摘要
分布式发电被公认是降低能耗、提高电力系统稳定性和灵活性的有效途径,目前已成为电力工业重要的发展方向之一。与传统大电网联网相比,采用永磁(同步)风力发电机的分布式直流并网系统因电网支撑能力有限,在并网可控性、稳定性和可靠性等方面性能较差。因此,宽运行范围下的机侧有效控制、直流侧电压稳定性研究和系统可靠运行是分布式直流并网变流器研究和实现的关键技术。
本文围绕上述的关键技术,重点针对宽运行范围下的永磁风力发电机变速恒压控制和机侧变流器交流侧电感、直流侧电容参数的选优,基于辅助支撑电容结构的直流斩波电路设计和恒压控制,系统级联恒压控制和系统效率优化等方面进行了理论分析和实验研究。主要包括以下内容：
(1)研究了基于永磁风力发电机和机侧变流器的宽风速运行范围下的变速恒压控制。首先确定永磁风力发电机励磁分量计算和补偿的原则,通过调整电机电枢电流,在一定范围内达到对电机输出电压水平的控制；其次对永磁风力发电机的功率因数控制进行分析,给出了满足功率因数可调要求的限制条件；最后对变流器输入交流侧电感和直流侧电容参数的选优做了深入研究和对比分析。
(2)为改善机侧变流器电压调节能力的不足和提高直流侧电压跟踪性,设计了基于辅助支撑电容结构的升降压直流斩波电路。控制策略上考虑了前端PWM变流器的电压调整特性,采用电压复合控制的方式实现了直流侧输出电压的快速调节和稳定并网。详细地阐述和分析了直流斩波电路中电感和滤波电容的计算过程及其对系统性能的影响。
(3)针对分布式直流并网变流器拓扑结构和控制上的特点,推导了机侧PWM变流器和直流斩波电路占空比描述的级联表达式,并依此设计了占空比控制的级联协调恒压控制模式。分析了影响变流器母线电压稳定性的因素,兼顾系统的效率优化问题,得到了考虑母线电压稳定性能、以直流侧功率为寻优目标的简单、可行的效率优化方案。
(4)分析了直流电压跌落的原因以及对系统造成的影响,基于宽风速运行范围分析了永磁风力发电机的多状态运行模式,提出了基于机侧动态电压解耦补偿控制的多模式优化控制方法,拓宽了输入范围,保证了直流并网的可靠性。
(5)详细地给出了永磁风力发电机分布式直流并网变流器的工程设计和实现过程。该类变流器的研究相对较少,本文以100kW变流器作为验证样机,在完善硬件设计的同时,对上述提出的控制方案和策略进行仿真分析和相关实验验证,使系统的性能指标满足：输入转速变化范围达到额定转速的20%～120%,交流侧功率因数可调范围-0.95～+0.95,直流并网电压调整率小于额定电压的3%。
Distributed generation is regarded as the most effective way to lower the energy depletion, improve power system stability and flexibility, and have been one of developing trend for power industry. The controllability, stability and reliability of connected grid are worse than conventional large electricity grid because of the limited support ability to grid for distributed dc grid system with permanent magnet (synchronous) wind generator. Hereby, the effective control of generator-side with wide operation rang, the study of dc-side voltage stability and system reliability operation are the key techniques for distributed dc grid converter study and implementation.
The aim of this dissertation is to focus on the key techniques above, the theoretical analysis and experimental research are carried out such as the variable speed constant voltage control of permanent magnet wind generator with wide operation rang and generator-side converter parameters optimum selected for inductance and capacitance, the dc chopper circuit design and constant voltage based on the structure of auxiliary surport capacitor, the cascade constant voltage control and the system efficiency optimization and so on. The dissertation includes the following aspects:
(1) The variable speed constant voltage control with wide operation rang based on permanent magnet wind generator and generator-side converter are studied. The principles of excitation compensation calculation and compensation defined firstly, and the generator voltage level is controlled within the range through change the armature current of permanent magnet wind generator. Secondly, the power factor control of permanent magnet wind generator is analyzed, and the constraint conditions of satisfying power factor adjustable requirements are given. Finally, the deeply study and comparative analysis of parameters optimum selected for inductance and capacitance are designed in detail.
(2) A dc chopper topology structure with auxiliary surport capacitor is designed in order to improve the voltage regulation capacity and the tracking performance of dc-side voltage. The composite control schemes is achieved considering forward PWM converter voltage control characters, and accomplish the fast adjustable of dc-side output voltage and stable grid. The calculative process and the influence of system for dc chopper circuit inductance and capacitance are expatiated and analyzed.
(3) In view of the topology structure and control characteristic of distributed dc grid coverter, the cascade expression of generator PWM converter and dc chopper is deduced based on the duty cycle expression, and the cascade coordination constant voltage control mode is designed. The influence factors of dc-side voltage stability are analyzed, and considering the efficiency optimization of system, the simple, feasible efficiency optimization scheme is obtained considering dc-side voltage stability and takes the optimal goal for dc-side power.
(4) The reason and influence of distributed dc grid voltage sags is analyzed, and multiple states operation mode of permanent magnet wind generator is analyzed based on wide wind speed operation range. The dynamic voltage decoupling compensation control with multiple states operation mode of generator-side is introduced to widen the scope of input and ensure the reliability of the grid connected.
(5) The design and implementation process of distributed dc grid converter are given in detail. The study of this coverter is relatively few, so the 100kW converter implemated as a prototype, the simulation analysis and relevant experimental results above control schemes and strategies ate validated base on the perfect hardware design. The performance satisfied the requirements:the generator rotational speed ranges from rating speed 20% and 120%, the power factor can be adjusted from -0.95 to +0.95, dc voltage voltage regulation factor lower rating voltage 3%.
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