Research On Full-scale Grid-connected Power Conversion Technology For Direct-driven Wind Generation System

Direct-driven wind generation system is considered to be an important research direction in wind generation technology area due to its superiorities of being gearless, no slip rings and brushes, flexible power control ability, etc. However, as to MW direct-driven wind turbine, the power converter and core control technologies are still relied on importing in our country, which significantly restricts further development of wind energy industry. Therefore, this dissertation gives theoretical study on full-scale grid-connected power conversion technology in direct-driven wind generation system. The design principles and control strategies of line-side converter and generator-side converter are both discussed comprehensively. Simulations and experiments demonstrate the correctness of the research results. Following are the major research works of this dissertation.Firstly, modeling and control of line-side converter have been discussed. Mathematical model and equivalent circuit of line-side converter are established; The working principle, advantages and disadvantages of voltage oriented control, direct power control, virtual flux oriented control and virtual-flux-based direct power control are analyzed and compared in detail.Secondly, line-side harmonic analysis and suppression have been discussed. As to harmonic analysis aspect, a general harmonic analysis algorithm for line-side converter is proposed, and based on which, the variation rule of harmonic distribution under different PWM strategies or modulation parameters are compared and analyzed; As to harmonic suppression aspect, a novel design method of line-side LCL filter is proposed. The design procedures as well as cooperating calculation criteria of each filtering component are explained in detail.Thirdly, control of line-side converter with neutral point connected has been discussed. Mathematical model and equivalent circuit of topology with neutral point connected is established; Zero-sequence current control and dc-link voltage balancing control strategies are studied; The unification of SPWM and SVPWM algorithm in neutral-point-connected topology (3-dimention vector space) is proposed and proven, and based on which, the line-side harmonic suppression ability and dc-link voltage utilization problem of neutral-point-connected converter are analyzed comprehensively. Fourthly, control of parallel connected line-side converter has been discussed. Working principle and line-side harmonic elimination effect between directly parallel and interleaving parallel scheme are analyzed and compared; The generating mechanism of high frequency circulating current and low frequency oscillation circulating current under interleaving parallel scheme are both studied. Working principle of interleaving parallel control strategy based on 2d-SVPWM, 3d-SPWM and improved SVPWM are analyzed comprehensively.Fifthly, generator-side converter control based on reactive-rectify topology has been discussed. Current control strategy based on voltage feedforward scheme of boost converter has been studied; Expression of input current reference value for wind energy maximum power point tracking is obtained; Unified algorithm of total ripple current for N-cell interleaving parallel boost converter is proposed, based on which, unified design criteria for input inductance of N-cell interleaving parallel boost converter is obtained.Sixthly, generator-side converter control based on active-rectify topology has been discussed. Mathematical model and equivalent circuit of permanent magnetic synchronous generator is established; Working principle, advantages and disadvantages of zero d-axis current control, constant mutual flux linkage control and maximum efficiency control are analyzed and compared.As to simulation demonstration aspect, firstly, simulation model of 1.5MW direct-driven wind generation system based on reactive-rectify topology has been built, which demonstrates the effectiveness of system design and control scheme including maximum power point tracking control, ripple analysis algorithm of interleaving parallel boost converter, control strategy during constant speed phase, etc. Secondly, simulation model of 1.5MW direct-driven wind generation system based on active-rectify topology has been built, which demonstrates the steady state and dynamic response performance of control strategy of full-scale power converter.As to experiment demonstration aspect, firstly, 15kW line-side converter prototype has been built, which demonstrates the accuracy and feasibility of both general harmonic analysis algorithm and LCL-filter design algorithm. Secondly, MW energy feedback experimental platform for wind generation power converter has been built, which verifies the steady state as well as dynamic response performance of both line-side converter and generator-side converter control strategy, and demonstrates the correctness of theoretical research results including line-side converter zero-sequence current control, grid-side current harmonic suppression by interleaving parallel, circulating current control strategy in parallel scheme, boost converter ripple current algorithm, voltage feedforward based current control strategy, etc.