INVESTIGATION OF EOG COMBINED WITH GRID FAILURE IN THE CASE OF OFFSHORE WIND TURBINES

1. INVESTIGATION OF EOG COMBINED WITH GRID FAILURE IN THE CASE OF OFFSHORE WIND TURBINES Niels Jacob Tarp-Johansen Presented at EWEC2007, 7-10 May 2007, Milan, Italy

2. Outline • Motivation • Background of present load case • Presentation of an alternative approach • A preliminary example

3. Motivation • The load case with EOG combined with electrical failure is a design driver for foundations for offshore wind turbines – DLC 2.3 • This is at least valid for modern MW pitch-regulated machines • It might be different for stall turbines, but these are not considered at state-of-the-art for offshore conditions is pitch-regulated turbines

4. Background for present load case for grid failure • The gust size seems to be obtained by distributions independent of 10-min mean wind but conditional on start wind speed • In other words: it seems that an appropriate weighting of distribution of start wind speed, gust size and e.g. 10-min mean wind speed is missing. • Response has been considered to some extend, as the slope of the gust has been regarded. • The shape has support in data. The appropriateness of the full coherence appears not to have been validate and has been accepted as conservative. • Finally some change of gust size has taken place to ensure that together with the probability of operational events like starts and stops a 50-yr return period event was obtained

5. Alternative approach: Aim and Idea of the method 1/2 • Aim: to formulate a general method which considers response. The method shall determine wind, and sea state conditions to apply in combination with the event of electrical failure • Idea: • The starting point is statistics of the rate of electrical failures and the jpdf. Of climate parameters: U, I, Hs, … • Then one determines the ‘normal’ combinations of electrical failure and climate parameters • One finalizes by performing stochastic response simulations for these normal conditions • So: the method does not aim at determining abnormal combinations of gusts and electrical failure

6. Alternative approach: Aim and Idea of the method • Three effects are included in the proposed method relative to DLC 2.3 • Replacing coherent gust with simulation of turbulent field • (Influence of eigenfrequency relative to gust duration) • Relaxing demand on worst case phasing of gust with electrical failure • Determining climate parameters in situations with electrical failure by different rationale

7. Alternative approach – Theory: Poisson model 1/2 • It is assumed that the events of electrical failure follows a Poisson process with constant intensity • An electrical failure may potentially lead to structural failure • Consequently the events where electrical failure leads to structural failure constitute a Poisson process too with constant intensity

8. Alternative approach – Theory: Poisson model 2/2 • An event with return period T is determined by requiring • This gives us the target probability of structural failure for characteristic values to be used in design against electrical failure

9. Alternative approach – Theory: Failure probability 1/2 • The structural failure probability depends on • Loads and strengths • Loads in turn depends on climate, design, and the applied control strategy in case of electrical failure. • This may be expressed through • Dependence between climate and electrical failure is accounted for by the choice for f (V,I,Hs,…)

10. Alternative approach – Theory: Failure probability 2/2 • Solve the equation with respect to design variables z • This can be done iteratively by use of • Extrapolation • FORM or IFORM • …

11. Standardisation – how? • Extrapolation may be allowed – this is probably less problematic than it sounds • An alternative similar to the idea behind ETM + stochastic response simulations may be developed • An alternative EOG dependent on turbine characteristics may be devised • The two first options are the preferred ones • Potentially the EOG should be included in extra/alternative load cases that aims at verifying the control system behaviour

12. A preliminary example 1/2 • We focus on the wind only: IEC 61400-1 model IB • Use IFORM extended to include response • NB most distributions are close to normal implying expectably small error • Compare results with DLC 2.3 • Assumptions: electrical failure on the one side and wind and sea state conditions on the other side are statisticallyindependent • storm events that potentially lead to grid loss (overhead lines clashing) will be geographically separated from offshore farms • ship dragging an anchor over the sea bottom may cause damage to the cable • Pitch speed: 7.5 deg/sec, tower frequency = 0.38 Hz

13. A preliminary example 2/2 • Simulations are performed in this way: • For a number of combinations of U and I • For each combination 100 simulations are carried out • The same 100 seeds are used for each U, I. • Simulation until transients have died out • Extremes after electrical failure is detected • Overturning moment.

14. A preliminary example: conclusions • The proposed method yields characteristic response about a factor of 2 smaller than DLC 2.3 - practically independent of the rate of electrical failure (4 – 50 yr-1) • BUT ... Safety level … • Safety factors must be added • Electrical failure + EOG is abnormal, that is gf = 1.1 • For the method presented here the situation is normal. Thus probably gf = 1.35 (but this remains the be proven) • Consequently one has FEOG, design ≈ 1.5 Felec. failure, design

15. Further work … • Simulation with other turbines: • Control strategy • Tower frequencies • Separate the three effects • Replacing coherent gust with simulation of turbulent field • Relaxing demand on worst case phasing of gust with electrical failure • Determining climate parameters in situations with electrical failure by different rationale • Safety factor assessment • Discuss modelling of dependence between grid failure and climate parameters

16. Acknowledgements • Discussions with colleagues inside and outside DONG Energy • Public Service Obligation founds from EnergiNet.dk