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Thursday, 13 March 2014
11:15 - 12:45 Innovative concepts for drive train components
Science & Research  


Room: Ponent
Session description

New developments in wind turbines need innovation and advances in technology in the field of wind turbine drive trains. This session focuses on topics related to transmissions and generators

Lead Session Chair:
Emilio Gomez-Lazaro, Universidad de Castilla-La Mancha. Renewable Energy Research Institute, Spain
Joseph Burchell The University of Edinburgh, United Kingdom
Co-authors:
Joseph Burchell (1) F P Ozan Keysan (1) Markus Mueller (1)
(1) The University of Edinburgh, Edinburgh, United Kingdom

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Presenter's biography

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

Joseph Burchell is a PhD researcher at the University Of Edinburgh, Scotland. He received his Masters in Mechanical Engineering from Edinburgh in 2009 and went on to work in the Building Services and Sustainability sector until he returned to Edinburgh’s School of Engineering to carry out a PhD on the structural development of HTS generators for Offshore Wind applications.

Abstract

Proposed structure for a HTS generator for direct drive offshore wind turbines

Introduction

The drive for larger offshore turbines with greater outputs is curtailed by the physical size of the generator and the ability to install devices in remote areas with often changeable weather conditions. High Temperature Superconducting (HTS) machines offer the technology able to replace large permanent magnet Direct Drive (DD) generators with more compact machines.
However some theoretical HTS designs often neglect the practicality of the physical support structure required to tolerate the huge magnetic attraction forces produced by increased field flux. Offshore wind turbines require structurally robust machines with low operation and maintenance costs as well as lightweight, material efficient designs. [1]

Approach

This paper will focus on the structure developed for a transverse flux machine topology. An example of this topology is shown in figure 2 [2].
A homopolar claw pole rotor design was chosen to create a varying flux on stationary armature coils from a stationary Superconducting (SC) field core. Claw pole machines are capable of delivering higher power densities than conventional machines they also minimize flux leakage and reducing the volume of SC tape required compared with an air cored machine [3]. The stationary superconducting coil reduces the complexity of the cooling and excitation couplings. However due to the use of iron, soft magnetic composites (SMC) or other ferromagnetic materials within the design, very high magnetic attraction forces are present with in the air gaps.

The magnetic attraction force present in this generator topography is explained by Maxwells stress formula and is proportional to the square of the magnetic flux present. Figure 3 illustrates the exponential increase of attraction forces produced by flux levels many times stronger than those found in conventional permanent magnet generators. The rigidity of the support structure is of upmost importance in maintaining a constant air gap between the armature and field core back iron and the rotating claw poles. However in achieving this rigidity the structure should not negate the benefits of producing more compact machine design.

Using Finite Element Analysis (FEA) the proposed structure has been analysed and illustrates whether any significant movement in the air gap is present. The evolution of the design will be explained and a comparison will be made with more traditional Permanent Magnet (PM) structures to differentiate the use of materials as well as composition. The nature of SC operation will be addressed and preloaded assemblies will be proposed as means to save mass whilst maintaining structural integrity.


Main body of abstract

This paper will address the design of a ridged support structure for a proposed transverse flux HTS DD generator design. An over view of the design is presented and the optimised structure is described, further explanation of optimisation processes can be found in [4]. Full results will be available with the full paper submission.

The possibility of producing HTS machines a third of the size and mass of conventional PM DD generators [1] allows for a decrease in the structural mass of the tower, nacelle and foundation there by cutting installation and construction times. High magnetic field strength is one of the benefits that enable SC technology to achieve higher power outputs; however topologies relying on ferromagnetic materials to direct flux tend to experience high forces acting on the structure. The maximum value of the air gap flux density is limited by natural permeability of the material. The topology of the HTS machine in this study uses Vacoflux50, a material with a Saturation point of roughly 2.2 T. The flux present in the air gap is in the region of 1.8 T – 1.9 T. Although these forces are curtailed by the saturation level and flux leakage, resulting forces in the air gap can be as high as 2000 kN/m2. To make these topologies competitive with air cored machines, structural mass, complexity and size must not diminish the benefits which SC technology offers.


Figure 5 portrays one possible installation technique for the HTS transverse flux machine on to the structure of a wind turbine [5]. This paper will explore how a conventional PM structure shown in figure , [6], compares with an optimised spoked generator structure for the machine outlined in figure 6 .

The structure is said to be optimised once the deflections in the generator air gap satisfy the design criteria. For this study deflection larger than 20% of the air gap value is considered a design failure. Forces were applied to the appropriate faces of the proposed structure mimicking magnetic attraction forces. Stress and deformation analysis showed areas which required reinforcement or removal; through this method an optimised structure was created. {7}

The incorporation of structural compression struts into the cad design, improved the performance of the spokes by roughly a quarter. The construction industry has used structural struts and tension bars for decades, in this case high strength carbon chrome steel bar was utilised. The ultimate yield strength of the bar is 1030 N/mm2 therefore supplying great rigidity for low mass. [7] The compression of the struts can be individually tailored at each spoke to counteract forces which may affect the air gap.

PM structures are under load at all times whereas utilising SC technology means that the structure will only experience loading when the SC reaches its critical temperature and is excited by a current. Although this will occur the majority of the time, any variations within the air gap could be monitored and extreme displacements trigger a shut off of the magnetic field. A safety procedure such as this is yet another benefit of this technology. The ability achieve auto compression control within the struts to aid with fluctuation within the air gap will be studied at a later date.


Conclusion

This paper presented a support structure for a transverse flux HTS machine for offshore wind turbines. The aim of the paper is to question to what extent conventional PM generator structures can with stand the high attraction forces produced within the air gap; to produce a potential design and compare it with an optimized PM design; to investigate the potential benefits in using novel structural elements and to aid industry by cutting mass and material costs. FEA simulations of a support structure were used to explain the evolution of a strong and lightweight support frame and compared to conventional PM structural design. The structural challenges created by this relatively new technology were investigated and solutions proposed to aid in the fabrication and installation for offshore wind applications.

The proposed transverse flux HTS DD generator design was found to be more ridged and lighter than traditional designs. The possible benefits of constructing a machine which does not require full structural integrity at all times were discussed and safety assessed. Future considerations will included the addition of drive train dynamics including harmonic loading, vibrations and displacements as well as unbalanced magnetic forces to determine the ability of this design to withstand the offshore turbine environment. Additionally the ability to alter the compression within tension struts to aid with fluctuation within the air gap will also be reviewed.

The benefits HTC technology can offer the wind industry are numerous. The move to install the technology will only come when the industry has faith that these benefits are achievable. Full analysis of the SC topic by researchers, including cryogenics, superconducting composites, novel materials and generator structural integrity will lead to greater confidence and reduce the concern of entering the next big step for power generation.



Learning objectives
The learning objectives of this study are to improve the knowledge base for High Temperature Superconducting machine design. Gain further recognition for this technology and aid industry in gaining confidence in its efficiency, practicality and construction.


References
[1.] M. Lesser, J. Muller, Superconductor Technology Generating the Future of Offshore Wind Power, Renewable Energy World Conference, Cologne, Germany (2009) 1–10.
[2.] O. Keysan, M. Mueller, A Transverse Flux High-Temperature Superconducting Generator Topology for Large Direct Drive Wind Turbines, Superconductivity Centennial Conference, (2011) 1-6.
[3.] O. Keysan, M. Mueller, A Homopolar HTSG Topology for Large Direct-Drive Wind Turbines, IEEE Transactions on Applied Superconductivity (2011) 1–9doi:10.1109/TASC.2011.2159005
[4.] A. Zavvos, ‘Structural Optimisation of Permanent Magnet Direct Drive Generators for Wind Turbines’, PhD thesis, January 2013, University of Edinburgh
[5.] O. Keysan, ‘Superconducting Generators for Direct Drive Generators”, PhD thesis, Submitted October 2013, University of Edinburgh
[6.] D. Bang, H. Polinder, G. Shrestha, and J. A. Ferreira, “Review of generator systems for direct-drive wind turbines,” in Proc. Eur. Wind Energy Conf. Exhib., Belgium, 2008, pp. 1–11
[7.] Macalloy “Post Tensioning System”, Concrete Engineering, www.macalloy.com/brochures/post-tensioning, November 2011.