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

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Outline

Motivation

Background of present load case

Presentation of an alternative approach

A preliminary example

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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

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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

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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:

1. The starting point is statistics of the rate of electrical failures and the jpdf. Of climate parameters: U, I, Hs, …

2. Then one determines the ‘normal’ combinations of electrical failure and climate parameters

3. 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

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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

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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

the rate of electrical faults per unit timeef

| Pr{structural failure | electrical fault}sf efp

|sf sf ef efp

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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

|

1 1sf sf ef

ef

pT T

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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,…)

1 2

| 1 2

, , , , , ,

1 2 1 2

Pr{structural failure | , , , , , , }

( , , , , , , )s

sf ef s

V I H R R

s s

p V I H R R

f V I H R R dVdIdH dR dR

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Alternative approach – Theory: Failure probability 2/2

Solve the equation with respect to design variables z

1 2

| 1 2

, , , , , ,

1 2 1 2

Pr{structural failure | , , , , , , }

( , , , , , , )s

sf ef s

V I H R R

s s

p V I H R R

f V I H R R dVdIdH dR dR

This can be done iteratively by use of

Extrapolation

FORM or IFORM

…

05

1015

2025

30

0

1

2

3

40.1

0.2

0.3

0.4

0.5

V [m/s] [m/s]

norm

aliz

ed r

espo

nse

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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

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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 statistically independent

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

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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.

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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 f = 1.1

For the method presented here the situation is normal. Thus probably f = 1.35 (but this remains the be proven)

Consequently one has FEOG, design ≈ 1.5 Felec. failure, design

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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

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Acknowledgements

Discussions with colleagues inside and outside DONG Energy

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