harmonic challenges and mitigation in large offshore wind … · mitigation more sophisticated...

Harmonic Challenges and Mitigationin Large Offshore Wind Power Plants

HARMONY Symposium

DONG Energy Wind Power,Łukasz Hubert Kocewiak

Harmonics and Stability in Power Electronic Based Power Systems

AalborgWednesday, August 26th 2015

IntroductionHarmonics in wind power plants

Harmonic problemsHarmonic sources and effects

Harmonic mitigationPassive vs. active filtering

SummaryNeed for further research

and standardization

2

Outlinetable of contents

Core activities Development, construction and operation of

offshore wind farms

Market position Market leader in offshore wind, has built

38% of European capacity Strong pipeline of projects in lead-up to 2020)

Business drivers Wind conditions Availability Electricity prices and subsidies Industrialisation and maturing of value chain Securing supplies via framework agreements and partnerships Partnerships with industrial and financial investors

ROCE targets 2016: 6-8% 2020: 12-14%

Strategic targets for 2020 Installed offshore wind capacity (before divestments): 6.5 GW Reducing offshore wind Cost of Energy to below EUR 100/MWh3

1 The percentages indicate the proportion of the Group that each business unit represented in 2012.2 Intragroup revenue means that the business units’ combined revenue exceeds consolidated revenue.3 Cost to society based on projects in the UK where investment decisions will be made in 2020.

DONG Energy Wind Powercompany profile

Active Filtering in Wind Power PlantsMotivation

HARMONY Symposium

Harmonic emission from wind turbines

Amplified harmonics

from the grid

1

3

Resonances amplifying harmonics

4

Controllers harmonic stability

problems

2

2015-08-26, Aalborg

200 400 600 800 1000 1200 1400 1600 18000

500

1000

1500

2000

|Yeq

| [S

]

200 400 600 800 1000 1200 1400 1600 1800-100

-50

0

50

100

Y

eq []

f [Hz]

20

40

60

80

200 400 600 800 1000 1200 1400 1600 18000

50

100

150

|Yeq

| [S

]200 400 600 800 1000 1200 1400 1600 1800

-100

-50

0

50

Y

eq []

f [Hz]

2

4

6

8

10

12

14

Horns Rev 2 Karnice

Conclusions• Long HVAC cables can affect significant series

resonances• Wind farm aggregated for different number of

connected wind turbines

• Different wind turbines can operate differently with wind farms

• Distribution network introduces higher impedance in comparison to the transmission system

5

Wind Farm Structuresresonance phenomena

HARMONY Symposium2015-08-26, Aalborg

Active Filtering in Wind Power PlantsMitigation

Passive filter at the onshore substation

Passive filter at the offshore substation

Passive filter in wind turbines

Active filter in wind turbines

Active filter in group of wind

turbines

Active filter at the point of common

coupling

1

2

3

5

4a

4b


Active Filteringpros and cons

Pros Cons

– Already existing technologies such as STATCOMs can be utilised for it active filtering at the connection point

– Improving technology not commonly applied in wind farms

– Possibility of online retuning or with a short downtime and thus reduced risk of design uncertainties

– Extra effort in terms of harmonic stability is needed

– Wide control potential (e.g. selective harmonic compensation, wide band high-pass active filtering, etc.)

– Limited harmonic range due to hardware constraints and thus hybrid solutions might be preferable

– Network impedance changes during operation can be addressed

– Component sizing issues and limited DC-link voltage utilisation

– Control method can be tuned for each of wind farms independently taking into consideration grid code issues as well as wind farm structure

– Negligible losses for series connected active filters such as wind turbines


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iq

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αβ

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e jθe -jθ

θ θ

-ωL

ωL

id*

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psT

K1

1

vgq

vgd

v*

vα*

vβ*

iα

iβ

i

psT

K1

1 ++

-

e -j7θ

θ

e j5θ

iα*

iα*

iβ*

iβ*

s

Ki7

s

Ki7

s

Ki5

s

Ki5

e j7θ

e -j5θ

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+

+++

+

+

+

+ +

+

+

+

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+

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+

+

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+-

-

-

-

id7*

id5*

iq5*

iq7*

+ω

-5ω

+7ω

Case: Control in rotating reference frame

Control: Harmonic compensation needed if background distortions are present (i.e. 5th, 7th, ...)

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 10

5

10

I 5 [

%]

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 10

5

10

15

TH

Di [

%]

t [s]

Conclusions: Harmonic current dependent on background and

selective harmonic control Command signal distorted to compensate current Current quality improved due to the control Converter output voltage more distorted

8

Active Filtering in Wind Turbinesharmonic current control


Active Filter Functionalitiesexperience from Anholt Offshore Wind Farm

Conclusions Changes in the converter internal impedance can

affect harmonic emission Emission can be reduced without loss of stability and

converter oversizing Harmonic rejection capability improved

Benefits Harmonic mitigation measure without extra

passive components, Flexibility, Reduced cost of electricity.

id

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id(+)*

iq(+)*

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+++

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+ω

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-jθe

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θ θ

ωL

-ωL

id(-)*

iq(-)*

vgq(-)

vgd(-)

+++

++++-

+-

-ω

++

++

vα*

vβ*

i

psT

K1

1

i

psT

K1

1

2ω0

2ω0

ωN1

i

psT

K1

1

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 20 40 60 80 100 120 140

V1

7[%

]

P [MW]

H17, PCC #2, before

H17, PCC #2, after


1 2 3 4 5 6 7 8 9 10 11 12 13

3.5

4

4.5

5

5.5

Number of turbines

VG

M [

dB

]

1 9 17 25 33 41 49 57 65 73 81 89

2

3

4

5

Number of turbines

VG

M [

dB

]

1 9 17 25 33 41 49 57 65 73 81 89

25

30

35

Number of turbines

GM

[d

B]

1 2 3 4 5 6 7 8 9 10 11 12 13

24

26

28

30

32

34

Number of turbines

GM

[d

B]

VGM

1

GM

1

S

1

PM

Re(DG)

Im(DG)

-1

Horns Rev 2

Karnice

Conclusions• Gain margin does not show stability issues for low

frequencies• Vector gain margin gives supplementary information

about the stability

• Vector gain margin can detect low-frequency resonance

• Vector gain margin and gain margin are equal when in case of boundary stability

PM ≥ 30°GM ≥ 3dBVGM ≥ 3dB

Wind Farm Stability AssessmentNyquist criteria and stability margins


Cables The underground export cable is typically physically

installed in flat formation which introduces asymmetry in positive and negative sequence impedance between phases. Therefore of the whole three-phase system would be more sophisticated.

Sequence It is assumed that positive and negative sequence

control of the grid-side converter in the WTs are the same and thus the Nyquist plot is symmetrical. Typically for steady-state operation this assumption can be valid for the sake of simplification.

Coupling It was assumed that the positive and negative sequence

controller are decoupled and thus the WT for harmonic and stability studies can be expressed as a simplified single-phase equivalent system.

Tolerances WPP components tolerances is also difficult to

incorporate into the model. Fortunately such

parameters as transformers copper losses are typically available from test reports and cable capacitance tolerances are provided by the suppliers and adjusted in order to find the best match between simulations and measurements.

Data WPP components such as transformers or cables

frequency-dependent characteristics were not available during the studies. Thus commonly accepted by the industry modelling approach was assumed.

Operation The sequence of WTs disconnection was assumed based

on the most probable wind direction however it does not necessarily reflect the reality.

Methods The proposed harmonic analysis methods are based on

linearity assumptions and such issues as frequency coupling in harmonic load flow or non-linearities (e.g. controller saturation, transformer saturation curve, etc.) were not included in the system representation.

Uncertaintiesto be addressed by R&D activities


DEVEX (DEVelopment EXpenditure) Lower risk during WPP development, e.g. reducing uncertainties

during filter design thereby reducing the needed contingency reserves,

Shorter development (design and implementation) period, Simplified procurement process and technical requirements of

harmonic filters, Simplified design reduce need for coordination within the project

and with stakeholders.

CAPEX (CAPital EXpenditure) Better risk management, e.g. reduced contingency reserve, Less passive components in the WPP system (e.g. shunt reactors,

passive filters), Smaller reactive power compensation need (in most cases only

cable system compensation) and smaller STATCOM size, Active filtering can be included in already existing components

such as WTG converters or STATCOMs causing no significant additional costs,

Reduced onshore substation size, Reduced risk of curtailment due to insufficient filter design and

resulting with grid-code compliance issues, Reduced issues with passive components lead time,

More optimized onshore substation layout, Additional space for potential future changes at the substation

due to e.g. changes in the external network, Facilitate connection to neighboring WPPs within extended

offshore clusters without significant electrical infrastructure topology change,

Reduced passive filter size implying possibility of compensation by dynamic reactive power compensation plants during the energization,

Smaller passive filter and avoidance of static shunt compensation can help avoid severe zero-miss phenomena,

Reduced problems with air core reactor electromagnetic field exposure.

OPEX (OPerational EXpenditure) Reduced downtime due to filter failure, Adaptive harmonic mitigation in case of changes in the external

network, Reduced risk of curtailment due to passive filter failure, Application of active filtering can help eliminate the risk of G5/4-1

and G5/5 non-compliance during the WPP energization process, Reduction of power losses in the infrastructure components.

Active Filtering in Wind Power Plantsbenefits in relation to the cost of electricity reduction


Active Filter Functionalitiesfor Power Converters in Wind Power Plants (R&D Activities)

Type of collaboration PSO- financed public research programme. ELFORSK research and development programme

under the Danish Energy Association.

Technical scope Analyse and develop advanced filter solutions based

on combinations of passive filters and active filters. Develop active filter solutions applicable for large

commercial wind power plant projects.

Partners Aalborg University, Denmark (University), DONG Energy Wind Power, Denmark (Wind Power

Plant Developer), Vestas Wind Systems A/S, Denmark (Wind Turbine

Manufacturer), ABB AB – FACTS/RT (Transmission System

Components Manufacturer), Energinet.dk, Denmark (Transmission System

Operator).

Benefits Wind power plant optimization, Uncertainties and risk management, Flexibility, Reduced cost of electricity.


Models Improvement of models, e.g. wind farm components for time and frequency analysis

Methods Better harmonic emission and stability assessment techniques applicable to waveforms affected by

power electronics

Mitigation More sophisticated harmonic mitigation methods such as active filtering

Design More optimized overall electrical infrastructure design as well as robust control solutions

Interdisciplinary approach Harmonic analysis/optimization should be introduced in other areas of electrical engineering, e.g.

power system, control, power electronics engineering, etc.

Research Directionsvaluable to the industry


Questions?

harmonic challenges and mitigation in large offshore wind … · mitigation more sophisticated...

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