模块化多电平换流器型高压直流输电的启动控制策略研究

发布时间：2018-05-07 22:15

本文选题：高压直流输电 + 电压源换流器　；参考：《湖南大学》2014年硕士论文

【摘要】：基于电压源换流器的高压直流输电（Voltage source converter-high voltagedirect current，，VSC-HVDC）采用全控型电力电子器件IGBT，取代常规直流输电中构成换流器结构的半控型器件晶闸管。该技术突出了全控型电力电子器件、电压源换流器和脉冲宽度调制三大技术特点，解决了常规直流输电中存在的波形质量较差、需要无功补偿、不能向无源系统供电等诸多瓶颈。目前已投入运行的VSC-HVDC系统大多采用两电平或三电平换流器，通过功率器件串联方式实现高电压和大容量，却存在串联器件难以均压和开关损耗较大等问题。模块化多电平换流器（Modular multilevel converter，MMC）是应用于高压直流输电的一种新型电压源换流器拓扑，每个桥臂由多个彼此独立子模块（Sub-module，SM）串联而成，子模块由IGBT和电容构成。在系统正常运行前需对SM进行预充电，使电容电压和直流母线电压由零上升到额定值，以完成MMC-HVDC系统的启动。MMC-HVDC的启动是一个复杂的暂态过程，合适的控制方式对提高启动速度、增强稳定性、抑制过电流和过电压至关重要。本文阐述了利用外部直流电源从直流侧进行预充电的方式，即他励模式，指出了这种方式存在的诸多不足。为了弥补这些弊端，本文介绍了MMC的工作原理，推导了MMC启动时的电压、电流和功率方程，建立了相应的数学模型和等值电路，为启动控制策略的研究和控制系统的设计提供了必要的理论依据。在详细分析MMC启动动态过程的基础上，本文提出了利用交流电网从交流侧对SM进行预充电的控制方式，即自励模式，并将此过程分为不可控和可控两个阶段。给出了最小限流电阻的选择方式，既避免了冲击电流又最大限度地减弱了电阻对启动速度的影响；不可控阶段结束点电压低于理论终值，避免了后期电压缓慢上升的过程；可控阶段基于载波移相调制（Carrier phase shifted modulation，CPSM），提出了电流控制、电压控制和子模块平衡控制策略。在实现对SM中电容快速充电的同时，能够有效抑制启动过程中出现的过电流和过电压，并兼顾系统的平衡，以实现可靠和高效的预充电。最后在Matlab/Simulink中搭建了5电平模型，通过电压电流双闭环控制与仅考虑电压单闭环控制的对比可以看出，本文提出的启动控制策略具有良好的有效性和可行性。
[Abstract]:Voltage source converter-high voltagedirect VSC-HVDC based on voltage source converter replaces the thyristor of half-control device in conventional HVDC transmission using full-control power electronic device IGBT. The technology highlights the three technical features of full control power electronic devices, voltage source converters and pulse width modulation, and solves the problem of poor waveform quality in conventional HVDC transmission, which requires reactive power compensation. Can not supply power to passive systems and many other bottlenecks. At present, most of the VSC-HVDC systems in operation use two-level or three-level converters to achieve high voltage and large capacity by means of power devices in series, but there are some problems such as the difficulty of voltage sharing and the large switching loss of series devices. Modular multilevel converter (MMC) is a new voltage source converter topology used in HVDC transmission. Each arm is composed of several sub-modules, Sub-Modules and SMs, which are composed of IGBT and capacitor. Before the normal operation of the system, SM should be precharged to make the capacitor voltage and DC bus voltage rise from zero to rated to complete the start-up of the MMC-HVDC system. MMC-HVDC is a complicated transient process, and the proper control mode can improve the startup speed. Enhancing stability and suppressing overcurrent and overvoltage are essential. In this paper, the precharge mode of external DC power supply from DC side, that is, the mode of external excitation, is described, and the shortcomings of this mode are pointed out. In order to remedy these disadvantages, this paper introduces the working principle of MMC, deduces the voltage, current and power equations of MMC, and establishes the corresponding mathematical model and equivalent circuit. It provides the necessary theoretical basis for the research of startup control strategy and the design of control system. Based on the detailed analysis of the dynamic process of MMC startup, this paper proposes a control method of pre-charging SM from AC side by AC power grid, that is, self-excitation mode, and divides the process into two stages: uncontrollable and controllable. The method of selecting the minimum current limiting resistance is given to avoid the impact current and weaken the influence of the resistance on the starting speed, the end voltage of the uncontrollable stage is lower than the theoretical end value, and the process of the slow rise of the late voltage is avoided. The controllable phase is based on carrier phase shifted modulation.Current control, voltage control and sub-module balance control are proposed. In order to realize the reliable and efficient precharge, the overcurrent and overvoltage in the starting process can be effectively suppressed while the capacitor in SM can be charged quickly, and the balance of the system can be taken into account. Finally, a five-level model is built in Matlab/Simulink. The comparison between voltage and current double closed-loop control and single-loop voltage control shows that the startup control strategy proposed in this paper is effective and feasible.
【学位授予单位】：湖南大学
【学位级别】：硕士
【学位授予年份】：2014
【分类号】：TM721.1

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