A study of the feasibility of connecting large offshore wind turbines in electrical clusters

• Main Author: Elliott, Douglas W.
• Published: University of Strathclyde 2014
• Subjects: 621.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.632684

A comprehensive study of the motivation and methods of implementing electrical clusters of up to five wind turbines is presented. The merits and pitfalls of three potential technologies for implementing such clusters are investigated through the extensive simulation of each, from which it is concluded that passively rectifying the outputs of PM generators and connecting them in parallel, using a multi-terminal DC collection network, is the preferred option; allowing a degree of independence to occur between the turbines, whilst maintaining a high energy transfer efficiency. The process of generator output rectification is investigated thoroughly, demonstrating that the distortion of the generator terminal voltages allows the generator speeds to slip by up to 33% from the cluster synchronous speed (derived from the cluster common point voltage). The maximisation of the rectifier commutation overlap length to increase the generator slip capability is demonstrated to reduce the generator power factor, but also reduce the winding current and torque total harmonic distortion at maximum power output. In addition it is found that the use of the commutation overlap voltage distortion to provide generator slip, allows a significant energy transfer efficiency benefit to be achieved over the use of additional resistance for the same purpose. The resulting generator slip capability is proven to benefit the energy capture efficiency of the clustered turbines and also provide sufficient damping of the wind turbine drive-train natural oscillations. The operation of the system and the capability of the wind turbine rotational speeds to slip from the cluster synchronous speed are verified experimentally using a bench scale test-rig, incorporating two small wind turbine generators.