A novel approach to underfrequency load shedding

The research and development of underfrequency load-shedding protection tends to rely on an adaptive approach. Unfortunately, adaptive schemes are very dependent on an active power imbalance determination, based on a measurement of the frequency first derivative. Therefore, their effectiveness might be seriously reduced by a lack of data or the wrong estimation of certain parameters that are needed to accurately define the relation between the frequency first derivative and the active power imbalance. Consequently, it is reasonable to attempt to avoid having to calculate the imbalance. A novel approach to underfrequency load shedding is presented in this paper, using the frequency second derivative as a source of information for a forecast of the frequency trajectory. A Newton-method-based approximation and the interpolation of the frequency second derivative are continuously performed in order to forecast the minimum frequency value using a numerical integration.     

Scheme design considering network topology and multi-attribute decision-making for under frequency load shedding

Power system emergency control is one key defense strategy in contingencies for protecting the system from cascading blackout. Under Frequency Load Shedding (UFLS) is one such strategy to ensure system stability by shedding load to retrieve balance between power supply and demand. Novel UFLS scheme design and a scheme optimization approach are proposed in this paper to find the optimal load-shedding schemes for different network partition resulted from contingencies. To obtain all possible UFLS schemes for a certain area, a candidate scheme set design algorithm based on value assigning of scheme parameters is proposed and then a relatively complete candidate scheme set is constructed. Considering network splitting caused by protection, several subsystems may exist from the reconstruction of independent network areas. The concept of homological area is defined and a graph-based method is used to analyse system topology change. Then, a multi-attribute decision-making (MADM) algorithm is introduced for order preference by similarity to an ideal solution (TOPSIS) for global optimizing candidate schemes. Optimal schemes for an area in both isolated area and homological area cases can be derived from all feasible UFLS schemes by MADM method. Simulation results demonstrate that the UFLS schemes can effectively restore system frequency in different network topologies.     

Application of long-term simulation programs for analysis of systemislanding

This paper describes the main results and conclusions from the application of two different long-term stability programs to the analysis of a system islanding scenario for a study case developed by Red Electrica de Espana (REE), based on the Spanish system. Two main goals were to evaluate the performance of both simulation programs in the long-term stability area, and to analyze the influence of some important control and protection elements (tie-line loss-of-synchronism relays, underfrequency load-shedding, load-frequency control, and power plant dynamics). Conclusions about modeling and computational requirements for system islanding (frequency) scenarios and use of long-term stability programs are presented     

Power system computer controlled load shedding

System conditions leading to power deficits and their identification are analyzed and the methods for load-shedding calculations are presented. Different criteria on which to base the decision-making process are discussed with the aim of defining an overall load-shedding protection scheme, based on real-time system conditions as processed by computer control centers. The basic theory to develop a new concept of load shedding is presented. Load-shedding schemes oriented to minimize the frequency transient fall compatible with limited amounts of disconnected load are sought. Real-time measurements of loads are used in a process to select, for each block of power, those feeders to be disconnected in an eventual emergency. The concept of area transient error (ATE) is introduced in order to look at the possibilities of using it as an indicator of system condition. A first load-shedding scheme applied in one area is based on the ATE. A second scheme, used as a backup, is applied throughout the system and is based on the frequency decay rate and a third crierion based only on power is presented. Tests on a sample system show the effectiveness of the proposed control strategy.     

Protective relay setting of the tie line tripping and load sheddingfor the industrial power system

This paper presents the proper underfrequency relay settings to enhance the operation of industrial power systems with cogeneration facilities. A cogeneration unit was installed in the plant in 1996 to supply the power demand of the plant. The cogeneration unit always faces the shutdown problem when a severe fault occurs on the nearby Taipower network. It is, therefore, necessary to investigate plant protective relay settings to prevent a whole plant blackout when the contingency occurs. A transient stability analysis has been performed by considering both the detailed models of the cogenerators with the governor and exciter control systems and the external utility power system. The underfrequency relay settings for tie line tripping and load shedding are designed to prevent the tripping of the cogeneration units so that the electricity service to the critical loads of the industrial customer can be maintained in case serious external disturbances such as short circuit faults occur. In this study, the scheme of underfrequency relay settings has been developed for the power system of a large synthetic rubber manufacturer with a large cogeneration unit. Three study cases have been selected for the transient stability analysis to verify the effectiveness of the proposed protective relay setting     

冰灾期间贵阳地区低周减载方案

讨论了低周减载对于电网安全稳定的重要性以及贵阳供电局调度中心制定低周减载方案的技术。实践证明,在冰灾期间根据电网情况及时地调整低周减载方案是确保电网安全稳定运行的重要的手段。     

Optimal load-shedding using local information

This paper describes an optimal load-shedding policy based on quadratic programming using only some power system state variables after severe generation outages. When generation outage is severe, the imbalance between supply and demand causes a declining frequency. Utilities generators cannot be operated excessively at frequencies above normal. Some of the load must be shed to prevent system damage. Optimal load-shedding policies have been studied using all state variables of the power system. However, in real power system operations, it is difficult to obtain remote information in that fraction of a second following generation outages. An optimal load-shedding method is constructed in this research using quadratic programming (QP) under the assumption that all power system state variables can be available. However, these state variables cannot be easily accessible (as already mentioned). A suboptimal load-shedding scheme based on the local state variables is then studied. The proposed load-shedding method is based on the aforementioned optimal one. The change in line-power flows and the amount of generation power outage are used as accessible data at each load point. Incorporating this local information into the optimal loadshedding method based on the QP method, the proposed load-shedding method is established here. The effectiveness of this proposed method is illustrated by two examples and simulation results on a model power system show that the method is encouraging.     

广州电网复杂故障阻隔策略研究

广州电网作为特大型中心城市电网对供电可靠性要求很高。梳理了可能导致广州电网大面积停电的三类风险并提出了应对策略。针对多回直流同时闭锁失步解列装置拒动导致系统失步的风险,提出了主动解列片区电网的措施;针对500 k V变电站全停故障下孤网频率失稳风险提出了联切负荷策略。针对线路三相短路主保护拒动局部电网失稳的风险提出了解列机组的措施。基于实际电网的算例验证了所提策略的有效性。     

Imperialistic Competitive Algorithm Based Unit Commitment Considering Risk of Cascading Blackout

Abstract—The generation scheduling of different units in the power system is one of the procedures that affects the cascading blackout. Generators are usually scheduled based on the solution of the unit commitment problem, which should minimize the running cost of the power system. In this article, a new objective function is presented to consider the load-shedding effect on the power generation scheduling. The simulations are carried out in MATLAB software (The MathWorks, Natick, Massachusetts, USA), showing that it may lead to high risk of blackout in some conditions if the generation scheduling is done only by considering the cost of generation.     

基于模糊多属性决策的黑启动方案优选

为在电力系统恢复初期从多个可行的黑启动方案中选出最优方案,应用模糊多准则决策理论和方法建立了方案优选模型。由于将黑启动方案的评价指标体系中的定性和定量指标及权重全部转化为三角形模糊数,克服了定性与定量评价指标不具可比性的缺点。采用模糊折衷型决策算法,对惠州电网的多个黑启动方案排序,选择出了最佳的黑启动方案。其计算结果和调度人员的经验相符合,表明用模糊多属性决策方法对黑启动方案的优化可行。该法可为快速做出各种恢复决策提供科学、直观的决策参考。     

An innovative PC based simulator for power system studies

This paper describes a PC based simulator for training students/engineers in the area of power system control and operation. The simulator models a generation and transmission system to meet a prespecified daily demand with an objective of minimizing the system operating cost. Transmission constraints, overcurrent relays, frequency dynamics and load-shedding schemes are also incorporated as part of the simulator. Sample training scenarios and an auto-assessment based on a prespecified grading scheme are presented.     

基于递归子问题树法的含DG配电网最优孤岛搜索

配电网发生大面积停电时,单个或几个分布式电源形成孤岛是保障重要负荷、提高系统可靠性的重要手段。为此,提出将孤岛搜索问题转化为若干个祖先约束背包问题（PCKP）,在配电网树模型中直接引入离散可卸负荷,利用基于子问题树的递归算法求解得到初始孤岛;并对初始孤岛进行校验和融合,形成最优孤岛方案。校验中考虑了孤岛的潮流、电压和电压稳定约束。该算法能同时兼顾连通性和最优性,形成的孤岛能有效保障重要负荷不失电,且复杂度只与节点数呈数性关系。通过算例证明了该算法的有效性。     

Operation and management of the electric system for industrialplants: an expert system prototype for load-shedding operator assistance

This paper presents the analysis carried out for designing and developing a prototype expert system capable of managing the electrical system of an industrial plant. The electric network of the plant, including significant gas turbine and steam turbine generation facilities, is presented. The modeling phase of the system for successive analysis by simulation using a commercial power system code is also reported. The load-shedding problem is then considered and the related engineering aspects are briefly recalled. The guidelines for the realization of a load-shedding scheme for industrial electrical plants based also on advanced artificial intelligence tools such as expert systems are proposed. The expert system is used online to prepare lookup load-shedding tables capable of accounting for actual process conditions. Offline validation of the proposed shedding is then operated by static and/or dynamic simulation. The use of a prototype rule-based expert system is expected to permit the achievement of a better flexibility in the realization of load-shedding schemes for improving the process continuity and management     

基于WAMS的自适应低频减载动态优化策略

在《电力安全事故应急处置和调查处理条例》(599号令)中,提出了在故障处理过程中的负荷切除量等同于故障损失负荷量,这要求电网在安全稳定运行的前提下尽可能地少切或不切负荷,以降低事故控制代价。因此,提出计及负荷频率调节效应、P-V特性和负荷重要度的减载贡献因子来有效指导低频减载过程中的选址定容。通过分析电压突变因素对不平衡功率的影响以提高功率缺额计算式精度,依据系统频率变化率的梯度变化逐轮次地动态优化减载量,以期充分发挥系统频率的自恢复调节能力。IEEE 39节点系统仿真分析表明,所提出的低频减载动态修正优化策略,能够在减少切负荷控制代价的同时改善系统频率恢复水平,从而兼顾频率紧急控制的经济性与可靠性要求。     

天津电网黑启动中启动厂用负荷的仿真校验

现代电网的一些潜在威胁若处理不当,可能会诱发恶性连锁反应而导致系统崩溃。近些年来,国内外大停电事故的频繁发生,为我国电网的运行安全敲响了警钟。电力公司除了加强电网调度和安全运行管理外,还应及时地开展电力系统黑启动的试验研究。电力系统全停后的黑启动恢复是一个非常重要的问题,黑启动过程与正常运行方式有极大的不同,针对天津电网黑启动方案,对被启动电厂中大型辅机启动时对小系统的冲击、带厂用负荷时对系统频率和电压的影响做了详细的仿真计算,并给出了图形化的结果。通过仿真计算校验了天津电网黑启动方案的可行性,为日后的现场试验和方案的顺利实施提供了依据和建议。同时也可以为其他电力公司黑启动预案的设想和制定提供有效的参考。     

A shot in the dark

Investigations of recent blackouts indicate that the root cause of almost all of these major power system disturbances is voltage collapse rather than the underfrequency conditions prevalent in the blackouts of the 1960s and 1970s. The operation of today's power system with generation frequently being remote from the load centers has made the power system very dependent on the utility transmission system. When transmission lines trip, voltage drops at the load center, whereas frequency may remain normal until a complete system collapse occurs. Utilities have begun to recognize this problem and are installing UVLS programs. Also the industrial customers that have in-house generation that operates in parallel with the utility need to recognize the problem. These industrial cogeneration customers should consider the use of undervoltage separation schemes in addition to their existing underfrequency schemes to address the voltage collapse scenario. This article proposes undervoltage separation logic schemes that can be easily installed within digital relays to enhance the security of an undervoltage separation to prevent false operation due to slow-clearing system faults. The article also indicates the required point of installation of these relays so that they properly measure system voltage.     

基于DEA/AHP模型的电力系统黑启动有效方案评估

电力系统全部停电后的黑启动过程是一个非常复杂的问题,科学合理的黑启动方案对于大停电后的快速恢复有着重要作用.把基于数据包络分析(DEA)的层次分析法(AHP)(DEA/AHP)模型用于电力系统黑启动方案的评估排序.该方法充分利用DEA和AHP的特点,避免了人为设定AHP判断矩阵的主观性和随意性,克服了DEA不能在一个尺度上对方案进行全排序的缺点.针对天津电网,用该方法对候选黑启动方案进行评估,得出了最优方案,验证了该方法的可行性和有效性.     

基于输电断面N-1静态安全潮流约束的联切负荷方案

电网中的一些输电断面受线路安全电流限制不能满足N—1原则时，为了保障系统的安全稳定运行，一般采取线路过载联切负荷措施。文中将基于输电断面N—1静态安全潮流约束的联切负荷方案分解为两个子问题，即确定联切负荷节点问题和实施联切负荷措施问题，基于直流潮流给出了整数规划数学模型及算法。算例采用实际电网中的联切负荷方案制定问题，结果表明，提出的分析方法简捷、可行，能够适用于实际系统。     

A New Optimal Under-frequency Load-shedding Method Using Hybrid Culture–Particle Swarm Optimization–Co-evolutionary Algorithm and Artificial Neural Networks

Optimum under-frequency load shedding during contingency situations is one of the most important issues in power system security analysis; if carried out online fast enough, it will prevent the system from going to a complete blackout. This article presents a new fast load-shedding method in which the amounts of active and reactive power to be shed are optimized with a dynamic priority list by using a hybrid culture–particle swarm optimization–co-evolutionary algorithm and artificial neural network method. The proposed method uses a five-step load-shedding scenario and is able to determine the necessary active and reactive load-shedding amounts in all steps simultaneously on a real-time basis. An artificial neural network database is established by using offline N – K (K = 1, 2, and 3) contingency analysis of the IEEE 118-bus test system. The Levenberg–Marquardt back-propagation training algorithm is used for the artificial neural network, and the training process is optimized by using a genetic algorithm. The artificial neural network database is updated based on new contingency events that occur in the system. The simulation results show that the proposed algorithm will give optimal load shedding for different N – K contingency scenarios in comparison with other available under-frequency load-shedding methods.