【作者】 陈实；

【导师】 邰能灵；

【作者基本信息】 上海交通大学 ， 电力系统及其自动化， 2017， 博士

【摘要】 随着我国日益增长的能源需求与国家鼓励政策的实施,新能源发电系统正逐步并入电网。新能源发电系统运行方式灵活,并网方式多样,既可以采用分散式接入的形式,也可以采取大规模接入远距离传输的形式。不同于传统同步发电机组,新能源发电系统包含大量电力电子器件,其故障特性由故障期间控制策略决定,传统故障特征分析方法难以满足新能源发电系统并网后的故障分析要求。在新能源发电系统渗透率逐步提高的背景下,其复杂的故障特征给传统交流电网继电保护带来了严峻的挑战,原有保护原理以及保护整定原则对于新能源发电系统接入后的电网存在适应性问题。作为电网安全稳定运行的第一道防线,提升新能源并网后继电保护的动作性能已成为促进新能源发电系统进一步发展的关键性问题。本文选取双馈异步风电机组和逆变型电源作为典型研究对象,基于这两类电源正常运行和故障情况下的控制策略,根据并网导则的要求,研究其故障序阻抗特性。在此基础上,对含逆变型电源接入的联络线选相元件的动作特性进行了分析。结合自适应保护的特点,提出含风电及逆变型电源并网的联络线电流保护、距离保护、差动保护改进方案。具体研究内容如下:(1)分析双馈异步风电机组和逆变型电源的故障阻抗特征。根据新能源发电系统并网导则要求,新能源发电系统故障情况下需具备低电压穿越能力,并在一定程度上向系统提供辅助服务,如动态无功支撑等。本文利用Crowbar电路实现双馈异步风电机组的低电压穿越,采用正序分量控制策略实现对逆变型电源的故障后控制。在此基础上,分析了这两类电源的故障电流以及故障序阻抗特征,着重比较两类电源在正负序阻抗方面存在的差异,为后续研究提供理论依据。(2)针对逆变型电源并网联络线逆变型电源侧选相元件存在的问题,提出低电压与阻抗判断的综合选相判据。以采取正序分量控制的逆变型电源为研究对象,对含大规模逆变型电源接入的联络线系统侧和逆变型电源侧选相元件的动作性能进行分析。结果表明,系统侧相电流差突变量选相元件和对称分量选相元件能正确选相,而逆变型电源侧的两类选相元件可能误选相,为提高该侧选相元件性能,提出低电压与阻抗判断的综合选相判据。(3)引入电流保护自适应修正系数,提出自适应电流I段和II段保护方案。双馈异步风电机组在Crowbar电阻投入后正负序阻抗不再相等,传统自适应电流保护若在计算保护定值时只考虑故障类型以及系统运行状态对保护定值的影响,风电机组接入点上下游线路电流保护存在误动或拒动的可能。在电流保护自适应修正系数的基础上,提出自适应电流I段和II段保护方案,确保电流保护的正确动作。(4)提出含双馈异步风电机组的距离保护II段自适应保护及定值整定方案。由于风电机组独特的故障序阻抗特性,风电机组接入点下游线路发生不对称故障时,上游线路保护实际测量到的阻抗要比距离保护II段整定阻抗大,II段保护存在拒动的可能。在正、负、零序分支系数的基础上,结合自适应保护的实时在线计算功能,引入距离II段自适应分支系数,提出了距离保护II段的自适应保护及定值整定方案。在不失选择性的前提下,使距离保护II段在下级线路具有更大的保护范围,提高保护的灵敏性。(5)提出基于序分量的逆变型电源接入联络线电流差动保护方案。逆变型电源的弱馈特性导致传统电流差动保护在逆变型电源接入联络线区内故障时保护灵敏度低。在区内故障下,采取正序分量控制的逆变型电源联络线两端负序电流存在显著差异。通过弱化正序和零序电流在故障全电流中的作用,定义了计算电流的概念,并引入差动系数,利用该系数在区内、外故障和正常情况下的幅值差异提出基于序分量的电流差动保护方案。仿真表明,该保护方案具有较好的动作性能。最后,对本文的研究内容进行了总结,并对未来的研究方向进行了展望。更多还原

【Abstract】 With the rising energy demand and the implementation of some favorable energy policies in China,renewable energy resource systems are gradually connected to the power system.The operating modes and integration methods of renewable energy resource systems are flexible and diverse.They can be connected into the network in the form of distributed generators.On the other hand,they can be in large-scale use and be transferred over long distances.Unlike traditional synchronous generators,there are a large number of power electronic devices in the renewable energy resource system.The fault characteristics of these systems are determined by the control strategy during the fault period.It becomes difficult for system operators to analyze the fault characteristic utilizing traditional methods.With the increasing penetration of renewable sources,their complex fault characteristics bring a lot of challenges to the relay system.The existing protection schemes and setting principles may not be adaptive to the change of the power system with the connection of renewable energy resources.Relay protection is the first choice to guarantee the safety and reliability of the utility network.How to improve the performance of relays is a key problem to promote the development of renewable energy resource systems.Based on the recent grid codes,control strategies under normal operating and fault conditions for renewable energy resource systems are firstly analyzed.The renewable energy resource systems include doubly-fed induction generators(DFIGs)and inverter-interfaced generators(IIGs).The sequence impedance characteristics of these systems are unveiled.Then the thesis focuses on the behavior of the phase selectors for the transmission line emanating from IIGs.With the utilization of the adaptive protection,the current protection,the distance protection,and the current differential protection are improved respectively when the transmission line is connected with wind generators and IIGs.The rest of this thesis is as follows:1.The fault impedance characteristics of DFIGs and IIGs are summarized.All the newly released grid codes require renewable energy resource systems to have the ability of low voltage ride-through(LVRT).Meanwhile,DFIGs and IIGs should provide some ancillary services to the utility grid,such as dynamic reactive power output.Crowbar circuit is the most popular method to achieve the LVRT of wind generators,whereas the positive sequence control strategy is always adopted to control IIGs during the fault period.In this case,the fault currents and equivalent sequence impedances of these two sources are analyzed,and the difference between the sequence impedances is also described.The following proposed protection schemes are all based on the fault analysis in this chapter.2.A criterion for the phase selector on the IIG side is proposed to overcome the failure of the phase selector.This criterion is based on the low voltage and the impedance comparison.When the transmission line is connected with the positive-sequence controlled IIGs,the performance of the phase selectors on both sides of this line is analyzed.The phase selectors on the system side,i.e.,the phase selector of the sudden-change of two-phase current difference and the phase selector of the phase comparison of symmetrical components,perform well.However,the phase selectors on the IIG side fail to select the fault phase.To solve this problem,the potential criterion for the phase selector on the transmission line connected with IIGs is proposed.This criterion detects the low voltage and compares the impedance measured by the relay with the predefined value.3.With the introduction of the adaptive correction coefficients,an adaptive current protection for the instantaneous overcurrent protection and the time delay instantaneous overcurrent protection is proposed.After the crowbar circuit is activated,the equivalent positive-sequence impedance of DFIGs is not equal to the negative-sequence impedance any more.If the setting principle for the adaptive current protection just considers fault types and prevailing system parameters,those current relays are very likely to mal-operate or fail to operate.With the help of the adaptive correction coefficients,an adaptive current protection is proposed to ensure the performance of relays.4.An adaptive distance protection for the zone 2 protection is proposed when DFIGs are integrated into the grid.Due to the exclusive sequence impedance characteristic of DFIGs,the impedance seen by the zone 2 protection on the upstream line of the connection point of wind generators is greater than the predefined setting,when an unbalanced fault occurs on the downstream line.There exists a high probability that the conventional adaptive zone 2 protection of distance relays refuses to operate.Based on the positive-,negative-,and zero-sequence branch coefficients,an adaptive branch coefficient is proposed to calculate the zone 2 setting of distance relays.With the utilization of the adaptive protection,the proposed scheme can realize the online real-time calculation of setting of zone 2.Without losing the selectivity of relays,a better coverage of zone 2 can be achieved,and the sensitivity of the protection is enhanced.5.A sequence-component based current differential protection is proposed for the transmission line emanating from IIGs.Due to the weak-infeed characteristic of IIGs,the conventional current differential protection is not sensitive to the internal faults in some extreme situations.When the positive sequence control strategy is adopted to control IIGs during the fault period,there is a remarkable difference between the negative-sequence components of fault currents on both sides of the line under an internal unbalanced fault.Hence,the proposed scheme reduces the contribution of the positive-and zero-sequence components of fault currents and then defines the calculation currents on both sides of the line.With the introduction of a differential coefficient,the protection scheme discriminates the internal faults accurately from the external faults and the normal operating conditions by means of the magnitude of this coefficient.Simulation results validate the effectiveness of the proposed scheme.Finally,the conclusion of the thesis is conducted,and the prospective researches concerning the protection for transmission lines connected with renewable energy resource systems are proposed.更多还原