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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
Deok-je Bang Korea Electrotechnology Research Institute, Korea, Republic of
Co-authors:
Deok-je Bang (1) F P Ji-Won Kim (1) Pil-Wan Han (1) Dae-Hyun Koo (1) Henk Polinder (2) Jan Abraham Ferreira (2)
(1) Korea Electrotechnology Research Institute, Changwon, Korea, Republic of (2) Delft University of Technology, Delft, The Netherlands

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Presenter's biography

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

Deok-je Bang received B.Sc. and M.Sc. degrees in mechanical engineering, Pukyong National University, Korea in 1996 and 1998, respectively. He received Ph.D. degree at Electrical Power Processing Group, Delft University of Technology in The Netherlands in 2010.
From 1998 to 2006, he worked in research area of electrical machines. In 2011, he was principal researcher and team leader at Wind Turbine Research Division, Hyundai Heavy Industries. Since 2012, he has been working at Electric Motor Research Center, Korea Electrotechnology Research Institute in Korea.
His research interests include large direct-drive machines for wind and tidal turbines.

Abstract

New bearingless generator with buoyant rotor for large direct-drive wind turbines

Introduction

The aim of this paper is to validate a new bearingless generator concept that would enable to significantly reduce both structural mass and bearing failures of large direct-drive wind generators. Large direct-drive wind generators have disadvantages of large mass and high cost compared to geared generators. In wind turbines, bearing failures have been a continuing problem and account for a significant proportion of all failures. It is thus required to significantly reduce both the structural mass and bearing failures in order to make large direct-drive wind generators more attractive for industry use.

Approach

Scaling up the power of direct-drive wind generators, the structural part of the generators becomes a dominant part of the total mass of the generators [1][2]. In wind turbines, bearing failures have been a continuing problem and account for a significant proportion of all failures. Bearing-related downtime is among the highest of all components of wind turbines. [3] Therefore, it is necessary to significantly reduce structural mass and bearing failures of large direct-drive wind generators in order to make those generators more attractive for industry use.
In order to reduce the mass of large direct-drive wind generators, different configurations of the generators have been proposed such as a lightweight ironless permanent magnet (PM) generator, a PM generator with the bearings close to the air gap, and a high temperature superconducting (HTS) generator. However, if a generator for large direct-drive wind turbines needs very small tolerance or very special material in constructing, the concept will not be attractive as far as cost is concerned even despite a lightweight structure.
In order to make a generator construction cost competitive and to reduce bearing wear resulting in bearing failures of generators, it was discussed to apply magnetic bearings or bearingless drives instead of mechanical bearings for large direct-drive wind generators.
In previous researches by the author of this paper, a new ring-shaped bearingless PM wind generator with a buoyant rotor has been proposed as a solution to significantly reduce structural mass and bearing failures. The ring-shaped buoyant rotor construction without shaft and torque arms would enable to significantly reduce the structural mass of large direct-drive wind generators by shortening the mechanical load paths of the generators. Bearingless PM generator would enable to significantly reduce bearing failures occurred by bearing wear since there is no physical contact. [4]
In this paper, the proposed generator concept is validated by experimental setups built to realize two concepts: a buoyant rotor concept integrated with magnetic bearings and a new bearingless drive concept. In order to assess the mass of the proposed generator concept, electromagnetic and structural parts of the generator are designed for 10MW wind turbines rated at 8.6rpm.

Main body of abstract

- Ring-shaped buoyant rotor concept
Using the buoyancy force, a very heavy structure floats easily in fluid. Therefore, the use of the buoyancy force for supporting the heavy rotating structure of large direct-drive wind generator is introduced to the proposed generator. The construction of the buoyant rotating part and the stationary part of the generator is sketched as shown in Fig. 1.

- New bearingless drive concept
A significant feature of the bearingless drive compared to the electric machine with the magnetic bearing is that the bearing winding is integrated into the electric machine. Conventional bearingless drives need to control both the bearing force with bearing winding and the torque with torque winding. [5]
However, the new bearingless machine drive in this paper does not need the power consumption to support the rotating part against the gravity because the part is supported by buoyancy. Additionally the new bearingless drive concept needs only one winding to produce both the bearing force to control the air gap length and the torque. [4]
Fig. 2 depicts a cross-section of linearized bearingless PM machine. The rotor of the bearingless PM machine consists of PMs and iron cores. The stator consists of the slotted iron cores and the windings.

Fig. 3 depicts an air gap control diagram of the new bearingless PM machine with current controllers. When the air gap 1 is larger than the air gap 2, the difference can be relayed to the current controllers by the gap sensors, and the air gap 1 can be reduced by increasing the attractive force in the air gap 1 by controlling the currents in stators.

- Experimental validation-1: Buoyant rotor concept integrated with magnetic bearings
In order to realize the buoyant rotor concept, the experimental setup has been constructed with a buoyant rotor, stator, magnetic bearings, seals and motor driving unit as shown in Fig. 4 and Fig. 5. Three U-cores and windings are equipped on each side of the stator, and I-cores are circumferentially equipped on the both sides of the buoyant rotor.


Fig.6 represents the air gap control results in the two cases: the buoyant rotor not rotating and rotating. Three lines in Fig.6 are the air gap displacement wave forms from the gap sensors equipped close by the magnetic bearings. The maximum displacement of the air gap in the axial direction is 0.5mm, which is 10% of the air gap length, at 45rpm (rotor linear speed: 2m/s).

- Experimental validation-2: New simplified bearingless drive concept
In order to realize the new simplified bearingless drive concept, a permanent magnet linear machine with double-sided air gaps is built. Fig. 7 depicts front view and upper view configurations of the machine. The stator consists of two-sets of three-modules with iron cores and windings. The mover consists of permanent magnets and iron cores for two-sides. Moving directions of the mover are X-axis for the air gap control and Y-axis for the thrust position control. Fig. 8 depicts the linear machine built to test and to realize the new bearingless drive concept. Fig. 9 represents the control diagram for both air gap control in X-axis and position control in Y-axis. Fig. 10 represents the experimental results, which are the current wave forms of both U-phase currents for two-sides and air gap displacement. The maximum displacement of the air gap in the direction of X-axis is 0.2mm, which is 6% of the air gap length, in operating at 0.2m/s speed and 0.2m stroke.




- Generator mass assessment for 10MW direct-drive wind turbines
In order to identify the mass competitiveness of the proposed generator, the electromagnetic and structural parts of the generator are designed for 10MW wind turbines rated at 8.6rpm, and the total mass of the generator is compared with the mass of DFIG 3G and HTSG.
Fig. 11 and Fig. 12 represent the FEA models of the electromagnetic part and the structural part, respectively. In the final paper, relevant equations and 3D-FEA design results of the electromagnetic and structural parts of the proposed generator will be included. Fig. 13 shows the total mass of wind generators with the power ratings of 5, 10 and 20MW. The total mass of the proposed 10MW generator is estimated at 200tonnes, which is comparable with the mass of geared generator and is 20 to 50tonnes heavier than high temperature superconducting generator (HTSG).


Conclusion

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This paper discussed new concepts of large direct-drive wind generators which enable to significantly reduce the structural mass and bearing failures. The following generator concepts have been proposed to overcome the disadvantages of large direct-drive generators:
- a ring-shaped generator without a main shaft and without torque arms
- multi-sets of generator systems
- a generator with a buoyant rotor
- a generator with a new simplified bearingless drive

The new concepts of bearingless generator with a buoyant rotor have been validated by the experiments to realize both the buoyant rotor concept integrated with magnetic bearings, and a new simplified bearingless drive concept.

In the experimental setup of the buoyant rotor concept integrated with magnetic bearings, the air gap control results have been obtained in the two cases: the buoyant rotor not rotating and rotating. The maximum displacement of the air gap was less than 10% of the air gap length.

New simplified bearingless drive concept has been validated by the experimental setup of a permanent magnet linear machine with double-sided air gaps. The maximum displacement of the air gap was less than 6% of the air gap length.

Both the electromagnetic part and the structural part of the new direct-drive generator have been designed for 5 MW, 10 MW and 20 MW wind turbines by the 3D-FEA. From the design, it was concluded that the proposed direct-drive generator was lighter than traditional direct-drive PM generators at all power ratings. When scaled up to larger than 9 MW, the proposed generator becomes lighter than a geared generator (DFIG 3G).


Learning objectives
- To state problems of generator systems for large offshore wind turbines
- To define the most suitable generator system
- To propose and validate a new solution to make large direct-drive wind generators more attractive for industry use


References
[1] A.S. McDonald, M.A. Mueller and H. Polinder, “Comparison of generator topologies for direct-drive wind turbines including structural mass”, in Proc. of the International Conference on Electrical Machines(ICEM), pp. 360.1-7, September 2006.
[2] D. Bang, H. Polinder, G. Shrestha, and J.A. Ferreira, “Review of generator systems for direct-drive wind turbines”, in Proc. EWEC (European Wind Energy Conference & Exhibition), Brussels, Belgium, March 31 - April 3 2008.
[3] J.K.H. Shek, D.G. Dorrell, M. Hsieh, D.E. Macpherson and M.A. Mueller, “Reducing bearing wear in induction generators for wave and tidal current energy devices”, in Proc. of the IET Conference on Renewable Power Generation, pp. 1-6, September 2011.
[4] D. Bang, “Design of transverse flux permanent magnet machines for large direct-drive wind turbines”, Ph.D. dissertation, Delft University of Technology, Delft, The Netherlands, 2010.
[5] A. Chiba, T. Fukao, O. Ichikawa, M. Oshima, M. Takemoto, and D.G. Dorrell, “Magnetic Bearings and Bearingless Drives”, London, U.K.: Newnes, 2005.