Page 479 - Wind Energy Handbook

TOWER 453

(1) Fixed yaw A yaw brake is provided in the form of one or more callipers acting
on an annular brake disc and is designed to prevent unwanted yaw motion
under all circumstances (see Figure 7.37). This can require six callipers on a
60 m diameter machine. During yawing, the yaw motors drive against the brake
callipers, which are partly released, so that the motion is smooth.

(2) Friction damped yaw Yaw motion is damped by friction in one of three different
ways. In the first, the nacelle is supported on friction pads resting on a
horizontal annular surface on the top of the tower. The yaw drive has to work
against the friction pads, which also allow slippage under extreme yaw loads.
This system was employed on the 500 kW Vestas V39 and the 3 MW WEG LS1.
In the second, the nacelle is mounted on a conventional rolling-element
slewing bearing, and the friction is provided by a permanently applied brake,
using the same configuration as for fixed yaw. Optionally, the pressure on the
brake pads can be increased when the machine is shut-down for high winds.
In the third, the nacelle is supported on a three-row roller-type slewing
bearing (see Figure 7.21(d), but with the rollers replaced by pads of elastomer
composite to generate friction.

(3) Soft yaw This is hydraulically-damped fixed yaw. The oil lines to each side of
the hydraulic yaw motor are each connected to an accumulator via a choke
valve, allowing limited damped motion to and fro to alleviate sudden yaw
loads. This system is used on the 300 kW WEG MS3, which has a two-bladed,
teetered rotor, but experiences significant yaw loads when teeter impacts occur.

(4) Damped free yaw A hydraulic yaw motor is used as before, but the oil lines to each
side of the motor are connected together in a loop via a check valve, rather than
being connected to a hydraulic power pack. This arrangement prevents sudden
yaw movements in response to gusts, but depends on yaw stability over the full
range of wind speeds. Unfortunately, yaw stability in high winds is rare.

(5) Controlled free yaw This is the same as damped free yaw, except that provision is
made for yaw corrections when necessary. This strategy was adopted success-
fully on several Windmaster machines, including the two-bladed, fixed-hub
750 kW machine.

Friction damped yaw is the strategy most commonly adopted.

7.9 Tower

7.9.1 Introduction

The vast majority of wind turbine towers are constructed from steel. Concrete
towers are a perfectly practicable alternative but, except at the smaller sizes, they
require the transfer of a substantial element of work from the factory to the turbine