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

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

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

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**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**Outline**• Motivation • Background of present load case • Presentation of an alternative approach • A preliminary example**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**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**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**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**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**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**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,…)**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 • …**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**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**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.**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**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**Acknowledgements**• Discussions with colleagues inside and outside DONG Energy • Public Service Obligation founds from EnergiNet.dk