Evaluation and selection of AC transmission lay-outs for large offshore wind farms

 ABSTRACT:

This paper studies different energy transmission solutions for AC offshore wind farms. This transmission of energy is based on AC submarine cables that present a strong capacitive behavior. Therefore, an analysis is necessary to determine transmission characteristics such as, the number of submarine cables, voltage or rated power. For that purpose, three different transmission configurations will be considered: unique HVAC, various HVAC and MVAC, combined with three submarine cables of different characteristics. By using a design procedure, it is shown that based on the electric characteristics provided by the manufacturer of the submarine cable, it is possible to determine the most efficient energy transmission solution, from the perspective of the submarine cable. Different variables will be taken into account, including transmission current, active power losses, the cost of the transmitted energy and the reactive power compensation required. In addition, the consequences of the selected transmission solution to other more general aspects of the wind farm such as, necessity of the offshore platform or local inter turbine network are also discussed.

KEYWORDS:

1.      Wind energy

2.       Transmission of electrical energy

3.       AC-cable

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAMS:

  

Fig. 1: General layout of HVAC offshore wind farm.

          

Fig. 2: General layout of MVAC offshore wind farm.

            

Fig. 3: General layout of offshore wind farm with multiple HVAC connections.

EXPECTED SIMULATION RESULTS:

  

Fig. 4: Module of the current through the submarine cable vs cable length. With compensation at both ends (red) and onshore compensation (blue). a) 5x30MW-36kV configuration. b) 150MW 150Kv

Fig. 5: Active power losses for 50km cable length, with compensation at both ends (red) and onshore compensation (blue) a) 150MW-150kV, 2x75MW-87kV, 3x50MW-66kV y 5x30MW-36kV configurations b) 150MW-220kV

Fig. 6: a) Rayleigh distribution for different average wind speeds b) Generated power on wind farm on function of the wind speed.

                     

Fig. 7: Energy transmission cost for different layouts.

CONCLUSION:

In agreement with built wind farms, a MVAC transmission system is the best option near to shore. This is because submarine cables are very expensive. With big cable lengths the cables costs do not compensate the money saved in the offshore platform. With short cable lengths (<20Km) MVAC connections are better than other layouts. Moreover, at 150 MW rated power MVAC configuration can be the best option to 60Km cable length. However in this case the clusters are of (40-50MW) and the submarine cables operates at 70-80% (or more depending the cable length) of their load capability. This can cause an inadmissible voltage drop in the transmission system or other harmful effects. In this paper only conduction losses in the submarine cables have been considered, armor losses or dielectric losses have also not been taken into account. But this simplification affect to cable parameterization and not to layout selection procedure. 220kV HVAC system is not the best option for any cable length. But the cable used in this evaluation has 3 times higher resistive component than other cables.

REFERENCES:

[ 1 ] S. Lundberg, "Wind farm configuration and energy efficiency studies series DC versus AC layouts," Thesis, Chalmers University of Technology 2006.

[ 2 ] S. Lundberg, "Evaluation of wind farm layouts," EPE Journal (European Power Electronics and Drives Journal), vol. 16, pp. 14-20, 2006.

[ 3 ] Å. Larsson, A. Petersson, N. Ullah, O. Carlson, “Krieger’s Flak Wind Farm”, Nordic wind power conference, May 2006

[ 4 ] S.D. Wright, A.L. Rogers, J.F. Manwell, A. Ellis, “Transmision options for offshore wind farms in the united states,” AWEA 2002

[ 5 ] S. Chondrogiannis, M. Barnes, “Technologies for integrating wind farms to the grid (Intering report)”, DTI 2006.

Analysis and Comparison of Medium Voltage High Power DC/DC Converters for Offshore Wind Energy Systems

ABSTRACT:

Offshore wind farm with an internal medium-voltage dc (MVDC)-grid collection connected HVDC transmission may be an option to harvest offshore wind energy. High-power MV dc/dc converters with high-step-up conversion ratios are the key components for the internal MVDC grid. In this paper, a high efficiency step-up resonant switched-capacitor converter for offshore wind energy system is studied, which is characterized by the soft-switching condition for all switches and diodes. This significantly reduces switching losses and higher switching frequency

is feasible to reduce the overall system volume and weight. The comparisons with other two kinds of topologies are also presented; moreover, the possible specification requirements of high power MV dc/dc converters are analyzed and set. The operation principle of the proposed converter has been successfully verified by simulation and experiment results.

KEYWORDS:

1.      High power

2.      Medium-voltage dc (MVDC) converter

3.      MVDC grid

4.      Offshore wind farm

SOFTWARE: MATLAB/SIMULINK

BLOCK DIAGRAMS:

 Fig.1. Layouts of three kinds of electrical systems for offshore wind farms. (a) AC system. (b) AC/DC system. (c) DC system.

CIRCUIT DIAGRAM:

Fig. 2. Configuration of the proposed ZCS RSC converter.

EXPECTED SIMULATION RESULTS:

 

Fig. 3. Idealized waveforms of Fig. 2.

 

Fig. 4. Simulation waveforms of an 8-level SCR converter.

Fig. 5. Experimental waveforms under full-load condition. (a) V_gs and V_ds of Q₁ , currents of L_p1 and L_{n 1} . (b) V_gs and V_ds of Q₁ , currents of L_p5 and L_p2 . (c) V_gs and V_ds of Q₁ , currents of D_p12 and D_p11 . (d) V_gs and V_ds of Q₁ , currents of D_p52 and D_p51 .

CONCLUSION:

High-power MV dc/dc converters with high-step-up conversion ratios are the key components in MVDC-grid collection systems for offshore wind farms. This paper has studied the possible specification requirements of high power MV dc/dc converters. A high efficiency step-up resonant switched-capacitor converter for offshore wind energy system has been proposed, which significantly reduces switching losses, increases switching frequency and minimizes the overall system volume. The operation principle and detailed design of the main circuit are presented. The experimental results from the prototype have confirmed the feasibility of the proposed converter.

REFERENCES:

[1] P. K. Steimer and O. Apeldoorn, “Medium voltage power conversion technology for efficient windpark power collection grids,” in Proc. IEEE Int. Symp. Power Electron. Distrib. Gener. Syst., Jun. 2010, pp. 12–18.

[2] S. M. Muyeen, R. Takahashi, and J. Tamura, “Operation and control of HVDC-connected offshore wind farm,” IEEE Trans. Sustainable Energy, vol. 1, no. 1, pp. 30–37, Apr. 2010.

[3] O.Martander, “DC grid for wind farms,” Licentiate of Engineering Thesis, Dept. of EPE, Chalmers University of Technology, Landala, Sweden, 2002.

[4] C. Meyer, M. H¨oing, A. Peterson, and R. W. De Doncker, “Control and design of DC grid for offshore wind farms,” IEEE Trans. Ind. Appli., vol. 43, no. 6, pp. 1474–1482, Nov./Dec. 2007.

[5] J. Robinson, D. Jovcic, and G. Jo´os, “Analysis and design of an offshore wind farm using a MV DC grid,” IEEE Trans. Power Deliv., vol. 25, no. 4, pp. 2164–2173, Oct. 2010.