Keeping turbines spinning in the wind
Landmark International Standard for wind turbine gearboxes
In spite of their often relatively simple appearance, wind turbines are sophisticated machines. Converting mechanical wind energy to electrical power requires a number of components. One of these, the gearbox, is essential for uprating the low speed of the main shaft attached to the rotor hub. IEC TC (Technical Committee) 88: Wind turbines, has just published an International Standard covering design requirements for wind turbine gearboxes.
From low to top gear
Anyone observing wind turbines may wonder how such an apparently slow moving rotor can produce electricity. This is made possible by the drivetrain which consists of the rotor's main shaft, the gearbox and a high speed shaft.
The gearbox converts slowly rotating, high torque (turning force) power from the wind turbine rotor and shaft to high speed, low torque power. This is then transferred to a high-speed shaft connected to an electrical generator.
Gearboxes are an essential part of wind turbines. They must be reliable and have a design lifetime similar to that of the wind turbine. For wind turbine classes I to III (high to low wind), this must be "at least 20 years", according to IEC 61400-1, Wind turbines – Part 1: Design requirements.
IEC 61400-4, Wind turbines – Part 4: Design requirements for wind turbine gearboxes, is an essential addition to the IEC 61400 series of International Standards for wind turbines. It has been prepared by IEC TC 88 in co-operation with ISO TC 60: Gears.
This first edition constitutes a technical revision of ISO 81400-4, which it cancels and replaces. Content has been extensively expanded and includes the following significant technical changes:
- extension of the scope to wind turbines above 2 MW rated power
- considerations for converging differing approaches to reliability in gear, bearing and wind turbine standards
- new clause on wind turbine loads specific to drivetrains
- new clause on testing and validation of new gearbox designs
- updated bearing selection tables for different locations in a wind turbine gearbox
- expanded design considerations on the use of bearings based on avoiding standard failures
- new clause on considerations and requirements in the design and analysis of gearboxes structural elements
- updated considerations and requirements for lubricants and lubrication systems
The proof of the design is in the testing
Good design is important, but possible design flaws may appear only after equipment has been in use for a while – sometimes this may be after quite some while. Such flaws can be eliminated using design verification. IEC 61400-4 identifies overall and specific test criteria and plans for all elements that need testing and also lists the gearbox and drivetrain design changes that require further testing.
Detailed criteria for workshop prototype, field, production and robustness tests are given as well as indications of what lubrication systems to use. Where mechanical systems are concerned, proper operation, service and maintenance are central to reliability and durability and to ensuring that the gearbox remains operational for its full expected lifetime.
To ensure this is the case, IEC 61400-4 gives detailed guidelines in terms of service, maintenance and inspection, commissioning and run-in, transport, handling and storage, installation, repair, condition monitoring and lubrication requirements – including oil type, oil test and analysis
Comprehensive supporting material
Extensive documentation including terms, definitions and conventions, symbols and dozens of figures is available and ensures this International Standard provides all of the information needed by manufacturers and suppliers of gearboxes and all associated elements along the entire length of the production line. It is also invaluable to the wind turbine industry in general, since gearboxes form a central element of any wind turbine installation
Powerful sales argument
IEC 61400-4 is a landmark International Standard and the first that applies to gearboxes used in wind turbines above 2 MW rated power, which are currently being deployed throughout the world.
Wind turbine manufacturers, who regularly reference several IEC 61400 International Standards as guarantees of their products’ certification, will undoubtedly be adding a mention of IEC 61400-4 to their sales documentation.
As TC 88 Chairman Sandy Butterfield told IEC Global Visions: "Ultimately in the wind energy business all wind turbines end up being certified; it's a market imperative. All wind turbine designers who know that their product is going to be shipped internationally will design to IEC standards because they can't possibly design to the range of national standards that are out there"