Modeling and Control of the Modular Multilevel Converter (MMC) based Solid State Transformer (SST) with Magnetic Integration

Solid state transformer(SST)can provide more advanced functionalities compared with conventional transformer,and has great potential in smart grid application.Recently,the SST with medium frequency(MF)isolation link and magnetic integration feature has been proposed,which can reduce the system volume and thus increase the power density.However,the magnetic integration also introduces strong coupling between the line frequency(LF)and MF variables,which poses a great challenge on modeling and control issues.This paper proposes a modeling and control method for an SST with magnetic integration and mixed-frequency modulation.A mathematical model based on dual d-q references is deduced,and then a cascaded control system is designed according to the model.Parameters of the controller for the variables at one frequency are properly designed to avoid disturbance from the variables at the other frequency.The simulation and experimental results show good decoupling effect and satisfactory dynamics performance of the proposed control system.

基于比例重复控制的MMC-SST子模块电容纹波电压抑制策略

基于模块化多电平变换器的固态变压器(modular multilevel converter based solid state transformer,MMC-SST)是实现交直流混合配电网柔性互联及能量多向流动的关键装备。针对固态变压器输入级MMC子模块电容纹波电压过大,导致装置的体积和成本增加的问题,提出一种基于比例重复控制的MMC-SST改进纹波电压抑制策略。首先利用基于比例重复控制的电容电压闭环得到调整后的功率移相角。然后,通过双有源桥变换器将子模块电容纹波功率传递到低压直流母线,从而有效抑制MMC子模块的各频次纹波电压,达到减小电容值的目的。最后,仿真结果表明在网侧电压对称或不对称工况下,基于比例重复控制的MMC-SST子模块电容纹波电压抑制策略均具有良好的纹波电压抑制能力。

Development of a Medium Voltage, High Power, High Frequency Four-Port Solid State Transformer

The power and voltage levels of renewable energy resources is growing with the evolution of the power electronics and switching module technologies.For that,the need for the development of a compact and highly efficient solid-state transformer is becoming a critical task in-order to integrate the current AC grid with the new renewable energy systems.The objective of this paper is to present the design,implementation,and testing of a compact multi-port solid-state transformer for microgrid integration applications.The proposed system has a four-port transformer and four converters connected to the ports.The transformer has four windings integrated on a single common core.Thus,it can integrate different renewable energy resources and energy storage systems.Each port has a rated power of 25 kW,and the switching frequency is pushed to 50 k Hz.The ports are chosen to represent a realistic industrial microgrid model consisting of grid,energy storage system,photovoltaic system,and load.The grid port is designed to operate at 4.16 k VAC corresponding to 7.2 kV DC bus voltage,while the other three ports operate at 500 VDC.Moreover,the grid,energy storage and photovoltaic ports are active ports with dual active bridge topologies,while the load port is a passive port with full bridge rectifier one.The proposed design is first validated with simulation results,and then the proposed transformer is implemented and tested.Experimental results show that the designed system is suitable for 4.16 k VAC medium voltage grid integration.

Sliding mode control of solid state transformer using a three-level hysteresis function

The solid state transformer(SST) can be viewed as an energy router in energy internet. This work presents sliding mode control(SMC) to improve dynamic state and steady state performance of a three-stage(rectifier stage, isolated stage and inverter stage) SST for energy internet. SMC with three-level hysteresis sliding functions is presented to control the input current of rectifier stage and output voltage of inverter stage to improve the robustness under external disturbance and parametric uncertainties and reduce the switching frequency. A modified feedback linearization technique using isolated stage simplified model is presented to achieve satisfactory regulation of output voltage of the isolated stage. The system is tested for steady state operation, reactive power control, dynamic load change and voltage sag simulations, respectively. The switching model of SST is implemented in Matlab/ Simulink to verify the SST control algorithms.

电网电压不平衡下MMC-SST反馈线性化滑模控制策略

将模块化多电平变流器(modular multilevel converter,MMC)投入到固态变压器(solid state transformer,SST)系统中时通常采用比例积分微分(proportional integral derivative,PID)控制方法,但这种策略存在参数选取繁杂、动态性能较差的缺点。为了提高系统的动态性能并简化参数选取,提出了MMC-SST反馈线性化滑模控制策略。首先建立了MMC-SST整体仿真模型。然后建立了采用反馈线性化滑模控制的MMC-SST控制模型。最后利用MATLAB/Simulink平台将所提的控制方法与常规PID控制策略作了仿真比较,验证了所提反馈线性化滑模控制策略具备参数选择容易、动态性能优良的优势。

不平衡电压条件下VSG-SST控制策略

当低压配电网电压不平衡时,产生的负序分量将引起传统VSG控制中SST输出级发生电流不平衡和功率振荡现象,为此该文提出基于改进VSG的T型三电平输出级联合控制策略,以提高输出级电能质量。首先,对不平衡电压下VSG电流不平衡及功率振荡进行机理分析;其次,基于VSG控制算法和瞬时功率理论设计新型电流基准发生器,与正负序电流调节器实现级联控制,以此保障SST输出电流平衡及有功/无功功率恒定,实现不同运行工况的可靠切换;最后,对正负序分量分离方法和中点电位平衡算法进行分析研究,并对控制策略进行仿真验证。结果表明,所提联合控制策略有效可行,能保障电压三相不平衡工况下SST输出级并网电流功率质量。

基于LQIR的MMC-SST输入级控制策略研究

针对现有固态变压器(Solid-State Transformer,SST)输入级控制算法复杂,且参数整定困难等不足,文章提出一种基于线性二次型积分调节(Linear Quadratic Integrate Regulator,LQIR)的最优控制策略。首先根据固态变压器输入级的简化等效电路建立了基于dq旋转坐标系的数学模型;然后,推导了换流器交、直流侧的状态空间方程,并提出了基于LQIR控制器的新型综合控制策略;最后,在Matlab/Simulink中搭建基于模块化多电平变换器(Modular Multilevel Converter,MMC)型SST的仿真模型,并与传统比例积分(Proportion Integration,PI)控制进行对比,在几种典型测试工况下验证了所提控制策略的可行性与优越性。

Effects of high-pressure heat treatment on the solid-state phase transformation and microstructures of Cu_(61.13)Zn_(33.94)Al_(4.93) alloys

The phase transformation activation energy of the Cu61.13Zn33.94A14.93 alloys, which were treated at 4 GPa and 700 ℃ for 15 minutes, was calculated by means of differential scanning calorimetry curves obtained at various heating and cooling rates. Then, the effects of high-pressure heat treatments on the solid-state phase transformation and the microstructures of Cu61.13Zn33.94A14.93 alloys were investigated. The results show that high-pressure heat treatments can refine the grains and can change the preferred orientation from （111） to （200） of α phase. Compared with the as-cast alloy, the sample with high-pressure heat treatment has finer grains, lower β＇→β and/β→β＇ transformation temperature and activation energy. Furthermore, we found that high cooling rate favours the formation of fine needle-like α phase in the range of 5-20℃/min.

SOLID-STATE PHASE TRANSFORMATION OF WELDED METAL IN CONTROL OF WELDING DISTORTION PROCESS

Based on the tests of a build-up welding at plate edge (BWPE) and amulti-layer build-up welding on plate (MBWP), the article studies on the solid-state phasetransformations which affect welding distortion process and on the influence rule of transformationstarting temperature (TST) of welded metal to the welding residual distortion. The weldingdistortion can be decreased or controlled by the transformation volume expansion caused bysolid-state phase transformation of welded metal during the cooling. The test results of BWPE showthat when TST is at about 190 deg C, the bending distortion of welded specimen is the smallest, andits displacements at free end are decreased to 58 percent and 67 percent compared with those ofconventional welding electrodes A102 and E5015, which TST are less than room temperature and equalto 758 deg C respectively. The test results of MBWP show that when TST were at 100 approx 250 deg C.the welded specimen would appear reversible bending distortion compared with those of A102 andE5015. The maximum deflection value of reversible bending distortion in 8 mm thick plate is -2.94 mmat about 170 deg C of TST. The test results provide a valuable method to decrease or to controlwelding residual distortion.

Effect of high Pressure on the Solid-State Phase Transformation Microstructure of Cu-Zn Alloy

The solid-state phase transformation microstructure of Cu-Zn alloy under different high pressure were investigated by means of SEM and XRD. The results show that the α phase with smaller grain size, different shape and random distribution appears in the Cu-Zn alloy, when the solid-state phase transformation generation in the Cu-Zn alloy under 25～750 ℃ and 3～6 GPa high pressure, and the volume fraction of transformation phase decreases with increasing pressure, under high pressure (6 GPa), the changes of microstructure of Cu-Zn alloy is not obvious. In addition, the effect of high pressure on the solid-state phase transformation microstructure of Cu-Zn alloy was discussed.

Two Theoretical Approaches in Solid-State Nuclear Magnetic Resonance Spectroscopy

We present the theories used in solid-state nuclear magnetic resonance and the expansion schemes used as numerical integrators for solving the time dependent Schrodinger Equation. We highlight potential future theoretical and numerical directions in solid-state nuclear magnetic resonancesuch as the Chebychev expansion and the transformation of Cayley.

固态变压器中隔离双向DC/DC变换器扩展移相下虚拟电流控制策略

该文以固态变压器后级隔离双向DC/DC变换器为重点研究对象,提出一种扩展移相下虚拟电流控制策略。通过分析后级变换器在扩展移相传输功率下相移量关系以获得最小电流应力最优解来提高变换器的传输效率,减少器件损耗。同时通过虚拟电流控制思想在线估算传输功率来提高系统动态响应性能,减少电流传感器使用。最后,对该文所提出的扩展移相下虚拟电流控制与扩展移相控制进行实验对比分析,验证了扩展移相下虚拟电流控制策略具有降低电流应力,显著提高变换器的动态响应的优点。

适用于中高压配电网的MMC-SST型电能质量综合治理方法

本文提出了一种采用模块化多电平变流器的固态变压器,并将其应用于中高压配电网的电能质量综合治理。首先,分析了模块化多电平变流器的固态变压器的拓扑结构与运行特性;其次,设计了其输入级、隔离级和输出级的控制策略;最后,构建了相应的Matlab/Simulink系统仿真模型。结果表明,在网侧电压瞬变、网侧电压不平衡、负载瞬变、谐波污染等工况下,模块化多电平变流器的固态变压器能始终保持交、直流侧的电压稳定,输入侧功率因数为1,且具有电压及电流动态响应快,抗负载扰动能力强等特点,能有效解决中高压配电网存在的电能质量问题。

高压条件下锆及其合金ω相变研究进展

本文综述了锆及其合金在高压环境静水压力条件下ω相变的研究进展,系统阐述了锆及其合金固态相变类别,包括马氏体相变、块状相变、第二相析出及ω相变的机理及析出行为,重点介绍了高压诱导ω相的相转变条件、相变对组织性能的影响及高压ω相形成机制的研究现状。从研究进展可知,当压力大于3 GPa时,高压将诱导锆合金中ω相的形成;当基体中存在ω相时,微观组织发生显著细化;当含有ω相的晶粒尺寸小于100 nm时,基体硬度及强度增大,塑性减小;ω相可显著提升锆合金的超导性能;目前高压ω相形成机制存在三类观点,分歧主要集中在中间相形成及α‖ω的取向关系。基于当前研究现状,本文指出了锆合金高压诱导ω相转变机理需进一步完善,并对后续锆合金ω相的研究方向进行了展望。

大功率高频变压器关键技术与发展趋势

大功率高频变压器是固态变压器的核心部件,在未来新能源并网、海上风电、中压配网、轨道交通和数据中心供电中需求广泛。该文综述了大功率高频变压器研究的关键问题与发展趋势,论述了大功率高频铁芯的材料、损耗、结构设计与制造现状,分析了绕组材料与结构设计,指出了大功率高频铁芯与绕组的关键问题。同时讨论了高频高压绝缘与散热问题,分析了绝缘失效、可靠性与散热设计中的矛盾问题。最后阐述了大功率高频变压器在设计、测试与应用等方面研究现状,讨论了其高效高功率密度设计方法,深入分析了大功率高频变压器在效率、绝缘以及温升等性能测试方面存在的关键问题。

源荷波动时含固态变压器的有源配电网动态无功优化方法

针对分布式电源并网引起的双向潮流导致网损增大以及分布式电源、负荷的波动导致节点电压波动等问题,文章基于固态变压器(Solid State Transformer,SST)两侧电力电子变换器的脉冲宽度调制技术,提出了一种控制潮流的方法。该方法首先建立了含SST的有源配电网动态无功优化模型;然后以多时刻的有功网损和电压波动为优化目标,采用改进多目标粒子群算法对SST的一、二次侧的电力电子变换器的调制角和调制系数等多个控制变量进行求解;最后建立仿真模型并与基于有载调压变压器的有源配电网动态无功优化方法进行比较。结果证明了所提方法在降低配电网网损和维持节点电压稳定方面的优越性。

12Cr13马氏体不锈钢热变形行为及微观组织分析

基于变形温度950~1150℃、应变速率0.1~10 s^(-1)等温热压缩实验数据,构建12Cr13钢的流变应力模型和热加工图,通过研究热变形中金属的微观组织演变规律,并优化工艺,使马氏体大角度晶界以及晶界密度均显著增加。研究结果表明:在低温低应变速率(950℃/0.1 s^(-1))条件下,原子空位扩散和位错滑移困难,此时动态再结晶不充分,动态回复占主导变形机制;然而在高温中应变速率(1100℃/1 s^(-1))条件下,再结晶晶粒数目显著增多,表现为锯齿状不连续动态再结晶特征,实现了晶粒细化,提高了材料的强韧性;此时δ铁素体相占比较少,δ铁素体相呈现等轴状分布,且晶界棱角形貌稳定存在。进一步研究发现,变形温度和应变速率同样对马氏体钢固态相转变影响显著,较高的变形速率对δ铁素体相的析出产生正面影响,而变形温度的影响归功于固态相变的驱动力。

一种提升直流固态变压器全工况动态性能的新型线性化控制策略

该文针对以输入串联-输出并联的双有源桥变换器为核心的直流固态变压器的控制策略展开研究。传统的由输出电压控制和输入均压控制构成的双环控制系统,各控制环的环路增益特性会随负载等工况发生显著变化,控制器参数的设计难以兼顾全工况下的动态性能。为此,该文利用双有源桥变换器的一阶模型特性,基于直流固态变压器的大信号模型,开发了一种新型的线性化控制策略,以实现输出电压控制和输入均压控制的环路模型的线性化,从而简化控制器参数的设计,并增强变换器对不同工况的适应能力。同时,该文所提出的控制策略保留了比例-积分控制的简洁性和鲁棒性,且实现了对负载的补偿作用。仿真和实验结果证实了所提的控制策略可以显著降低直流固态变压器的环路模型和动态特性对工况的敏感程度,有利于实现全工况范围内动态性能的综合优化。

考虑电能质量优化的MMC-SST输入级控制策略

针对配电网中固态变压器(SST)网侧电压三相不平衡以及谐波污染时,电流负序分量激增影响网侧电流波形质量进而危及系统正常运行等问题,文中在SST输入级提出了一种可有效抑制电流负序分量的新型双序控制策略。首先介绍了模块化多电平变换器型固态变压器(MMC-SST)的运行原理及输入级数学模型,采用改进型二阶广义积分-正交信号发生器(SOGI-QSG)对网侧电压、电流等电气量进行正、负序分量的分离与提取。引入正交中间变量建立比例复数积分(PCI)控制器的数学模型,并在两相静止坐标系(αβ坐标系)中的正、负序模型下提出一种基于PCI的新型双序控制策略,对网侧电流的正、负序分量进行分序控制。最后,在Matlab/Simulink中搭建MMC-SST仿真系统,并与传统的比例积分(PI)控制、改进的PI双序控制进行对比,结果验证了所提基于PCI的双序控制策略的可行性与优越性。

Fe-4Al钢中团簇状有序相析出行为研究

采用热膨胀仪对低密度含铝Fe-4Al钢进行了淬火试验,即将试样加热至800℃,随后分别以1、10和100℃/s的速率淬火。采用光学显微镜、X射线衍射仪、透射电子显微镜和差示扫描量热仪研究了Fe-4Al钢的显微组织、DO3有序相的转变顺序、淬火速率对显微组织的影响及淬火加热过程中的相变行为。结果表明:Fe-4Al钢中团簇状DO3有序相按DO3→B2→A2的顺序转变;淬火速率对DO3有序相形态的影响较小,DO3有序相始终呈团簇状,但较大的淬火速率推迟A2→B2的转变。