研究了基于多电平变流器(MMC)的两端柔性高压直流(HVDC)输电系统.首先建立了MMC数学模型,分析了MMC内部环流产生的原因以及环流危害,接着研究了HVDC输电系统的工作原理和控制策略.系统的控制逻辑可分为3级,其中最低层控制策略为换流站内的阀级控制,包含MMC的调制方法和环流抑制策略.重点研究了换流站级的双环PI控制和阀级的环流抑制.通过搭建输电系统模型实现了正常工况下理论分析和软件仿真论证,同时设计了非正常工况下经过正负序分离后的双环PI控制方法,实现了电网电压不平衡时系统的稳定控制.

模块化多电平变流器(MMC)用于高压直流输电(HVDC)系统中时,通常采用PI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.为了提高高压直流输电(HVDC)系统的动态性能、简化参数选取,提出背靠背(B2B)型模块化多电平变流器-高压直流输电(MMC-HVDC)系统的Lyapunov控制策略.首先,建立B2B型MMC-HVDC数学模型;然后,设计Lyapunov函数控制策略;其次,在控制部分加入环流抑制、移相载波调制等,以完整地实现系统功能;最后,在Matlab/Simulink平台上验证了Lyapunov控制策略具有参数选择容易、动态性能优良的特点.

The Modular Multilevel Matrix Converter can realize the three-phase AC-AC transformation. Its outstanding advantages are easy to modularize, high reliability and low harmonic content, which can be used in high-voltage and large-capacity driven system. Aiming at the shortcomings of traditional PI control, such as slow stability, easy overshoot and poor dynamic performance, this paper proposes a new nonlinear Flatness-Based-Control strategy based on Differential Flatness theory, which has the advantages of fast response, no overshoot, no static error in tracking and high dynamic performance. It can greatly improve the control effect of M3C input-side and output-side current. And under the operating conditions of input-side frequency changes, the Flatness-Based-Control strategy can still maintain the extremely low impact of the system, and the overall control effect is better. At last, the correctness and superiority of this method are verified by experiments under different working conditions.

电力电子变压器(PET)具有瞬时功率调节、谐波抑制等优点,输电线路经过PET向无源网络供电是柔性输电的一个重要应用领域.将模块化多电平变换器(MMC)技术与PET相结合,使得PET应用于高电压、大容量的输电和配电系统成为了可能.首先,本文针对MMC-PET输入级在电网故障时产生的正负序电流,推导出基于欧拉-拉格朗日(EL)模型的内环正负序电流的无源控制器(PBC);接着,将MMC技术应用于MMC-PET中间隔离级高压侧,提高其中间隔离级的供电可靠性,并采用移相调压的控制策略;再后,针对MMC-PET输出级,设计了基于EL模型的内环电流无源控制器.最后,在Matlab/Simulink中建立一个向无源网络供电的MMC-PET系统,在电网电压出现故障时,对向无源网络供电的MMC-PET系统进行了软件仿真实验验证,实验结果表明MMC-PET系统的控制策略具有良好的动态控制性能.

针对三级式电力电子变压器(power electronic transformer,PET)中间隔离级DC-DC变换器双侧的模块化多电平换流器(modular multilevel converter,MMC)控制存在的问题,提出了基于价值函数独立的模型预测控制(model predictive control,MPC)的MMC控制方法,对多个控制目标分别建立独立的价值函数,无须配置MPC中价值函数的权重,解决了价值函数的权重配置难问题,且减小了 MPC的计算量.另外,针对现有的三级式PET中间隔离级的DC-DC变流器可靠性低、灵活度差、适应范围较小等问题,设计了基于MMC的PET中间隔离级DC-DC变换器拓扑,通过采用双高频变压器的结构,提高了供电的灵活性,实现了 PET不间断供电能力.最后搭建系统模型平台,并通过实验验证了所提控制策略的正确性和有效性.

Solid state transformer (SST) has attracted wide attention due to its advantages of active and reactive power control, harmonic cancelation, and power factor correction. The input stage of SST based on modular multilevel converter (MMC) topology can be applied to the transmission systems in the high voltage range. Under the positive and negative sequences system, this paper proposes an unbalanced-grid-fault ride-through nonlinear control method based on the input stage and output stage of MMC-SST. The control strategy of input stage aims at minimizing the fluctuation of DC voltage on the input side and reducing the negative input stage's sequence current. The control strategy of output stage focuses on controlling the active power oscillation of output stage. To further reduce the impact of unbalanced grid fault, a PI-based phase-shift control of the isolation stage during the fault is proposed. To verify the proposed control strategy, a three-stage MMC-SST model during the fault is built in Matlab/Simulink. The simulation results show that the control strategy proposed in this paper has the advantages of eliminating transient power quality problems in both grids.

模块化多电平矩阵变换器(modular multilevel matrix converter,MMMC)具备模块化的优点,能实现三相AC-AC的变换,且可用于高电压大功率的场合.但传统的载波移相调制策略在MMMC的模块数为偶数时,会导致桥臂上两两H桥子模块的输出电压发生100％的重合,从而致使桥臂上H桥子模块不能被高效使用.另一方面,桥臂上H桥子模块总输出电平数也会因此锐减,使得MMMC的工作效率低下.为了解决上述问题,提出一种新型的调制策略(即双载波调制策略)能够解决此问题且能大大提高MMMC 的输出效果.通过理论分析、推导和在MATLAB/Simulink仿真平台上进行两种调制策略的仿真对比,验证了在MMMC的模块数为偶数时,在同等条件下,采用新型调制策略的输出效果优于采用传统调制策略的输出效果,且新型调制策略仅需传统调制策略一半的模块数就能达到与传统的调制策略一样的效果.因此新型的调制策略具有更高的经济性和应用价值.

针对传统Z源逆变器(Z-source inverter,ZSI)电容电压应力大和升压能力不足等缺陷,提出了一种改进型准Z源逆变器(quasi ZSI,QZSI)拓扑结构.首先,对改进型Z源三电平逆变器的稳态工作原理进行深入分析,得到直流链输出电压和电容电压表达式,并与传统Z源三电平逆变器进行对比.理论推导出改进型Z源网络的无源参数,结合改进简单升压控制策略,说明改进型Z源逆变器整体升压能力.最后,通过在Matlab软件仿真和硬件平台上实验结果验证了改进型Z源逆变器具有良好的升压能力,降低电容电压应力以及抑制启动冲击效果.相比于传统的Z源逆变器,提出的改进型Z源逆变器具有电容电压的应力更小、输出电压的升压更高、电感电流的冲击更小等优点.

模块化多电平矩阵变换器(M3C)在风力发电中具有突出优势,可实现从低频交流电到工频交流电的AC/AC变换.为了解决比例-积分(PI)控制调节参数多、谐波含量高等问题,文中依据无源控制理论,分析了M3C输入侧数学模型的无源性及稳定性,提出其无源控制策略.通过MATLAB/Simulink建模仿真,验证了所提无源控制策略的正确性和优越性,并且模拟了输入侧频率变化、输出侧负载变化时,通过无源控制策略实现M3C的变频及变负载运行.相比于PI控制,所提无源控制响应更快、调节参数更少、谐波含量更低,整体控制效果更好.

电网电压不平衡时,电流电压波动较大,基于模块化多电平变换器(MMC)的统一电能质量调节器(UPQC)采用简单的PI控制难以调节电能质量.针对MMC-UPQC在电网电压不平衡的运行状态,提出一种基于正负序分离MMC-UPQC的微分平坦控制(DFBC)方法,它能够综合治理电压和电流的电能质量问题.首先,根据MMC-UPQC的拓扑结构,建立其在不平衡电网下的数学模型,分析MMC-UPQC的内部特性,验证MMC-UPQC的平坦性和稳定性;然后,根据正负序分离方法,采用无需锁相环方法对检测量进行分离,基于微分平坦控制理论,搭建结合前馈参考轨迹和误差反馈补偿的微分平坦控制器,并将其应用到多电平、高电压的MMC-UPQC电能质量补偿系统中,综合解决电网电压不平衡状态下的电网电能质量问题;最后,通过实验验证了基于所提微分平坦控制器的MMC-UPQC系统解决电压暂升、暂降和注入谐波问题的有效性和优越性.

To solve defects of the traditional Z-source topology, an enhanced three-level neutral-point-clamped quasi-Z-source inverter (3L NPC QZSI) topology is proposed in the paper. New topology combines the advantages of three-level neutral-point-clamped inverter with the advantages of quasi-z-source network. Compared with traditional topology, new topology can effectively enhance inverter boost capability, reduce the capacitor voltage stress and suppress start-up shock current. All the conclusion mentioned above have been confirmed by simulations and experiment.