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A high efficiency electrical conversion system with variable reluctance generator for variable speed wind turbines

Exploitable results

A current trend is to equip wind turbines with drive trains without gearbox (direct drive), but these have typically had high material and labour costs. Variable reluctance generators (VRG) can demonstrate several advantages over the standard induction generator in direct drive applications. Firstly, the VRG rotor consists only of iron laminations, making it robust and easy to manufacture. Secondly, the windings on the stator are assembled and encapsulated prior to mounting and finally the overall design is compact compared to the traditional 'pancake' direct drive generator. As reluctance machines always require a controllable power electronic converter (PEC), variable speed operation is enabled at no additional cost. The feasibility of VRGs for use in direct drive, variable speed wind turbine generators has been investigated. The design of a 20 kW direct drive VRG and the building, control and testing of an electromechanical conversion system consisting a PEC for variable speed wind turbine applications were also undertaken. The layout and operating conditions of the PEC were chosen according to the foreseen operational characteristics of the target wind turbine and the working principals of reluctance machines. The target wind turbine had a net power rating of 18 kWe and operates at speeds of between 60 and 120 rpm in moderate wind speeds. A generator side PEC, consisting of a 4-phase rectifier, dc-link and inverter was then designed and constructed. The rectifier can be used for both low speed and high speed VRGs. The rectifier has been incorporated into an 18 kW variable speed electronic conversion system, designed and built for an existing high speed variable reluctance system. A computer controlled test rig was used to drive the PEC. Functional system tests were supported by an integrated control model derived from design data and lumped together with the generator model. This integrated control model enabled dynamic analysis, optimizes energy capture and assesses overall electric losses. It was used for the design of the controls. The validity of this model was checked experimentally in the test-rig operated under steady state conditions. Transient tests on 3 levels of complexity were used to assess the stability and the controller performance of the conversion system. The results indicate that a small-size variable speed wind turbine with VRG has an electric efficiency similar to that of systems with synchronous or induction machines and has good controllability. Variable speed operation was successful between 1000 and 1500 rpm, which covered the major part of the power range (2 to 18 kWe). The overall electric efficiency ranges from 80% at 2 kWe to 90% at 18 kWe, while the machine efficiency varies between 82% and 92%; similarly rated synchronous and induction machines have an efficiency of greater than 90% at rated load. Tracking to changing wind speed proved to be sufficiently fast for the realization of expected variable speed benefits under real turbulent field conditions. Below 1000 rpm high acoustic noise levels were observed. These are probably due to the mechanical design of the reluctance machine for constant speed operation at 1500 rpm. Equipment cost is not reduced when reluctance machines are operated at constant rotor speed. Hence a constant speed wind turbine with reluctance machine is not a viable option.

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