【作者】 张静；

【导师】 徐政；

【作者基本信息】 浙江大学 ， 电力系统及其自动化， 2009， 博士

【摘要】 电压源换流器型直流输电(VSC-HVDC)是一种以电压源换流器和脉宽调制等技术为基础的相对较新的直流输电技术。由于具备向无源系统或者弱交流系统供电、有功功率和无功功率能快速独立控制、易于构成多端网络和环境友好等优异性能,VSC-HVDC在输电和配电领域都有着非常广泛的应用前景,吸引了国内外工业界和学术界的广泛关注。随着大功率电力电子器件的发展,VSC-HVDC的造价和运行、维护费用逐步降低,在世界范围内,VSC-HVDC工程建设方兴未艾,其传输容量和电压等级逐步提高,我国相关科研院所和公司也投入了很多力量进行相关技术的研究。在这样的背景下本文主要就VSC-HVDC的控制策略等进行了研究,期望能对我国VSC-HVDC技术的发展有一定借鉴意义。论文主要内容如下：1.概括性地讨论了VSC-HVDC控制系统的分层结构(系统级控制、换流器级控制和触发级控制等),其中换流器级控制是本学位论文研究的重点。通过在交流侧和直流侧引入合理的基值,建立了VSC-HVDC在交流系统对称和不对称运行条件下的标幺值数学模型,此模型为后续的主电路参数选择及控制器设计提供了便利。2.讨论了VSC-HVDC主回路中连接电抗器和直流电容器这两个主要电气设备参数的初步选择方法。3.为了改善系统故障等条件下VSC-HVDC的运行特性和生存能力,提出了两种换流器级控制策略：(i)设计了一种新型广义直流电压控制策略,外环有功功率控制器(APC)被设计成一个广义直流电压控制器(GDCVC),当采用直流电压控制器的换流站由于某种原因不能有效控制直流电压时,控制直流电压的任务由采用GDCVC的换流站自动、平滑地接替；(ii)设计了一种包含“依赖于直流电压的电流指令限流单元”(VDCOL)的控制策略。当直流电压异常时,VDCOL根据直流电压信息按照预先设计的规律快速修正传递给内环电流控制器的有功电流指令。这两种控制策略,都能降低暂态过程中交流系统和直流系统间的有功功率不平衡,从而达到抑制系统的直流过电压和欠电压水平、改善系统运行性能、降低系统被迫退出运行概率等目标,且两端换流器间不需要进行通信联系。4.实际运行的VSC-HVDC系统中存在不确定性因素,如连接电抗器、直流电容器等参数会偏离其设计值,交流系统的运行状态也会改变。这些不确定性会恶化控制系统性能,有时甚至使系统失去稳定。为了降低不确定性对控制系统品质的影响,提出了一种基于定量反馈理论(QFT)的VSC-HVDC换流器级鲁棒控制系统设计方法。QFT是一种工程化的鲁棒控制器设计方法,基于此设计方法设计了VSC-HVDC的内环电流控制器和外环功率控制器。5.为了减缓VSC-HVDC系统启动及重新启动时对自身和电网的冲击,针对目前应用较广泛的两电平和二极管箝位型三电平换流器拓扑结构,提出了一种简单、适用于VSC-HVDC系统的两段式自励启动方法。在启动的第一阶段,串接限流电阻器对直流侧电路充电。若设定的时间内,电容器电压不能达到设定值,启动转入第二阶段,此时换流站在直流电压控制器的作用下对电容器继续充电,直至达到预设值,从而为换流器从启动控制模式切换到正常运行模式做好准备。文中提出的启动方式既能够把直流电压快速提升到设定值,同时又能有效避免启动过程中可能出现的过电压和过电流。通过数字仿真的方法来验证上述控制策略和算法的有效性。更多还原

【Abstract】 Voltage Source Converter based HVDC (VSC-HVDC) is a relatively innovative technology and has many advantages over the classic HVDC in many aspects. With the advances in semiconductors and the cost reduction for installing and operating, VSC-HVDC is expected to find increased use both at transmission and distribution level for its high controllability, environmental benefits and easy expandability in near future. It enables fast control of active and reactive power independently and can delivery power to the weak or passive network on the islands and oil platforms. The rating and voltage level of VSC-HVDC are increasing gradually. The design and installations of VSC-HVDC are in the ascendant.The dissertation is organized as follows.(1) A general discussion about the hierarchy of VSC-HVDC control system, namely the system control, the converter control and the fire control, is presented. The dissertation focuses on the converter control. Based on the properly selected base values, a per unit model in the dq synchronous rotating frames under balanced and unbalanced operating conditions for VSC-HVDC is addressed.(2) The design of dc capacitors and interfacing reactors is an important part for the design of a VSC-HVDC. The sizing of the dc capacitors and interfacing reactors is generally investigated.(3) To enhance the operating performance and the ability to survive the faults, two refined converter control strategies for VSC-HVDC have been introduced and investigated:(i) a novel Active Power Controller (APC) is designed as a Generalized DC Voltage Controller (GDCVC), which takes over the function of DC Voltage Controller (DCVC) automatically and smoothly when the DC link voltage can not be maintained by DCVC for some reasons.(ii) a DC Voltage Dependent Current Order Limiter (VDCOL) is included between the outer active power control loop and the inner current control loop. VDCOL changes the inner active current orders according to the DC link voltage. When the DC voltage is abnormal, VDCOL rapidly modifies the current orders, which are derived from the outer control loops, before been send to the inner current control loops according to pre-set characteristics. With the two proposed converter control strategies, the active power unbalance between the ac and dc system can be both reduced during transients. Therefore the DC undervoltage and overvoltage can be effectively suppressed and the risk of tripping the whole system is reduced.(4)There are uncertainties in the practical VSC-HVDC system, such as the interfacing reactors and dc capacitors may deviate from their rated values for many reasons. These uncertainties may deteriorate the performance without dedicatedly designed controllers. To deal with these uncertainties and meet the performance specifications, a design methodology for robust control of VSC-HVDC using quantitative feedback theory (QFT) is proposed. In order to obtain a high performance, an inner-outer cascaded control scheme of quantitative feedback theory is utilized to derive the inner current loop controllers and outer power loop controllers.(5) In order to reduce the undesired impacts and damages on the VSC-HVDC system and the connected AC system during the startup and re-startup procedures, a two-stage self startup procedure is proposed for two-level and diode-clamped three-level VSC-HVDC system. During the first stage, the resistors are inserted to limit the inrush current during the charging. If the DC voltage can not reach the preset value after predetermined time, the startup procedure switches to the second stage and the capacitors will be further charged with a DC voltage controller. The startup procedure completes till the DC link voltage reaches the preset value and then the converters are ready to change from the startup mode to the normal operating mode.Digital simulation has been used to demonstrate the feasibility of the proposed control strategies and algorithms.更多还原