【作者】 李爱民；

【导师】 蔡泽祥；

【作者基本信息】 华南理工大学 ， 电力系统及其自动化， 2010， 博士

【摘要】 高压直流输电线路保护的正确动作率一直较低,大量直流线路故障是由直流控制系统响应动作,造成直流闭锁,引起不必要的直流停运,给电网安全运行带来重大影响。一方面由于直流线路故障本身的复杂性,目前缺乏有效的理论分析体系和保护整定校验方法,单纯依赖实际直流工程数值仿真系统的研究模式难以揭示深层机理和作用边界;另一方面,线路故障后直流线路保护的暂态响应特性与控制系统强相关,而对直流系统这种特有复杂性对线路保护影响的研究较少。本文结合我国直流输电工程重大需求,在直流线路故障解析、控制保护动态响应特性与协调、保护新原理及方案改进等方面开展研究,选题具有重要的学术价值和工程实用意义。本文主要研究工作包括:1)在系统研究直流线路行波在故障点、模量间及线路末端的折反射规律的基础上,推导了基于无畸变传输线模型的单极和双极直流线路行波解析解,用以研究平波电抗器、直流滤波器、故障电阻等因素对直流线路行波的影响机理。研究表明:直流滤波器使行波波头由阶跃波变成衰减的振荡波,从而使其变化率和幅值都减小;过渡电阻和故障距离主要影响行波波头的幅值大小。基于实际直流工程的EMTDC仿真结果表明,本文提出的行波解析解可用于实际工程中的行波保护整定计算。2)提出了基于模量极性比较的单端行波保护新原理,有效地提高了直流线路行波保护耐受过渡电阻的能力。现有行波保护以行波波头变化率和变化幅度为判据,其定值必须大于区外故障引起的波头变化率以保证其选择性,制约了对过渡电阻和故障距离的反映能力。本文基于故障极线路与健全极线路反向行波的差异构成判据,以极性比较代替幅值比较,大大提高了对高阻接地故障的灵敏性。应用参数灵敏度分析了该保护原理的相关因素和影响机理,给出了整定原则,基于实际直流工程的EMTDC仿真结果验证了该行波保护原理的有效性。3)针对差动保护闭锁判据无法区别线路故障与区外故障的问题,提出了基于差流频谱特性差异的闭锁判据,EMTDC仿真结果表明,该判据能够准确识别线路故障与区外故障,且受过渡电阻,故障距离影响较小,有效地减少了差动保护动作时间。4)基于实际直流工程的控制系统EMTDC模型,研究了直流控制系统对线路故障的影响机理,指出整流侧低压限流的电压修正环节及暂态电流控制环节可能导致直流线路电压电流持续波动,造成线路差动保护拒动。本文提出在电压修正环节引入整流侧换流母线电压闭锁逻辑,可以有效地避免差动保护受控制系统作用的不利影响。5)结合现场故障录波与EMTDC仿真,研究了微分欠压保护动作特性和动作边界,提出将线路电流闭锁判据引入微分欠压保护,以防止其在区外交流系统连续故障时误动作。6)基于当前直流工程线路保护采样率水平和通道条件,提出了提高直流线路保护正确动作率的改进方案,在实际直流工程的详细EMTDC模型基础上,建立了包含现有方案和改进方案的EMTDC仿真平台,对比验证了改进方案的有效性。本文的研究工作得到了国家自然科学基金项目(50807017)与南方电网合作项目的资助,部分研究成果已在南方电网实际直流工程保护改造中得到应用,验证了本文工作的正确性与有效性,取得了重要的经济和技术效益。更多还原

【Abstract】 A large number of unnecessary HVDC outages are induced by the HVDC line faults because of the low level of sensitivity of HVDC line protections, leading to a significant negative influence on the safety of the power system. On the one hand, there is a lack of effective analytical theory and setting calculation method due to the complexity of HVDC line faults. It is difficult to reveal the underlying principle and the border of influencing factors just by the numerical simulation of the HVDC system. On the other hand, the transient characteristics of the HVDC line fault and thus the line protections are affected by the HVDC control system, which is very complicated and still a hard nut to crush. In this paper, the analytical theory of the HVDC line fault, dynamic characteristics of the protections and the control system and their coordination are studied and an improved protection scheme is proposed to meet the significant demand of the HVDC system, which is not only valuable in the academic area but also utility in the engineer field.The work is summarized as following:(1) Analytical treatment is applied to indicate the reflection principles at the fault position, between the modal quantities and at the terminals of the HVDC line. Based on the distortionless line model, the analytical solutions of the traveling waves on both the monopole system and the bipolar system are developed to reveal effects of the smoothing reactor, DC filters, fault resistance on the traveling waves caused by the line fault. It is shown that the wave-front of the voltage at the terminal is consist of a exponential decay component and a sinusoidal component and thus the amplitude and the derivative of the wave-front both attenuate, although the initial incident wave has a step waveform. The initial amplitude mainly depends on the fault resistance. Furthermore, the change rate of the voltage which is the dominating discriminant of the existing traveling wave protection can be obtained for setting calculation of the traveling wave protections of the HVDC line. The simulation results obtained from the EMTDC model of the real HVDC system shows the effectiveness and credibility of the setting method.(2) A novel protection principle based on the polarity comparison of modal back waves is proposed in the paper, which is effective to improve the capability of detecting the high resistance line faults. The threshold of the change rate of the traveling waves which is the dominating discriminant of the existing traveling wave protection must be greater than the ones caused by the external faults. Thus, the response of the existing traveling wave protections to the high resistance and the far distance line faults. The polarity comparison instead of amplitude comparison is used to improve sensitivity of the traveling wave protection. The simulation results based on the EMTDC show the effectiveness of the proposed protection principle.(3) The operation time of the existing DC Line Differential Protection (DLDP) is too long as the blocking criterion can’t distinguish the DC line fault from the AC system failure. A novel blocking criteria, according to different spectrum of the differential currents between the DC line fault and the AC system failure, is presented. Simulation results obtained by the EMTDC show that the proposed criteria can discriminate the line fault and the faults in the AC system availably and rarely affected by the fault impedance and the fault distance, while it does shorten the operation time of the DLDP.(4) The effect of the control system on the HVDC line faults is studied using the EMTDC model of real HVDC system. It is shown that the continuous fluctuation of HVDC line current may be induced by voltage adjusting function and transient current control function of the Voltage Dependent Current Limit (VDCL) at the rectifier, which leads to miss-trip of DLDP. For this issue, a criterion using the voltage of the commutation bus at the rectifier is proposed to avoid the unfavorable influence of the control system on the DLDP.(5) The dynamic characteristics of the Derivative and Under Voltage Protection (DUVP) is presented by means of the analysis of field data and the simulation results. The DUVP may mal-operate because of the harmonic distortion of DC voltage and in-coordination between the AC main protection and the DUVP when the cascading failure happens in the AC system. To address the problem, the DC current criteria is added to the DUVP.(6) A novel HVDC line protection scheme, based on the sampling frequency and channels of the existing HVDC line protection. A simulation system including both the existing scheme and the proposed one is developed based on the EMTDC models of an real HVDC system. The comparison of the two schemes shows the validity of this paper’s effort.This work was supported by the National Natural Science Foundation of China (50337010) and the co-operative researches with China Southern Power Grid(CSG). Some of the results has been applied in the modification of the HVDC projects in CSG, which shows effectiveness and credibility of this paper’s efforts.更多还原