【作者】 连霄壤；

【导师】 江道灼； 何奔腾；

【作者基本信息】 浙江大学 ， 电力系统及其自动化， 2012， 硕士

【摘要】 多电平变换器因具有低器件开关应力,高输出波形质量,高转换效率等优点,广泛应用于高压大功率用电领域。基于静止无功补偿器(STATCOM)的无功补偿技术在提高负荷中心电压稳定性,改善电能质量等方面效果明显,具有显著的应用前景。模块化多电平技术是多电平技术发展的最新成果,为高压大功率场合电力电子装置的研制提供了新的思路。本文采用理论分析与仿真验证相结合的研究手段,将模块化多电平技术应用于动态无功补偿领域,重点对模块化多电平STATCOM控制策略进行研究,相关研究内容为：(1)对模块化多电平换流器研究所涉及的三个重要问题,即调制策略、电容均压策略和环流抑制策略展开讨论。本文将多电平换流器的调制策略与电容均压策略统一分析,通过对已有多电平调制策略的比较,提出了一种基于分级排序子模块电容均压策略及相应的优化方法,该方法可在维持子模块电容电压均衡的条件下有效降低开关频率,提高了换流器的工作效率。对模块化多电平换流器环流问题的研究中,本文从环流产生机理入手,分别对正序、负序网络中的环流特性展开研究,并基于广义坐标旋转变换的概念,提出了适用于正序以及负序网络环流抑制控制器的设计思路,该控制器可在不增加桥臂限流电抗的条件下,有效抑制环流的波动分量。(2)通过对STATCOM无功补偿原理的分析,分别建立三相静止坐标系以及两相同步旋转坐标系下的STACOM数学模型和等效电路。同时针对STATCOM数学模型中dq分量耦合,无法实现独立无功控制的缺点,提出了一种基于状态反馈的解耦控制策略。该策略通过引入dq电压前馈实现解耦控制,可获得无功补偿电流良好的追踪特性。(3)针对模块化多电平换流器各桥臂由大量子模块级联而成,易出现可靠性问题的特点,通过对级联多电平换流器故障容错控制方案的分析,提出了适用于模块化多电平静止无功补偿器的故障容错控制方案。该方案通过将冗余子模块设置在热备用状态,可以在子模块发生故障时迅速投入冗余模块,从而保证静止无功补偿器的正常工作。此外该方案还进一步根据发生故障模块数的不同,设定不同的控制目标,通过适当提升子模块电容电压参考值的方法,最大化的提高了静止无功补偿器的工作范围。(4)基于瞬时对称分量理论和广义同步旋转坐标变换关系,分析交流系统电压平衡和电压不平衡时系统的功率平衡关系,分别建立三相静止坐标系和两相同步旋转坐标系下的换流器低频动态数学模型。为提高不平衡工况下同步相位与瞬时对称分量的检测精度,提出基于广义同步旋转坐标变换的同步相位检测方案。该方案既可消除负序分量对锁相性能的影响,又可实现可分量的提取,为不平衡工况换流器的控制提供了保证。本文另一个研究重点为不平衡工况下模块化多电平换流器的控制与保护问题,本文提出的不平衡工况下的控制策略可分别实现对负序电流或者直流侧电容电压波动的进行有效抑制,保障了换流器的正常运行。(5)在对不平衡工况下模块化多电平换流器环流抑制控制器设计方案的讨论中,本文首先是基于广义同步旋转坐标变换,将不平衡工况下换流器的功率控制与内部环流控制统一讨论。在保证换流器出口侧的有功功率稳定的前提下,通过广义同步旋转坐标变换解除了正序和负序网络之间的耦合关系,并将不平衡工况下的网络分解为正序.正序和负序-负序网络进行分析,实现了正、负序网络的独立控制。本文其次是分别根据环流在正序及负序网络的特性,基于广义同步旋转坐标变换的概念,提出基于前馈交叉解耦的不平衡工况下环流控制器的设计思路和实现方法。该控制器可在不对称工况下,无需增加桥臂限流电抗即可有效抑制环流的波动分量。更多还原

【Abstract】 Multilevel conveter topology has great potential in high voltage and large power applications, considering its significant features such as lower switching stress, higher efficeintcy, and better output waveforms. Dynamic reactive power compensation based on Static Synchronous Compensator (STATCOM) plays an active role in ensuring voltage stability of power load centers and improving power quality, thus has important realistic meaning and engineering value. Modular Multilevel Converter (MMC) is lastest result of Multilevel Converter which provides new ideas for R&D work of high power converter. In this dissertation, the control strategy of STATCOM based on MMC topology is investigated by means of theoretical analysis and simulation validation. The main contests and related conclusions are as follows:(1) Based on the analysis on the structure and operation principle of MMC, Three main important issues are taken into discussion which includes modulation strategy, capacitor voltage balancing and circulating current suppression. A comparision of existing modulation strategies sutiable for multilevel converter is made, a novel capacitor voltage balancing algorithm based on hierarchical sorting and related optimization have been presented. The proposed algorithm ensures voltage balancing among sub-modules while reduces swiching frequency and improves efficieny of MMC. As to circulating current suppression, based on the analysis on its generation mechanism, feature of circulating current under both positive and negative networks is investigated. According to the concepet of decoupled double synchronized transformation, the method of ciruculating current suppression controller is proposed, which efficiently suprreses the fluctuation without increasing arm inductance.(2) Based on the operation principle of reactive compensation, the mathematical model and equivalent circuit are created for STATCOM in both three phase coordinates and dq two phase synchronous coordinates. As to the coupling of dq component, a kind of statefeedback decoupled PI controller is presented. This control strategy can eliminate coupling between dq components by introducing dq components feed-forward, simulation results verify fast dynamic and non-steady error reactive power trace.(3) Considering quite large amount of sub-modules (SMs) used in MMC topology, attention is drawn on the reliability of operation. Based on the analysis of existed fault-tolerant design for H-bridge converters, a novel fault-tolerant control strategy suitable for MMC-based STATCOM is proposed. According to proposed strategy, all redundant SMs are set in hot stand-by state, once fault occurs, redundant SMs will rapidly take place of the faulted SMs, thus ensures the STATCOM under normal condition. Furthermore, according to different number of faulted SMs, control strategy adopts different objectives which can greatly improve operating range of STATCOM.(4) Based on the concept of instanous symmetrical component and synchronized transformation, power balance relation between the AC and DC side of MMC are analyzed under both balanced and unbalanced ac voltage conditions, corresponding low frequency dynamic mathematical models of MMC are developed in dq synchronous reference frame. In order to increase detection accuracy of synchronous phase and instantaneous symmetrical components, a novel detection method based on the concept of decoupled double synchronized transformation is presented. The proposed methed can not only eliminate the adverse effects of negative sequence componets, but realize sequence component extraction as well. Control strategy for MMC under unbalanced condition is proposed which can eliminate negative sequence components or ensure DC voltage stable without fluctuation, thus enlarge the operating region of MMC.(5) A novel control strategy for suppressing circulating current of modular multilevel converter (MMC) under unbalanced conditions is proposed. Based on decoupled synchronous transformation, dynamic behavior of converter can be expressed in both positive- positive and negative-negative networks. Mechanism and dynamic performance of circulating current in both networks will be further discussed. Based on the concept of decoupled double synchronous transformation, circulating current suppressing controller suitable for both networks is presented which can eliminate coupling between two netwroks by introducing dq components feed-forward. The proposed control strategy can efficiently suprress the fluctuation without increasing arm inductance under unbalanced condition.更多还原