【作者】 赵晓东；

【导师】 徐殿国； 李彬彬；

【作者基本信息】 哈尔滨工业大学 ， 电力电子与电力传动， 2024， 博士

【摘要】 海上风电具有资源丰富、风速稳定、利用小时数高等优势,成为了世界各国可再生能源发展的重要方向。近年来,随着近海风能资源开发逐渐饱和,海上风电正走向深水远海。目前远海风电主要采用中压交流汇集、高压直流送出方案,但该方案存在模块化多电平(modular multilevel converter,MMC)换流阀体积重量大、交流汇集海缆成本与损耗问题突出,难以适应海上风电机组大型化、汇集大范围化的发展趋势。中压直流汇集、高压直流送出的全直流方案可有效降低汇集损耗、海缆成本以及海上平台体积重量,是支撑海上风电大规模开发的有效途径。然而,全直流海上风电场目前仍处于探索阶段,尚缺乏关键装备的支撑。本文以“汇集装备→送出装备→全直流系统”的研究思路,以“高效率可调压风电机组、轻型化低成本海上平台”为核心目标,聚焦拓扑创新,同时对全直流系统的协同控制技术展开研究。高效率可调压风机直流变压器是直流风电机组的核心装备。针对现有谐振变换器存在电压调节时零电流软开关失效、谐振腔电压电流对输出电压波动敏感和短路故障电流易失控的挑战,本文探索基于谐振腔能量钳位的风机直流变压器。首先,从拓扑结构和运行原理出发,通过对电路运行模态的分析,揭示输入输出电压不匹配条件下的软开关机理。通过二极管对谐振电容电压的钳位作用,解决了输出电压波动导致的瞬态过电压和过电流现象,具备良好的动态抗扰特性。在此基础上,通过分析传输功率与开关频率、输入输出电压之间的关系,提出基于定脉宽变频调制的功率调控方法,解决了软开关与可调压之间的矛盾。此外,通过结合二极管钳位作用和定脉宽变频调制的恒定伏秒积特性,揭示钳位谐振风机直流变压器的故障自限流机理。所提风机直流变压器为全直流系统的构建与故障隔离提供了汇集装备支撑。送出直流变压器是实现中压直流汇集与高压直流送出的桥梁,用于将风电机组数十千伏的汇集电压提升至数百千伏送出。针对现有MMC型送出直流变压器存在体积重量大的问题,开展轻型化设计和效率优化方法研究。其中,轻型化是首要目标,通过消除输入输出直流滤波电容和减小子模块电容容值来实现,效率优化则是在满足轻型化的前提下降低电流有效值和器件损耗。首先,基于MMC型送出直流变压器的基本电压电流关系,确定电流控制自由度和优化设计准则。以消除输出高压直流滤波电容为目标,明确交流电流设计约束,基于最小化电流有效值的原则提出交流电流波形优化设计方法。然后明确满足桥臂能量平衡、输入电流平滑连续和桥臂电容容值最小化的环流设计约束,以运行效率最优为目标提出基于卡罗需-库恩-塔克条件(Karush-Kuhn-Tucker conditions,KKT条件)的非线性优化设计方法,得到环流优化设计波形。所提方法可以消除输入输出滤波电容,实现子模块电容容值最小化,同时优化运行效率,为送出直流变压器的轻型化提供了理论依据。针对MMC型送出直流变压器任意时刻总有一半子模块处于闲置状态,导致功率器件数目多、体积重量大、器件损耗高的问题,本文将晶闸管、二极管阀组导通损耗低、功率密度高、串联技术成熟的优势与子模块桥臂电压电流波形高度可控的能力相结合,提出一种混合型送出直流变压器。首先,对混合型送出直流变压器换流机理展开研究,分析子模块桥臂与晶闸管、二极管阀组的换流路径,探索通过子模块桥臂的灵活调控,实现晶闸管、二极管阀组的低应力开通和可靠反压关断,并主动限制换流过程中的电压、电流变化率。在此基础上,通过三相桥臂间的协同配合,实现对晶闸管与变压器换流过程的解耦调控。通过梯形波电流设计,实现三相电流波形相位交错,保证任意时刻三相阀组电流之和为恒定的直流电流,从而抵消输入、输出直流电流谐波,实现直流端口无滤波器设计。所提拓扑在器件成本、转换效率、体积重量等方面具有显著优势,为海上平台的轻型化提供了技术基础。基于前述章节提出的钳位谐振风机直流变压器和混合型送出直流变压器,构建全直流系统典型场景,明确系统容量、汇集线路数目以及各电压等级序列选取依据,确定全直流系统稳态控制策略。在故障保护方面,分析了全直流系统在不同直流短路故障下的特性,结合风机直流变压器和送出直流变压器固有的故障隔离特性,提出基于机械开关的故障隔离方法,实现保护设备轻型化。针对全直流系统启动问题,在电路上提出双向功率传输的混合型送出直流变压器,同时提出使直流风电机组具备功率反向传输能力的低成本电路改进方法,在控制上提出风电机组与送出直流变压器的协同启动方案,为风电机组提供启动所需能量。更多还原

【Abstract】 Offshore wind power has several advantages such as abundant resources,steady wind speeds,and high utilization hours,making it an important spotlight for the development of renewable energy worldwide.In recent years,as nearshore wind energy resources have gradually saturated,offshore wind power is advancing towards deeper water and longer distances.Currently,the medium-voltage alternating current(MVAC)collection and high-voltage direct current(HVDC)transmission scheme is the primary method for deep-sea and loang-distance offshore wind power.However,this approach faces challenges such as the large volume and weight of modular multilevel converters(MMCs),and significant cost and loss issues with AC cables,making it less suitable for the trends of high-capacity wind turbines and large-scale collection in offshore wind power development.The all direct current(All-DC)scheme utilizing medium-voltage direct current(MVDC)collection and HVDC transmission can effectively reduce collection losses,cable costs,and the volume and weight of offshore platforms,providing a promising solution for supporting the development of offshore wind power.However,the All-DC offshore wind farm is still in the exploratory stage and lacks key equipment.This dissertation adopts the research route of“collection equipment→transmission equipment→All-DC system”with the core objectives of“high-efficiency and voltage-adjustable DC wind turbines,lightweight and cost-effective offshore platforms”.It emphasizes topological innovations and investigates coordinated control technologies for All-DC system.High-efficiency,voltage-adjustable wind turbine DC transformers are the core equipment of DC wind turbines.Clamping-resonant wind turbine DC transformer is investigated to address the major challenges of existing resonant converters,including loss of zero current soft switching during voltage regulations,sensitivity of resonant voltage and current to voltage fluctuations,and divergence of short-circuit fault currents.First,from the perspective of circuit configurations and operating principles,the circuit operating modes are analyzed,revaling the mechanism for achieving soft switching under voltage mismatches.By clamping the resonant capacitor voltage through diodes,the transient overvoltage and overcurrent phenomena in the resonant tank caused by output voltage fluctuations are mitigated,demonstrating good dynamic disturbance resistance.Building on this,a power linear regulation method based on fixed pulse width variable frequency modulation is proposed,addressing the contradiction between soft switching and voltage adjustability.Furthermore,by combining the diode clamping effect with the constant volt-second integral of fixed pulse width variable frequency modulation,the proposed clamped-resonant wind turbine DC transformer intrinsically has fault current limiting capability.This study provides equipment support for the construction and fault isolation of All-DC systems.The transmission DC transformer is the bridge enabling MVDC collection and HVDC transmission,raising collection voltages of tens of kilovolts to hundreds of kilovolts for transmission.To address the issues of large volume and weight in existing MMC-based transmission DC transformers,this dissertation focuses on lightweight design and efficiency optimization methods.The primary objective is lightweight design,achieved by eliminating input and output DC filtering capacitors and reducing the capacitance of SMs.Efficiency optimization aims to reduce the root mean square(RMS)value of current and device losses while meeting lightweight design requirements.Firstly,based on the fundamental voltage-current relationship of MMC-based DC transformers,the degrees of freedom for current control and optimization design criteria are established.To eliminate the output high-voltage DC filtering capacitor,AC current design constraints are clarified,and an AC current waveform optimization design method is proposed to minimize its RMS value.Subsequently,the circulating current design constraints for achieving arm energy balance,ensuring smooth and continuous input current,and minimizing SM capacitance are established.A nonlinear optimization design method based on the Karush-Kuhn-Tucker(KKT)conditions is proposed with the goal of optimal operational efficiency,resulting in an optimized circulating current waveform.The proposed method can eliminate input and output DC filtering capacitors,minimize SM capacitance,and optimize operational efficiency,providing theoretical support for the lightweight design of transmission DC transformers.In existing MMC-based DC transformer,half of the SMs are always bypassed at any given time,resulting in a large number of power devices,large volume and weight,and high device losses.To address these issues,this dissertation proposes a hybrid DC transformer.The proposed hybrid topology combines the low conduction loss,high power density,and mature series connection technology of thyristor valve with the high controllability to voltage and current waveforms of SM arms.The research reveals the essence of the current-commutation mechanism,analyzing the commutation paths of SM arms and thyristor/diode valves.By flexibly controlling the SM arms,low-stress turn-on and reliable reverse voltage turn-off of thyristor valves are achieved,while actively limiting the voltage and current change rates during the commutation process.Furthermore,coordinated operation among three-phase SM arms achieves decoupled control of thyristor and transformer current commutation processes.The design of trapezoidal wave currents ensures that the sum of three-phase thyristor/diode currents at any time is a constant DC current,canceling input and output DC current harmonics and eliminating the need for DC filters.The proposed topology offers significant advantages in terms of device cost,conversion efficiency,volume,and weight,providing a new approach for the lightweight of offshore platforms.Based on the previously proposed clamped-resonant wind turbine DC transformer and hybrid transmission DC transformer,this dissertation constructs typical scenarios for All-DC system,defining system capacity,the number of collection lines,and the selection criteria for each voltage level,thereby establishing the steady-state control strategy for the All-DC system.Regarding fault protection,the fault characteristics of the All-DC system under different DC short-circuit faults are analyzed.Combining the inherent DC fault blocking capability of wind turbine DC transformers and transmission DC transformers,a fault isolation method based on mechanical switches is proposed to achieve lightweight protection equipment.To address the startup issue of All-DC systems,a hybrid transmission DC transformer with bidirectional power transmission capability is proposed.Simultaneously,a low-cost circuit improvement method is suggested to enable reverse power transmission capability in DC wind turbines.In terms of control strategies,a coordinated startup scheme for DC wind turbine and DC transformer is proposed to supply the energy required for wind turbine startup.更多还原