Review of generator systems for directdrive wind turbines

1
Review of generator systems for direct-drive wind
turbines
Deok-je Bang , Henk Polinder , Ghanshyam
Shrestha , Jan Abraham Ferreira Electrical
Power Processing, DUWIND Delft University of
Technology The Netherlands d.j.bang_at_tudelft.nl
2
Objective

• Review direct-drive generators
• Indentify promising concepts for direct-drive

3
Contents

• Introduction
• Different generator systems
• Why direct-drive concept is heavy?
• Electromagnetic structure for direct-drive
• Mechanical structure for direct-drive
• Expected suitable concepts
• Summary

4
1. Introduction

• Direct-drive
• reduces gearbox failure
• but heavy, large and expensive
• Geared generator system mostly used on the market
• Trends of WTs towards
• up-scale
• offshore
• Direct-drive systems require
• light construction
• low cost
• high reliability

5
2. Different generator systems
6
3. Why direct-drive concept is heavy ?

• Increasing WT power,
• Direct-drive generator is large, heavy and
  expensive

7
4. Electromagnetic structure for direct-drive

• Strength of PM machine
• high energy yield and light weight
• no additional power supply for the field
  excitation
• improvement in the efficiency
• higher reliability without slip rings
• higher power/weight ratio
• compared to EE machines

8
AFPM machine producing the magnetic flux in
axial direction with PMs
9
RFPM machine producing the magnetic flux in
radial direction with PMs
10
TFPM machine the magnetic flux is perpendicular
to the direction of rotor rotation
11
Comparison
12
Comparison

• Cost advantage of TFPM machine KTFPM_cost
• (Dubois 2002)

13
Large direct-drive wind turbines of different
manufacturers
For large direct-drive, ? RF type mostly
used ? AF type not used over 1 MW ?
TF type not used
14
5. Mechanical structure for direct-drive
Conventional structure
15
Lightweight structure
Zephyros Z72
Spooner et al (05)
Tavner et al (06)
NewGen (07)
16
Direct-drive generator mass comparison

• McDonald (06)
• 2, 3, 5 MW RFPM machine with optimum
  Kradls /Dg
• Polinder, Bang (07)
• 10 MW RFPM machine with Krad0.16

? Conventional PMSG DD (with optimum Krad) heavy
compared to geared system ? Decreasing Krad the
active mass is decreased, but the inactive mass
and total mass are increased. ? New configuration
is required to reduce both the active and
inactive material.
17
6. Expected suitable concepts

• Electromagnetic structure
• material reduction concept with short flux path
  
• ? RF, AF ?, TF potential

18
6. Expected suitable concepts

• Electromagnetic structure
• material reduction concept with short flux path
  
• ? RF, AF ?, TF potential

• 10 MW RF TF PM generator
• Force density 40 kN/m2
• Pole pair 160
• Pole pitch 0.1 m
• Dg 10 m
• air gap 10 mm
• phase 3

19
6. Expected suitable concepts

• Electromagnetic structure
• loss reduction concept with small loss
• (In low speed machine, copper loss is
  dominant than core losses)
• ? (HT) superconducting coil potential

20
6. Expected suitable concepts

• Structure to maintain the air gap
• Less material to maintain the air gap length
• (1) light generator structure

(3) generator which can control the air gap
without
magnetic bearing
(2) generator with additional magnetic bearing

21
6. Expected suitable concepts

• Practical issue
• Concept with
• Modular structure for easy production,
  transportation and assembly in the field
• Each module can work individually
• Flexible and lightweight connection - instead of
  heavy, stiff and accurate main shaft

22
7. Summary

• Comparison of different generator systems
• Definition of the most suitable generator
• ? PMSG DD with minimum cost
• Promising PM machines for large direct-drive
• RF attractive for large direct-drive
• TF potential for light construction
• Promising mechanical structures
• Conventional structure optimum Krad required
• Lightweight structure easy production and low
  cost required
• Expected suitable concepts
• Active part TFPM generator with short flux path
• Inactive part Air gap controlled by the
  generator
• without magnetic bearing
• Others Modular construction for easy production
  and transportation

23
Thanks for your attention !