Research On The Multi-Pulse Flexible-Topology Thvristor Rectifier Based On The Predictive Current Control Strategy

Wind energy is a green energy with abundant storage, thus exploiting and utilizing it can not only alleviate the present energy crisis, but also effectively reduce the pollution to the environment. As the wind-turbine-generator system (WTGS) is developing towards the trends of large scale, and the power of the single generator is increasing continuously, those are big challenges to the present power converter used in the wind energy conversion system (WECS). Multi-pulse thyristor rectifier (MPTR) owns its advantages of high power capacity, high voltage and regulable output voltage, thus the problem of WTGS enlarging against the capacity restraint of the power converter can be solved if the MPTR is introduced in the WECS. In order to meet the requirement of wide voltage range of the wind generator, it is necessary to modify the MPTR and thereby a new converter topology named multi-pulse flexible-topology thyristor rectifier (MPFTTR) is generated. Based on the predictive current control strategy, this thesis concentrates on the research of the proposed MPFTTR, including the topology, the operating principle, the control strategy and the engineering application.First, a topology-switching strategy based on the optimal efficiency is proposed to the12-pulse FTTR after deeply analyzing its operating principle. Meanwhile, the effect of the diodes which are parallel with each rectifier is discussed. When the12-pulse FTTR operates in the series mode, the performance is a little different from that of the conventional12-pulse series thyristor rectifier due to the existence of diodes. This thesis studies the series mode in detail and then the operating range of the rectifier is proposed from the point of reducing the total harmonic distortion (THD) of the input current.In order to obtain fast current response and smooth transition between two operating modes, the predictive average-current control strategy is proposed to the12-pulse FTTR. Then the principle of the predictive current control strategy is illustrated in detail. The performance comparisons between the proposed control strategy and the conventional PI controller are carried out to demonstrate the fast response and tracking abilities. In the case of coping with sudden change of the load current or the topology switching, the proposed control strategy is better than the PI controller.Second, an18-pulse FTTR is proposed to cover the shortage of12-pulse FTTR and meet the increasing trends of the phase numbers in the permanent magnet synchronous generator (PMSG). There are two switches in the18-pulse FTTR and these two switches have three combinations, corresponding to three operating modes: parallel mode, series mode and hybrid mode. The18-pulse FTTR has a wider range of operating voltage due to its additional operating mode, which contributes to mitigating the input current THD and improves the power factor when powering a light load. To achieve this aim, a topology-switching strategy is designed according to the topology characteristics of the18-pulse FTRR. The concept of voltage vector is used to deal with the voltage across the inductor, and the predictive average-current control strategy is also proposed to the rectifier. Based on the proposed control strategy, the fast current response as well as the smooth switching is also obtained, and simulation and experimental results verify the effectiveness of the control strategy. In view of the practical requirements, the fault-tolerant ability of the18-pulse FTTR is verified. Simulation results of the single-rectifier fault and short-circuit fault show the rectifier has good redundancy.Third, a general structure of the N-pulse FTTR is proposed based on the previous studies, and then the number of diodes and switches can be determined according to the relationship between pulse numbers and winding numbers. Therefore, the transition of the topology switching is studied. It is found that the topology switching which is carried out at appropriate instant will not only reduce the switch losses, but also reduce the duration time of the transition. Namely, if the topology switching occurs at the natural commutation point of the thyristor, it will be finished immediately, otherwise, the topology switching will be finished in a period of time.Fourth, as the predictive current control strategy presented in the thesis is sensitive to the variation of inductance, an on-line parameter correction method based on the least square method (LSM) is proposed to identify the inductance and the internal resistance of the dc-side inductor, and then the identified parameters are input to the predictive current controller to modify the current prediction. Simulation and experimental results demonstrate that the proposed LSM can modify the dc-side inductance and eliminate the current errors.Fifth, on the basis of sufficient studies, we try to apply the N-pulse FTTR to the high power WECS, which is composed of a multi-phase PMSG, the N-pulse FTTR and two three-level inverters in parallel. The control strategies of the generator-side and the grid-side converter are discussed respectively in terms of normal and faulty operations. Thus the topology switching strategy of the generator-side converter is proposed according to the characteristics of the WECS. After that, the maximal point power tracking method is discussed. This method can reach the maximal power point adaptively even in the presence of wind variations, thus it can be used in a variety of WECS.