harmonic challenges and mitigation in large offshore wind … · mitigation more sophisticated...
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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
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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
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5
4a
4b
HARMONY Symposium2015-08-26, Aalborg
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
HARMONY Symposium2015-08-26, Aalborg
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-
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id7*
id5*
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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
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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
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Active Filtering in Wind Turbinesharmonic current control
HARMONY Symposium2015-08-26, Aalborg
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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2ω0
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ωN1
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0 20 40 60 80 100 120 140
V1
7[%
]
P [MW]
H17, PCC #2, before
H17, PCC #2, after
HARMONY Symposium2015-08-26, Aalborg
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
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Number of turbines
VG
M [
dB
]
1 9 17 25 33 41 49 57 65 73 81 89
25
30
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Number of turbines
GM
[d
B]
1 2 3 4 5 6 7 8 9 10 11 12 13
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Number of turbines
GM
[d
B]
VGM
1
GM
1
S
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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
HARMONY Symposium2015-08-26, Aalborg
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
HARMONY Symposium2015-08-26, Aalborg
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
HARMONY Symposium2015-08-26, Aalborg
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.
HARMONY Symposium2015-08-26, Aalborg
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
HARMONY Symposium2015-08-26, Aalborg
Questions?