【作者】 赵杰；

【导师】 李辉；

【作者基本信息】 重庆大学 ， 电气工程， 2017， 硕士

【摘要】 海上风电场低频输电(low frequency alternating current,LFAC)由于减小了输电频率,可以成倍地提高输电线路的输电能力,与直流输电相比,不需要建立海上换流站,极大节约投资和运维成本。模块化多电平变流器(modular multilevel converter,MMC)集成度高、输出电能质量较好、开关频率低,应用于大容量海上风电场长距离低频输电领域极具优势。然而MMC低频工况下面临桥臂环流增大,子模块损耗增大,损耗计算不准确的问题,影响其运行可靠性。为了进一步提高MMC可靠性水平,论文开展MMC损耗的改进计算方法以及桥臂环流抑制的研究。本文主要内容包括:(1)针对低频输电用MMC电容电压波动增大影响输出电能质量的问题,对电容电压波动进行抑制。首先,结合MMC的拓扑机构介绍了MMC的工作原理和改进的载波移相调制策略;其次,采用低频工况下子模块电容电压平衡控制策略对电压波动进行抑制;最后,搭建MMC低频仿真模型对抑制效果进行了验证。(2)针对MMC在低频输电工况下环流增大导致MMC损耗计算不准确的问题,提出了考虑环流的低频输电用MMC改进损耗计算方法。首先,分析了MMC器件损耗分布不均现象;其次,考虑环流对开关器件投入概率的影响,推导了开关器件的损耗的解析表达式;最后,基于PLECS/Simulink联合仿真平台搭建了21电平低频输电系统电热耦合模型,将理论计算值与耦合模型仿真值进行了对比验证。(3)针对MMC在低频输电工况下桥臂环流增大的问题,提出一种基于虚拟阻抗的环流抑制策略。首先,推导了二、四次环流幅值表达式;其次,根据环流幅值与桥臂阻抗成反比,提出了基于虚拟阻抗的环流抑制策略;最后,建立21电平低频输电用MMC及其控制模型,并与常规环流抑制方法进行比较验证。(4)为了分析基于MMC的海上风电场低频输电性能,首先建立了直驱永磁同步发电机组并入低频输电网仿真模型;其次,提出了低频输电用MMC的有功和无功解耦控制策略;最后,建立基于B2B-MMC的海上风电场低频输电系统仿真平台,对稳态情况和电网故障情况下输电系统的运行性能进行了仿真分析。论文提出的改进的损耗计算方法可以为低频输电用MMC损耗的准确计算与分析提供支持,提出的桥臂环流抑制策略可为低频输电用MMC控制策略改进提供技术支撑,建立的基于B2B-MMC海上风电场低频输电仿真平台可以为低频输电应用提供技术参考。更多还原

【Abstract】 Offshore wind farm low frequency AC transmission can increase power transfer capability according to decrease transmission frequency.There is no need for offshore converter station compared with HVDC,which can save investment and operational costs greatly.LFAC technologies will be a trend in offshore wind farm connecting to grid area.MMC is suitable for LFAC transmission for its highly modularity,low harmonic ratio and low switching frequency.However,there are some problems when MMC is used in LFAC,such as bridge arm circulating current growing,submodular power loss increasing and loss calculation inaccuracy,which affects the MMC’s operation reliability.To further improve the reliability of MMC used for offshore wind farm LFAC transmission system focusing on the power loss calculation of MMC and circulating current growing problem,the improved power loss calculation method of MMC and circulating current suppression strategy were studied in this paper.The main contents of this thesis are followings:In view of the MMC sub module capacitor voltage fluctuation aggravated affecting power quality of AC side in LFAC system,a balanced control stratgy of sub module was adopted to suppress capacitor voltage fluctuation.Firstly,operation theory is introduced combined with the topology of MMC and improved carrier phase-shifted modulation algorithm is used to modulate MMC.Secondly,a balanced control stratgy of submodular was adopted to suppress capacitor voltage fluctuation.Finally,21 levels MMC LFAC simulation model is established to verify the effectiveness of the balanced control stratgy of submodular.In view of the MMC circulating current growing under LFAC condition resulting in power loss calculation inaccuracy,an improved MMC power loss calculation method is proposed considering circulating current.Firstly,the phenomenon of MMC losses nonuniform distributions is analysed in theory.Secondly,based on the probability of the inserted sub module,average and effective values of current flowing through the four switching devices are deduced.Then MMC junction temperature can be calculated based on equivalent thermal network of IGBT modules.Finally,21 level FAC MMC simulated model is established based on PLECS/Simulink platform to verify the calculated value.As with the application of conventional module multilevel converter(MMC)in low frequency alternating current transmission(LFAC),circulating current growing may be occurred,a new MMC circulating current suppression strategy is proposed based on Virtual impedance.First,the relationship expression of 2nd and 4th circulating current amplitude with impedance and frequency is derived in this paper by analyzing circulating current generation mechanism.Then,a suppression strategy of circulating current based on virtual impedance is proposed to suppress 2nd and 4th circulating current amplitude.Finally,21 level LFAC MMC simulated model is established to verify the effectiveness of the proposed strategy compared with traditional PI circulating current suppressing method based on coordinate transformation.In order to analyse the performance of offshore wind farm LFAC transmission system with MMC,firstly,directly driven wind turbine with permanent magnet synchronous generators control model is built.Secondly,MMC decoupled control strategy of active power and reactive power is proposed to achieve active power and reactive power decoupling control and DC bus voltage stable.Finally,B2B-MMC offshore wind farm LFAC transmission system connected to grid simulation model is established to analyse the performance of offshore wind farm LFAC transmission system.Improved power loss calculation method established in this paper can provide support for the accurate calculation and analysis of MMC loss.The proposed circulating current suppression strategies can provide technical support for the improvement of the control strategy of MMC used for LFAC,as well as the improvement of the reliable operation of MMC.The established B2B-MMC offshore wind farm LFAC transmission system connected to grid simulation model can provide technical reference for LFAC applications.更多还原