Impact of Electrical Topology, Capacity Factor and Line Length on Economic Performance of Offshore Wind Investments
2. Description of Electrical Topologies Considered
2.1. AC Offshore Wind Energy System
2.2. DC Offshore Wind Energy System
2.3. Electrical Design for Offshore Wind Farm Collection System
3. Annual Energy Yield and Electrical Losses Estimation
3.1. Power Electronics Losses
3.2. Transformer Losses
3.3. Collection and Transmission Lines Losses
3.4. Annual Energy Losses
4. Economic Analysis
4.1. Major Investment Indicators Considered For Economic Assessment
4.2. Cost Calculation for AC and DC Offshore Wind Energy Projects
4.3.1. Wind Turbine Cost
4.3.2. Support System Cost
4.3.3. Electrical System Cost
- Collection system cable cost—the wind turbines are connected to each other as well as to the onshore substation through submarine cables. The core (conductor) of the cables can be either stranded copper or aluminum. Due to the surrounding sea environment, sufficient electrical insulation is needed around the conductor. The subsea cable insulations are made of different dielectric materials. Among the two common ones are mass impregnated paper and Cross Linked Polyethylene (XLPE) Polymeric cables [42,43]. There are additional layers exist for shielding and mechanical strength purposes. For the DC configuration, XLPE cables are mostly used with VSC topology  and were therefore selected for this study. For the AC configuration, 3 core XLPE type cables were selected from available manufacturer datasheets. Reference  provides DC cable cost formulation for different voltages. Reference  presents a cost function for a 30 kV voltage level cable. The cost model used is given as follows :Typical cable cross-sections were taken from a manufacturer’s  publicly available datasheet and depending on the selected voltage and power levels, cable costs are estimated as in Table 2. The cable cross sections were selected such that they carried the maximum power output of the wind turbines. It was considered that the selected radial collector system has a single cable in a row and carries the power of minimum 3 and maximum 13 wind turbine outputs in one feeder. These numbers were determined by considering the physical layout design as well as cable cross sections ampacity levels. An additional 40 m supplementary cable for each turbine was considered as recommended in Reference . The total cable cost for DC collection system was calculated asThe cost function for 3 core XLPE cables given in Reference  was used to calculate the cable costs as follows:The cables were considered to be buried under on the seabed. The burying cost in Reference  was used as 273 k€/km. The total burying cost for the collection system was calculated by considering all the cable lengths used in the system as
- Onshore DC/DC converter substation—one of the factors affecting the DC offshore wind system cost is converter costs. The percentage of the converters over the total cost varies based upon the system topology and transmission distance. Converter costs are found to be around 20% in References [49,50]. The DC configuration becomes more economical over the AC configuration once the converter cost is covered by the cable costs. In DC offshore wind systems, the onshore and offshore substations house the converters and a few other components such as reactors, filters and DC breakers. In addition, the offshore substations cost includes the platform cost as well. In this paper, since there were two onshore substations considered, platform cost was disregarded.Obtaining the exact cost figures for substation converter stations is very difficult. Reference  investigated many studies and proposed € 150/kW. Reference [45,52] presented 1 SEK/VA price for the converters. Reference  argues this is because of having different insulation levels at different voltages and proposes three different cost figures for different power ratings. Reference  states that this cost includes the cost of valves, filters and other necessary parts. The average of these figures given in the literature was used in this study, which is € 194.23/kVA by assuming that all the substation component costs are included. The onshore substation in the island is a DC/DC converter that steps up the voltage. It is assumed that the converter station includes a series connection of valves for the total power rating. The total price is calculated as
- Onshore AC substation and power factor correction costs—since the substation has many components including transformers, switchgeras, backup generators and so forth, the cost is considered as a lump sum that is a function of the installed wind power capacity. Based on the cost model in Reference , a cost of 50 k€/MW was used for the calculations. The cost models for power factor correction devices (i.e., SVC, STATCOM, shunt reactors) are given as follows :Thus, the total substation cost is found  by
- Transmission line cost: The total power of 100 MW and 300 MW collected from wind turbines are delivered to onshore substation with the monopole collector system in DC collection system. The DC/DC converter in onshore substation steps up the collector voltage from 30 kV to 150 kV for HV transmission system. A bipole with two conductor system given in Reference  is considered for the transmission system from the onshore substation on the island to the other onshore substation on the mainland. The system has 150 kV voltage and two identical cables deliver the power as an underground (1.7 km) and subsea system (17.3 km). No overhead lines are considered for the transmission system. The two cables connecting the two substations share the total power due to being a bipole system and their cross-sections were considered based on the maximum current. The transmission cable cost that was calculated earlier was used for per km. Additional 100 km supplemental cable was considered and the total cable cost was calculated as:Since all the transmission cables are underground and subsea cables, a burying cost of 273 k€/km given in Equation (18) was used. In this study, close laying structure of cables was considered for the bipole system. The two cables were considered to be close to each other and buried together to have a single burying cost. The literature does not present any cost model for a single core XLPE cable used in the HVAC transmission system for high power delivery. It is also difficult to get the cost information from a manufacturer due to it being sensitive information for business operations. Therefore, it was assumed that a single core cable cost is 40% of the 3-core for the same current rating because of better insulation requirements for high voltage. The HVAC transmission system includes three single core cables, hence, the total cost for transmission system cable is tripled.
- Onshore DC/AC converter substation—the onshore substation on the mainland is a DC/AC substation that converts 150 kV DC to 154 kV AC national transmission voltage. Although this is a DC/AC converter, the price of € 194.23 /kVA used for the DC/DC converter in Equation (19) was used for the calculations. Many converters are connected together for the total power as considered earlier. Since the total cost is given as per power, the total cost considered with total power is in Equation (19).
- Grid connection cost—although the cost of the substation in the mainland was considered earlier, it was assumed that there is an additional grid connection cost at the point of common coupling in order to be connected to the 154 kV AC national electric transmission system. The cost for both AC and DC configuration is given as a function of total delivered power in Reference  as
4.3.4. Project Development, Management and Other Costs
5. Results and Discussion
5.1. Losses Assessment
5.2. Economic Assessment
5.3. Sensitivity Analysis
- The studied OWF was found to be economically viable for both AC and DC configurations with 13 and 14 years of DPBPs for the AC and DC options, respectively. The estimated LCOEs for the AC and DC OWF configurations range from 88.34 $/MWh to 113.76 $/MWh and from 97.61 $/MWh to 126.60 $/MWh, respectively. LCOEs for both options slightly change even though the wind farm size was increased by three-fold.
- Losses in the AC and DC configurations range from 3.75% to 5.86% and 3.75% to 5.34%, respectively, while LCOEs vary between 59.90 $/MWh and 113.76 $/MWh for the AC configuration and 66.21 $/MWh and 124.15 $/MWh for the DC configuration.
- It was found that the transmission line length parameter is more sensitive in loss estimation while the capacity factor parameter is more sensitive in LCOE estimation.
- It was proved that the superiority of AC configuration over the DC option in terms of LCOE decreases as capacity and transmission line length increase.
- It was also shown that the advantage of DC configuration over the AC option in terms of losses increased as capacity factor and transmission line length increased.
Conflicts of Interest
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|Parameter||5SNA 3600E170300||5SNA 1200G450300||5SNA 0750G650300|
|Voltage (kV)||Topology||Cable Cross-Section (mm)||Price (€/m)|
|AC Configuration||100 MW||300 MW|
|Total line losses||1,612,692||0.41||4,191,070||0.36|
|Power electronics including transformer||13,256,069||3.39||39,768,206||3,39|
|Total energy losses||14,868,760||3.80||43,959,276||3,75|
|DC Configuration||100 MW||300 MW|
|Total line losses||1,198,297||0.31||3,264,131||0.28|
|Total energy losses||14,750,420||3.77||43,962,129||3.75|
|AC 100 MW||DC 100 MW||AC 300 MW||DC 300 MW|
|Capacity factor at PCC (%)||41.24||41.20||41.24||41.24|
|Net AEP (kWh/year)||361,344,935||361,194,688||1,084,598,234||1,084,598,234|
|Annual Revenue with base FIT (million$)||26.38||26.37||79,18||79,18|
|Annual Revenue with max FIT (million$)||39.75||39.73||119.31||119.31|
|OPEX (million$/20 years)||5.71||6.37||17.17||18.98|
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Kucuksari, S.; Erdogan, N.; Cali, U. Impact of Electrical Topology, Capacity Factor and Line Length on Economic Performance of Offshore Wind Investments. Energies 2019, 12, 3191. https://doi.org/10.3390/en12163191
Kucuksari S, Erdogan N, Cali U. Impact of Electrical Topology, Capacity Factor and Line Length on Economic Performance of Offshore Wind Investments. Energies. 2019; 12(16):3191. https://doi.org/10.3390/en12163191Chicago/Turabian Style
Kucuksari, Sadik, Nuh Erdogan, and Umit Cali. 2019. "Impact of Electrical Topology, Capacity Factor and Line Length on Economic Performance of Offshore Wind Investments" Energies 12, no. 16: 3191. https://doi.org/10.3390/en12163191