Introduction
After it has been ensured that our platform is stable under normal operation conditions, we had to examine how it would be connected to main grid. In this section we will analyse the components of transmission line as well as the control system, needed to synchronize both types of generation. The Figure 1 gives a better understanding of the concept, however in our case the transformer is not necessary:
Figure 1: Grid connection concept of HAPI [1]
Cables
The selection of the cables is a crucial factor of the connection with the grid. The platform as well as all the components installed on it are floating. Therefore, dynamic cables are required in order to keep the mechanical stresses induced on them within safe operating limits [2]. The critical point is the dynamic section of the cable because of the loads on the cables imparted by the motion in the turbine and mooring lines. The installation of these particular cables must be done by a specific cable laying vessel.
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Figure 2: Dynamic cables [3]
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Generally, the dynamic cables are characterized by excellent mechanical strength and they are not affected by twisting and bending moments. The XLPE insulation will protect the cables from the external damage which can be caused by other objects in the river [4]. Moreover, an intermediate buoy could be used in order to prevent the cables from being kinked near the riverbed.
Figure 3: Dynamic cables representation in HAPI platform
AC or DC?
Figure 4: AC vs DC cost comparison [6]
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When it comes to connection of an offshore system with the grid on the shore, usually there is a rival whether to use AC or DC cables [5]. Figure 4 shows that AC cost increase with a greater pace than the DC cost with distance. As it can be easily concluded our decision to use AC cables was straightforward. The nominal voltage of the cables is 400 V which is identical to the voltage output from the wind and the water current turbines.
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On-shore substation
The onshore substation is the linkage between our platform and the main grid. The main component of the substation is the 100 KVA frequency converter [7] which ensures that the frequency of the output signal is always within the accepted limits. Moreover is provides the same functions as the typical onshore electrical substations: switching devices to connect or disconnect equipment, protection equipment to respond to faults, and transformation to higher voltages for either transmission to shore or feeding an AC/DC converter station.
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Figure 5: The selected frequency converter
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Generally, the power output of the offshore-wind turbine fluctuates during the day due to changes in wind speed. On the contrary, the power output from the water current turbines is not expected to change dramatically in a period of a day. However, even the small fluctuations affect the frequency and the voltage amplitude. For that reason, both turbine types are equipped with output voltage control system that keeps the voltage constant when the wind or water stream velocity changes.
Connection to the grid
The electric grid of Egypt is considered to be quite weak in our selected location and many regions nearby face often electricity blackouts due to the increasing demand [8]. It is our responsibility to provide a steady voltage output that will not violate the flicker and harmonics limitations that are established by the Egyptian Electricity Authority. Our system will be connected to the local low-voltage substation as it is indicated in [9]. This will also enhance the distributed generation near this area and will gradually lead to a more stable electric grid.
References
[1] Easywindenergy.blogspot.co.uk. (2018). Wind Turbine Grid Connection. [online] Available at: http://easywindenergy.blogspot.co.uk/2013/03/wind-turbine-grid-connection.html [Accessed 12 Mar. 2018].
[2] Taninoki, r. (2017). Dynamic Cable System for Floating Offshore Wind Power Generation. SEI Technical review.
[3] Offshore Wind Industry. (2018). Dynamic cables operating at 66 kV for Windfloat Atlantic. [online] Available at: http://www.offshorewindindustry.com/news/dynamic-cables-operating-66-kv-windfloat [Accessed 6 Apr. 2018].
[4] Xiaoguang Qi and Boggs, S. (2006). Thermal and mechanical properties of EPR and XLPE cable compounds. IEEE Electrical Insulation Magazine, 22(3), pp.19-24.
[5] J. Green, Electrical collection and transmission systems for offshore wind power. Golden, CO: National Renewable Energy Laboratory, 2007
[6] Easywindenergy.blogspot.co.uk. (2018). Wind Turbine Grid Connection. [online] Available at: http://easywindenergy.blogspot.co.uk/2013/03/wind-turbine-grid-connection.html [Accessed 12 Mar. 2018].
[7] 100 kVA Frequency Converter, 5. (2018). 100 kVA Frequency Converter, 50Hz/60Hz/400Hz. [online] GoHz.com. Available at: http://www.gohz.com/100-kva-frequency-converter-50hz-60hz-400hz [Accessed 8 May 2018].
[8] Mahdy, M. and Bahaj, A. (2018). Multi criteria decision analysis for offshore wind energy potential in Egypt. Renewable Energy, 118, pp.278-289.
[9] Jeong, M., Kim, Y., Moon, S. and Hwang, P. (2017). Optimal Voltage Control Using an Equivalent Model of a Low-Voltage Network Accommodating Inverter-Interfaced Distributed Generators. Energies, 10(8), p.1180.
[2] Taninoki, r. (2017). Dynamic Cable System for Floating Offshore Wind Power Generation. SEI Technical review.
[3] Offshore Wind Industry. (2018). Dynamic cables operating at 66 kV for Windfloat Atlantic. [online] Available at: http://www.offshorewindindustry.com/news/dynamic-cables-operating-66-kv-windfloat [Accessed 6 Apr. 2018].
[4] Xiaoguang Qi and Boggs, S. (2006). Thermal and mechanical properties of EPR and XLPE cable compounds. IEEE Electrical Insulation Magazine, 22(3), pp.19-24.
[5] J. Green, Electrical collection and transmission systems for offshore wind power. Golden, CO: National Renewable Energy Laboratory, 2007
[6] Easywindenergy.blogspot.co.uk. (2018). Wind Turbine Grid Connection. [online] Available at: http://easywindenergy.blogspot.co.uk/2013/03/wind-turbine-grid-connection.html [Accessed 12 Mar. 2018].
[7] 100 kVA Frequency Converter, 5. (2018). 100 kVA Frequency Converter, 50Hz/60Hz/400Hz. [online] GoHz.com. Available at: http://www.gohz.com/100-kva-frequency-converter-50hz-60hz-400hz [Accessed 8 May 2018].
[8] Mahdy, M. and Bahaj, A. (2018). Multi criteria decision analysis for offshore wind energy potential in Egypt. Renewable Energy, 118, pp.278-289.
[9] Jeong, M., Kim, Y., Moon, S. and Hwang, P. (2017). Optimal Voltage Control Using an Equivalent Model of a Low-Voltage Network Accommodating Inverter-Interfaced Distributed Generators. Energies, 10(8), p.1180.
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