【作者】 张执超；

【导师】 王增平；

【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】 当前互联电网网架结构日趋复杂,使系统隐性故障和连锁故障发生的概率不断增多,解列事故频发；特别对于受端电网,解列事故会导致系统失去重要输电通道或联络线,出现大容量功率缺额,如果系统紧急控制措施有限,就很可能发生系统失稳。频率崩溃是造成系统大范围停电的重要原因之一,近些年发生的多起电网崩溃都有频率失稳的因素。本文结合国家自然科学基金(50837002)课题“集中决策与分布实现相协调的大电网后备保护系统研究”,深入系统地开展了“电力系统紧急状态下切负荷控制策略研究”的论文工作,主要创新性成果如下：分析系统运行状态的分类及对应采取的安全控制措施的基础上,深入研究了系统紧急状态的特征；以构建的单机带负荷系统为研究对象,建立了包括系统频率、电压和功角状态变量的代数微分方程组；借助Matlab编程求解系统稳定运行和紧急状态下的状态方程,仿真系统分别受到有功、无功功率扰动时,系统运行状态的变化情况。提出了预防控制与紧急控制作用相结合的系统稳定控制框架；电力系统受到大的有功扰动,预测和判断系统是否将进入紧急状态具有关键作用,进而及时地采取切负荷等有效控制措施,阻止系统中有功、无功功率平衡的破坏和维护系统的稳定。基于发电机有功功率扰动和频率变化之间的转子运动平衡方程,分析建立了单机系统的负荷频率控制原理框图,进而扩展到多机及多分区系统的频率响应建模研究；构建了考虑调速器、自动发电控制和联络线功率偏差控制的系统频率响应动态模型图,推导了系统受扰后频率轨迹变化的方程,用于研究系统的频率稳定变化。研究表明,扰动初期的频率变化率只与系统初始的不平衡功率有关,功率缺额越大,频率下降速度越快；系统的稳态频率偏差受到功率缺额和频率调节效应的共同影响；频率的动态过程不仅和有功缺额的大小有关,与扰动位置,电网结构都是密切相关的。建立了具有紧急控制作用模块的系统频率响应动态模型,包括系统参数,频率保护的门槛值,系统的受扰功率,紧急控制措施和互联线路的潮流变化对频率稳定的影响。利用建立的地区系统频率响应动态模型,从中长期的角度预测系统受扰后频率的轨迹变化和频率稳定控制的可行性。提出了一个新自适应UFLS控制策略,所建立的系统频率响应动态模型应用于系统紧急控制和保护策略的制定；新策略考虑和低频调速及系统备用容量的快速释放相协调,根据系统扰动自适应地确定减负荷的数量和动作级数,避免了负荷的过切或欠切；在考虑切负荷地点时,采取就地平衡扰动功率的原则,避免潮流转移引起级联事故的发生。在研究影响系统频率和频率变化率测量的因素基础上,推导了系统受扰后的有功功率缺额与频率变化率之间的准确函数关系式。进一步,在电压灵敏度分析的基础上,提出了利用电压灵敏度确定切负荷地点与相应切负荷量的自适应切负荷控制策略,目的在于考虑切负荷过程中,有效恢复系统有功平衡的同时,最大限度的实现无功的就地平衡；在系统的脆弱点,即电压和频率下降更多或无功需求更大的地点切负荷,可以提高系统的电压稳定边缘,降低了系统崩溃的风险。提出的切负荷策略有更强的自适应性,是切负荷量,切负荷地点和动作时间的函数关系,能够考虑系统网络拓扑的变化,更有利于系统稳定性的恢复,有应用的优势和价值。更多还原

【Abstract】 The increasing risk of hidden and cascading failures and splitting of power system brings new challenge as the grid structure is more and more complicated. The power shortage threatens in particular in receiving system when the tie line failures and the system instability can happen if the emergency control is limited. Frequency collapse is one of the most important reasons for blackouts in recent years.Combined the subject’the Coordinated Research of Centralized Decision-making and Distributed Implementation in Backup Protection for Large Power Grid’(No.50837002) which is supported by the National Natural Science Foundation of China. This thesis’Study on Load Shedding Strategy for Power System under Emergency Situations’ is carried on profoundly. The main innovative results are as follows:Based on the analysis of different power system operation states and the corresponding control methods, system’s emergency state is deeply researched. Taking the single machine single load model as the research model, the algebraic differential equation is established based on the system frequency, voltage and power-angle as state variables. Matlab is used to solve the equation of states both in normal and emergency system and to simulate the operation states when system is disturbed by active power and reactive power disturbance. When a large disturbance occurs, it is important to predict and verify whether the system is in the emergency state and thereby to take effective control measures in time, such as load shedding, to prevent the system from instability and to recover both balances of active power and reactive power.By analyzing the dynamic equivalent equation of relating active power disturbance and frequency variation, logic diagram of load frequency control is established in single machine system and multi-machine system with several partitions for further study of frequency response. The dynamic frequency response diagram is clearly established, with the governor, AGC and tie-line bias control considered in the system. The trajectory equation of frequency after the system disturbance is deduced and used in the paper to predict the frequency. It is summarized that at the beginning of the disturbance, frequency change rate is only related to the initial imbalance of power. The frequency decreases faster with a bigger active power shortage and the steady-state frequency deviation is both influenced by the power shortage and frequency regulation factor. The dynamic process of frequency is not only related to the capacity of the active power shortage, but also the disturbance place and power grid’s structure. Possessing the function of emergency control, the dynamic frequency response model is built in the paper, which considers system parameters, threshold value in frequency protection and influence on frequency caused by the disturbed power and changed power flow in tie line. By the dynamic frequency response model, the trajectory of frequency and possibility to control the frequency after a disturbance are also analyzed from the mid-term and long-time perspective. Moreover, a new adaptive under frequency load shedding method is proposed in order to apply the dynamic frequency response model to analyze emergency control and system protection dynamically. the load shedding method can be coordinated with under frequency governor control, the reserve capacity and automatic generation control. The new method can calculate the amount of shed load and shedding stage according to system disturbance, thus avoiding shedding over or less. When considering the shedding location, it is necessary to obey the rule that the disturbance power should be balanced locally to avoid transfer of power flow causing cascading accidents.Considering the existing measurement problems, the factors influencing the frequency and its change rate measurement are studied and a precise equation which reflects the relationship between active power imbalance and frequency change rate is presented. Furthermore, the voltage sensitivity is considered to decide the shedding location and coresponding shedding value, aiming to enable the local reactive power balance. It is more reliable to shed the load in the weaker point, namely the node that its voltage or frequency declines faster or needs more reactive power in the disturbance. This method can improve the voltage stability margin and reduce the risk of system collapse. The proposed load shedding method is more adaptive. It is’the functional relation to shedding amount, location and action time, and the change of grid topology is considered, which is more meaningful for system’s stability.更多还原