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| Fig. 1: Illustration of offshore wind farm. Piles (1) are driven into the seabed once a suitable place for the wind farm is found. Erosion protectors, similar to sea defenses, are placed at the base to prevent damage to the sea floor. The top of the foundation is painted a bright color to make it visible to ships and has an access platform to allow maintenance teams to dock. Once the turbine is assembled, sensors on the turbine detect the wind direction and turn the head, known as the nacelle, to face into the wind, so that the blades can collect the maximum amount of energy. The movement of the wind over the aerodynamically shaped blades (2) makes them rotate around a horizontal hub, which is connected to a shaft inside the nacelle (3). This shaft, via a gearbox, powers a generator to convert the energy into electricity. Subsea cables (4) take the power to an offshore transformer (5) which converts the electricity to a high voltage (33kV) before running it back 5-10 miles to connect to the grid at a substation on land (6). |
Wind energy has been considered by many to be the cheapest and yet least effective sources of renewable energy in the 21st century. However, since the 1990s, there has been a resurgence in renewable wind energy: installed capacity increased five-fold, wind turbine manufacturing underwent significant consolidation, and the development of offshore wind power began, too. During the start of the 21st century, that trend has continued, with European countries and manufacturers leading the way. Today, the wind turbines that make up the majority of the total generating capacity produce between 1.5 and 5 MW. [1]
Indeed, there is considerable hope that such offshore wind farms may be the solution (or at least a part of the solution) to the global energy crisis. However, one of the biggest issues regarding offshore wind farms is the transmission of power once the power is generated. Developing offshore wind farms means that there will most likely be limited maintenance access, limited space, and extremely variable environmental factors to go along with the typical issues in transmitting power from conventional sources. [2]
At the moment, the tentative solution is the use of high-voltage direct current (HVDC) technology. Scientists argue that HDVC can fully realize the potential of the developments in offshore wind farms, overcoming some of the major technical challenges facing traditional AC solutions while also being an economically attractive solution. HVDC allows the power to be sent and received at different frequencies, and the transmission distance using DC is not affected by cable charging current. Power losses across the transmission cables will be also be lower since there is a higher power transmission capability per cable. [2]
Naturally, with plans in the making to create up to 1000 MW offshore wind farms, the ability to produce electricity stably is also a concern, especially considering that offshore wind farms are typically built in areas that are environmentally volatile. The issue of network instability has to be addressed for offshore wind farms to be viable; the generator must be able to dynamically react to any network short circuit fault.
One of the proposed solutions is to use what's referred to as dynamic reactive compensation. Steady state and transient analysis of the power transmission indicates that voltage control at the terminal bus of the installation is key to ensuring stability. By utilizing voltage control through shunt-connected static VAr compensator (SVC) devices, the SVC (through a series of mechanical controls) can compensate when there is a significant change in voltage. [3]
Another possibility for improving dynamic stability is to use the turbine mechanical construction and mechanical torque control, thus still relying on conventional technology to develop an unconventional solution [3]. Arguably the most significant solution long-term is the development of a variable-speed wind farm, which would greatly enhance dynamic stability. [4]
© Firas Abuziad. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. F. Manwell, J. G. McGowan and A. L. Rogers, Wind Energy Explained: Theory, Design and Application, 2nd Ed. (Wiley, 2010).
[2] W. Lu and B.-T. Ooi, "Optimal Acquisition and Aggregation of Offshore Wind Power by Multiterminal Voltage-Source HVDC," IEEE Trans. on Power Delivery 18, 201 (2003).
[3] L. Gyugyi, "Dynamic Compensation of AC Transmission Lines by Solid-State Synchronous Voltage Sources," IEEE Trans. on Power Delivery 9, 904 (1994).
[4] A. Feijoo, J. Cidras and C. Carrillo, "A Third Order Model for the Doubly-Fed Induction Machine," Electric Power Systems Research 56, 121 (2000).