【作者】 刘俊磊；

【导师】 王钢；

【作者基本信息】 华南理工大学 ， 电力系统及其自动化， 2013， 博士

【摘要】 随着我国直流工程建设的快速推进，南方电网和华东电网已经形成了复杂的多馈入直流输电系统(multi-infeed direct current，MIDC)。在含有多条直流馈入的交直流混联大电网中，交直流系统间以及各馈入直流系统间所存在的极其复杂的相互作用关系，在故障情况下显得越发突出，其机理的揭示及相关问题的研究对于确保交直流混联大电网的安全稳定运行具有重要的理论和实用价值。本文围绕交流电网故障时，交直流系统间故障稳态相互作用分析计算方法和故障暂态过程对交流电网保护的影响机理展开研究工作。针对故障稳态，分析建立不引起换相失败的交流故障情况下直流系统等值模型，并基于其提出一种MIDC系统受端交流电网故障分析及谐波计算方法；研究直流输电系统谐波不稳定的机理以及提出直流谐波保护新判据。针对故障暂态，分析建立换相失败情况下直流系统等值工频及工频变化量电流动态相量模型，并基于其分析直流换相失败对交流电网继电保护的影响机理。主要内容如下：1．在分析交流系统故障情况下换流器动态开关特性和直流控制系统响应特性的基础上，建立了计及直流控制特性的直流系统等值模型。该模型反映了交流系统故障时由直流系统决定的直流系统注入交流系统电流的工频及各次谐波分量与换流母线电压的工频及各次谐波分量之间的关系，提供了多直流馈入交流电网故障分析所需的交直流系统间的接口模型。2．基于计及直流控制特性的直流系统等值模型，结合受端交流电网的拓扑结构和交流不对称故障的边界条件，提出了一种多直流馈入交流电网故障分析和谐波计算方法，能有效提高各种运行工况下直流系统注入交流系统电流以及换流母线电压的工频和各次谐波分量的计算精度。以CIGRE标准测试系统为基础搭建三馈入直流输电系统模型，将所提方法用于该模型的故障分析和谐波计算，并与PSCAD/EMTDC电磁暂态仿真软件的仿真结果相比较，结果表明所提方法计算简单，可靠收敛，精度较高，为MIDC系统故障分析、谐波抑制、滤波装置的配置和继电保护的整定配合等提供了定量分析依据。3．高压直流换流器谐波调制特性和换流变压器饱和特性的共同作用可以导致谐波不稳定现象的发生。对换流器的谐波调制特性和换流变压器饱和时的谐波变换特性进行分析，基于谐波在饱和换流变压器以及换流器交直流侧的闭环传变特性，提出了一种换流变压器铁心饱和型谐波不稳定判据。基于该判据，对换流器运行参数、换流变压器参数和交直流系统等值阻抗等影响因素进行了理论分析。利用电磁暂态仿真软件PSCAD/EMTDC建立测试模型证明了该判据简单，物理意义清晰，参数易得，且可定量评估换流变压器铁心饱和型谐波不稳定发生的程度，为高压直流输电系统谐波不稳定的风险评估和抑制措施等提供了理论依据。4．针对直流100Hz保护逻辑中存在识别换流阀故障和交流不对称故障的难题，在详细分析换流阀故障和交流不对称故障导致换流器电流开关函数产生工频负序分量机理不同的基础上，提出了利用两种故障情况下换流站6脉动换流器电流开关函数工频负序分量幅值比的不同来识别换流阀故障和交流不对称故障的新方法。该方法不仅能正确识别换流阀故障和交流不对称故障，而且能实现换流阀故障时故障桥的定位。基于贵广II回HVDC系统详细模型对该方法进行了仿真验证。结果表明，该方法简单可靠，具有工程实用价值。5．在交直流互联系统中，交流电网故障引发的直流换相失败可导致交流电网保护不正确动作。分析建立了换相失败情况下逆变器的开关函数模型和直流电流暂态变化模型；根据调制理论及卷积定理，推导出了换相失败情况下直流系统的等值工频及工频变化量电流的动态相量模型；通过分析表明该等值工频及工频变化量电流的变化特性有别于纯交流系统，可能造成交流电网故障引发换相失败时交流电网保护的不正确动作；最后，结合两种具体的交流电网保护，分析了直流换相失败导致交流电网保护不正确动作的机理，为交直流系统继电保护策略研究提供了理论基础。更多还原

【Abstract】 With the rapid advance of HVDC projects construction, the complex multi-infeed directcurrent (MIDC) system has been formed in China Southern Power Grid and East China PowerGrid. For MIDC system, especially in the case of ac faults, the interactions between ac systemand dc system and the interactions among each dc system are extremely complicatedprocesses. The mechanism of those processes and the study of related issue can provideimportant theoretical and practical value for the safe and stable operation of large scale ac/dchybrid system. Therefore, fault steady state interaction analysis and calculation method ofac/dc system and mechanism analysis of fault transient processes influence on ac powernetwork relay protection under ac network faults are studied in this dissertation. For faultsteady state, a dc system equivalent model under no commutation failure condition isestablished. And base on it, a MIDC system ac power network fault analysis and harmoniccalculation method is proposed. Besides, the mechanism of harmonic instability and the newcriterion of dc harmonic protections in HVDC transmission system are studied. For faulttransient processes, the dynamic phasor model of dc system equivalent power-frequencycurrent and equivalent power-frequency variation current under commutation failurecondition is deduced. And based on it, the mechanism of HVDC commutation failureinfluence on ac power network relay protection is analyzed.The main work is summarized as following:1. By consider the dynamic switching characteristics of inverter and the steady-stateresponse characteristics of dc control system under various fault-types comprehensively, a dcsystem equivalent model which considering the dc control characteristic is established. Themodel reflects the relationship between each order current which injected by dc system andeach order converter bus voltage, which determined by dc system. It provides the interfacemodel for MIDC system ac power network fault analysis and harmonic calculation.2. Base on the dc system equivalent model which considering the dc controlcharacteristic, a MIDC system ac power network fault analysis and harmonic calculationmethod is proposed with the topology structure of receiving-end ac system and the boundaryconditions of ac asymmetric fault. It can improve the computational accuracy of each ordercurrent which injected by dc system and each order converter bus voltage effectively. Basedon CIGRE model, a three-infeed HVDC power transmission model is built and the methodhas been tested widely for fault analysis and harmonic calculation. The results are comparedwith those obtained by simulation using PSCAD/EMTDC software. It is shown that the proposed method is accurate and effective, and provides the analysis basis of harmonicmitigation and protection analysis in MIDC system.3. The combined action of HVDC converter harmonic modulation characteristics andconverter transformer saturation characteristic can lead to converter transformer coresaturation instability. The author analyses harmonic modulation characteristics of converterand the frequency transformation feature of saturation converter transformer. Based on theclosed-loop variable characteristics of harmonic transfer back and forth through saturationconverter transformer and converters in the ac side and dc side, a new criterion to determinewhether the core saturation instability occurs is proposed. Based on the criterion, the effect ofthe inverter operation parameters, the parameters of the converter transformer and ac/dcsystem impedance on the core saturation instability is theoretically analyzed. The criterion isverified by dynamic simulation of several systems using PSCAD/EMTDC software. Thestudy results indicate that the proposed criterion simple, physical meaning clear, parametersaccessible, which can quantitative evaluation of harmonic instability degree of convertertransformer core saturation instability, and provides the theory basis for risk evaluation andsuppression measures of HVDC system harmonic instability.4. Discrimination between converter valve fault and ac asymmetric fault has been achallenging issue in dc100Hz protections. Based on the different mechanism in generatingpower frequency negative sequence component of converter current switching functionbetween converter valve fault and ac asymmetric fault, a new method based on powerfrequency negative sequence component amplitude ratio of converter station six-pulse bridgecurrent switching function is proposed to discriminate the converter valve fault and the acasymmetric fault. The proposed new method is not only can discriminate the converter valvefault and the ac asymmetric fault, but also can achieve the fault converter bridge locationunder converter valve fault condition. The method has been tested widely based on GuiguangII HVDC system. Result show that the proposed method is simple, reliable and has well usevalue.5. In the ac/dc interconnection system, the dc commutation failure caused by ac powernetwork fault can lead to maloperation of ac power network protection. In this paper, underthe commutation failure condition, a switching function model of inverters and a transientchange model of dc current are established. According to modulation theory and convolutiontheorem, the dynamic phasor model of dc system equivalent power-frequency current andequivalent power-frequency variation current are deduced. The analysis shows that thevariation characteristics of the equivalent power-frequency current and equivalent power-frequency variation current are different from those of pure ac system, which may leadto maloperation of ac power network protection under the dc commutation failure caused byac power network fault. At last, combining the two specific ac power network protection, themechanism for maloperation of ac power network protection caused by dc commutationfailure is analyzed, which provides a theoretical basis for research on protection strategy ofac/dc interconnection system.更多还原