【作者】 张达敏；

【导师】 吕征宇； 张德华；

【作者基本信息】 浙江大学 ， 电气工程， 2013， 博士

【摘要】 风能是一种储量非常可观的绿色能源,对它的开发利用不仅可以缓解当前的能源危机,还可以有效减少对环境的污染。伴随着机组的大型化,单机功率在不断提高,全功率变流器需要处理的功率也越来越大,这对当前风电系统主流的变流器拓扑是一个巨大的挑战。多脉波相控整流器具有功率容量大、耐压等级高和输出电压可调的优点,将它引入风电系统可以解决机组大型化与变流器功率容量有限的矛盾。为了适应风力发电机大范围的工作电压要求,需要对多脉波相控整流器的拓扑结构进行适当的改进,由此产生了多脉波变拓扑相控整流器。基于电流预测控制策略,本文主要对多脉波变拓扑相控整流器(FTTR)的拓扑结构、控制策略和工程应用等方面展开研究。在对12脉波FTTR工作原理深入分析的基础上,提出了基于效率最优的拓扑切换策略及具体的拓扑切换条件。研究并联二极管对12脉波FTTR串联工作模式的影响,由于并联二极管的存在,12脉波FTTR工作于串联模式时与普通的12脉波串联相控整流器的特性不完全相同。当负载电压较高时,两者完全等价；当负载电压较低时两者不等价,表现在两者的电流THD值不同,详细分析了导致该现象的原因,从减小输入电流THD的角度出发提出12脉波FTTR的工作区间。为了获得快速的电流动态响应和平滑的拓扑切换过程,提出了平均值电流预测控制策略,并详细分析了预测控制策略的工作原理。与常规的PI控制器的对比实验表明电流预测控制器具有快速的电流动态响应能力和电流跟踪能力,在应对电流突变或拓扑结构改变的能力明显优于PI控制器。针对12脉波FTTR的不足和多相发电机相数逐渐增多的趋势,提出一种18脉波FTTR的拓扑结构。18脉波FTTR具有两个切换开关三种组合方式,因此具有三种工作模式：串联模式、并联模式、混合模式。与12脉波FTTR相比增加了一种工作模式,因此具有更宽的工作电压范围,并在轻载和重载时都能保持更低的电流THD和更高的功率因数。根据18脉波FTTR的拓扑结构特点,对串、并联两种模式设计了合适的拓扑切换策略,实现不同工作模式的切换。利用电压矢量的概念对串、并联模式的电感电压进行统一的研究。基于电流预测控制,18脉波FTTR在串、并联两种模式下具有快速的电流响应能力,并且两种模式之间的切换快速、平滑,仿真和实验验证了电流控制策略的有效性。考虑到工程实践的需要,研究了18脉波FTTR的容错能力,分别对其单组整流器故障和负载短路故障进行研究,表明18脉波FTTR具有较好的冗余度,可以应对常见的短路故障。提出了N脉波FTTR的一般结构,根据脉波数与移相变压器绕组数的关系可以确定移相角、二极管和辅助开关的个数。电流预测控制器根据电压矢量相位关系图在适当的时刻触发相应的晶闸管,实现电感电流的预测控制。深入研究了N脉波FTTR的拓扑切换过程,选择合适的切换时刻,不仅可以减小开关损耗,还可以减小切换的过渡过程时间。当拓扑切换恰好发生在晶闸管的自然换流点,则拓扑切换瞬间完成,拓扑切换的过渡过程时间约为零；当拓扑切换发生在电感电流上升或下降阶段,拓扑切换需要经历一段较短的时间才能完成工作模式的切换。针对电流预测控制算法对电感参数敏感的问题,提出利用最小二乘法在线辨识系统的电感及其内阻参数,并将辨识后的电感及其内阻代入电流预测控制器中参与电流预测。仿真和实验结果表明,使用最小二乘法校正后的电流预测控制器能够消除电流稳态误差。在对N脉波FTTR充分研究的基础上,将具有宽工作电压范围的N脉波FTTR应用于大功率风力发电系统中,提出新的风力发电系统拓扑：多相发电机和N脉波FTTR组成风力发电系统的前级,并联的三电平逆变器组成后级。针对正常工作和电网电压跌落两种情形分别研究了机侧变流器和网侧变流器的控制策略。结合风力发电的特点,提出了N脉波FTTR的拓扑切换策略并确定拓扑切换点的转速。最后讨论了N脉波FTTR追踪最大功率点的策略问题。以最优功率曲线为参考,将当前功率与最优曲线对应的功率进行比较,使当前功率不断接近最优功率,该方法可以在风速变化的情况下自适应地追踪最大功率点,具有很好的通用性。更多还原

【Abstract】 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.更多还原