Wind turbines normally comprise one or more energy generating units, each energy generating unit comprising a rotor comprising a hub carrying one or more wind turbine blades. The wind acts on the wind turbine blades, thereby causing the hub to rotate. The rotational movements of the hub are transferred to a generator, either via a gear arrangement or directly, in the case that the wind turbine is of a so-called direct drive type. In the generator, electrical energy is generated, which may be supplied to a power grid.
Some wind turbines are provided with two or more energy generating units in order to increase the total power produced by the wind turbine, without having to provide the wind turbine with one very large, and therefore heavy, energy generating unit. Such wind turbines are sometimes referred to as ‘multirotor wind turbines’.
In multirotor wind turbines the energy generating units may be carried by a load carrying structure which is, in turn, connected to a tower structure. Thereby at least some of the energy generating units are not mounted directly on the tower structure, and they may have a centre of gravity which is displaced with respect to a longitudinal axis defined by the tower structure. When the wind acts on energy generating units mounted in this manner, thrust forces will be created, which will in turn cause loads to be introduced in the load carrying structure, and possibly at connection points between the load carrying structure and the tower structure.
In order to allow the rotors of the energy generating units to be directed into the incoming wind, the load carrying structure(s) may be mounted on the tower structure in a rotatable manner via a yawing arrangement. Thereby the load carrying structure can perform yawing movements relative to the tower structure, thereby ensuring that the rotors are orientated in a correct manner with respect to the incoming wind. The yawing movements may be performed actively, e.g. by means of one or more yawing motors. Alternatively, the yawing system may be of a self-yawing kind which automatically ensures that the rotors are orientated correctly. This may, e.g., be obtained by providing the load carrying structure with a wind vane or the like.
Prior art self-yawing mechanisms are known to operate reliably most of the time. However, under certain conditions, e.g. under turbulent wind conditions, there is a risk that these self-yawing mechanisms suddenly ‘flip’, i.e. rotate the entire load carrying structure approximately 180°, and the load carrying structure is subsequently slowly rotated into a correct orientation. This introduces undesirable loads on the wind turbine, and the energy production of the wind turbine is reduced while the load carrying structure is rotated into the correct orientation.
EP 2 463 513 A1 discloses a wind power generating device comprising a tower column, a first wind generating set and a second wind generating set. The first wind generating set is installed at a position on the tower column near the top, and generates a first torque on the tower column during rotation for power generation. The second wind generating set is installed at a position on the tower column below the top, and generates a second torque on the tower column which at least partially counteracts the first torque.
JP 2006-322383 discloses a wind power generating device comprising two upwind rotors and a stabilizing fin. The stabilizing fin ensures that the wind generating device is self-yawing.
Examples of multi-rotor wind turbines are further known from GB 2 443 886 A and FR 2 868 483.