Reliability Study, Elforsk report 07:65

Titel: Reliability Study – Analysis of Electrical Systems within Offshore Wind Parks, Elforsk report 07:65
Författare: Bengt Frankén STRI AB
Utgivare: Elforsk, Vindforsk
Årtal: 2007
Ämnesord: offshore
Sökord/Keywords: offshore, reliability method, energy
Rapport



Sammanfattning/Abstract:
In this report a reliability optimization method is presented that may be used for investment decisions concerning sub-sea cable systems of offshore wind parks. The method is based on reliability computations in different designs of the collection grid for the wind park. The method is using reliability data of involved components such as failure rates, repair times and switching times.

The method consists of three distinctive stages:

• In the first stage, the expected annual energy not supplied is derived for the basic configuration. In principle, the basic configuration can be any configuration, but a configuration without any redundancy could be an appropriate choice. The expected annual energy not supplied is calculated.

• In the second stage, redundancy is built into the collection grid. The choice of redundancy is based on the contribution of each component to the expected annual energy not supplied. The difference between the energy not supplied in the basic and in the new configuration is the additional energy that can be supplied.

• The third stage is an economical evaluation where the additional en-ergy that can be supplied is converted to additional income per year or over a whole life-cycle. At this stage the method is using assumptions regarding the energy price and the number of years in a life-cycle.

The method can be used for comparison of different configurations or for comparison of additional income versus additional investment in redundancy. The method can also be used to estimate the expected annual energy produc-tion of an existing wind park or an existing design.

The method is applied for case studies of three different sizes of offshore wind parks: small; medium-size; and large. A typical topology without redundancy for each size is used as basic configuration. The experiences from the case studies can be summarized in the following conclusions:

• The main contribution to the expected annual energy not supplied is due to the long repair time of components at an offshore location.

• Redundancy is introduced in the form of spare capacity in sub-sea ca-bles and additional cables and transformers.

• Two levels of redundancy should be distinguished based on the type of switchgear used. Remote-controlled load-switches in combination with remote indication of faulted segment will result in a restoration time between several minutes and one hour. Circuit-breakers with appropri-ate protection equipment will reduce the number of interruptions.

• The additional gain of installing circuit-breakers is limited whereas the costs are typically very high. The costs may include the costs of switchgear able to withstand the higher fault currents.

• The gain of installing remote-controlled load-switches is significant as it reduces the duration of a production stoppage from several weeks or months to one hour or less.

• There is an optimal number of load-switches, above which additional ones only increase costs and complexity without significant further gains in expected annual energy production.

The method described in this report is a probabilistic method, which is inher-ently associated with uncertainty. Some care should be taken in comparing rather accurately known investment costs with uncertain gain in annual pro-duction. A small difference in total costs between two design alternatives should not be seen as significant and a base for an investment decision. There are, however, no general rules for how to handle this and a further discussion on this is beyond the scope of this report.

A change in input parameters (failure rate, expected repair time, investment costs, value of non-delivered energy) may impact the preferred design under the method described in this report. As several of the input parameters are in itself uncertain, this would introduce an additional uncertainty in the final de-cision. However, it is
generally accepted in power system reliability that the outcome of the comparison is not impacted when the most-likely value is used for all input parameters and when the difference between the design is not too small.