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Published in IET Generation, Transmission & Distribution Received on 6th March 2008 Revised on 2nd July 2008 doi: 10.1049/iet-gtd:20080112
N 2 1 security in optimal power ﬂow control applied to limited areas G. Hug-Glanzmann G. Andersson
Power Systems Laboratory, ETH Zurich, 8092 Zurich, Switzerland E-mail: email@example.com
Abstract: Blackouts in recent years have demonstrated that a reliable and secure power system is a key component of an efﬁcient economy. Therefore control devices such as ﬂexible AC transmission system devices (FACTS) are placed in the system and utilised to improve the security of the system. A method to determine appropriate settings for these devices is optimal power ﬂow control. As the area of inﬂuence of a FACTS device is usually limited, it is sufﬁcient to include only a reduced area in the optimisation problem. Here, such an optimal power ﬂow problem is formulated where the considered area is deﬁned using sensitivity analysis. To include N 2 1 security as an objective, a current injection method is applied, which facilitates the determination of the system state in the case of a line outage, without having to carry out a full-load ﬂow simulation.
The overall goal of the power supply industry is to assure continuous access to electrical power of desired quality for all their costumers. In this regard, a reliable power system is indispensable. Consequently, considerable efforts are put into improving the system security of which a fundamental aspect is the consideration of possible outages and the minimisation of their consequences. To avoid major harmful incidents, preventive and corrective actions are important. Preventive actions are used to keep the system in a state where an outage in the system does not lead to cascading failures possibly resulting in a blackout, and corrective actions come into play when a failure has happened and has to be mitigated. This paper concentrates on preventive actions by formulating an optimal power ﬂow problem  to determine the optimal settings of the control devices with the objective to ensure N 2 1 security. A system is N 2 1 secure if any element in the system may fail without overloading any other element . Here, the focus lies on line outages and the prevention of overloaded lines. When a line fails, the state of the system changes and in order to investigate N 2 1 security, this state has to be 206 & The Institution of Engineering and Technology 2009
determined. One possibility is to carry out power ﬂow simulations for every considered outage. For system studies, this is an adequate procedure but if N 2 1 security should be included as an objective into an optimal power ﬂow problem, this results in a considerable increase in decision variables and constraints. Another option is to apply a current injection method where a line outage is simulated by the introduction of current injections at the buses of the system [3, 4]. For the determination of these current injections, and, consequently, the voltages and currents in case of an outage, only the initial system state before the outage is needed. An additional aspect which is discussed in this paper is the formulation of the optimal power ﬂow problem for a reduced area of the power system ....