Summary and Comparison

Different redundancy degrees are considered and compared. The incurred reliability costs are estimated in British pounds or million pounds per MWh/year (£ million per MWh/year). This data is also not applicable, so for each level, choose four proper

Chapter 6 Reliability Enhancement of Offshore Wind Farms by Redundancy Analysis 167

values to compare with the required extra device costs. The extra device quantity, redundancy degree, increased cost and EENS costs are shown below: Level 1 − Table 6.6 and Figure 6.11, Level 2 − Figure 6.12, Level 3 − Table 6.7 and Figure 6.13.

Table 6.6: Level 1 - Device Cost Increase and EENS with Different Redundancy ng-g nr γ1 Cost Increase (£ million) EENS (MWh/year)

0 0 1.000 0 70662 2 4 1.032 4.506 52632 4 7 1.060 8.511 40602 5 9 1.076 11.216 33504 7 12 1.104 13.719 24774 10 18 1.152 17.728 12622 13 23 1.196 21.036 4470

ng-g – the number of group-to-group redundant cables;

nr – the number of inner group redundant cables.

1 1.05 1.1 1.15 1.2 0 5 10 15 20 25 30 35 40 Redundancy degree C os t I nc re as e a nd R el ia bi lity C os ts ( £ millio

n) _{Device Cost Increase}

£200 per MWh/year £250 per MWh/year £333 per MWh/year £500 per MWh/year

γ1

Figure 6.11: Collection grid level – level 1 cost and reliability analysis (different £ per MWh/year

Chapter 6 Reliability Enhancement of Offshore Wind Farms by Redundancy Analysis 168 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4 0 0.5 1 1.5 2 2.5 Redundancy degree C os t I nc re as e a nd R elia bilit y C os ts ( £ mi llio

n) Device Cost Increase £20 per MWh/year £50 per MWh/year £100 per MWh/year £200 per MWh/year

γ2

Figure 6.12: Platform transformer level – level 2 cost and reliability analysis (different £ per

MWh/year values represent different conditions of cost incurred on average for an MWh loss per year).

Table 6.7: Level 3 - Device Cost Increase and EENS with Different Redundancy

Redundant Cable length (km) Switchgear No. γ1 Cost Increase (£ million) EENS (MWh/year) 0 0 1.000 0 282744 3.75 2 1.081 13.375 88269 11.50 4 1.234 20.750 9600 22.25 8 1.455 41.906 0 1 1.1 1.2 1.3 1.4 1.5 0 20 40 60 80 100 Redundancy degree C os t I nc re as e an d R eli ab ility C os ts ( £ mill io n)

Device Cost Increase £50 per MWh/year £100 per MWh/year £200 per MWh/year £333 per MWh/year

γ3

Figure 6.13: Transmission system level – level 3 cost and reliability analysis (different £ per

MWh/year values represent different conditions of cost incurred on average for an MWh loss per year).

Chapter 6 Reliability Enhancement of Offshore Wind Farms by Redundancy Analysis 169

Collection grid (level-1) redundancy design has many options so here we use seven points; platform assessments (level-2) use two points to show the linear relationship; for transmission system (level-3), due to the limited options, four points are shown. From the above comparison, if the EENS loss information is available, the optimal redundancy degree can be found at the point of reliability cost curve across the increased device cost curve. The total maximal redundancy γ = 1.196×1.4×1.455 = 2.436 can be considered as the full redundancy condition.

In [6.19], it is mentioned that the fault likelihood and the associated costs are assumed to be lower than the costs for the additional devices. Therefore, redundancy is not taken into consideration. This may be true for small wind farms. But the comparison results show that redundancy is necessary for large-scale offshore wind farms due to economic aspects.

This systematic design method is in favour of comparing numerous options for complex offshore wind farm electrical system design. In addition, the results of AC and DC wind farms can be compared to explore the difference related to the diverse cost distribution among equipment, foundations and space, and individual device reliabilities. Hence it will be helpful for DC wind farm design, notwithstanding the disadvantage of high-cost DC devices. However, key to this method is accurate offshore wind farm operation statistics and detailed AC and DC equipment costs for accurate optimisation results.

6.6 Conclusion

The growing scale of future offshore wind farms makes reliability enhancement important during the planning and design phases. After analysing the importance and necessity of redundancy in wind farm collection and transmission systems, a detailed systematic redundancy design method is proposed and described from both technical and economic standpoints. The syntheses of cost and reliability measures are defined. The final degree of redundancy can be achieved using reliability and economic loss statistics. Results show that the balance between reasonable investment in

Chapter 6 Reliability Enhancement of Offshore Wind Farms by Redundancy Analysis 170

redundancy and the reliability of offshore wind farms can be analytically reached. More practical operational statistics and economic analysis are required for future modern wind farm applications, especially for large-scale DC offshore wind farm scenarios.

Chapter 6 Reliability Enhancement of Offshore Wind Farms by Redundancy Analysis 171

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