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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Electrical aspects and grid integration' taking place on Thursday, 13 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

MIKEL DE PRADA IREC, Spain
Co-authors:
MIKEL DE PRADA (1) F P JOSE-LUIS DOMÍNGUEZ (1) FRANCISCO DÍAZ GONZÁLEZ (1) ANDREAS SUMPER (1) ORIOL GOMIS-BELLMUNT (1)
(1) IREC, SANT ADRIA DE BESOS, Spain (2) CITCEA-UPC, BARCELONA, Spain

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Presenter's biography

Biographies are supplied directly by presenters at EWEA 2014 and are published here unedited

M. de Prada received the degree in industrial engineering from the School of Industrial Engineering of Barcelona (ETSEIB), Technical University of Catalonia (UPC), Barcelona, Spain, in 2010. He was an academic visitor of the National Research Energy Laboratory (NREL) in Golden, Colorado, USA in 2012. He is currently pursuing a Ph. D. degree in electrical engineering. His research interest are modelling and control of electrical machines and power converters and its application in
renewable energy generation systems.

Abstract

Technical and economic assessment tool for offshore wind power plants based on a collector grid connected to a single VSC-HVDC converter

Introduction

In recent years, wind power generation has experienced tremendous growth throughout the world, thanks to the environmental benefits, technological advance of wind power and government incentives [1].

Trends point to a growing importance of Offshore Wind Power Plants (OWPPs) mainly because the wind speeds at sea are potentially higher and more constant than on land which leads to a much higher energy production and there is less space limitations, which allows the possibility of using larger wind turbines [2].


Approach

Nowadays, environmental and social aspects are forcing Offshore Wind Power Plants (OWPPs) to be constructed further from shore, (which usually leads to deeper waters) and the trend is expected to continue in the coming years [3]. Several studies have demonstrated that if the distance between an OWPP and its grid connection point at the Point of Common Coupling (PCC) exceeds a certain critical distance, HVDC transmission becomes the most appropriate solution, since reduce cable energy losses and decrease reactive power requirements [4,5]. Regardless of whether HVDC technology is based on Line Commutated Converters LCC-HVDC or Voltage Source Converters VSC-HVDC, both options require a single large power converter (SLPC) at the connection point of the OWPPs, allowing a centralized control for the whole WPP.

This paper presents a tool to assess the performance, from the technical and economic point of view, of two proposed OWPP configurations in comparison with the conventional OWPP scheme. These novel concepts are based on removing the individual power converters of each wind turbine and connecting a cluster of wind turbines or an entire OWPP to a single large power converter which operates at variable or constant frequency, respectively.

The tool has been exemplified with a study case in order to analyze the cost-effectiveness of each OWPP configuration.

Wind power plant concepts analyzed

- Conventional topology MPCs: This scheme consists of a wind power plant with multiple power converters (MPCs), one for each wind turbine, and another VSC-HVDC or LCC-HVDC for the entire wind farm . This proven topology [6] guarantees that the wind farm generates the maximum power available from the wind, regardless of wind speed variability in wind farms, since it enables independent speed control for each wind turbine.


- Proposed topology SLPC: This scheme is shown in . The wind power plant is based on removing the individual power converters of each wind turbine and connecting a turbine cluster (or an entire WPP) to a single large power converter (SLPC) by means of a centralized control. According to this approach, two WPP topologies are studied depending on whether the SLPC operates at variable or constant frequency (SLPC-VF or SLPC-CF).


Main body of abstract

Description of the tool

As it can be seen in , the methodology used for the technical and economic assessment of the proposed OWPP topology is based on seven steps which the user must execute sequentially.

- STEP 0 – DESCRIPTION OF THE TOOL:

This step is purely informative and describes the main objective of the tool with figures and text.

- STEP 1 – WPP LAYOUT DEFINITION:

In this step, the layout of a specific wind power plant has to be defined in order to compare both WPP topologies (conventional and proposed) in a same scenario. Thus, a completely generic WPP layout is defined so that it can be determined not only considering a matrix rectangle, but also specifying the coordinates of each wind turbine.

- STEP 2 – WIND CONDITIONS DEFINITION:

In this step, certain wind conditions are established for the WPP previously defined. These wind conditions are characterized by specific wind speeds and wind directions. The wind speeds are obtained from the Weibull distribution function at the wind power plant location, while the wind directions are computed according to the wind rose function. It is assumed that the wind strikes the blades of each turbine under the same wind direction. It is also considered that the incoming wind speeds depends on its wind direction so that the Weibull distribution parameters used depends on each wind direction.

- STEP 3 – WAKE EFFECT CONSIDERATION:

Once the WPP layout and its wind conditions are defined, the next step consists in computing the wind speeds of each wind turbine. In order to obtain more realistic values of wind speeds, the wake effect amongst wind turbines is considered. A comprehensive wake model considering single, partial and multiple wakes within a wind power plant and taking into account different wind directions, is used.

- STEP 4 – OPTIMUM ELECTRICAL FREQUENCY CALCULATION:

To maximize the power generation of the aforementioned SLPC-VF scheme, the optimal grid electrical frequency depending on the measured wind speeds in all the wind turbines is computed.

- STEP 5 – TECHNICAL ANALYSIS:

According to the output data resulted from step 1 to 4, a technical analysis is carried out to evaluate the suitability of the proposed OWPP topology. In order to provide an accurate assessment, different types of losses have been taken into consideration. These losses can be classified into two main groups: the steady-state losses (CP losses and power flow losses) and the unavailability losses of the system due to the failure of certain equipment (corrective maintenance losses) or the partial or total stop of the installation during a fixed time for preventive maintenance purposes (preventive maintenance losses).

- STEP 6 – COST ANALYSIS:

Finally, in the step 6, this tool performs a cost assessment to evaluate the total cost of the three WPP topologies analyzed (MPC, SLPC--VF and SLPC--CF). An accurate model considering both the initial investment costs and the operation and maintenance (O&M) costs has been developed.

Case study

In order to determine which OWPP (MPC, SLPC-VF or SLPC-CF) is more cost-effective, the tool has been applied to a particular case study.
The OWPP examined consists of a unique cluster composed by 20 wind turbines with a rated power of 5MW and a rotor diameter of 126m. It is laid out in a regular matrix of 5 rows and 4 columns (or strings). The spacing between two nearby wind turbines is 7 rotor diameters (D) in both directions.



Conclusion

Table and table show the results of the technical and economic analysis, respectively, for the three configurations evaluated.

In this paper, a technical and economic analysis tool for OWPPs is presented. In particular, this tool is intended to be helpful in assessing the possible benefits of adopting two new proposed OWPP concepts instead of the conventional OWPP topology.
These novel concepts proposed are based on removing the individual power converters of each wind turbine and connecting a turbine cluster (or an entire WPP) to a single large power converter (SLPC) which operates at variable (SLPC-VF) or constant (SLPC-CF) frequency, respectively. These proposed schemes are specially appropriated for OWPPs with an HVDC transmission link.
By means of the presented methodology, a comprehensive technical and economic assessment has been applied to a case study in order to determine the cost-effectiveness of each concept.
According to the results obtained, SLPC-VF is presented as an appealing OWPP alternative, since a total cost saving of up to 6% compared to the conventional MPC WPP topology can be achieved. Thus, although the optimal operation point of each turbine cannot be assured due to the inherent configuration of the proposed scheme, the absence of dedicated power converters for each turbine brings a reduction in capital costs, as well as, in maintenance and power flow losses, so that economic benefit can be realized.
Likewise, the paper demonstrates the effectiveness of using the optimum electrical frequency calculation algorithm for variable frequency operation, as can be seen by comparing the total WPP cost resulted for SLPC-VF or SLPC-CF schemes.



Learning objectives
This paper presents a comprehensive methodology to analyze different offshore wind power plants topologies, from the technical and economic point of view. All types of losses (operational, and from corrective and preventive maintenance) are included in the study. The cost analysis takes into account not only the capital expenditures of each components but also the costs associated to these aforementioned losses.


References
[1] Global wind statistics 2012, Tech. rep., Global Wind Energy Council (GWEC) (February 2013).

[2] Offshore wind toward 2020. On the pathway to cost competitiveness, Tech. rep. Roland Berger Strategy consultants GmbH (April 2013).

[3] J. Ladenburg, Visual impact assessment of offshore wind farms and prior experience applied Energy 86 (3) (2009) 380-387.

[4] N. B. Negra, J. Todorovic, T. Ackermann, Loss evaluation of HVAC and HVDC transmission solutions for large offshore wind farms, Electric Power Systems Research 76 (11) (2006) 916-927.

[5] N. Kirby, M. Luckett, L. Xu, W. Siepmann, HVDC transmission for large offshore wind farms. iee ac-dc power transmission, no. 485, IEE ACDC Power Transmission, London, 2001.

[6] Pena R, Clare JC, Asher G. Doubly fed induction generator using back-to-back PWM converters and its application to variable-speed wind-energy generation. IEE Proceedings Electric Power Applications 1996;143 (3):231-41.