During the design phase of wind turbine (WT) systems, simulation models are used in order to design and optimise the WT system behaviour and its components. Since (offshore) WTs are complex systems that interact with various environmental conditions and other technical systems, it is a considerable challenge to develop simulation models in such a way that all relevant influences are reproduced properly.

Overall, the content of this chapter comprises approaches for modelling and simulation of WT systems, including hands-on experience in their applications and suitability. Due to the huge diversity of types and fidelity levels of simulation models, as introduced in section 2.1, the subsequent elaborations are limited in scope and mainly focus on aerodynamic and mechanical models of WT systems and their components. Since the mechanical behaviour of a WT is not only driven by the structure itself but also strongly affected by environmental influences and resulting loads on the system, the modelling of environmental conditions is addressed first (section 2.2). The subsequent description of WT system and component modelling starts from a medium-fidelity level in the general and fully coupled modelling of WTs (section 2.3), including aero-, hydro- and structural dynamics, and pointing out the modelling of structural components while just touching upon the modelling of other components. The final focus, then, lies on drivetrain models (section 2.4), comprising details on modelling approaches and best practices. A short summary and conclusions are provided at the very end (section 2.5).

The yaw system of a wind turbine is responsible for orientating the wind turbine rotor towards the wind. This chapter is intended to provide an insight into common yaw system concepts and designs. The focus is on active and friction-damped yaw systems with electro-mechanical drives, which are the most common concepts in multimegawatt upwind turbines onshore and offshore.

First, the fundamentals are described, which are necessary for the general understanding of yaw systems. Second, the design loads are discussed in detail to create the basis for the subsequent sections. Third, common yaw system concepts and component designs with their advantages and disadvantages are presented. Finally, a yaw system is dimensioned for the Fraunhofer IWES wind turbine IWT-7.5-164 and some design aspects are discussed in more detail.

In 1997, the Swiss gear company MAAG Getriebe AG was taken over by the Danish company F.L. Smidth-Fuller Engineering A/S in Valby, Copenhagen. As a young engineer at MAAG, the author was not aware at the time of how these two companies would have a long-term influence on his career path. The combination of gears and wind turbines became a fascinating challenge.

Both companies come from quite different backgrounds. F.L. Smidth's three-bladed wind turbine on the island of Bogø (built in 1942) already looked very similar to the classic 'Danish' wind turbine. It was part of a combined wind-diesel system that provided power to the island, with a rotor diameter of 24 m and rated power of about 60-70 kW.

Max MAAG, the founder of the company of the same name in Zurich, discovered the profile addendum modification in 1908 and thus made very strong teeth in the root circle possible for the gearing world. The company ZF in Friedrichshafen, co-founded by Max MAAG, was one of his first customers of these strong teeth, for modern drive units in airships. Today, ZF is one of the most important manufacturers of high-performance wind power gearboxes up to 15 MW.

The fascination for wind power increased for the writer in 2001 at the first meeting of the wind gearbox industry at NEC Micon in Randers DK. Load gearboxes in wind turbines suddenly got a bad reputation around the turn of the millennium. The massive increase in the number of units overtaxed the manufacturers. An increase in production was attempted by dangerously increasing the grinding speed of profile grinding machines. Massive grinding burns in the tooth root are the worshipping consequences. The faster profile grinding process compared to the gear generating process became the big nightmare of several gear manufacturers overnight.

This is how the author experienced his entry into the wind world as a young gearbox designer. And where does the industry stand today? Some 20 years later, the author of these words was asked to write a chapter in a German wind reference book about the concepts and design of wind power gearboxes. Without any academic degree worth mentioning, but with a large backpack of collected experience, I would like to write this chapter for those young people who have also succumbed to the fascination with gears and wind power.

Manufacturing a gear wheel is handcraft! Every gear must be perfectly designed, produced, measured and installed so that it does not become the weakest link in the chain. This handcraft can only be learned in a gear manufacturing company. The knowledge of the interrelationships in the design, manufacture and operation of gear drives cannot yet be fully represented in modern computer programs. Less critical engineers quickly become enthusiastic about these tools. There is a great danger that they will unreservedly believe the numbers and figures in the solution template. But especially the expensive damage cases of the last 20 years in wind power gearboxes show how close the borders between success and failure are. Was it external circumstances, such as higher loads than assumed, or was it the company's own failures in the manufacture of the gearbox that led to the premature failure of the turbine component? This question must be approached very openly and neutrally in the case of any damage. Every gearbox failure in a wind turbine is one too much. A betrayal of the sustainability of wind energy and nerve-wracking for the operator of the turbine. The following lines are intended to further encourage curiosity and enjoyment of wind power gearboxes and their reliable construction and to help wind power remain a success story in the renewable energy family.

The main cooling system of a wind turbine is responsible for the complete temperature management of the drivetrain system.

The main shaft and suspension system may be described as the heart of the drivetrain, as it enables the rotation of the rotor and transfers the entirety of wind, inertia and weight loads from the rotating system to the stationary system. In the event of a rotor bearing failure, commonly the entire drivetrain has to be dismantled for bearing replacement, which involves extremely high costs.

In Chapter 5, the various drivetrain concepts are explained in detail and the corresponding fixed and floating bearing arrangements of the rotor shaft are shown. The following chapter builds up on this, referring to the selection of the rolling bearing types according to the selected drivetrain concept, their respective advantages and disadvantages, as well as the various calculations done around rolling bearings.

Typically used state-of-the-art bearing types are outlined and their applicability and usefulness for various existing drivetrain concepts are discussed.

After that the bearing calculation process is described with consideration of the special requirements for rotor bearings. The focus lies on the applied calculation models, relevant load assumptions and load case generation.

Section 10.3 dives deeper into the design of tapered roller bearings (TRB) and shows the influences of boundary conditions and parameters of different calculation models by means of a calculation example.

After the discussion of bearing reliability or typical damages to rotor bearings, the chapter concludes with an outlook on future developments.