An AC OPF-based Heuristic Algorithm for Optimal Transmission Switching

peer reviewedThis paper focuses on reducing generators dispatch cost by means of transmission line switching. The problem is formulated as a mixed-integer nonlinear program (MINLP) optimal power flow (OPF). A scalable heuristic algorithm is proposed to break-down the complexity of the problem due to the huge combinatorial space. The algorithm aims at providing the sequence of lines to be removed from service, one at the time, until no further decrease in the dispatch cost can be obtained. It identifies the line candidate for removal at each step by exploiting the (continuously relaxed values of) lines breaker statuses at the solution of a relaxed OPF problem. The algorithm thus relies on solving a sequence of OPF problems formulated as nonlinear programs (NLPs). The effectiveness of the approach is demonstrated on the IEEE118-bus system. Results show that the approach can provide good quality sub-optimal solutions with relatively small computational effort and by removing only few lines from service

Optimal Transmission Switching in Power Systems with Large-scale Renewable Resources

In the past decade, there has been a rapid growth for renewable generations in power systems worldwide. However, the natural intermittency of wind and solar causes a variable output for renewable generations in power systems. Under this new situation, the optimal network topology of a power system can vary with time. This research focuses on an emerging topology control technology, optimal transmission switching, to improve the flexibility and efficiency of power systems with large-scale renewable generations. Novel optimization and stability enhancement approaches for optimal transmission switching are developed considering the grid uncertainties caused by the highly variable renewable generations and load fluctuation. Three major problems of optimal transmission switching are resolved in this research. First, novel optimization approaches are developed to calculate accurate switching plans for optimal transmission switching actions. Different from the existing approaches, the proposed approaches are focused on the alternating current optimal power flow for a better accuracy. New elements like renewable generations and energy storage devices are included in the optimization problems. In addition, grid uncertainties are taken into consideration and stochastic programming is used in the decision-making process. A scalable decomposition approach is proposed to solve the stochastic programming problem of the alternating current optimal power flow based optimal transmission switching. Second, transient stability issues in the transmission switching actions are investigated and transient stability enhancement methods are proposed. And a new transient stability index, critical switching flow, is proposed for transmission switching actions. Based on the new quantitative index, a preventive stabilizing redispatch scheme is developed. The proposed scheme considers the grid uncertainties in the day-ahead planning and can be applied to avoid unstable switching actions in the online operation. Third, the cyber-security issues associated with transmission switching actions are analyzed. The potential threat of false data injection attack is discussed. The cyber-attack may compromise the state estimator and make a risky switching action stable in the online stability check. As a result, a catastrophic instability will be led by the cyber-attack. The countermeasure is given to defend the cyber-attack. Numerical results on the different test systems justify the proposed approaches in this research. The developed approaches will facilitate the implementation of optimal transmission switching in the real world

Aumento de la seguridad en sistemas de potencia en condiciones no convencionales de operación mediante modificaciones topológicas de la red

RESUMEN : En este trabajo se presenta una herramienta de identificación de escenarios topológicos de la red de transmisión, que permite disminuir el número de restricciones violadas en un sistema eléctrico de potencia (SEP). Esto con el objetivo de definir nuevas estrategias para aumentar el nivel de seguridad del SEP ante condiciones de red degradada, por indisponibilidad de recursos de generación, transmisión o de los sistemas de protección. La herramienta propuesta utiliza la flexibilidad que ofrecen los diferentes tipos de configuración de subestaciones para modificar la topología de un SEP. Se plantea encontrar combinaciones de posiciones de los interruptores (abiertos o cerrados), buscando que la nueva topología de la red eléctrica brinde mayores beneficios en términos de seguridad que la red considerada inicialmente. Los escenarios identificados a través de esta herramienta se valoran en términos de aumento de seguridad y nivel de complejidad de implementación de las soluciones encontradas. La seguridad se mide como el número de contingencias N−1 que pueda soportar el SEP. Se realizó un pre procesamiento de la información asociada a las subestaciones, interruptores, seccionadores y los elementos tipo rama del SEP, con el objetivo de almacenarla en forma estructurada. Con esto se creó una plataforma desde la cual se implementaron tres métodos de búsqueda: un Aleatorio Puro, Recocido Simulado y Búsqueda Local Iterada. La aplicación de los algoritmos de búsqueda sobre el sistema de pruebas confirmó que se puede soportar un número mayor de contingencias cuando se realizan cambios topológicos; lo que significa un aumento en la seguridad del SEP respecto al caso base sin reconfiguraciones. Las simulaciones determinaron soluciones similares respecto a las acciones que se debían tomar independientemente del método de búsqueda utilizado. La forma en que se desarrollaron los métodos de búsqueda permitió realizar una exploración sobre subestaciones tipo anillo y considerar configuraciones sobre los diámetros de las subestaciones tipo interruptor y medio donde los elementos que comparten diámetro se pueden aislar de la subestación. Otro elemento diferenciador del método propuesto es la manera en la que se hace la evaluación de contingencias; pues este trabajo considera la aclaración de fallas con los interruptores adyacentes a la misma, diferente a como se considera en otros trabajos donde las contingencias se representan retirando la correspondiente rama del SEP evaluado.ABSTRACT: This work presents a tool for identifying the topological scenarios of the transmission network, which allows the reduction of violated constraints in an electrical power system (EPS). This with the aim of defining new strategies to increase the security level of an EPS when dealing with network degraded conditions, due to the unavailability of generation, transmission or protection systems resources. The proposed tool takes advantage of the flexibility offered by different types of substation configurations to modify the topology of a SEP. It is proposed to find combinations of switches positions (open or closed), looking for the resulting topology of the electrical network to provide greater benefits in terms of safety than the initially considered network. The scenarios identified through this tool are evaluated in terms of increased security, and level of complexity of implementation of the solutions found. The security is measured as the number of contingencies N−1 that the SEP can support. A pre−processing of the information associated with substations, switches, disconnectors and branch type elements of the SEP was carried out with the aim of storing them in a structured way. With this, a platform was created from which three search methods were implemented: Pure Random Search, Simulated Annealing and Iterated Local Search. The implementation of the search algorithms on the test system confirmed that a greater number of contingencies can be supported when making topological changes; which means an increase in the security of the EPS with respect to the base case without reconfigurations. Simulations determined similar solutions regarding the actions that should be taken regardless of the search method used. Way in which the search methods were developed allowed to perform an exploration on ring−type substations and to consider configurations on the diameters of the switch−type and medium−type substations where the elements that share a diameter can be isolated from the substation. Another differentiating element of the proposed method is the way in which the contingency assessment is made; since it considers clarification of faults with adjacent switches, different from how it is considered in other works where contingencies are represented by removing the corresponding SEP branch evaluated