Towards increased reliability of power converters in wind turbines

Katharina Fischer, Andreas Reuter

12 November 2014

© Fraunhofer 1

Short profile of Fraunhofer IWES North-West

© Fraunhofer

Managing Director: Prof. Dr.-Ing. Andreas Reuter

Research spectrum: Wind energy from wind physics to grid connection

Overall budget 2013: around 14 million € Staff: 150 employees Previous investments in the establishment of the institute: € 50 million

Strategic Association with ForWind and the German Aerospace Center (DLR)

2

Wind EnergyResearch Alliance

WEA26%

BOP20%

P&D4%

T&I14%

O&M33%

Rückbau3%

WEABOPP&DT&IO&MRückbau

: Windenerg ieanlag e: BalanceBof Plant: Planung BundBEntwicklung: Tranport and Installation: BetriebBundBWartungWEA

26%

BOP20%

P&D4%

T&I14%

O&M33%

Rückbau3%

WEABOPP&DT&IO&MRückbau

: Windenerg ieanlag e: BalanceBof Plant: Planung BundBEntwicklung: Tranport and Installation: BetriebBundBWartung

Life cycle cost Scottish Enterprise 2011

(complemented)

Reliability is key

© Fraunhofer 3

!  Operation and Maintenance (O&M) cost sums up to ~1/3 of the wind turbine life cycle cost (LCC)

!   Large portion of O&M cost: Maintenance cost (service and repair)

!   In addition: opportunity cost due to unavailability

Reliability is among the key factors for a further COE reduction

Maintenance

Rent

Other

Operations

Insurance Offshore Onshore (new) Onshore (old)

O&

M c

ost [€c

ent/k

Wh]

Sou

rce:

Svo

boda

(201

3)

Reliability)study)for)variable1speed)wind)turbines)(RELIAWIND)project,)200812011):)

Challenge: Power-converter reliability

! High failure rates of power converters cause considerable maintenance cost and production loss

! Impact on turbine availability offshore more severe than onshore

! Problem concerns virtually all OEMs and wind-turbine topologies

! Failure causes are presently mostly unknown

! Urgent need for solutions

PITCH)SYSTEM)21%)

FREQUENCY)CONVERTER)

13%)

YAW)SYSTEM)11%)GENERATOR)

ASSEMBLY)7%)

LV)SWITCHGEAR)6%)

GEARBOX)ASSEMBLY)

5%)

SENSORS)4%)

COMMUNICATION)SYSTEM)4%)

SAFETY)CHAIN)4%)

MV)SWITCHGEAR)

3%)

TOWER)3%)

OTHER)19%)

Failure)rates)

PITCH)SYSTEM)23%)

FREQUENCY)CONVERTER)

18%)

YAW)SYSTEM)7%)

GENERATOR)ASSEMBLY)

11%)LV)SWITCHGEAR)

3%)

GEARBOX)ASSEMBLY)

5%)

SENSORS)4%)

COMMUNICATION)SYSTEM)4%)

SAFETY)CHAIN)2%)

MV)SWITCHGEAR)3%)

TOWER)2%) OTHER)

18%)

Down=me)

© Fraunhofer 4

Project “Investigation of converter failure in wind turbines“ (CONFAIL)

! Participants:

! T. Thiringer, R. Karlsson, Chalmers Univ. of Techn., Göteborg ! T. Stalin, Vattenfall Wind Power, Stockholm ! J. Wenske, Fraunhofer IWES, Bremerhaven ! H. Zelaya, ABB Corporate Research, Västerås ! H. Ramberg, Volvo Car Corporation, Göteborg ! G. Wetter, Swerea IVF, Göteborg

! Project leader: K. Fischer (Chalmers, now Fraunhofer IWES)

! Funding: Swedish Energy Agency / Vindforsk III

! Focus on two different wind-turbine models (type 1: DFIG, type 2: IG with full power converter) at offshore and onshore sites, in total ~150 turbines, up to 4 years of data, low-voltage converters

© Fraunhofer 5

CONFAIL: Considered generator/converter topologies

© Fraunhofer 6

DFIG with partially rated converter (ca. 30% of PWT): 6 power modules (WT1)

IG with full power converter (100% of PWT): 18 power modules (WT2)

[Sou

rces:)C

ONFA

IL)project)(2

012),)Sem

ikron])

IGBT-based converters in back-to-back configuration

Converter module used in wind turbines

Exterior view Driver)board)

DCB)with)IGBTs)and)diodes,)covered)with)po`ng)compound)

Heat)sink)(water1cooled))

Disassembled)module)

AC)terminals)

[Sou

rce:)CONFA

IL)project)(2

012)])

Exterior)view)

© Fraunhofer 7

Root-cause analysis for power converters: Multi-track approach

Failure-data analysis, correlation with

environmental factors

Operating environment in the field

Measurements on failed modules, forensic analysis

Root-Cause Analysis workshops: Failure mechanisms and causes, countermeasures

© Fraunhofer 8

Literature: Focus on DFIG / thermomechanical failure

Generator-side converter in DFIG considered most critical, due to high currents at low frequencies ⟹ severe load and thermal cycling ⟹ thermomechanical stress ⟹ bond-wire and solder fatigue

Figures: Simulated junction temperature and power dissipation characteristics of a 1200V/50A-IGBTin inverter operation for different fundamental output frequencies [Konrad (1997)]

© Fraunhofer 9

CONFAIL: Power-converter failure rates, seasonal impact

© Fraunhofer 10

WT type 1 (DFIG, 6 modules), offshore WT type 2 (IG with FPC, 18 power modules), offshore

[Sou

rce:)CONFA

IL)project)(2

012)])

CONFAIL: Effect of generator/converter topology and site

Wind park

Turbine model Site Avg. failure event rate [1/turbine/yr]

Avg. failure event rate [1/module/yr]

Site A WT1 Offshore 0.13 0.020

Site B WT1 Onshore n.a. n.a.

Site C WT1 WT1* Onshore 0.15 0.025

Site D WT2 Offshore 0.39 0.021

Site E WT2 Onshore n.a. n.a.

Site F WT2 Onshore n.a. n.a.

Site G WT2 Onshore n.a. n.a.

Average converter failure rates for the turbine models and sites of interest

Similar failure rates per module in spite of different turbine topologies and sites

[Sou

rce:)CONFA

IL)project)(2

012)])

DFIG)

IG))with))FPC)

© Fraunhofer 11

Correlation of converter failure with external factors

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0,2)

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0,8)

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May.10)

Jul.10)

Sep.10)

Nov.10)

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Mar.11)

(a))FailureAevent)rate)[1/turb./yr])

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25)

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Mar.09)

May.09)

Jul.09)

Sep.09)

Nov.09)

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May.10)

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Sep.10)

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Jan.11)

Mar.11)

(b))Amb.)temperature)(monthly)avg))[°C])

0)20)40)60)80)100)

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Mar.09)

May.09)

Jul.09)

Sep.09)

Nov.09)

Jan.10)

Mar.10)

May.10)

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Sep.10)

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Jan.11)

Mar.11)

(c))Rela=ve)humidity)(monthly)avg))[%])

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(d))Wind)speed)(monthly)avg))[m/s])

Correlaeon)of)monthly)number)of)failures)with)average)T,)RH)and)vwind)?))

[Sou

rce:)CONFA

IL)project)(2

012);))

Clim

ate)data:)D

anish

)Metereo

logical)Insetute])

© Fraunhofer 12

WT)type)1,)offshore

WT)type)2)(IG)with)FPC))offshore

) month1scale month1scale day1scale Ambient)temperature + o o Rela=ve)humidity 1 + o Wind)speed o + ++)(but)no)corr.)with)explosion)

failure) Lightning)strikes ? ++ ++)(esp.)strong)with)explosion)

failure)

Correlation of converter failure with external factors

Found correlations:

Correlation of converter failure with lightning strikes?

[Sou

rce:)CONFA

IL)project)(2

012)])

© Fraunhofer 13

Field measurements inside converter cabinets

(a) Temperature and humidity loggers, (b) thermo-sensitive tape, (c,d) tape positions on modules

[Sou

rce:)CONFA

IL)project)(2

012)])

© Fraunhofer 14

Operating environment inside converter cabinets

Example)case:))WT)type)1,)site)A,)offshore)

Main findings: ! No problematic temperature levels found ! Risk of condensation failure during start-up

after long standstill confirmed )

[Sou

rce:)CONFA

IL)project)(2

012)])

© Fraunhofer 15

Forensic analysis of damaged power modules

[Sou

rce:)CONFA

IL)project)(2

012)])

Possible explanations e.g. electrical overstress due to lightning event, air gap reduction by foreign object

Evidence of high-voltage sparkover between PCB cooling fin and a screw (heat sink)

© Fraunhofer 16

Forensic analysis

Degraded thermal grease between DCB and heat sink Possible causes: ageing, dry-out, pumping effects, …

Spots on DCB and aluminium heat sink: Mechanisms of ”fretting corrosion” (due to thermo-mechanically induced relative movements in combination with mechanical pressure) ⟹ Overheating due to increased thermal resistance to heat sink?

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00keV

0

8000

16000

24000

32000

40000

48000

56000

64000

Counts

O

Al

Si

P

SSCl

ClNi

Ni

Ni

Ni

Cu

Cu

Cu

Cu

Zn

Zn

ZnAu

Au

Au

Au Au

[Sou

rce:)CONFA

IL)project)(2

012)])

© Fraunhofer 17

Analysis of converter driver-boards

Contamination with salt and corrosion products found on one driver-board

Optical microscopy

SEM and EDS

X-ray miscroscopy

[Sou

rce:)CONFA

IL)project)(2

012)])

© Fraunhofer 18

Microscopy inspection of the bond wires and die attach

No indications of the “classical” converter reliability issues, (bond-wire or solder damage) found

[Sou

rce:)CONFA

IL)project)(2

012)])

© Fraunhofer 19

Outlook

! CONFAIL pre-study: ! Classical converter reliability issues (solder degradation, bond-wire damage)

seem to play a minor role in wind turbines at present

! Indications for insufficient protection (salt, condensation, foreign objects) ! Indications for electrical overstress (correlation with lightning events)

! Next step: from indications towards clear evidence & solutions ! Clarify converter-failure causes and mechanisms based on extended failure-data

analysis, directed field measurements and hardware analysis ⟹ extend the scope to further wind-turbine models, additional topologies, MV technology, sites with different climatic conditions, …

© Fraunhofer 20

R&D cluster on reliable power electronics for wind turbines (Fraunhofer-Innovationscluster Leistungselektronik für regen. Energieversorgung)

Project partners:

!   Fraunhofer IWES, Fraunhofer ISIT

!   4 universities

!   20 industry partners

Duration: 2013 – 2016 Budget: 8 M€

Research partners: Industry partners:

Project focus and objectives:

! Improving reliability and availability of

frequency converters in wind turbines

!   Root-cause analysis, solutions for existing

and new turbines

!   System behaviour in dynamic operation

! Condition monitoring for electronics

!   Fault-tolerant generator/converter concepts

© Fraunhofer 21

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