In wind turbines (WTs), a leading cause of outages and unreliable performance is damage to drive trains, often manifested as failed gearboxes. Significant contributors to these failures include torque excursions during operation and drive train wear (e.g. chatter or fretting) when the WT is not in service.

MPR has assisted New World Generation (NWG) in the development of a new wind turbine drive train technology aimed at addressing these issues. The NWG technology is a simple and reliable friction drive which replaces the expensive and failure-prone gear box in existing designs. The new design is more reliable because it inherently limits the torque transmitted through the drive such that values of torque and torque rate of change above pre-determined design levels are not achieved. In addition, the new design reduces the potential for drive train wear when the WT is not in service by directly coupling the parking brake to the main rotor shaft.

Full Image The turbine with nacelle open and closed. A view of the turbine with the nacelle open, showing the individual parts, and with the nacelle closed.

As outlined below, the friction drive is mated with other design features (based on existing technologies) to manage torque and achieve optimum reliability:

• Variable Speed Rotor and Electrical Generator Operation - Variable speed rotor and generator operation allows energy from wind gusts to be temporarily stored as increased kinetic energy of the rotating blades without increasing torque transmitted to the generator. As a result, there is a reduction in torque variations seen by the mechanical drive train. Increasing the speed of the WT rotating components is not limitless; however, energy storage by such means is essentially instantaneous.
  
  
• Pitch Control of Wind Turbine Blades - Pitch control of the WT blades allows control of torque (and power) of the rotor as wind speed changes. Although pitch control is relatively rapid, it is not instantaneous. During a gust when wind speed changes faster than the speed changes faster than the pitch control can keep pace, other controls (variable speed and friction drive) manage the torque to maintain design levels.
  
  
• Power Electronics to Condition and Control Electricity Produced - Power Electronics (PE) help to manage the torque on the mechanical drive system by controlling the generator load and “back torque” resisting the motion of the WT. PE controls are very fast, e.g. within a few electric current cycles (about 16 ms/cycle). PE also allow the system to operate with variable speed yet deliver power and synchronize to a fixed-frequency AC grid (60 Hz or 50 Hz depending on location).
  
  
• Friction Drive - The friction coupled mechanical drive allows torque peaks to be avoided by slippage between the friction components when torque exceeds a prescribed level. A gust driven torque not managed by the above controls is dissipated by friction heating when the slippage occurs. This safety system is instantaneous and slippage only occurs during a torque overload. The friction safety feature is not used as a regular control feature because of energy dissipation, associated wear of the friction components, and loss of efficiency.
  
  In addition, by combining use of the above features with multiple generators, the new design increases the efficiency and availability of the wind turbine.
  
  
• Multiple Generators to Allow Wide Ranging Power Production - Multiple small generators allow wide range electrical power production at relatively high efficiency. For instance, a 1.5 MW wind turbine with six generators, each 250 kW, could operate at the 250 kW power level in low wind with a single generator functioning at full rated power, which is its best efficiency point. As wind speed increases, additional generators are brought into service, maintaining an overall high generating efficiency over a wide range of wind speed.
  
  
• Independent Load Paths - Multiple independent generators provide the ability to run the turbine even if one of the generators or its associated drive is out of service. With conventional turbines, generation cannot be continued while the generator or gearbox is down for service or repair.

Installing the CWind prototype. The prototype has been installed and also serves as Proof-of-Concept. This prototype is the culmination of three months work.

MPR designed and oversaw the fabrication of a prototype mechanical drive (as described above) for a 65 kW WT drive train based on the NWG technology. The figures show the prototype configuration with the nacelle cover retracted (open) and in place. The final figure shows a view from the ground of the installed prototype. The detailed design of the prototype was completed in less than three months to meet the demands of the development schedule. The WT prototype serves as a Proof-of-Concept demonstration of the innovative friction drive concept. The prototype serves as a test-bed to address technical uncertainties in the development of a larger scale production wind turbine in the 1.5 to 2 MW range. The successful implementation of this technology will provide wind turbines that have a lower cost of electricity.

For further information on this article, a copy of the latest MPR Profile or our engineering services, contact Larry Cundy.