed zone by
adding gearbox to the drivetrain, the frequency increases accordingly, too. As
a result, it is made essential that the lamination level should be escalated in
order to mitigate iron losses which are prone to a rise due to not sinusoidally
distributed flux densities.
Table 4 the material properties of stator laminations can be found.
Stator laminations
Material M330-50A, (non grain oriented magnetic steel)
Specific losses, P!",!"#$!! 3.3 W/kg at 50 Hz and 1.5 T
Mass density, !!,!"#!! 7650 kg/m3
Table 4 Material properties of stator laminations [7]
In order to project a better picture of how the lamination ratio differs in
accordance with the generator speed in low-speed and medium-speed PM
generators Figure XIII is provided. Detailing the projection, the formula
Aydin, 7/21/13 19
shown in eq. ( 2.11 ) is established to calculate the ratio of lamination level for
the specific PM generator speed.
To expound on Figure XIII and referring to Figure XIV, the level of
lamination usage for low-speed PMGs amount to approximate number of
14%, whereas by the effect of previously in detail explained increasing
generator speed the lamination usage enlarges to 33.7%, 32.9% and 32.2% of
the total generator system weight respectively for 3 MW-126 rpm, 4 MW-114
rpm and 5 MW-105 rpm PMGs with a single-stage gear ratio 9.
Figure XIII Lamination stack mass in percentage in PMGs as a function of generator speed
!"#$%!"#$%"&$'% != !!.!"#$ ∗ !"! !"#"$%&'$!!"##$ − !.!"#! ( 2.11 )
Ratio: in %
Generator speed: in rpm
Based on The Switch (1.65 MW-150 rpm and 3.3 MW-136 rpm) [9] medium-
speed PMGs’ technical specifications and drawings, after linear up-scaling in
accordance with the increase of generator torque the specific weight of 119.6
0.00"
10.00"
20.00"
30.00"
40.00"
50.00"
0.00" 100.00" 200.00" 300.00" 400.00"
Perc
enta
ge (%
)
Generator speed (rpm)
Lamination usage in PMGs
Aydin, 7/21/13 20
kg/kNm is estimated for medium-speed PMSG featuring a single-stage gear as
shown in eq. ( 2.12 ).
!"#"$%&'$!!"#$%&!"#$_!" != !!!".! ∗ !"#$%!!"#$%&!"#"$%&'$! ( 2.12 )
Generator weight: in kg
Rated torque: in kNm
Figure XIV Material shares in the generator of PMSG_1G
Before continuing with cost estimation the material shares should be
calculated for which correlation method is used between current commercial
direct-drive PMSGs’ and medium-speed PMSGs’ weight distributions. The
result of the correlation is presented in Figure XIV and as observed the
structural part forms a little more than the half of total weight for all three
different turbine ratings. Portions of copper and magnets follow the second
weightiest part, lamination stack, from behind with 3.4% and 8.3% in the
mentioned order.
3.4"33.7"
54.5"
8.4"3.4"
32.9"
55.4"
8.3"3.5"
32.2"
56.1"
8.3"
0.0"
20.0"
40.0"
60.0"
80.0"
100.0"
120.0"
140.0"
160.0"
180.0"
magnet lamination structure copper
Wei
ght (
%)
PMSG_1G weight distribution
5"MW" 4"MW" 3"MW"
Aydin, 7/21/13 21
By using eq. ( 2.12 ), firstly the weight estimation for the generator of
PMSG_1G is achieved. Later, by focusing on the material shares in Figure
XIV and the April 2013-based material specific prices in Table 5, cost
calculation model returns the results, both of which are displayed in Figure
XV.
Figure XV PMSG_1G generator weight and cost estimation
2.2.2. Weight'and'cost'estimations'of'PMSG_2G'
“There is a significant potential for two stage planetary gearboxes with higher
gearbox ratios to lower drivetrain costs. Conventional two stage planetary
gearboxes are facing design restrictions limiting their gearbox ratio to about
40” [8] state Schmidt et al. in their research paper presented at annual event of
EWEA in Copenhagen in 2012. In this study, the gearbox ratio is set at 30,
which leads generator speeds to 420 rpm, 380 rpm, 349 rpm for 3 MW, 4 MW
and 5 MW respectively. The increasing generator speed and decreasing
generator size results in lessening copper usage in terms of weight. However,
300000.0"
400000.0"
500000.0"
600000.0"
700000.0"
800000.0"
900000.0"
1000000.0"
0.0"
10000.0"
20000.0"
30000.0"
40000.0"
50000.0"
60000.0"
70000.0"
3 MW-126 rpm
4 MW-114 rpm
5 MW-105 rpm
Gen
erat
or C
ost (€)
Wei
ght (
kg)
PMG_1G generator weight and cost estimation
Weight" Cost"
Aydin, 7/21/13 22
in terms of its ratio of weight in the total generator system it shows a slight
climbing slope, which is projected in Figure XVI.
Figure XVI Copper winding usage in percentage in PMGs as a function of generator speed
Detailed comprehension of Figure XVI conveys that the copper consumption
in low-speed and medium-speed PM generators shift between 7% and 9% of
the total generator weight. Exact figures under this study scheme range from
8.9% to 9%.
Based on The Switch (3.12 MW-414 rpm) [9] and ABB (7MW-400 rpm) [10]
medium-speed PMGs’ technical specifications and sketches, after linear up-
scaling in accordance with the increase of generator torque the specific weight
of 178 kg/kNm is determined for medium-speed PM synchronous generator
weight which features 2-stage gear. As observed from eq. ( 2.13 ), hereby the
particular meaning is generator in current PMSG_2G drivetrain weighs 178 kg
per kNm of generator rated torque.
!"#"$%&'$!!"#$%&!"#$_!" != !!"# ∗ !"#$%!!"#$%&!"#"$%&'$! ( 2.13 )
Generator weight: in kg
Rated torque: in kNm
0.00"
2.00"
4.00"
6.00"
8.00"
10.00"
0.00" 100.00" 200.00" 300.00" 400.00"
Perc
enta
ge (%
)
Generator speed (rpm)
Copper Usage in PMGs
Aydin, 7/21/13 23
Figure XVII Material shares in the generator of PMSG_2G
As it is performed for PMSG_1G’s material weight distribution, the same
correlation method is carried out here for the material shares estimation in the
generator of PMSG_2G system. The figures what may be perceived from
Figure XVII, show how the lamination stack level surges to higher records
and closely compete with the weight ratio of structural part, which is already
expected due to the previously touched on reasoning and trend given in Figure
XIII. Respective values related to lamination levels are 43.4%, 42.6% and
41.9 % of total generator weight for 3 MW-420 rpm, 4 MW-380 rpm and 5
MW-349 rpm in the mentioned order. Moreover, it is spotted that ratio of
magnet usage shrinks to 3% with a slight decrease compared to its 3.4% share
in PMSG_1G. However, this ratio, on the contrary, shifts from 8.3% to 9% for
the copper level, which can be tracked in Figure XVI.
3.0"43.4" 44.6"
9.0"3.0"
42.6" 45.4"
9.0"3.0"
41.9"46.1"
8.9"
0.000"
20.000"
40.000"
60.000"
80.000"
100.000"
120.000"
140.000"
160.000"
magnet lamination structure copper
Wei
ght (
%)
PMSG_2G weight distribution
5"MW" 4"MW" 3"MW"
Aydin, 7/21/13 24
Figure XVIII PMSG_2G generator weight and cost estimation
Formerly executed process for weight estimation of the generator of
PMSG_1G system holds here, too. Initially, the weight estimation for the
generator of PMSG_2G is achieved by operating eq. ( 2.13 ). Subsequently,
by centering on the material shares in Figure XVII and the April 2013-based
material specific prices in Table 5, cost calculation model yields the
outcomes, both of which are displayed in Figure XVIII.
What is worth to add at this point is, when the Figure XV is analogized to
Figure XVIII, it is detected that the weight of PM generator run in PMSG_2G
system weighs 45% of the PM generator run PMSG_1G system, which
triggers the cost die-down for PMSG_2G’s generator and set its price to 40%
of PMSG_1G system’s generator.
2.3. Design'and'economic'model'of'low;speed'wind'generators'
As it is commonly known, the increasing turbine ratings resulted in growing
numbers of failures in the gearboxes, which yielded the interest on direct-
drive wind turbines amongst the manufacturers. However, the interest brought
100000.0"
150000.0"
200000.0"
250000.0"
300000.0"
350000.0"
400000.0"
0.0"
5000.0"
10000.0"
15000.0"
20000.0"
25000.0"
30000.0"
3 MW-420 rpm
4 MW-380 rpm
5 MW-349 rpm
Gen
erat
or C
ost (€)
Wei
ght (
kg)
PMG_2G generator weight and cost estimation
Weight" Cost"
Aydin, 7/21/13 25
with itself the concerns about significant weight of low-speed direct drive
generator (see Figure XIX), of which the weightiest part was constituted by its
structural mass in order to deal with the magnetic! attraction force between its
stationary and moving parts.
Figure XIX Low-speed PMSG system outline incl. full-scale power converter [59]
2.3.1. Weight'and'cost'estimations'of'PMSG'_DD'
Even though there are many new alternative PMG designs set by different
relative positioning of the active materials within to decrease the attraction
force, which will accordingly lessen the structural mass, in this study off-the-
shelf PMG types will be utilized.
Figure XX Structural mass in percentage in PMGs as a function of generator speed
In Figure XX, it is noticeable that the percentage of structural mass in a
directly-driven PMG varies between 70%-80% of the total generator weight,
whereas the percentage can deepen to the range of 40%-50% for medium-
0.00"10.00"20.00"30.00"40.00"50.00"60.00"70.00"80.00"
0.00" 100.00" 200.00" 300.00" 400.00"
Perc
enta
ge (%
)
Generator speed (rpm)
Structure usage in PMGs
Aydin, 7/21/13 26
speed PMG. The projection done in Figure XX is formed on the analysis of
permanent magnet generators in the current market.
Bang et al. estimates 25 kg/kNm [5] for PM generator specific weight in
rough design phase. However, the current applications in the market show that
the weight should be scaled with 28 kg/kNm of specific weight in function
with turbine rated torque, which is referred to The Switch’s technical
specifications of its low-speed permanent magnet generators [6] and Alstom
Power’s experience in the direct-drive turbines’ market.
!"#"$%&'$!!"#$%&!!_!"# != !!" ∗ !"#$%!!"#$%&!"#$%&'! ( 2.14 )
Generator weight: in kg
Rated torque: in kNm
Figure XXI PMSG_DD weight as a function of rated power
As shown in eq. ( 2.14 ) and demonstrated with Figure XXI up-scaling
follows a linear trend for PMSG_DD weight calculation.
20000"
40000"
60000"
80000"
100000"
120000"
140000"
2" 2.5" 3" 3.5" 4" 4.5" 5" 5.5"
Wei
ght (
kg)
Rated power (MW)
PMSG_DD weight calculation
Aydin, 7/21/13 27
In order to create a cost calculation model, in addition to the ratio of structural
part, the other materials’ weight distribution in permanent magnet generator
should be estimated, firstly. Concerning weight allocation, as seen in Figure
XXII structural part is most dominant, where the lamination takes up to
14.4%. Copper with its nearly 7% and permanent magnets with approximately
4.3% of total generator weight have only small shares.
Here, it should be noted that for a detailed, concrete weight distribution and
related cost estimation for specific directly-driven PMG:
• air gap diameter and width
• number and dimensions of pole pairs
• dimensions of teeth, yokes, slots and end-windings
• active length of generator
• copper, iron and additional losses
• magnet placement order
• efficiency features and reactive power figures
should be taken into account.
Figure XXII Estimated generator weight distribution for directly-driven PMG
0.00"
10.00"
20.00"
30.00"
40.00"
50.00"
60.00"
70.00"
80.00"
magnets lamination structure copper
Wei
ght (
%)
PMSG_DD weight distribution
Aydin, 7/21/13 28
Due to highly dynamic magnet prices, a general relation between torque of the
generator and the total cost cannot be provided. Total generator cost
estimation should be performed with reference to material features and its
specific costs.
As it is well known the magnet prices have displayed extreme price volatility
over the last 2 years period, which made the cost estimations tougher for the
long-term perspectives. Owing to the reason that price trend shows fluctuating
levels, in this study following figures valid for April 2013, as it is presented in
Table 5, are used:
Materials Specific costs
Permanent magnet 167 €/kg
Structure, bearings, cooling 15 €/kg
Copper wire 15 €/kg
Lamination stack 3 €/kg Source: provided by Alstom Power
Table 5 Material specific costs in April 2013
After clearing up in previous steps the weight and the material specific cost
issues, cost estimation of PMSG can be performed and the total cost values for
3 MW, 4 MW and 5 MW generators can be taken out of Figure XXIII, which
conveys also the material costs with respect to turbine rated power.
Aydin, 7/21/13 29
Figure XXIII PMSG_DD total cost as a function of rated power and materials cost-
breakdown
Viewing both Figure XXII and Figure XXIII an explicit image is derived that
even though the share of permanent magnets in weight distribution is limited
with 4.3%, the share in cost-breakdown positions at the second high-level
with its 36% following 56.5% of structural part which leads the highest cost in
PMSG.
3. Cost'and'Weight'Calculation'of'Wind'Turbine'Gearboxes'
3.1. Design'and'economic'model'of'single;stage'gearbox'
In order to cover up the specific disadvantages of gearless low-speed wind
turbine concept, e.g. large diameter, heavy weight and high-priced generator,
an alternative concept, PMSG_1G, developed during the second half of the
1990s by the renowned German engineering consultancy aerodyn
Energiesysteme could be a solution.
2" 3" 4" 5" 6"
0"
500,000"
1,000,000"
1,500,000"
2,000,000"
2,500,000"
0"
200,000"
400,000"
600,000"
800,000"
1,000,000"
1,200,000"
1,400,000"
magnets lamination structure copper
Rated Power (MW)
Tota
l Cos
t (€)
Mat
eria
l Cos
t (€)
PMSG_DD cost-breakdown and total cost estimation
3"MW" 4"MW" 5"MW" Total"cost"
Aydin, 7/21/13 30
As it may be surmised the weight of a single-stage gearbox varies with the
changing gear ratio. Conventional planetary-type single stage gearbox with
gear ratio in the order of 6 is limited to the maximum ratio of 6.3. However, in
this study, an unorthodox single-stage gear arrangement, or with the informal
definition “one-and-a-half-stage” gearbox is used. The gearbox features a
step-up ratio of 9 and resembles the gear system used in Areva M5000
concept, which is manufactured by Moventas.
a) b)
Figure XXIV Proposed gear design for single-stage gearbox (a) in reference to M5000 (b) [16]
Figure XXIV displays the proposed design with 10 planet wheels in total for
single-stage gearbox, diameter of which varies between 1.5 – 3 m,
approximately, for 3 MW, 4 MW and 5 MW applications.
Aydin, 7/21/13 31
GR: gear ratio
Figure XXV PMSG_1G weight estimation as a function of rated power and gear ratio
The reason the gear ratio of 9 is chosen is that when gear ratio of 6 is chosen,
PM generator weight shows an increase with 33.3%, which triggers escalation
in sum of generator and gearbox weight with 3.3% and causes cost expansion
in sum of generator and gearbox price with 19.2%, averagely. Owing to this
effect and already known fact that the PM prices have registered dynamic
price volatility recently, gear ratio of 9 is highly recommended to be utilized
in PMSG_1G system, which will also decrease the dependency on permanent
magnet market by avoiding unfortunate economic consequences. More
detailed insight is provided with Figure XXV and Figure XXVI.
0.0"
20000.0"
40000.0"
60000.0"
80000.0"
100000.0"
120000.0"
3"MW" 4"MW" 5"MW"
Wei
ght (
kg)
PMSG_1G weight estimation vs. gear ratio
Generator"(GR"9)" Generator"(GR"6)"Gearbox"feat."GR"9" Gearbox"feat."GR"6"Total"weight"(GR"9)" Total"weight"(GR"6)"
Aydin, 7/21/13 32
GR: gear ratio
Figure XXVI PMSG_1G (gearbox + generator) cost estimation as a function of rated power and gear ratio
Weight estimation is derived from Chen et al.’s suggestions for gearbox
weight and power loss calculation in [14] and Li et al.’s cost function diagram
in [15] is used as a basis for the cost estimation.
Lately, General Electric has patented a drivetrain design comprised of a
single-stage gearbox, which includes plurality of planet gears and a stationary
gear mounted to the mainframe in order to support the weight of rotor hub
[18].
3.2. Design'and'economic'model'of'two;stage'gearbox'
0"
200,000"
400,000"
600,000"
800,000"
1,000,000"
1,200,000"
1,400,000"
1,600,000"
1,800,000"
3"MW" 4"MW" 5"MW"
Cos
t (€)
PMSG_1G cost estimation vs. gear ratio
Generator"(GR"9)" Generator"(GR"6)"Gearbox"feat."GR"9" Gearbox"feat."GR"6"Total"cost"(GR"9)" Total"cost"(GR"6)"
Aydin, 7/21/13 33
Figure XXVII 2-stage gear technology from Winergy [16]
Rising concerns about optimum drivetrain configurations led the wind turbine
manufacturers and component developers to split torque-handling task
between the generator and gearbox, which brought about embedding 2-stage
gearbox in the drivetrain. The motivation supporting this decision was
including more gear stages could be a more-reasonably-priced-and-reliable
option. Based on that approach, companies like Gamesa, WinWind,
Fuhrländer, DeWind and W2E took initiatives to implement 2-stage gearbox
in their product ranges aiming at lower design weight and cost, yet risking the
reliability issues to be caused by the gearbox unit and the financial
tentativeness induced by aforementioned dynamic PM pricing. In Figure
XXVII an example outline of 2-stage gearbox is exposed.
When further looked into the topic it is observed that setting the optimum gear
ratio forms one of the key entanglements, about which Schmidt et al. conclude
“Conventional two stage planetary gearboxes are facing design restrictions
limiting their gearbox ratio to about 40” [8]. Taking that into account, hereby
in the research, the gear unit with step-up ratio of 30 is investigated.
Aydin, 7/21/13 34
Figure XXVIII 2-stage gearbox weight and cost estimation
In reference to Schmidt et al.’s proposal in [8], it is deduced that two speed
gearbox weight in PMSG_2G concept can be scaled as given by eq. ( 3.1 ).
!"#$%&'!!"#$%&!"#$_!" != !!.!" ∗ !"#$%!!"#$%&!"#$%&'! ( 3.1 )
Gearbox weight: in kg
Rated torque: in kNm
Validity of eq. ( 3.1 ) is cross-checked with Moventas FD3000 gearbox which
runs on 3000 kNm mechanical torque.
For the cost estimation, 12 €/kg [8] is taken as the specific cost and together
with weight values are projected in Figure XXVIII. Here it is noted that the
specific cost of 2-stage gearbox is higher than specific cost of single-stage
gearbox, which can be explained by highly precise parts’ exorbitant pricing of
production.
160,000"
200,000"
240,000"
280,000"
320,000"
360,000"
400,000"
440,000"
0"
5000"
10000"
15000"
20000"
25000"
30000"
35000"
40000"
3 MW 4 MW 5 MW
Gea
rbox
Cos
t (€)
Wei
ght (
kg)
2-stage gearbox weight and cost estimation
Weight" Cost"
Aydin, 7/21/13 35
Figure XXIX Conventional drivetrain vs. PMSG_2G
An application of 2-stage gearbox featuring PM generator is provided in
Figure XXIX pointing out at how the nacelle volume can be resized to smaller
designs. Furthermore, the use of 2-stage gearbox in the drivetrain can help to
puzzle out negative OPEX impacts caused by dreadful gear unit reliability.
3.3. Design'and'economic'model'of'three;stage'gearbox'
Most typically it is seen in the market that three-stage gearboxes incorporate a
simple planetary first stage, either accompanied by a second simple planetary
and a parallel offset stage or only two parallel offset stages comprised of bull
gear and pinion gears. Along with that, there are other riveting alternative
designs in the current market, outlines of which are depicted in Figure XXX.
Aydin, 7/21/13 36
Figure XXX Several three-stage gearbox concepts [19]
Hereby, it should be disclosed that the model of the three-stage gearbox used
in this study holds the same for PMSG_3G, DFIG_3G and SCIG_3G systems
and runs with a step-up ratio of 107.
The weight calculation is realized by polynomial functioning of low speed
shaft torque in reference to gearbox series of Alstom Power’s suppliers. Later,
weight function is proof-checked with the mass figures of Wikow W3000 [20]
and ZF’s integrated rotor side gearbox [21] and presented by eq. ( 3.2 ) and
Figure XXXI.
!"#$%&'!!"#$%&!" !
= !!.!!"# ∗ !"#$%!!"#$%&!"#$%&'! − !".!"#∗ !"#$%!!"#$%&!"#$%&' + !"#$$!!
( 3.2 )
Gearbox weight: in kg
Rated torque: in kNm
Figure XXXI 3-stage gearbox weight estimation as a function of rated turbine torque
10000"15000"20000"25000"30000"35000"40000"45000"
0" 1000" 2000" 3000" 4000" 5000"
Wei
ght (
kg)
Rated Torque (kNm)
3-stage gearbox weight estimation
Aydin, 7/21/13 37
For the cost estimation of three-stage gear unit, specific cost of 103.41 €/kNm
is determined after the analysis of present price offers from various gear
suppliers. Cost related figures can be obtained from eq. ( 3.3 ) and Figure
XXXII.
!"#$%&'!!"#$!" != !!"#.!" ∗ !"#$%!!"#$%&!"#$%&'! ( 3.3 )
Gearbox cost: in €
Rated torque: in kNm
Figure XXXII 3-stage gearbox cost estimation as a function of rated turbine torque
When a glance is taken on Figure XXVIII and Figure XXXII a compelling
detail is discovered that two-stage and three-stage gearbox costs for 3 MW, 4
MW and 5 MW turbine applications roam about the same price levels which
is due to the previously mentioned fact in Chapter 3.2, highly precise parts’
exorbitant pricing of production in two-stage gearbox system and additionally
due to high availability of three-stage gear units in supplier’s market resulting
in lower prices.
150,000"
200,000"
250,000"
300,000"
350,000"
400,000"
450,000"
500,000"
0" 1000" 2000" 3000" 4000" 5000"
Cos
t (€)
Rated Torque (kNm)
3-stage gearbox cost estimation
Aydin, 7/21/13 38
Here it is, also, necessary alluding to one of the key failure initiator, which is
as the turbine rated power is up-scaled from 3 MW to 5 MW, the lubrication
system’s vitality gains more liability, namely to prevent the scattered
fragments from stacking in first-stage ring gear and delivering less thicker oil
to high-speed third-stage. In this context, housing two detached lubrication
systems or the use of an external oil tank by excluding the internal oil sump
can create more feasible and trustworthy gearbox even though this will bring
more design cost with it.
3.4. Design'and'economic'model'of'four;stage'gearbox'
Figure XXXIII 4-stage differential gearbox design from Bosch Rexroth [23]
Currently, in the market, there is only one key supplier, Bosch Rexroth, who
provides drivetrains with four-stage gearboxes. The gear unit put on in Figure
XXXIII illustrates a sketch of one of the Bosch Rexroth REDULUS GPV-D
series gearboxes, which comprises two planetary stages, one differential and
one parallel shaft helical gear stage, where the 3rd planetary stage acting as
differential gear summarizes the speed from one carrier and a non-stationary
ring gear on the sun gear [24]. More in detail, the speed rates of revolving ring
Aydin, 7/21/13 39
gear and planet carrier of the differential stage result in a cumulative speed of
the sun gear where the split power in the first planetary stage is brought
together again [23].
At present, Vestas uses the same technology gearbox, GPV-570 D [22] with
gear ratio of 113.2 and input torque of 2462 kNm, in V112-3.0 MW.
In this study a gear ratio of 128 is applied on the four-stage gearbox and
related to weight estimation, technical specifications of GPV 531 D and GPV
570 D are taken as reference yielding the following formula, as seen by eq. (
3.4 ).
!"#!"#$!!"#$%&!"#$_!" != !!".!" ∗ !"#$%!!"#$%&!"#$%&'! ( 3.4 )
Gearbox weight: in kg
Rated torque: in kNm
And again based on those two reference gearboxes, specific cost for four-stage
gearbox is resolved as 113.4 €/kNm which is given by eq. ( 3.5 ).
!"#$%&'!!"#$!"#$_!" != !!!".! ∗ !"#$%!!"#$%&!"#$%&'! ( 3.5 )
Gearbox cost: in €
Rated torque: in kNm
After performing eq. ( 3.4 ) and eq. ( 3.5 ) the end-result values for weight and
cost estimation are presented in Figure XXXIV.
Aydin, 7/21/13 40
Figure XXXIV 4-stage gearbox weight and cost estimation
4. Cost'and'Weight'Calculation'of'Wind'Turbine'Converters'
Power electronic converters are the interfaces between the load and the power
network decoupling the generator frequency from the grid frequency. An
overview concerning the two main types of power electronic converters for
wind turbines is depicted in Table 6, including the advantages and the
disadvantages of both technologies.
Converter Type Advantages Disadvantages Partial-scale less semiconductors crowbar circuit on rotor side
smaller size higher number of dTj at low frequency
semiconductor power losses up to 0.4-0.6% of the generated power at full power condition control complexity
line-side inductance is only 2-4% (10-15% of the rotor power) max. power is 120-130% of generator power
susceptible to grid disturbances due to direct connection of stator winding to grid
reduced converter cost restricted overcurrent limit in rotor-
200,000"230,000"260,000"290,000"320,000"350,000"380,000"410,000"440,000"470,000"500,000"
10000"
15000"
20000"
25000"
30000"
35000"
40000"
45000"
3"MW" 4"MW" 5"MW"
Gea
rbox
Cos
t (€)
Wei
ght (
kg)
4-stage gearbox weight and cost estimation
Weight" Cost"
Aydin, 7/21/13 41
side converter
improved efficiency due to reduced losses
double-size semiconductors due to rotor-side converter operation at low frequency and low voltage
Full-scale (multilevel)
simple generator-side converter and control
power loss up to 3% of the generated power
quality/harmonics/reactive power/flicker control
line-side inductance of 10-15% of the generated power
allowance of operation at higher DC voltage
higher voltage amplitude voltage unbalance on DC link
larger output power high LVRT capabilities
unequal loss distribution between the outer and inner switching devices in a switching arm may lead to derated converter power capacity
midpoint voltage fluctuation of dc bus
Table 6 Comparison of partial-scale and full-scale power converter [33][37][38][39]
4.1. Design'and'economic'model'of'partial;scale'power'converter'
As commonly known, a partial converter rated at approximately 1/3 Pn is
connected to the wound rotor asynchronous machine’s rotor windings via slip
rings and brushes in DFIG applications setting the speed range at ±30% of
generator synchronous speed. Here, the IGBT-based AC/DC/AC converter
uses FOC (field oriented control) and PWM (pulse width modulation) practice
to lessen the harmonics existing in the DFIG system by being connected to the
3-phase electrical power network directly via stator windings and at the same
time controlling the rotor frequency. Related to those facts, in order to protect
the grid and to own ride-through capabilities a reliable slip ring unit together
with efficient cooling and correct brush material are key obligatories. For
further insight on partial converter placement in DFIG system Figure XXXV
can be viewed.
Aydin, 7/21/13 42
Figure XXXV Outline of DFIG system together with partial-converter
Setting a fixed specific weight for partial-scale converters is a perplexing
topic to deal with which stems from varied applications of module setup,
generator/grid current, grid apparent power, dimensioning etc. by different
manufacturers. In Figure XXXVI low-voltage (690 V AC) partial-scale
converters’ weight figures [25][27][27] provided by ABB, Ingeteam and
Woodward are projected.
Aydin, 7/21/13 43
Figure XXXVI Commercial LV partial-scale wind converters’ weight as a function of
generator rating
Taking into account aforementioned reasoning and mass variations presented
in Figure XXXVI, in order to standardize the weight estimation for partial-
scale wind converters, specific weight of 0.75 kg/kW is determined as shown
in eq. ( 4.1 ).
!"#$%&'%&!!"#$%&!"#$%"& != !!.!" ∗ !"#$%&'!!"#$%&! ( 4.1 )
Converter weight: in kg
Turbine rating: in kW
According to state-of-the-art doubly-fed converters in the market and Alstom's
experience with various price offers from different suppliers, 23.4 €/kW is
used as a specific cost for partial converter cost calculation, as shown in eq. (
4.2 ) and in Figure XXXVII.
!"#$%&'%&!!"#$!"#$%"& != !!".! ∗ !"#$%&'!!!"#$%! ( 4.2 )
Converter cost: in €
1000"2000"3000"4000"5000"6000"7000"8000"
1000" 2000" 3000" 4000" 5000" 6000"
Wei
ght (
kg)
Generator Rating (kW)
Partial-scale wind converters
Woodward" ABB" Ingeteam"
Aydin, 7/21/13 44
Turbine rating: in kW
Figure XXXVII Partial-scale power converter weight and cost estimation
4.2. Design'and'economic'model'of'full;scale'power'converter'
Full-scale power converter (see Figure XXXVIII) allowing complete
separation between drivetrain and the grid, yet on the other hand, providing
lower LCOE having additional losses due to handling the whole generator
output power flow within, corresponds to full variable speed control in SCIG,
PMSG and WRSG systems.
0"500"1000"1500"2000"2500"3000"3500"4000"
3 MW 4 MW 5 MW 0"
20,000"
40,000"
60,000"
80,000"
100,000"
120,000"
140,000"
Wei
ght (
kg)
Cos
t (€)
Partial-scale converter weight
and cost estimation Cost" Weight"
Aydin, 7/21/13 45
Figure XXXVIII Main HW diagram of B2B multilevel IGBT full power converter
Weight estimation of full power converter is performed with specific weight
of 1.46 kg/kW as a function of turbine rating, in reference to weight figures in
[25][27], which makes full power converter 95% heavier than partial-scale
converter, averagely.
!"#$%&'%&!!"#$%&!"## != !!.!" ∗ !"#$%&'!!"#$%&! ( 4.3 )
Converter weight: in kg
Turbine rating: in kW
Full power converter price is scaled with 40.95 €/kW as a function of turbine
rating, which sets a 75% higher price value than partial-scale converter.
!"#$%&'%&!!"#$!"## != !!".!" ∗ !"#$%&'!!"#$%&! ( 4.4 )
Converter cost: in €
Turbine rating: in kW
Further insight is provided in eq. ( 4.3 ), eq. ( 4.4 ) and Figure XXXIX.
Aydin, 7/21/13 46
Figure XXXIX Full-scale power converter weight and cost estimation
One of the key differences between partial-scale and full-scale power
converter is generator-side converter’s rated power goes beyond that of the
grid-side converter in partial-scale power converter due to the fact that
generator-side converter tends to react strongly in case of grid disturbances.
However, in contrary, in full-scale power converters, the size and rating of the
generator- side and the grid-side converter are typically identical [28].
Hereby, it should be also mentioned that a robust partial converter e.g. xDFM
of Ingeteam (see Figure XL) equipped with DFIG and additional electric
generator can correspond to 3-phase electrical power network compatibility
advantages of a full-scale power converter system, eliminating the power
transformer from nacelle and providing back-up power from the exciter-like
running generator.
0"1000"2000"3000"4000"5000"6000"7000"8000"
3 MW 4 MW 5 MW 0"
50000"
100000"
150000"
200000"
250000"
Wei
ght (
kg)
Cos
t (€)
Full-scale converter weight and cost estimation
Cost" Weight"
Aydin, 7/21/13 47
Figure XL Sketch of xDFM topology [29]
Another alternative to remove the transformer from the nacelle calls for
application of a medium-voltage power converter, which is further discussed
in Chapter 7.
5. Transportability'of'Drivetrain'Components'
Increasing power ratings unveil further transport headaches for onshore
turbine market. Manufacturers holding over logistic issues to last phase of
their design processes, wind farm developers trying to stick to construction
schedules, governmental transportation limitations owing to abnormal loads
and volumes, infrastructure inadequacies and possible route clearances, severe
damage created on transportation roads, shortage of trucks and cranes for
offloading at construction sites and ports, produced environmental effects due
to road conveyance are the main grounds which trigger this headache.
a)
Aydin, 7/21/13 48
b)
Figure XLI a) Modular heavy haul trailer with special low-deck [30], b) Scheurle 15-axle intercombi trailer carrying WinWind WWD-3 [32]
Transportation retains high impact on the choice of customers, namely the
selection of customer may target the manufacturer, which is located close to
its establishment site as a means to save on freight costs shaped by remoteness
of manufacturer’s geographical setting.
Shipping by trailers, as shown in Figure XLI, has some complications such as
awaiting routing permissions for many days/weeks from local authorities,
fulfilling varied requirements of multiple countries’ transport regulations
provided that the freight goes over border, setting up road policing escorts,
availability of proper transport equipment in the country of origin or else
awaiting for its importation from nearby countries, arranging rightly
configured trucks and trailers to keep within the bridge and tunnel clearances,
arising maneuverability concerns as cornering the bends (see Figure XLII),
and lack of well trained labor force to use trucks.
Aydin, 7/21/13 49
Figure XLII Horizontal swept path analysis of nacelle transportation [31]
Shipping by rail can yield remarkable cost cut-down and time saving by
lending a hand to green development realized with fewer trips on condition
that the scope of the project requires numerous trips on trailers to the site.
On the other hand, shipping by barge/sea vessel (see Figure XLIII) offers
extremely low-cost, yet slow transportation options. The fact here is that
shortage of rail tracks and accessible ports within reach of the foundation site
makes the road transport inevitable to finalize the freight at its end-station.
Aydin, 7/21/13 50
Figure XLIII Nacelle and tower deployment of DFIG_3G wind turbine on a sea vessel in an
Alstom Power project
Ease of conveyance provided by transporting in one-piece on most common
load-carriers in the market is the desire of every manufacturer. However, not-
light-enough and oversized nacelle designs require disassembling before
transportation to be reassembled on-site, bringing in cost add-ons and
significant risks during critical components’ assemblage e.g. segmented
PMSG_DD generators, converter/transformer contained side housings.
Since the allowed dimensions vary in different provinces/countries, hereby
just to draw a picture of the principal requirements following Table 7 is
produced in reference to necessities of a standard DFIG_3G.
Requirements for accesses and roadways nacelle dimensions L: 10 m, W: 4.4 m, H: 4.1 m
nacelle weight 84 tons (+ 4 tons side housings) transportation equipment
Low loader with self-steering wheels on rear axle/modular with self-steering wheels on rear axle
crane equipment
lattice boom installed chain chassis (CC) or truck chassis (TC) crane
height reduction of convoy up to 25 cm crane weight max. 15 tons/axle transportation weight
max. 14 tons/axle (more weight = more axles = more length)
ground resistance and deformability test max. 120 MPa ground bearing capacity 25 tons/m2 minimum operating area of the main crane 22 - 28 m bridges 4.7 - 4.8 m (Europe), 4.5 m (Eastern countries)
Aydin, 7/21/13 51
Source: provided by Alstom Power
Table 7 Transportation requirements for nacelle of DFIG_3G
For the specific transportation cost estimation, transport and erection cost-
breakdown of recent Alstom project (see Table 8) in Europe, is utilized.
Specific cost estimations are projected with eq. ( 5.1 ) and eq. ( 5.2 ) being
regarded as representative figures for European zone. Abnormal sizes and
harsh territory specifications will let these estimations escalate to higher
values, dramatically.
4 WTG DFIG_3G – (4 x 90 Tons) NACELLE TRANSPORTATION PRICE
Off shore (k€)
On shore (k€)
Transport from factories to port (200 km) 13 Handling in port 9 Sea freight from port to port close to site (640 NM) 20 Handling in destination port 9 Transport from port to site (200 km) 13
Total Transportation 42 22
Cranes for erection of complete WTG 75
Total 140 Source: provided by Alstom Power
Table 8 Transportation cost model for a recent wind farm project in Europe (2013)
!"#$%&'"(#()'$!!"#$!"#$% != !!.!"# ∗ !"#$%%$!!"#$%& ∗ !"#$%&'(! ( 5.1 )
Transportation cost: in €
Nacelle weight: in tons
Distance: km
!"#$%&'"(#()'$!!"#$!"#$ != !!.!"" ∗ !"#$%%$!!"#$%& ∗ !"#$%&'(! ( 5.2 )
Transportation cost: in €
Nacelle weight: in tons
Distance: NM (nautical miles)
Aydin, 7/21/13 52
As before mentioned comparison of eq. ( 5.1 ) and eq. ( 5.2 ) proves the fact
that shipping by sea vessel is a cheaper transport solution than shipping by
truck. Here it should be noted that the specific cost estimations do not cover
the handling costs e.g. loading and off-loading of truck/ship.
6. O&M'frequencies'and'costs'
Expectation of higher availability crowned by high degree of reliability calls
for additional operation and maintenance work in the drivetrain, resulting in
better operational safety, yet intense frequencies and extortionate service costs.
Nonetheless, the service costs to attain high availability lowers the risk of
more expensive repairs and unscheduled replacements caused by catastrophic
failures in actuality.
In this case, the scope of the O&M carries a heavy significance. As plotted in
Figure XLIV, maintenance strategies are subcategorized to reactive and
preventative methods, where the former is realized by involving crisis
management and the latter by comprising of time-based pre-emptive
maintenances, which are listed in Table 9 for a DFIG system’s drivetrain.
Aydin, 7/21/13 53
Figure XLIV Scope of wind turbine O&M [40]
Here, the potential problem is set intervals for maintenance and inspections
are too long to spot the hitch in time, as seen in Table 9, and also induced by
late alerting, preventative maintenance substitutes with reactive maintenance,
thereafter, letting the drivetrain and personnel safety be at stake. In order to
prevent these, following to-do list is prepared:
# Geographical conditions should be taken into account in order to
attain promised turbine lifetime
# Owners should be in charge of the turbines rather than the
manufacturers, and should perform life-cycle-cost analysis by keeping
track of all SCADA and service reports
# High investment on constant utmost vigilance provided by real-time
conditional monitoring system (CMS) should not be avoided in the aim
of setting a trade-off and cutting down on risky reactive maintenance
frequencies and costs
Aydin, 7/21/13 54
# Careful alignment of drivetrain components should be a vital
prerequisite
# Downtime cause sorting should be performed in order to give rise to
higher availability
# Ways of providing expeditious quality service should be improved
# Better cooperation among turbine producer, component manufacturer
and sub-supplier should be sustained by sharing more insight with
each other and arranging recurrent brainstorming meetings
Tendency to cause more component failures over the progressing time effects
the full-service O&M costs in a way to shadow the same climbing trend. In
other words, first year costs after commissioning are much lower than the
possible costs in drivetrain’s twentieth year of lifetime. Namely, due to the
hardship of predicting specific annual cost, this figure is claimed to be 19.2
k€/MW per year, averagely, in 2012 based on contractual data submitted from
leading players in the onshore wind energy sector worldwide [41].
Aydin, 7/21/13 55
Source: provided by Alstom Power
Table 9 Preventative O&M frequencies of DFIG system’s drivetrain
Zone Activity+(months) 3 6 9 12 15 18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69 72 75 78 81 84 87 90 93 96 99 102 105 108 111 114 117 120 123 126 129 132 135 138 141 144 147 150 153 156 159 162 165 168 171 174 177 180 183 186 189 192 195 198 201 204 207 210 213 216 219 222 225 228 231 234 237 240Gearbox Gearbox(Oil(replacement X X X X XGearbox Gearbox.(Vent(filter(replacementGearbox Cooling(System(for(Gearbox.(General((inspection X X X X X X X X X X X X X X X X X X X XGearbox Cooling(System(for(Gearbox.(Oil(filter(substitution X X X X X X X X X X X X X X X X X X X XGearbox Rotor(bearings(inspection(and(greasing X X X X X X X X X X X X X X X X X X X XGearbox HSS(coupling(retightening X X X X X X X X X X X X X X X X X X X X XGearbox LSS(re@tightening X X X X X X X X X X X X X X X X X X X X XGearbox Gearbox(General(Inspection(and(oil(test X X X X X X X X X X X X X X X X X X X X XGearbox LSS(coupling(inspection X X X X X X X X X X X X X X X X X X X XGearbox Vring(Inspection(and(Substitution X X X X X X X X X X X X X X X X X X X XGenerator General(inspection X X X X X X X X X X X X X X X X X X X XGenerator Automatic(grease(feeding(system(inspection X X X X X X X X X X X X X X X X X X X XGenerator Bearing(manual(greasing X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X XGenerator Rotary(connectors(maintenance X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X XGenerator Generator(re@tightening X X X X X X X X X X X X X X X X X X X X XGenerator Connection(boxes(inspection(and(re@tightening X X X X X X X X X X X X X X X X X X X X XGenerator Generator(vibrations(inspection X X X X X X X X X X X X X X X X X X X XGenerator Alignment(inspection X X X X X X X X X X X X X X X X X X X XGenerator Support(inspection X X X X X X X X X X X X X X X X X X X XGenerator Cooling(system(Inspection X X X X X X X X X X X X X X X X X X X XGenerator Cooling(system.(Accumulator(pressure(inspection X X X XGenerator Cooling(system(coolant(Substitution X X X XConverter General(inspection X X X X X X X X X X X X X X X X X X X XConverter Cabinet(cleaning X X X X X X X X X X X X X X X X X X X XConverter Components(and(connections(inspection X X X X X X X X X X X X X X X X X X X X XConverter Security(system(inspection X X X X X X X X X X X X X X X X X X X XConverter Fan(&(Heat(exchangers(Inspection X X X X X X X X X X X X X X X X X X X XConverter Cooling(System(Inspection X X X X X X X X X X X X X X X X X X X XConverter Cooling(system(coolant(replacement X X X X
Aydin, 7/21/13 56
7. Grid'Integration'&'Transmission'
Fulfilling grid codes, namely, being dynamically stable, providing efficient
FRT (fault ride-through) capability, regulating active power, meeting higher
penetration levels and reactive power demands, is the key and most important
detail defining a grid integration process (see Figure XLV).
Figure XLV Schema of power network interconnection
Further in depth, withstanding voltage dips together with fast active and
reactive power restoration reinforced by active power ramping, network-
dependent reactive power injection and voltage/frequency out-range excursion
allowance in varying time-span and ratio under compiled grid code scheme of
each specific country, constitute the major interconnection issues faced by
wind farm operators. Figure XLVI illustrates the reactive power variation
capability at different regional scales.
Aydin, 7/21/13 57
Source: provided by Alstom Power
Figure XLVI Reactive power variation capability at different regions
Interconnections to weak power grids can end up in tripping relays leading to
undesired generator overheating which can be averted by STATCOM (static
synchronous compensator) or SVC (static VAR compensator)
encompassment, contributing to short-term voltage stability enhancement
together with grid strengthening.
a)
b)
Aydin, 7/21/13 58
c)
Figure XLVII Onshore HVDC configurations [47], a) underground cable circuit, b) overhead line circuit, c) back-to-back circuit
Depending on the need of specifications one or multiple of above presented
HVDC (high voltage direct current) transmission technologies (see Figure
XLVII) can be utilized. Another alternative, majorly preferred technology for
onshore platform, is HVAC (high voltage alternative current) functioning
together with FACTS (flexible alternating current transmission system),
which dynamically controls line series impedance, nodal voltage amplitude,
nodal voltage angular difference, shunt impedance and line current of high
voltage AC transmission lines [48]. Table 10 depicts the key features,
investment costs and surface occupation of two different FACTS
technologies, SVC and STATCOM.
Aydin, 7/21/13 59
Table 10 Key features, investment costs and surface occupation of FACTS technologies
[48]
It should be noted that the grid connection costs – costs of grid extensions,
staff and all related paperwork – differ in huge amounts within EU-27, which
is sketched in Figure XLVIII. Here, overall project costs stand for all the
expenses required in order to realize the project excluding the posterior O&M
costs. The remarkable point in Figure XLVIII is grid connection costs for
Sweden and Denmark indicate respectively 1% and 1.4% of overall project
costs, which is remunerated by TSO/DSO in Denmark while the connection
and transmission fees are capped by the authorities in Sweden [49].
Aydin, 7/21/13 60
Figure XLVIII Relative costs for connecting wind parks across EU-27 [49]
As the scope of the study covers the range of 3-5 MW onshore topologies, it is
useful to analyze different voltage applications in the drivetrain. Eventually, in
Table 11, this analysis comprising a comparison of different voltage
drivetrains is reported.
Drivetrain type Advantages Disadvantages Low-voltage cost efficient at low power levels larger & heavier cables more robust and mature technology per phase more cables
simpler and more rugged cooling installation complexity & labor cost
drinking water quality for cooling lower system efficiency heat sinks on ground potential higher nacelle weight
better Corona effects space taking due to parallel connection
better partial redundancy higher material cost less total cost increased switching frequencies service proven smaller filter, higher power density Medium-voltage greater power output expensive cable construction
lower current flow adding up to lower losses more insulation requirements
use of smaller cables, potential to reduce cost non-conductive liquid cooling
better efficiency with IGCT enabling less switching frequencies more complex cable terminations
Aydin, 7/21/13 61
lower content of copper less mature technology
possible transformerless design qualified technician for maintenance
less material cost more complex cooling requiring own internal cooling
improving cost to power ratio stringent material and de-ionized water for cooling
fewer parts enabling better reliability heat sink on voltage potential
smaller footprint more sensitivity to condensation and moisture
compact converter size and paralleling is not needed expensive converter
better heat dissipation added harmonic filtering reduced torque ripple
Table 11 Comparison of LV and MV drivetrains [34][35][36]
8. Reliability'
Reliability in drivetrain subassemblies is the key aspect for optimization of
OHS (occupational health and safety) level, drivetrain availability, O&M
methods, frequencies and costs, spare-part management and also for
optimization of component longevity and energy loss management.
Top failures Reasons Gearbox
Bearing
Cracking localized stresses caused by non-uniform cooling or unequal transformation of austenite to martensite
Abrasion
contamination of lubricant by hard, sharp-edged particles (sand, rust, machining chips, grinding dust, weld splatter, and wear debris)
Adhesion load concentrations Skidding too low loading
Gear
Fretting corrosion inadequate lubricant to replenish, permitting metal- to-metal contact and causing adhesion of surface asperities
Bending fatigue less cyclic stress than the yield strength of the material and greater number of cycles than 10,000
------------------------------ -------------------------------------------------------------------------------- Generator
Misalignment thermal growth and physical movement
Aydin, 7/21/13 62
Electrical discharges grounded stray electrical currents via generator bearing Bearing failure Lubrication, grounding ------------------------------ -------------------------------------------------------------------------------- Converter
Reverse torque Electrical transients in DFIG Thermal cycling IGBT operating close to synchronous speed in DFIG Condensation longer periods of standstill Electrical overstress lightning strikes Flashover insects
Table 12 Top failures in drivetrain [28][43][44][45]
In Table 12 dominant failures in the drivetrain subcomponents are listed, by
which it is concluded that the main gearbox failure causes root in gears and
bearings. Albeit randomly occurring, bearing failure in the planetary stage
prompts whole gearbox replacement, whereas bearing failure on HSS (high-
speed-shaft) can be resolved by possible replacement in its original place. As
presented in Figure XLIX bearing failures have ascendancy over gear failures
even though they are often not the root cause of failures [46]. Bearing faults
are concentrated in the parallel section whereas gear faults are more or less
evenly distributed between planetary and helical stages [40].
Here it should be noted that cylindrical roller bearings (CRB) and tapered
roller bearings (TRB) perform more efficiently than spherical roller bearings
(SRB) in the gearbox.
Figure XLIX Failure distribution within gearbox [40]
Aydin, 7/21/13 63
In generator, bearing failures take the lead as in the gearbox and constitute a
great portion of 58%, as it can be seen in Figure L. In order to prevent these
faults, the root cause -electrical discharges- should be mitigated by better
bearing insulation and shaft grounding embodiment. Nonetheless, mentioned
generator system modification is a costly solution, which will also decrease
the mean time between failures (MTBF). Specifically, the MTBF for
components of the PMG and full-scale converter is estimated at 8000 hours
whereas this value for components of the DFIG and partial-scale converter is
estimated at 1500 hours [35].
Figure L Failure distribution within generator [42]
Market feedback, as depicted in Figure LI, has shown that the control and
power converters seem to be more prone to failure even though the generator
and gearbox have the largest downtime [33]. In this context, it can be stated
that full-scale power converter incorporation in the PMSG_DD system results
in excessive failure rates compared to gearbox failures in geared drivetrain,
yet reacting to shorter mean-time-to-repair periods.
When closely inspected, reverse torque creation due to electrical transients,
semiconductor failures owing to thermal cycling, condensation caused by
Aydin, 7/21/13 64
longer periods of standstill, electrical overstress as a consequence of lightning
strikes and flashovers by reason of existence of insects in the converter
cabinets are observed to be most frequent failures (see Table 12).
As a means to provide viable countermeasures to reliability issues in the
drivetrain, following to-do list is prepared:
# Gearbox reliability should be scrutinized by model analysis,
dynamometer and field tests and further condition monitoring in the
light of well-handled failure databases [43]
# Focus should be moved on reliability issues at first hand rather than
on long-term full service contracts
# Turbine cut-in speed should be increased to 5.5-6 m/s in order to
prevent excessive wear of bearings caused by too low loading
resulting in skidding and oil film removal [44]
# Minimum loads as a function of speed variations should be looked into
more deeply [44]
# Preference should be given to losing energy in low wind speeds by not
operating the turbine and as a result gaining longer drivetrain lifetime
crowned by higher energy yields [44]
# Damage mitigation tactics for generator bearing should be improved
with reasonable pricing and higher effectiveness
# Industrial standardization level of PMSG_DD system’s generators
should be increased
# Enhancement of protection against insects should be provided in the
converter cabinets as well as over-voltage protection [28]
# Maturing of multiple cell power converter systems should be sped up
in the aim of reaching less power losses
Aydin, 7/21/13 65
Figure LI Annual failure rate and down time statistics [33]
9. New'Trends'
Since most of the failures in the gearbox are rooted in bearings, Ricardo began
to develop durable gearbox bearings for wind turbine gearboxes [52], under
their MultiLife concept. The mechanism is powered by low-pressure lubricant
oil to rotate the inner raceway a defined step (say 40 degrees) away from the
load path and hence prevents further damage to this portion of material
resulting in five times longer lifespan [53]. In this context, use of pre-loaded
tapered roller bearings in the first stage gearbox bearings highly observed to
alleviate premature wear issues due to skidding, which is most prevalent in
three point suspension arrangements [55].
Another trend followed in bearing industry is the increasing focus on
insulation of the generator bearings from stray electrical currents, which can
cause pitting and early failure [54].
In addition to more robust bearing developments, different gearbox trends also
set in, such as CVT (continuously variable transmission) gearboxes arranged
between the primary gearbox and the generator, with a purpose of speed and
torque controlling on the input shaft and providing faster response than blade
pitching [24]. New studies are performed on fluid power drivetrains utilizing
Aydin, 7/21/13 66
hydraulic transmission technology, which comes with benefits of modular and
high-torque-to-weight-ratio design along with CAPEX/OPEX reduction by
the help of synchronous generator adoption in addition to the power converter
and the transformer elimination, yet also with some important challenges e.g.
limited efficiency and availability [55][56].
One of the new generator trend follows the novel developments in BDFIG
(brushless doubly-fed induction generator) technology, which offers lower
CAPEX by utilizing partial-scale converter and excluding brush, slip ring and
carbon extraction system from the generator, which as a result helps avoiding
higher OPEX costs caused by brush and slip ring maintenance. However, the
difficulty lies down on the efficiency of this generator and needs to be
improved in order to gain higher upside in the market [4].
Another progress is recorded in ironless stator core design, which eliminates
the attraction forces between the rotor and stator, and further reduces the PM
content together with the overall generator mass, yet resulting in larger
diameters compared to conventional direct-drive generator [55][57].
An additional cutting-edge progress has been experienced in material science.
Specifically, highly dynamic PM market has directed the turbine
manufacturers to search for material substitutes such as Aluminum-Nickel-
Cobalt alloys, iron-cobalt nano structure, Holmium element and alternative
methods for PM use optimization, e.g. spraying and heat treatment [55].
Superconducting direct drive technology developments gained also speed
lately, owing to increasing need for maximum power per drivetrain. Currently,
three big projects are developed by the manufacturers e.g. American
Superconductor (HTS – High Temperature Superconductivity), General
Electric (LTS – Low Temperature Superconductivity) and Advanced Magnet
Lab (MgB2 based fully superconducting generator). The superconducting
direct drive technology has some advantages compared to PMG e.g. higher
Aydin, 7/21/13 67
magnetic field and smaller generator volume, however, the immaturity related
reliability and high cost issues together with low production capacities, tardy
material developments and required cryogenic temperatures form its big
demerits [61].
Ultimately, rated powers increasing over 3 MW has brought up the topic of
medium-voltage drivetrain applications, which minimize cable losses, make
generator design easier and allow the use of a robust medium-voltage
converter with high availability in the drivetrain [58].
Conclusion'&'Recommendations'
In this work, which is a new onshore platform study for Alstom Power, seven
different wind turbine topologies are compared with each other in major scope
of cost and weight estimations, also by taking into account the side parameters
such as reliability, grid integration, transportability and serviceability in order
to determine the best drivetrain configuration for future onshore wind turbines.
The conclusion of the study is direct-drive technology is neither by
economical nor by weight wise means feasible topology for onshore wind
turbines ranging between 3-5 MW (see Figure LII, Figure LIII, Figure LIV)
mainly due to high amount of structure usage (~74%) and high-priced PM
prices. Countermeasure can only be taken by leasing a PM reserve in
production countries to be able to have tariff fixation and hinder the effects of
volatile market. The fundamental reasoning follows the fact that ~4% of total
PMSG_DD generator weight is comprised of PM, which covers ~36% of the
total PMSG_DD generator cost (based on April 2013 PM pricing).
The same two major factors hold the same for the PMSG_1G system, which is
the decisive point to eliminate this topology from the selection of alternatives.
Aydin, 7/21/13 68
For topology of 3 MW (see Figure LII, it is observed that PMSG_4G records
the lightest configuration, whereas the DFIG_3G system indicates the most
economical option. At this stage, trade-off is achieved through
aforementioned side parameters, which namely evidence that inferior grid
code compliance together with more exorbitant O&M costs shadows the
DFIG_3G system in contrast to PMSG_4G.
PMSG_2G system due to having higher design costs, PMSG_4G system due
to compromising reliability and its related potential overall cost add-on during
its lifetime as a result of hosting 4-stage gearbox is out of the scope of
selection, which yields the conclusion that PMSG_3G and SCIG_3G systems
are the most feasible topologies for 3 MW category. For last decision the
scope of the study should be extended on predesign phases for both
configurations in order to analyze their real-time efficiencies, losses and AEPs.
Figure LII Comparison of seven drivetrain topologies for 3 MW category
For 4 MW and 5 MW topologies (see Figure LIII and Figure LIV), the
observation shows that SCIG_3G topology takes over the lightest design from
k€#356#k€#400#
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Weight%(tons)%
3%MW%drivetrain%weight%and%cost%
Generator# Converter# Gearbox# Cost#
Aydin, 7/21/13 69
PMSG_4G topology, where on the other hand DFIG_3G topology still keeps
its most economical position. However, the before given comments and
concerns for topologies of 3 MW category hold the same for 4 MW and 5
MW categories, which as a result indicate the SCIG_3G and PMSG_3G
systems as the most feasible topologies. Here the significant demerit is
observed at the overall drivetrain cost divergence of 84 k€ and 126 k€
between SCIG_3G and PMSG_3G systems, respectively for 4 MW and 5 MW
topologies. It should be added that one potential reason why SCIG_3G system
may not be preferable in this case despite of its most lightweight and second
most economical profile, has another significant problem which comes with
rated power up-scaling. Therefore, even though the first visualization directs
the decision to SCIG_3G, the predesign phase should be started in order to
analyze aforementioned parameters: real-time efficiencies, losses and AEPs
along with SCIG_3G system’s power up-scaling compatibility. Moreover, a
downturn on PM prices will make these two options more cost-competitive
with each other.
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Generator# Converter# Gearbox# Cost#
Aydin, 7/21/13 70
Figure LIII Comparison of seven drivetrain topologies for 4 MW category
Figure LIV Comparison of seven drivetrain topologies for 5 MW category
Overall, concerning the decision met for the best onshore wind turbine
topology – PMSG_3G – following recommendations have been developed:
# On land, keeping the gear in the drivetrain is more economical
solution than direct-drive technology over the long haul, as depicted in
Figure LII, Figure LIII and Figure LIV.
# Based on the comparison to be performed in the predesign phase
either PMSG_3G or SCIG_3G drivetrain configuration should be
selected for the new onshore platform and to support PMSG_3G
system selection, the options of PM reserve leasing in production
countries should be looked further into, to provide tariff fixation and
prevent the influences of volatile PM market prices on supply chain.
# Since the gear is kept in the drivetrain, more intense preventative
O&M intervals should be set to hinder drastic extortionate reactive
maintenances and unscheduled replacements.
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DFIG_3G# SCIG_3G# PMSG_1G# PMSG_2G# PMSG_3G# PMSG_4G# PMSG_DD#
Cost%(k
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5%MW%drivetrain%weight%and%cost%
Generator# Converter# Gearbox# Cost#
Aydin, 7/21/13 71
# Focus should be given to reliability at design phase rather than after
turbine sale long-term full-service O&M contracting which will return
more profit in the long run together with less downtime in the
drivetrain.
# Increasing of wind turbine cut-in speed should be considered which
would result in loss of energy in small amounts at low wind speeds but
help prolonging the lifetime of drivetrain.
# Further study on possibility of utilizing medium-voltage drivetrain
instead of low-voltage drivetrain should be performed, which would
yield more lightweight solution owing to possible omission of
transformer from the drivetrain and more efficient alternative on
account of lower losses.
# Transportation issues should not be delayed to face in the later project
phases, yet instead, compact and lightweight drivetrain should be
modeled in the design phase aiming at use of standard freight
transportation vehicles and capability of transporting in one-piece.
# Trade-off between reliability and cost cut-down should be set so that
higher prestige can be held, escorted by more customer preference.
Aydin, 7/21/13 72
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