双端有源MMC-HVDC系统的控制策略研究

多端模块化多电平变流器(Modular Multilevel Converter,MMC)一直是柔性高压直流输电系统(High Voltage Direct Current,HVDC)工程应用的重要部分,因此对其控制策略的研究很有实际意义。本文基于三相静止坐标系下MMC的数学模型,建立了dq旋转坐标系下的数学模型,根据瞬时功率理论设计出外环功率和内环电流的MMC-HVDC系统控制器。针对传统电容电压平衡策略的问题,从减少开关频率的角度提出了改进型的子模块电容均压方式。由于本文中真模型中相单元子模块过多,为使系统更稳定可靠运行采用了最近电平逼近策略(nearest level modulation,NLM)。最后在Matlab/Simulink仿真软件中搭建89电平双端有源MMC-HVDC系统模型,从改变控制器参考值、有功功率反转等角度对控制系统进行仿真分析对比,验证了MMC-HVDC系统控制器的可靠性和稳定性。     

井下柔性直流输电系统换流站设计

针对煤矿井下供电负荷、供电距离、供电系统有功损耗均较大,且负载侧存在电压偏低和闪变等问题的现状,将基于模块化多电平换流站的柔性直流输电(MMC-HVDC)系统应用到井下供电系统,重点介绍了该系统中换流站(MMC)的拓扑结构、数学模型、参数及控制器设计等。井下MMC-HVDC系统包含整流站和逆变站2个MMC,整流站采用快速电流内环控制和定直流电压、定无功功率外环控制的双闭环控制结构,逆变站采用定交流电压控制;MMC均采用载波移相脉宽调制和基于能量均分控制、电压平衡控制的均压策略。在PSCAD/EMTDC软件中对井下MMC-HVDC系统进行仿真实验,结果表明:井下MMCHVDC系统在稳态运行时具有良好的动态响应,实现了有功和无功功率独立控制,提高了直流输电效率;系统受到因井下负荷变化引起的扰动时可以较快的速度调节有功和无功功率,快速稳定直流输电电压,同时将逆变站输出电压幅值和相位稳定在额定值。     

MMC的优化开关频率法研究

模块化多电平换流器(Modular Multilevel Converter,MMC)不仅没有类似箝位型多电平换流器直流母线之间的直流电容器组,还具有级联型换流器"模块化"的优点,因此模块化多电平换流器在高压直流输电中具有广阔的应用前景。本文介绍了MMC的数学模型和工作原理,提出了一种优化载波移相SPWM,并在MATLAB/Simulink环境下搭建了基于开关频率优化PWM法的MMC仿真系统,仿真结果验证了所提方法具有直流电压利用率高、等效开关损耗小等优点。     

基于DC/DC变换器的柔性直流输电数学建模仿真

为缓解高质量电压对直流输电线路造成的抑制影响,充分增强柔性直流输电网络的可靠性与灵活性,建立基于DC/DC变换器的柔性直流输电数学应用模型.通过DC/DC变换器控制的方式,选择与变换器结构体相关的拓扑连接行为,再利用高频变压器原副边匝数比数值,实现基于DC/DC变换器的直流输电特性分析.设置MMC基本控制器,采取内外环控制同步实施的方法,完成输电网络环境中的三次谐波调制,实现基于DC/DC变换器的柔性直流输电数学应用模型的顺利应用.仿真结果表明,应用所提方法的柔性直流输电网络中的电压传输量、电流传输量均出现明显增大的变化趋势,输出功率稳定,可有效缓解高质量电压对直流输电线路造成的抑制性影响.     

应用于VSC-HVDC的模块化多电平变换技术研究

近几年来,电力电子技术快速发展,VSC-HVDC在电力电子行业中发挥了重要作用,特别是提出了采用级联子模块的模块化多电平变换器拓扑(Modular Multilevel Converter,MMC)。该技术的应用更便于集成和进行调试安装,方便开关控件增加装置容量以及控制好电压等级。同时,考虑到MMC具有直流母线,同VSC-HVDC的应用理念比较契合。但同时子模块电容电压不均衡与环流问题严重影响MMC对于精准度的控制,易发生一些突发的状况,造成保护机器的成本过高。     

一种新型三相电压型PWM整流器启动冲击电流抑制方法

针对三相电压型PWM整流器启动瞬间产生较大的冲击电流,致使系统稳定性、可靠性降低的问题,在整流器前端加入预充电电路,对预充电电路软启动过程和整流器启动过流的原因进行了详细分析,再对2种无预充电电路的母线电压缓给定方法和2种预充电电路结合母线电压缓给定方法抑制启动冲击电流的作用机理进行了对比分析,最终提出了预充电-分段二阶抛物线母线电压缓给定混合控制的最优抑制方法,最后通过搭建4种母线电压缓给定方法的仿真模型和30 kW整流器实验样机进行了验证。结果表明,采用预充电-分段二阶抛物线母线电压缓给定混合控制启动的整流器有效地抑制了冲击电流,网侧电流启动无过冲现象,直流侧母线电压无超调。     

并联全桥子模块MMC直流故障抑制性能仿真分析

基于并联全桥子模块(Paralleled Full-bridge Sub-modules,P-FBSM)的模块化多电平换流器(Modular Multilevel Converter,MMC)因具备良好的自均压能力和电流通流能力在高电平应用领域得到重视。目前针对P-FBSM-MMC的研究侧重于其自均压特性,而对直流故障阻断机理分析较少。文章为研究P-FBSM-MMC系统在发生直流侧双极短路情况下的故障电流抑制能力和恢复能力,首先分析P-FBSM-MMC在发生直流侧双极短路故障时的故障电流通路,然后利用PSCAD/EMTDC仿真软件搭建21电平P-FBSM-MMC和半桥子模块(Half-bridge Sub-modules,HBSM)MMC仿真模型,分析了P-FBSM-MMC在直流侧双极短路故障情况下的故障电流、直流侧电压以及故障恢复能力。通过与HBSM-MMC仿真结果对比验证得出结论：P-FBSM-MMC在发生直流侧双极短路故障情况下通过闭锁所有IGBT能够有效抑制故障电流,且直流电压能在解锁IGBT后的较短时间内恢复到额定值,对未来P-FBSM-MMC的研究有较大参考价值。     

基于载波混合SPWM控制模块化多电平变换器的研究

模块化多电平变换器(modular multilevel converter,MMC)是柔性直流输电的应用热点之一。针对传统脉冲宽度调制方式在MMC系统中存在使每个模块单元输出功率不等,输出波形性能降低等问题,提出了一种基于载波混合脉冲宽度调制策略(carrier-based hybrid SPWM,CBH-SPWM)。该调制策略同时包含了载波重叠和载波移相两种调制方法的特点。一方面提高了输出电压的性能,另一方面使MMC中每个模块输出功率相等。仿真结果验证了所提CBH-SPWM方法的正确性与有效性。     

MMC型换流器电容电压波动抑制方法研究北大核心CSCD

为了解决模块化多电平换流器(MMC)电容电压波动导致的换流器成本与体积增加等问题,提出了一种电容电压二倍频纹波抑制方法。首先从能量脉动角度推导了电容电压波动的数学模型,比较了不同二倍频环流对MMC电容电压波动抑制效果的影响;然后采用Clark变换将桥臂共模电压分解为αβ轴分量,利用准比例谐振控制器生成桥臂环流参考值,设计内环控制器对外环控制器生成的桥臂环流参考值进行跟踪,从而抑制子模块电容电压波动;最后在MATLAB/Simulink仿真系统上对所提控制策略进行仿真验证,通过对比传统环流抑制策略与所提控制策略对电容电压波动的抑制效果,证明了所提方法可有效抑制电容电压波动,改善输出电压的波形质量。     

基于紧急功率支援的直流输电系统研究

VSC-HVDC通过利用全控期间的电压源型换流器,在一定程度上,克服了传统直流输电的不足之处,同时实现了有功功率和无功功率的独立控制。本文通过在仿真软件PSCAD/EMTDC中搭建了基于电压源型换流器的三端直流输电系统,并设计采用了改进型PID控制算法与功率支援模块用以快速改变直流线路传输功率,通过动态电流限幅环节抑制交流侧故障过电流,针对逆变器交流侧的几种常见故障和直流线路故障时系统的运行状况进行了仿真分析,并对不同的故障设计了不同的运行方案,仿真结果表明该系统在多种故障情况下均能够有效的保持系统频率与电压的稳定。     

European Community policy and the market

Abstract This paper starts with some reflections on the policy considerations and priorities which are shaping European Commission (EC) research programmes. Then it attempts to position the current projects which seek to capitalise on information and communications technologies for learning in relation to these priorities and the apparent realities of the marketplace. It concludes that while there are grounds to be optimistic about the contribution EC programmes can make to the efficiency and standard of education and training, they are still too technology driven.     

一种自适应子融合集成多分类器方法

融合集成方法已经广泛应用在模式识别领域,然而一些基分类器实时性能稳定性较差,导致多分类器融合性能差,针对上述问题本文提出了一种新的基于多分类器的子融合集成分类器系统。该方法考虑在度量层融合层次之上通过对各类基多分类器进行动态选择,票数最多的类别作为融合系统中对特征向量识别的类别,构成一种新的自适应子融合集成分类器方法。实验表明,该方法比传统的分类器以及分类融合方法识别准确率明显更高,具有更好的鲁棒性。     

煤矿井下安全监控分站的设计及其仿真实现

为了设计一种具有低成本、低功耗、易操作、功能强且可靠性高的煤矿井下安全分站,针对煤矿安全生产实际,文章提出了采用MCS-51系列单片机为核心、具有CAN总线通信接口的煤矿井下安全监控分站的设计方案;首先给出煤矿井下安全监控分站的整体构架设计,然后着重阐述模拟量输入信号处理系统的设计过程,最后说明单片机最小系统及其键盘、显示、报警、通信等各个组成部分的设计;为验证设计方案的可行性与有效性,使用Proteus软件对设计内容进行仿真验证,设计的煤矿井下安全监控分站具有瓦斯、温度等模拟量参数超标报警功能和电机开停、风门开闭等开关量指示功能;仿真结果表明:设计的煤矿井下安全监控分站具有一定的实际应用价值.     

Explanation and prediction: an architecture for default and abductive reasoning

Although there are many arguments that logic is an appropriate tool for artificial intelligence, there has been a perceived problem with the monotonicity of classical logic. This paper elaborates on the idea that reasoning should be viewed as theory formation where logic tells us the consequences of our assumptions. The two activities of predicting what is expected to be true and explaining observations are considered in a simple theory formation framework. Properties of each activity are discussed, along with a number of proposals as to what should be predicted or accepted as reasonable explanations. An architecture is proposed to combine explanation and prediction into one coherent framework. Algorithms used to implement the system as well as examples from a running implementation are given.     

View-based model-driven architecture for enhancing maintainability of data access services

In modern service-oriented architectures, database access is done by a special type of services, the so-called data access services (DAS). Though, particularly in data-intensive applications, using and developing DAS are very common today, the link between the DAS and their implementation, e.g. a layer of data access objects (DAOs) encapsulating the database queries, still is not sufficiently elaborated, yet. As a result, as the number of DAS grows, finding the desired DAS for reuse and/or associated documentation can become an impossible task. In this paper we focus on bridging this gap between the DAS and their implementation by presenting a view-based, model-driven data access architecture (VMDA) managing models of the DAS, DAOs and database queries in a queryable manner. Our models support tailored views of different stakeholders and are scalable with all types of DAS implementations. In this paper we show that our view-based and model driven architecture approach can enhance software development productivity and maintainability by improving DAS documentation. Moreover, our VMDA opens a wide range of applications such as evaluating DAS usage for DAS performance optimization. Furthermore, we provide tool support and illustrate the applicability of our VMDA in a large-scale case study. Finally, we quantitatively prove that our approach performs with acceptable response times.     

Three Process Perspectives: Organizations, Teams, and People

This paper provides the author's personal views and perspectives on software process improvement. Starting with his first work on technology assessment in IBM over 20 years ago, Watts Humphrey describes the process improvement work he has been directly involved in. This includes the development of the early process assessment methods, the original design of the CMM, and the introduction of the Personal Software Process (PSP)^SM and Team Software Process (TSP){^SM}. In addition to describing the original motivation for this work, the author also reviews many of the problems he and his associates encountered and why they solved them the way they did. He also comments on the outstanding issues and likely directions for future work. Finally, this work has built on the experiences and contributions of many people. Mr. Humphrey only describes work that he was personally involved in and he names many of the key contributors. However, so many people have been involved in this work that a full list of the important participants would be impractical.     

A pragmatic multi-method investigation of discrepant technological events: Coping,attributions, and ‘accidental’ learning

Discrepant technological events or situations that entail a problem, a misunderstanding or a difficulty with the Information Technology (IT) being employed, are common in the workplace, and can lead to frustration and avoidance behaviors. Little is known, however, about how individuals cope with these events. This paper examines these events by using a multi-method pragmatic approach informed by coping theory. The results of two studies – a critical incident study and an experiment – serve to build and test, respectively, a theoretical model that posits that individuals use a variety of strategies when dealing with these events: they experience negative emotions, make external attributions, and adopt engagement coping strategies directed at solving the event, eventually switching to a disengagement coping strategy when they feel they have no control over the situation. Furthermore, users’ efforts may result in ‘accidental’ learning as they try to overcome the discrepant IT events through engagement coping. The paper ends with a discussion of the results in light of existing literature, future opportunities for research, and implications for practice.     

基于复小波噪声方差显著修正的SAR图像去噪

提出了一种基于复小波域统计建模与噪声方差估计显著性修正相结合的合成孔径雷达(Synthetic Aperture Radar,SAR)图像斑点噪声滤波方法。该方法首先通过对数变换将乘性噪声模型转化为加性噪声模型,然后对变换后的图像进行双树复小波变换(Dualtree Complex Wavelet Transform,DCWT),并对复数小波系数的统计分布进行建模。在此先验分布的基础上,通过运用贝叶斯估计方法从含噪系数中恢复原始系数,达到滤除噪声的目的。实验结果表明该方法在去除噪声的同时保留了图像的细节信息,取得了很好的降噪效果。     

The Distributed Time-Triggered Simulation Scheme: Core Principles and Supporting Execution Engine

Distributed real-time simulation is a young technology field but its practice is under increasing demands. In recent years the author and his collaborators have been establishing a new approach called the distributed time-triggered simulation (DTS) scheme which is conceptually simple and easy to use but widely applicable. The concept was initiated in the course of developing a new-generation object-oriented real-time programming scheme called the time-triggered message-triggered object (TMO) programming scheme. Some fundamental issues inherent in distributed real-time simulation that were learned during recent experimental studies are discussed along with some approaches for resolving the issues. An execution engine developed to support both the TMOs engaged in control computation and the TMOs engaged in DTS is also discussed along with its possible extensions that will enable significantly larger-scale DTSs.     

How do children do mathematics with LOGO?

Abstract  This paper considers some results of a study designed to investigate the kinds of mathematical activity undertaken by children (aged between 8 and 11) as they learned to program in LOGO. A model of learning modes is proposed, which attempts to describe the ways in which children used and acquired understanding of the programming/mathematical concepts involved. The remainder of the paper is concerned with discussing the validity and limitations of the model, and its implications for further research and curriculum development.