【作者】 蔡新红；

【导师】 赵成勇；

【作者基本信息】 华北电力大学 ， 电力系统及其自动化， 2014， 博士

【摘要】 模块化多电平换流器(Modular Multilevel Converter, MMC)是一种新型电压源换流器拓扑结构。相比于传统的2电平或3电平拓扑,基于MMC的高压直流输电(HighVoltage Direct Current, HVDC)系统因其开关损耗小、易于扩展、不依赖器件串联技术等优势,已成为电压源换流器型高压直流输电(Voltage Source Converter Based HVDC, VSC-HVDC)工程的建设趋势。MMC-HVDC系统控制保护策略的研究直接关系到MMC-HVDC输电系统的可靠性能及安全性能,具有明显的工程应用价值和现实意义。本文采用理论分析和仿真验证相结合的研究手段,重点研究了MMC-HVDC系统的稳态控制、故障控制、保护策略及其相关技术。(1)MMC-HVDC系统的数学模型为了研究MMC-HVDC系统的控制保护策略,建立了理想条件下和交流系统电网电压不平衡条件下MMC-HVDC系统的数学模型。基于MMC的运行工作原理,推导了理想条件下适用于暂态分析的MMC的开关函数数学模型和适用于稳态分析的低频动态数学模型。在此基础上,利用瞬时对称分量理论和同步旋转坐标关系,建立了交流系统电网电压不平衡时MMC的低频动态数学模型。并对这两种情况下MMC交、直流两侧的有功平衡关系进行了分析。数学模型的建立和功率平衡关系的分析是设计MMC-HVDC控制保护系统的基础。(2)MMC-HVDC系统控制保护体系框架控制保护系统是MMC-HVDC系统的核心之一,为了自上而下开展MMC-HVDC控制保护系统设计,构建了MMC-HVDC系统控制保护体系框架。首先阐述了MMC-HVDC控制保护系统的设计原则和主要功能,指出采用冗余配置和分层设计的必要性,其次在搭建了MMC-HVDC控制保护系统总体结构的基础上,提出了直流控制保护系统的设计和实现方案,建立了四层结构的MMC-HVDC控制保护体系框架,即直流系统控制层、极控制保护层、阀控层、子模块控制保护单元,并进一步明晰了各控制层的控制功能及各层相互关系,为直流控制保护系统的设计和研发奠定了基础。(3)(?)MMC-HVDC系统阀控层控制策略研究从MMC-HVDC系统的控制体系结构出发,重点研究了MMC-HVDC系统阀控层的MMC内部环流抑制、MMC调制及子模块电容电压平衡控制策略。为了抑制MMC内部环流,在MMC内部数学模型的基础上,采用二倍频负序旋转坐标变换和欧拉近似公式,设计了基于离散模型的环流抑制控制器(Circulating Current Suppressing Controller, CCSC),消除了桥臂电流的环流分量,减少了桥臂电流的畸变程度和电容电压的波动幅度。针对MMC的调制策略,重点介绍了最近电平逼近调制(Nearest Level Modulation,NLM)策略和载波移相调制(Carrier Phase Shifted Sinusoidal Pulse Width Modulation, CPS-SPWM)策略的原理及实现。为了均衡MMC各个子模块的电容电压,对传统排序法进行了改进,通过排序前引入不同的保持系数,改善了子模块频繁投切的状况,有效地降低了开关器件的开关频率和开关损耗。最后在PSCAD/EMTDC环境下仿真验证了所提出控制策略的有效性。(4)MMC-HVDC系统极控层控制策略研究极控层是MMC-HVDC系统的核心控制层,重点研究了理想情况下MMC-HVDC系统极控制层的控制策略。针对串级PI调节的双闭环控制系统参数调节困难、稳定工作区域小等缺点,将PWM整流控制领域中的无源控制理论应用于MMC-HVDC的控制器设计。基于MMC的无源性和欧拉-拉格朗日(Euler-Lagrange,EL)数学模型,通过阻尼注入方法,提出了由状态期望稳定平衡点、状态及状态误差设计无源控制规律的新方法,设计了内环电流无源控制器,实现了MMC-HVDC有功功率和无功功率的解耦控制。基于广泛应用于计算机控制中的离散控制,建立了旋转坐标系下MMC离散数学模型,设计了内环电流离散控制器。为了补偿离散控制的延时,设计了基于Smith预估器的内环电流离散控制器,改善了系统的暂态性能。最后对向无源网络供电的MMC-HVDC系统的整流侧和逆变侧控制器进行了设计,并在PSCAD/EMTDC环境下对上述提出的控制策略进行了验证,仿真结果表明,所提出的控制策略具有良好的动稳态控制性能,便于工程实际应用。(5)交流系统故障时MMC-HVDC控制保护策略研究MMC-HVDC交流系统故障时的控制保护策略是MMC-HVDC安全、经济运行的保障。将MMC-HVDC系统的控制策略研究从理想情况过渡到交流系统发生故障的运行工况下,研究了交流系统故障时MMC-HVDC系统的控制保护策略。首先分析了MMC-HVDC系统交流系统故障时的故障特征,以抑制负序电流为目标,设计了基于MMC欧拉-拉格朗日模型的正负序无源控制器,抑制了不对称故障引起的负序电流,仿真表明无源控制器具有良好的控制性能和限流能力。其次分析了MMC-HVDC系统两侧交流系统分别发生故障时对MMC直流电压控制的影响,并提出了相应的控制保护对策。仿真结果表明,设计的含直流电压控制环节的外环有功功率控制器和保护策略能够实现直流电压的控制和限流,提高了MMC-HVDC系统的持续运行能力及安全性。最后针对向无源网络供电的MMC-HVDC系统交流系统故障时的故障特性,提出了故障时的正负序无源控制策略及保护策略。最后在PSCAD/EMTDC环境下对所提出控制保护策略的有效性进行了仿真验证。更多还原

【Abstract】 Modular Multilevel Converter (MMC) is a new type of voltage source converter(VSC) topology, which has great potential in high voltage and large power applications. Compared to the traditional two-level or three-level VSC topologies, the MMC-based HVDC system with low requirements for power electronic devices and good scalability can be easily extended to high voltage and power levels, and has better steady-state and transient performances. So MMC has become the trend of the VSC-HVDC projects for its advantages of less switching losses and independent technology of devices in series.The control and protection strategies of MMC-HVDC directly affect its practical operations. Thus to research on the control and protection strategies is of evident application value and practical significances. In this dissertation, the control and protection strategies of MMC-HVDC and its related technologies under steady state and ac system fault conditions are investigated, by means of theoretical analysis and the simulation validations. The main contents of this dissertation are as follows:(1) The mathematical model of MMC-HVDC systemFor the purpose of investigating the control and protection strategies of MMC-HVDC, the mathematical models of MMC-HVDC under ideal conditions and unbalanced ac voltage conditions are studied and developed respectively. Under the ideal conditions, the high frequency mathematical models and the low frequency dynamic models of the MMC in the three-phase static coordinates are developed. Based on these studies, the low frequency dynamic model of MMC in d-q coordinates is established. Using the concept of the instantaneous symmetrical components, the low frequency dynamic mathematical models of MMC under unbalanced ac voltage conditions are developed in d-q synchronous reference frame. The active power balancing relation between the ac and dc sides of MMC are analyzed under both ideal and unbalanced conditions. Simulation results show that the double-frequency fluctuation will appear in the dc voltage and the active power under ac system asymmetrical fault conditions. Analysis of the mathematical model and power balancing are the basis for the design of MMC-HVDC control and protection system.(2) Control and protection system framework of MMC-HVDC systemIn order to design of MMC-HVDC control and protection system from top to bottom, the control and protection system framework of MMC-HVDC system is built. Based on the topological structure of MMC-HVDC, the design principles and main functions of its control and protection system are illustrated. Since the control and protection system is of great importance, the necessity of its redundant configuration and multi-layer design are pointed out. Its overall structure is given and a four-layer framework is proposed:dc system control layer, polar control and protection layer, valve control layer and sub-module control and protection unit. The control functions of each control layer and relationship among these layers are further introduced. The research has guiding significance for the design of MMC-HVDC control and protection.(3) Research on the control strategies of valve control layer of MMC-HVDC systemThree main important issues of valve control layer of MMC-HVDC are studied which includes modulation strategy, capacitor voltage balancing algorithm and circulating current suppression controller (CCSC). According to the inner mathematical model of MMC through a double fundamental frequency negative rotational frame to decompose the circulating currents to two DC components, a CCSC based on the inner discrete mathematical model of MMC is designed to eliminate the circulating currents among the three phase-units, and the designed CCSC can minimize the peak value and the distortion degree of the arm current significantly. The modulation principle of nearest level modulation (NLM) and carrier phase-shifted pulse width modulation (CPS-SPWM) are investigated. For the purpose of balancing the sub-module capacitor voltages, an improved capacitor voltage values based raking method was proposed to reduce the averaged switching frequency and the losses. Simulations in PSCAD/EMTDC validated its correctness and effectiveness.(4) Research on the control strategies of polar control layer of MMC-HVDC systemThe control strategies of polar control layer of MMC-HVDC under ideal conditions are studied. The control system of the MMC based on the traditional series PI regulator is analyzed, and the theory of the passive control is applied to design the controller of MMC-HVDC. Then based on the Euler-Lagrange (EL) mathematical model in synchronous d-q coordinates and its passivity, the passive control with expected state balance points, states and state errors is proposed by damp injection. The proposed control algorithm can realize the decoupled control of real and reactive power, and maintain very good static and dynamic performances. Using the theory of computer discrete control, a discrete inner-loop current controller based on Smith predictor is also designed and the decoupled control of active and reactive currents is realized rapidly. So the MMC controllers are greatly simplified and the control performances are improved. The passive controller of MMC-HVDC supplying passive networks are designed and validated under multi-conditions such as variable reactive power, voltage regulating and passive load disturbance, etc. Finally21-level MMC-HVDC model in PSCAD/EMTDC validated the effectiveness and correctness of all the proposed control methods.(5) Research on the control and protection strategies of MMC-HVDC system under ac system faultsThe control and protection strategies of MMC-HVDC are studied under ac system unbalanced conditions. First, the decoupled positive-negative sequence passive controller based on Euler-Lagrange mathematical model is proposed to suppress the negative sequence current. The controller with favorable control performance and current limitation can restrain the negative sequence current caused by unbalanced faults, and then the control performance of positive-negative sequence passive controller is compared to the traditional vector controller. Second, the influences of different ac side faults of MMC-HVDC on the dc voltage control of the MMC are analyzed. To restrain dc voltage fluctuation caused by ac system faults, an active power controller integrating dc voltage control is developed, leading to improve the sustained operation capability. Moreover the protection strategy under fault condition is proposed to ensure MMC-HVDC meet the requirements of its safety operation. The results show that the dc voltage is controlled and current limitation of MMC is realized. Finally, the control and protection strategies of MMC-HVDC supplying passive network under fault conditions are proposed. The simulation results in PSCAD/EMTDC show that the proposed controller and protection strategies have very good dynamic performances and are valuable to the practical system.更多还原