Direct drive superconducting wind generators - Wind Power Generation and Wind Turbine Design - page 306

Turbine Stopped Hours by Cause

Entire Unit
1%
Rotor
2%

Air Brake
5%

Not recorded
4%

Sensors
1%
Hydraulics
3%

Mech Brake
1%

Other
1%

Pitch Adjustment
5%
Main Shaft / bearing
1%

Electrical System
18%

Electrical Controls
2%

Windvane / anemometer
0%

Yaw system
2% Gearbox

42%

Generator
12%

Figure 1 : Down time for turbines in Germany.

report for the Kentish Flats offshore wind farm in the UK [8] reported that one-third
of turbines were unavailable during that period due to gearbox problems.
There have been a number of studies on gearbox reliability Ribrant and Bertling
[9] studied the cause of turbine failure in Sweden over the period from 1997 to 2005.
They found that, while the gearbox was not the most common cause of failure, the long
time to repair meant that it was responsible for the largest proportion of down time.
There have been a number of studies looking at how to improve gearbox reliability.
Musial et al. [10] have embarked on a long-term study to systematically investigate
gearbox reliability problems. In the meantime problems continue. Figure 1 shows the
proportion of turbine lost hours as a result of problems with specifi c components for
turbines in Germany during the third quarter of 2008, data from [11].
A logical result of these problems is that a number of wind turbine manufactures
and several independent studies have looked to direct drive where the generator
rotates at the same speed as the turbine blades. By their nature, these generators
also have fully fed converter system to connect to the grid. Polinder and van der
Pijl [12] made a comparison study between wind turbines with direct drive syn-
chronous, direct drive permanent magnet, single stage geared with both permanent
magnet and DFIG, and three-stage geared systems with DFIG generators.

2.3 Direct drive generators

A direct drive generator for a wind turbine is characterized by having a very low rota-
tional speed, and hence very high torque for a given power. Torque in an electrical