【作者】 郭晓明；

【导师】 何奔腾； 贺益康；

【作者基本信息】 浙江大学 ， 电力系统及其自动化， 2008， 博士

【摘要】 我国的风能资源丰富的区域大都地处于偏远,远离负荷中心,所建立的风电场与电力主干网的联接较弱,电网电压易波动、不平衡。电网异常条件下变速恒频双馈异步风电系统的运行与控制成为了研究的重点。系统中交流励磁变换器的控制策略决定了风电机组的动态运行特征,而直接功率控制其结构简单、动态性能好,近期巳逐渐成为交流励磁变换器的故障穿越控制研究的焦点。本文的研究工作主要是围绕双馈异步风电机组(DFIG)交流励磁电源的直接功率控制展开。在充分理解电网运行规程和变速恒频双馈风力发电机组运行特征后,对现有故障穿越技术进行了分析,确认针对各种电网故障情况下改进的DFIG控制策略可在其励磁变频器控制范围内可最大限度保证机组安全,实现故障穿越过程中多个控制目标,是弱电网条件下风电机组控制研究的重点内容。本论文主要研究工作集中在:(1)对传统直接功率控制(DPC)方法进行了研究。通过对矢量分析的结果表明,传统DPC的单一开关表和扇区划分方式会导致电压矢量出现周期性误差,最终致使DFIG风电机组网侧变换器输入电流含大量高次谐波,必须对其进行改进和优化。(2)对DFIG风电机组交流励磁变频器的网侧变换器进行了研究,提出了一种复开关表DPC方法,在DPC基本原理的基础上通过引入SVM技术提出了恒定开关频率DPC。复开关表DPC的开关表数据通过离线计算获得,主要逻辑部分由查表完成,故采用复开关表的DPC系统结构简单,硬件要求低。而恒定开关频率DPC通过引入了空间矢量调制(SVM)技术可使开关谐波集中在采样频率附近,虽适度增加了控制算法的复杂度但简化了风电机组配套的电力滤波器的硬件设计难度。(3)对DFIG风电机组交流励磁变频器的转子侧变换器进行了研究,提出了三种可用于电网电压波动条件下DFIG励磁变频器有效控制的恒定开关频率的DPC策略。在故障穿越的动态过程中这三种控制策略可分别实现转子磁链、转子电流和电磁转矩恒定的控制目标,从而分别实现缩短动态过程的振荡时间、保护交流励磁变频器和抑制机械转矩脉动的控制效果,从不同方面满足弱电网条件下DFIG风电机组的运行控制的要求。(4)使用对称分量法分析了弱电网条件下各种类型输电线路故障对风电场内DFIG风电机组机端电压的影响,估算了最严重故障条件下发电机机端正序电压跌落、负序电压分量的大小,以及不同故障类型下使DFIG产生最大、最小定子直流磁链的故障相位角。并以此为依据分析了当前变速恒频DFIG风电系统中常规矢量控制在不同类型电网故障穿越过程中的表现。(5)设计了一种基于二倍频陷波器的无时延正序分量提取方法。根据不对称运行条件下瞬时功率分析改造了本文提出的网侧PWM变换器的复开关表DPC方法、定开关频率DPC方法,以及DFIG的定开关频率DPC算法,使它们适用于不对称性电网故障穿越。(6)使用Matlab/Simulink建立了完整的双馈异步风电机组仿真模型,并对DFIG仿真模型进行了深入探讨。对网侧PWM变换器和DFIG风电机组的静稳、动态特性以及不对称运行进行了仿真。对一套15kW交流励磁变速恒频风力发电实验系统进行了改造使之适用于电网电压不对称运行研究,完成了包括网侧PWM变换器和DFIG完整样机系统的不对称运行实验。更多还原

【Abstract】 In China, as in other parts of the world, many of the best resources for wind power generation are located far away from load centers. A large number of wind farms, supplying distant load centers by long AC transmission lines, face numerous technical challenges. Since the Variable-Speed Constant-Frequency (VSCF) wind power generation system based on Doubly-Fed Induction Generator (DFIG) is very popular in the modern wind farms, the control strategies of DFIG, which dominates the dynamic performances of wind turbines attract more and more attentions to researchers and wind-farm managers . Recently, direct power control (DPC) of DFIG’ based wind farms becomes one of the research highlights, especially in the filed of grid fault ride through (FRT) operation due to its simple structure and good dynamic performance. This dissertation is mainly focused on the DPC strategy of DFIG-based wind power systems under weak grid conditions.The dissertation disccusses the advanced grid codes and the characteristics of DFIG, particularly its FRT capability at first. The conclusion reveals that an improved control strategy for DFIG, which is capable of protecting the wind turbines and the AC excitation converter from damages, and even geting enhanced system performance during FRT operation, is the key issue to the investigation of wind turbines driven DFIG connected to the weak network. As a result, the main contributions of the dissertation can be summarized and highlighted as follows:1. The traditional DPC scheme is studied and analyzed in detail firstly. In the traditional DPC, the simple switching table and the section-dividing method produce periodic errors to the output voltage vector, which leads to abundant AC current harmonics and needs to be improved and optimized.2. The DPC of grid-side converter of DFIG is investigated in detail. A multi-switching-table DPC strategy and a constant switching frequency DPC with Space Vector Modulation (SVM) approach are proposed. The former needs a simple structure and lower hardware requirement since it calculates the switching table off-line and is implemented mainly by looking up the table. The latter keeps the switching frequency around the sampling frequency with SVM, which increases the complexity of the control algorithm properly but effectively simplifies the design of AC filter coupled with generators. 3. The DPC of rotor-side converter of DFIG is studied and three selectable DPC strategies with constant switching frequency by using SVM technique are presented. Based on the relationship between DFIG state-variables and control variables, these DPC strategies aim at the controlling of DFIG rotor flux (RF-DPC), rotor current (RC-DPC) and electromagnetic torque (EMT-DPC), respectively. The RF-DPC is able to reduce the system oscillating time, the RC-DPC is capable of depressing the magnitude of rotor oscillating current effectively and the EMT-DPC provides the capability of eliminating electromagnetic torque vibration and, consequently diminishing the stress on turbine shaft, which can be selected according to the different requirement of wind farms connected to the weak network.4. The influences of different faults on the transmission line connected to the DFIG’s terminal of wind farms are analyzed by symmetrical component method. Under different types of fault conditions, positive and negative sequence components of the terminal voltage are measured. Besides, the fault angles leading to the maximum and the minimum stator dc flux respectively are estimated. On the above basis, the performance of conventional vector control scheme for DFIG’s FRT operation is evaluated.5. A zero-time delay extraction method for positive sequence components based on the second-order notch filter is presented. As a result, during unbalanced network conditions, the proposed multi-switching table DPC, constant frequency DPC for the grid-side PWM converter and constant frequency DPC for the DFIG wind turbine system are improved, which makes them suitable for the unbalanced grid fault ride through operation.6. An integrated simulation mode of a DFIG wind power generation system is built-up by using MATLAB/SIMULINK. Various simulations on grid-side PWM converter and DFIG system are carried out to validate the proposed control strategies. Meanwhile, an experimental platform of 15kW DFIG system is re-built for the tests particularly under unbalanced network voltage conditions.更多还原