面向交直流混合配电网的功率复合型模块化多电平固态变压器基础研究

Solid State Transformer (SST) based on modular multilevel converters features a variety of smart functions and is regarded as the key element in the ac/dc hybrid electrical distribution network in future. However, the factor of low power density is preventing SST from further development, where the complicated steps that the power is converted as well as the large number of power units are considered as the main reasons. Therefore, this project is to propose a new SST topology with reduced power conversion steps, reduced number of power units and passive devices using the idea of power integration, three-phase-four-arm structure and double half bridge power units respectively, aiming at enhancing the power density of SST. The fundamental research contents are conducted as follows: the frequency integrated modulation principle is analyzed and the integrated modulation method based on the universal state machine is presented; the feature of the active power balance is clearly explained and the corresponding control scheme based on state change enforcement is proposed; the rule how the low order power flows is revealed and the suppression method using the established high frequency power loop is provided; the coupling mechanism of multiple targets including the system efficiency is investigated and the proposed new SST is comprehensively optimized. As a result, this project is able to provide the theoretical foundation for improving the power density of SST in hybrid power distribution network and is of great research significance to promote its further application and to accelerate its industrialization.

基于模块化多电平变换器的固态变压器（Solid State Transformer, SST）具有多种智能化特征，是未来面向交直流混合配电网的关键装备。然而，现有SST变换级数多和模块数多，导致其功率密度偏低从而制约其进一步发展。本项目以提高SST功率密度为主要目标，拟采用功率复合思想减少变换级数和采用三相四桥臂及双半桥单元拓扑减少模块单元和无源元件个数，从而提出一种功率复用型SST新拓扑，并围绕该拓扑展开基础研究：分析频率复合调制原理并提出基于通用状态机的集中式调制方法；阐明集中式复合频率调制下有功功率平衡特征并提出基于状态强制切换的控制方法；揭示低频脉动功率流动特征并提出基于一体化高频通路的平抑方法；分析含系统效率在内的多目标耦合机制并进行综合优化。本项目能够为提高交直流混合配网用SST功率密度、运行特性提供理论基础，其研究成果有望推动SST在配电网的产业化进程，具有重要的研究意义。

模块化多电平固态变压器（Solid State Transformer, SST）是未来交直流混合配电网的关键装备。然而，现有SST的低功率密度成为了其发展中的瓶颈问题。针对该瓶颈问题，本项目从功率复合思想出发，主要研究内容和取得的成果如下：（1）提出了一种具有高功率密度的功率复合型模块化多电平SST拓扑结构，该拓扑功率变换级数少、无源滤波器用量少，和传统面向交直流混合配电网的SST相比功率密度显著提升；（2）围绕此功率复合型SST，提出了共载波调制方法，在此基础上分析了功率复合与解耦原理并提出了集中式频率复合调制方法，即载波/方波混合调制方法，在调制分配过程保证了子模块开关动作次数的最小化，在一定程度上提高了功率复合型SST的运行效率；（3）明晰了功率复合型SST的有功功率传递特征和分层平衡机理，分析了其电容电压在子桥臂内、子桥臂间、桥臂间、相桥臂四个维度产生不平衡的规律和原因，并基于此提出了针对功率复合型SST的四层式电容电压平衡控制方法；（4）剖析了功率复合型SST分别产生稳态功率脉动和动态功率脉动（振荡）的机理，并分别提出了基于一体化功率路径下波动功率传递的稳态功率脉动抑制方法和基于附加振荡抑制角的有源阻尼型动态功率脉动抑制方法，能够帮助减小装置无源器件体积，提高装置功率密度；（5）建立了功率复合型SST的损耗模型，与MMC+ISOP、MMC+DAB两种常见拓扑进行了对比研究，指出了所提功率复合型SST拓扑虽然在运行效率方面略有降低，但在功率密度和成本方面有较大优势；（6）针对该SST输入级的功率单元应用功率复合思想，提出了一种具有更高安全性的耦合电感型子模块，在不增加有源器件的条件下将子模块输出提高至三电平，功率复合型SST输入级整体输出增加至4N+1电平，进一步提高了谐波性能，减小了无源滤波需求，间接提高了功率密度。.相关研究成果在国内外期刊和重要会议上发表论文8篇，其中SCI收录2篇，EI收录3篇，申请项目相关国家发明专利12项，其中获得授权6项，已转让4项。培养硕士研究生10名，其中已毕业3名。