Research Of Design And Optimization For The Current Control System Of PMSG Based Direct-Driven Wind Energy Conversion System

Since increasing depletion of traditional fossil energy and severe degeneration of global ecological enviroment, the cleaning renewable energy power generation, especially the wind power generation, has been deemed as the research hot pot of energy utilization. The permanent magnet synchronous generator (PMSG) based wind energy conversion system (WECS), which is in possession of such merits as high reliability and efficiency, superior capability of riding through grid faults, decoupling between grid and PMSG, is gradually becoming the mainstream technology for large-scale MW-class wind power generation. The technology of current control, as the core of power electronic applications in power system, plays a decisive role for performances of the WECS. Therefore, focusing on the PMSG-based PMSG, this paper has made an in-depth study on desigining and opti-mizing of the current control system, including such factors as passive filter, current control-ler, phase-lock-loop and so on. Accordingly, by means of theoretical modeling, simulation analysis and experimental verification, this paper has made these research achievements as follows:(1) For the T-type LCL filter, this paper has proposed an inductor saved design method. First, to avoid over-desiging harmonic attenuation capability of the LCL filter, the harmonic spectrum of the SVM based converter voltage is precisely analyzed, and then an iterative calculation method for the converter voltage is presented. On this basis, the distortion factor (DF) is defined to weigh influences of different order harominc voltage on the grid current. Subsequentyly, objective-functions for designing the LCL filter are derived. Then, with the iteratively derived operating point which presents the maximum haromic influence, the diagramming method is applied to design the parameter combination of the LCL filter. By applying the proposed method, the harmonic attenuation capability of the LCL filter can be moderately designed so that the amount of inductor can be greatly saved.(2) For the high-order current control system of the LCL-type grid connected conver-ter, a design and optimization method which takes the influences of synchronous reference frame phase lock loop (SRF-PLL) has been proposed to design the current loop regulator parameters. First, by taking the digital control delay and grid line impedance into consider-ation, the continuous-domain static model of current control system is established, and its objective functions including phase margin (PM), gain margin (GM) and steady tracking error are then derived. On this basis, with quantified requirments of desired control per-formances, the satisfactory value range of these regulator parameters are obtained by means graphic analysis. Subsequently, with the influence of SRF-PLL being considered, transient model of the current control system is set up, and its characteristic equation is applied as the transient objective function. By doing this, the existing value range is further optimized so that a better-behaved regulator parameter combination can be derived. Besides, this paper has also proposed a positive sequence component of the capacitor brance volage based feed-forward method, so that the suppersion capability on disturbances from grid voltage of current control system is effectively improved. By applying these proposed methods, both the dynamic and static performances of grid current control and suitability to different grid are notably enhanced.(3) This paper has proposed an SRF-PLL based sensor-less vector control strategy using the enhanced predictive dead-beat algorithm for the PMSG. In the proposed control scheme, the estimated back electromotive force (EMF) with cascade compensator was first applied into the dead-beat controller to improve the system stability while enhance the system disturbance rejection capability. Then, an advance prediction of the stator current based on luenberger algorithm was used to compensate for the one-step-delay casused by digital control. Finally, the proposed SRF-PLL based sensor-less observation, which de-pends on the estimated back EMF, achieved fast and accurate observation of the rotor posi-tion angle and speed without encoder, realizing both the sensor-less orientation and speed feedback control. On this basis, by taking the digital control delay, motor parameter mis-matches and orientation errors, the precise sensor-less oriented vector control system in discrete domain was established.Then, the closed-loop characteristic equatioin can be derived, and the proposed predictive dead-beat controller can be designed and optimized by means of Jury criterion and root locus. By doing this, both the dynamic and static perfor-mances and robustness against parmater mismatches are effectively enhanced. (4) Finally, by taking the proposed design and optimization method, a10kW prototype of the PMSG-based direct-driven wecs was set up, and all the relevant experiment veri-fications were performed. It can be found that the experimental results are almost identical to the simulaition analysis results, which confirm the correctness of the presented theo-retical system modeling, and effectiveness of proposed design and optimization methods.