【作者】 张颖；

【导师】 荆龙；

【作者基本信息】 北京交通大学 ， 电气工程， 2019， 硕士

【摘要】 近年来,直流电网凭借在新能源并网及消纳方面的优势已成为国内外的研究热点,用于实现直流互联的直流变压器是构建直流电网的核心设备之一。传统隔离型直流变压器在传输功率时全部功率均经过换流器,使得变压器设计容量加倍、运行损耗高。为了满足直流电网对直流变压器高效率、低成本的迫切需求,本文提出了一种基于DC/DC串并联技术的新型直流自耦变压器(DC autotransformer,DC AT)拓扑,用于实现不同电压等级的直流线路互联及潮流调控。本文重点研究内容如下:受交流自耦变压器(AC autotransformer,AC AT)技术启发,结合DC/DC串并联组合系统的优势提出了新型DCAT拓扑,分析了其拓扑结构及工作原理。该DC AT拓扑的特点为所互联系统间有直接的电气联系,其明显优势在于仅部分功率经换流器传输,变压器设计容量小,运行损耗降低,可依据两侧直流系统的电压等级灵活地对子模块(sub-module,SM)进行串并联组合。为了确保DC AT系统稳定运行,研究了 DC/DC多模块串并联系统的拓扑结构及均压控制策略,重点研究了输入串联、输出串联型(Input-Series Output-Series,ISOS)组合系统,并提出一种基于电感电流补偿值与输入/输出电压双下垂特性的自适应均压控制策略,可实现ISOS系统输入/输出侧均压以及模块化运行。对于串并联复合系统,设计了相应的均压控制策略,通过原理分析并结合仿真和实验验证了控制策略的可行性。为了实现DC AT系统平稳启动,研究了预充电控制策略。限流电阻充电方式适用于DC AT的两侧均为有源电网工况;仅低压侧无源时,采用占空比逐渐增加的可控充电方式可实现低压侧的自启动;仅高压侧无源时,采取恒流充电控制策略可实现高压侧电压平缓上升,且具有电流冲击小的优势。最后对DC AT的整体控制策略进行了研究,采用分层控制思想,底层为移相调制及子模块均压策略,在移相调制中加入分组载波移相可实现系统的交错并联控制,中间层为子模块电压电流双闭环控制,顶层可根据不同场景工况采取定功率或定电压控制。在Matlab/Simulink里搭建了 DC AT拓扑,并在实验室搭建了由4台隔离型DC/DC变换器组成的实验平台,对所提出的控制策略进行仿真及实验验证,研究结果表明DC AT拓扑可稳定工作且可部分降低换流器容量。更多还原

【Abstract】 In recent years,DC grid has become a research hotspot at home and abroad by virtue of its advantages in new energy integration and absorption capability.The DC transformer used for interconnecting DC lines is one of the core equipments for constructing DC grid.When it comes to power transmission,the entire power will pass through the traditional isolated DC transformer,which will double its design capacity and increase its operation loss.In order to meet the urgent demand of efficient and low-cost DC transformers for DC grid,this paper proposes a new topology of DC autotransformer(DC AT)based on DC/DC series-parallel technology,which is able to connect DC lines of different voltage levels and realize power flow control.The main contents of this paper are as follows:Inspired by AC autotransformer(AC AT)technology,a new DC AT topology is proposed based on the advantages of DC/DC series-parallel combination system,and its topology structure and working principle are both analyzed.The DC AT topology is characterized by direct electrical connections between interconnected systems.Its obvious advantage is that only part of the power is transmitted by the transformer,which can reduce the design capacity and operation loss of the transformer.The sub-modules(SM)can be flexibly connected in series and parallel according to the voltage levels of the DC systems on both sides.In order to ensure the stable operation of DC AT,the topology structure and voltage sharing control strategy of DC/DC multi-module series-parallel system are studied.This paper focuses on the input-series output-series(ISOS)combined system and proposes an adaptive voltage sharing control strategy based on dual-droop characteristics of input and output voltages,which can realize the voltage sharing and modularization of the input/output side of ISOS system.For the series-parallel compound system,the corresponding voltage sharing control strategy is designed,and the feasibility of the control strategy is verified by theoretical analysis,simulation and experiment.For the sake of smooth starting of DC AT system,the precharge control strategy is studied.Charging by current-limiting resistance is suitable for the situation where the DC AT is used to connect active power grids on both sides.When the low-voltage side is passive,the self-starting of low-voltage side can be realized by using controllable charging mode of increasing the duty ratio step by step.When the high-voltage side is passive,the constant-current charging strategy can make the voltage at the high-voltage side rise smoothly,and it has the advantage of low current impact as well.Finally,the overall control of DC AT is achieved by means of hierarchical control.Specifically,the lower layer control is accomplished by implementing phase-shifting modulation and sub-module voltage sharing strategy,and the interleaved parallel control can be realized by adding grouping carrier phase shift.Besides,the middle layer adopts the voltage and current double closed-loop control,and the upper layer can employ constant power or voltage control according to different operating modes.A DC AT model is constructed in Matlab/Simulink,and an experimental platform consisting of four isolated DC/DC converters is built in the laboratory.The proposed control strategies are simulated in MATLAB and verified experimentally.The results show that the DC AT topology can work stably and partly reduce the converter capacity.更多还原