The main function of the power grid is to transfer electric energy from generating facilities to consumers. To have a reliable and economical supply of electricity, large amounts of electric energy often have to be transferred over long distances. The transmission system has a limited capacity to transfer electric power, called the transfer capacity. Severe system failures may follow if the transfer capacity is reached during operation. Due to uncertainties, such as the random failure of system components, the transfer capacity for the near future is not readily determinable. Also, due to market principles, and reaction times and ramp rates of production facilities, power flow control is not fully flexible. Therefore, a transfer limit, which is below the transfer capacity, is decided and preventative actions are taken when the transfer reaches this limit. In this thesis an approach to deciding an optimal strategy for power flow control through activation of regulating bids on the regulating power market is outlined. This approach leads to an optimal definition of transfer limits as the boundary between the domain where no bid should be activated and the domains where bids should be activated. The approach is based on weighing the expected cost from system failures against the production cost. This leads to a stochastic impulse control problem for a Markov process in continuous time. The proposed method is a novel approach to decide transfer limits in power system operation. The method is tested in a case study on the IEEE 39 bus system, that shows promising results. In addition to deciding optimal transfer limits, it is also investigated how the transfer capacity can be enhanced by controlling components in the power system to increase stability. / QC 20111010

As the Internet continues to evolve, without providing any performance guarantees or explicit feedback to applications, the only way to infer the state of the network and to dynamically react to congestion is through end-to-end measurements. The emph{available bandwidth} (avail-bw) is an important metric that characterizes the dynamic state of a network path. Its measurement has been the focus of significant research during the last 15 years. However, its estimation remained elusive for several reasons. The main contribution of this thesis is the development of the first estimation methodology for the avail-bw in a network path using end-to-end measurements. In more detail, our first contribution is an end-to-end methodology, called SLoPS, to determine whether the avail-bw is larger than a given rate based on the sequence of one-way delays experienced by a periodic packet stream. The second contribution is the design of two algorithms, based on SLoPS, to estimate the mean and the variation range, respectively, of the avail-bw process. These algorithms have been implemented in two measurement tools, referred to as PathLoad and PathVar. We have validated the accuracy of the tools using analysis, simulation, and extensive experimentation. Pathload has been downloaded by more than 6000 users since 2003. We have also used PathVar to study the variability of the avail-bw process as a function of various important factors, including traffic load and degree of multiplexing. Finally, we present an application of avail-bw estimation in video streaming. Specifically, we show that avail-bw measurements can be used in the dynamic selection of the best possible overlay path. The proposed scheme results in better perceived video quality than path selection algorithms that rely on jitter or loss-rate measurements.

The issue of angle instability has been widely discussed in the power engineering literature. Many control techniques have been proposed to provide the complementary synchronizing and damping torques through generators and/or network connected power apparatus such as FACTs, braking resistors and DC links. The synchronizing torque component keeps all generators in synchronism while damping torque reduces oscillations and returns the power system to its pre-fault operating condition. One of the main factors limiting the transfer capacity of the electrical transmission network is the separation of the power system at weak links which can be understood by analogy with a large spring-mass system. However, this weak-links related problem is not dealt with in existing control designs because it is non-trivial during transient period to determine credible weak links in a large power system which may consist of hundreds of strong and weak links. The difficulty of identifying weak links has limited the performance of existing controls when it comes to the synchronization of generators and damping of oscillations. Such circumstances also restrict the operation of power systems close to its transient stability limits. These considerations have led to the primary research question in this thesis, "To what extent can the synchronization of generators and damping of oscillations be maximized to fully extend the transient stability limits of power systems and to improve the transfer capacity of the network?" With the recent advances in power electronics technology, the extension of transfer capacity is becoming more readily achievable. Complementary to the use of power electronics technology to improve transfer capacity, this research develops an improved control strategy by examining the dynamics of the modes of separation associated with the strong and weak links of the reduced transmission network. The theoretical framework of the control strategy is based on Energy Decomposition and Unstable Equilibrium Points. This thesis recognizes that under extreme loadings of the transmission network containing strong and weak links, weak-links are most likely to dictate the transient stability limits of the power system. We conclude that in order to fully extend the transient stability limits of power system while maximizing the value of control resources, it is crucial for the control strategy to aim its control effort at the energy component that is most likely to cause a separation. The improvement in the synchronization amongst generators remains the most important step in the improvement of the transfer capacity of the power system network.