of Large Offshore Wind Farms Survey of Reliability of Large Offshore Wind Farms Nicola Barberis Negra nibne@dongenergy.com Ole Holmstrøm oleho@dongenergy.com

General information UpWind Integrated Wind Turbine Design FP6 Integrated project From March 2006 to February 2011 40 partners Work package 9 Survey of present status Power system requirements Different electrical concepts Extreme wind conditions

General information Offshore wind power – Research-related bottlenecks Mutual shadow effect between blocks of wind turbines Extreme structural loading of offshore wind turbines Interaction of large wind farms with waves and current Grid connection and reliability Environmental aspects Optimized operation and maintenance for offshore wind farms DAWE

Outline Introduction Current state of offshore wind farm How to assess wind farm reliability Reference

Outline Introduction Current state of offshore wind farm How to assess wind farm reliability Reference

Introduction How has reliability of wind farms been addressed? Overall power system (Grid Codes) System adequacy – Prediction/reserves System security Wind farm owner/operator Adequacy – Cost effective wind farm design Security – Grid Code compliance Power system adequacy wind treated like a ”variable negative load” addressed in terms of prediction and secoadary reserves Capacity credit = low if any With large offshore wind farms and high penetration: controllability of wind farms have been i intruduced. Some system support. BUT APART FROM THIS GRID CODES HAVEN’T MADE ANY SPECIFIC REQUIREMENTS. TSO have no concern about WF avaialbility, etc. Operation on market conditions. Power system security: fault ride through requirements was adressed quite early and have been introduced in most power systems. Winfd farms and wind turbines can meet requirements. Wind farm owner adequacy analyses of wind power has mainly been related to the cost effective wind farm design (i.e. optimisation of the installation costs and owners revenue) rather than the contribution (positive or negative) to the power system reliability.   Security: fullfil grid code.

Outline Introduction Current state of offshore wind farm How to assess wind farm reliability Reference

Current Installations

Typical Wind Farm Layout

Wind Turbine Large size for better production (>2 MW) Higher voltages Transformers Generators Protection (depends of Grid Code) Selectivity is applied individually Individual protection for under-/overvoltage Individual protection for under-/overfrequency Standard IEC 61400 for wind turbine design

Wind Turbine The first offshore wind turbines experienced several problems - “built-in” failures or design errors Simple systems and equipment placed offshore may experience new impact of climate, vibrations and intermittent operations Offshore wind farms include a large number of identical units Repairs to even simple problems may take disproportionately long time

Wind Turbine Power electronics and electronic equipment The experience shows relatively significant failure rates Active stall – Electronic reactive power control DFIG – Rotor side converters Full size converters Simple, robust and well-tried solutions should be preferred

Internal Distribution and Transmission Grid Typical industrial distribution system First wind resource, then electrical parts Cluster/string configuration (redundancy only for North Hoyle) Voltages at 33-36 kV Protection Faults cause tripping of interested radial Time to inspect and reestablish

Connection to Shore Belong to the wind farm or the TSO (i.e. Denmark) Small wind farm (below ~100 MW) 1 connection/cluster at MW level Large wind farm (above ~100 MW) Offshore substation 1 HV connection for the whole wind farm All HVAC solutions, HVDC are under study

Protections Depends of each national Grid Code (if available) System neutral earthing Different solutions for different parks Horns Rev is impedance earthed Nysted is with isolated neutral Lightning protection The number of lightning strokes experienced by offshore wind turbines is 3-4 times larger than land-based turbines Increased risk of damages

UpWind First Conclusions First offshore wind farms shows a need for increasing focus on design, procurement, quality, risk assessments and quality assurance New standards or practices for risk assessments and quality assurance of offshore installations are needed; existing IEC standards may not be sufficient Quantification of reliability parameters is still uncertain due to the few installations and the short operational experience (e.g. no cable failures)

Outline Introduction Current state of offshore wind farm How to assess wind farm reliability Reference Questions

Why to Assess Reliability of Wind Farms? Wind farm design must be optimize according to losses, reliability and economical aspects Increase of wind installations Energy penetration (Denmark) Installed capacity (Germany) Wind generation is part of large power systems and it must be controlled for power balance issues Models and data are required

Available Assessment Techniques Deterministic solutions First used approaches No uncertainties are included Probabilistic methods (sequential or not) Analytical models or Monte Carlo simulations Uncertainties are included Broader range of studies

Comparison of Techniques Analytical methods Mathematical models to represent the system Simplifications are needed Faster, but the model is almost a “black box” Monte Carlo simulations Easier to implement with less approximations Longer computation time All aspects can individually be analyzed

Aspects of Relevance Simulation of wind speed Wake effects of the park Wind turbine technology Power collection grid of the park Grid connection system Offshore environment Different wind speeds in the site Correlation of output power among different wind farms Hub height variations

Component availability data General Model 1. Simulation of wind speed 2. Wake effects of the park 9. Hub height variations a. Wind speed data d. Output Results c. Wind Farm b. Component availability data 3. Wind turbine technology 4. Power collection grid in the park 5. Grid connection system 6. Offshore environment 7. Different wind speed in the site 8. Correlation of output power among different wind farms

Reliability Data New technology  Difficulty in getting data Available data are based on values of land-based installations “guessed” at offshore locations Example of data

Outline Introduction Current state of offshore wind farm How to assess wind farm reliability Reference

Reference O. Holmstrøm, N. Barberis Negra, ‘UPWIND Deliverable D9.1 - Survey of reliability of large offshore wind farms. Part 1: Reliability of state of the art wind farms’, Report, May 2007. N. Barberis Negra, O. Holmstrøm, B. Bak-Jensen, and P. Sorensen, ‘Aspects of relavance in offshore wind farm reliability assessment’, IEEE Transaction on Energy Conversion, Vol. 22, No. 1, March 2007, pp.159-166. N. Barberis Negra, O. Holmstrøm, B. Bak-Jensen, and P. Sorensen, ‘Comparison of Different Techniques for Offshore Wind Farm Reliability Assessment’, 6th International Workshop on Large-Scale Integration of Wind Power and Transmission Networks for Offshore Wind Farms, October 26-28, 2006, Delft, The Netherlands. G.J.W. van Bussel and M.B. Zaarijer, ‘Reliability, Availability and Maintenance aspects of large-scale offshore wind farms, a concepts study’, Proceeding of MAREC 2001, Newcastle, England, 2001. A. Sannino, H. Breder, and E. K. Nielsen, ‘Reliability of collection grids for large offshore wind parks’, 9th International Conference on Probabilistic Methods Applied to Power Systems, June 11-15, 2006, Stockholm, Sweden, . DOWEC Team ‘Estimation of turbine reliability figure within the DOWEC project’, DOWEC project No. 10048, No. 3, October, 2003. ...

Thank you for the attention Nicola Barberis Negra nibne@dongenergy.com c/o Dong Energy A/S Kraftvaerksvej 53 7000 Fredericia Denmark Ole Holmstrøm oleho@dongenergy.com c/o Dong Energy A/S Kraftvaerksvej 53 7000 Fredericia Denmark