· · · Institutional Repository

Power sector vulnerability has been a key issue in society for over a decade. A component failure may trigger cascades of failures across the grid and lead to a large blackout. Complex Network approaches have shown a direction to study some of the problems faced by power grids and it is a continuing challenge thus far. Power grids have been studied for their structural vulnerabilities using purely topological approaches. A purely topological approach assumes that flow of power is dictated by shortest paths. However, this fails to capture the real flow characteristics of power grids. We have proposed a flow redistribution mechanism that closely mimics the flow in power grids using the \ac{PTDF}. With this mechanism we enhance the already existing cascading failure models to study the vulnerability of power grids. We apply the model to the European high-voltage grid to carry out a comparative study for a number of centrality measures. `Centrality' gives an indication of the criticality of network components. Our model offers a way to find those centrality measures that give the best indication of node vulnerability in the context of power grids, by considering not only the network topology but also the power flowing through the network. In addition, we use the model to determine the spare capacity that is needed to make the grid robust to targeted attacks.

Reliability evaluation of power system substations is of significant importance when performing asset management. Most of the studies about substation reliability only focus on the substation connectivity. The reaction of protection system is fully neglected, which cannot be true in reality. Failures of the protection system failure or the circuit breakers do have an effect on the substation reliability. In this thesis, the substation reliability with respect to protection failures is evaluated using the event tree method. The basic protection principles for substations are explained first. Then, the event tree analysis is also introduced. Two case studies will be analyzed in this thesis. The effects of different substation configurations on the reliability is analyzed and compared. Then, the reliability of a real substation, Maasvlakte 380kV substation in the Netherlands, will be evaluated using event tree methods. The failure results will be combined with a load flow scenario of Maasvlakte substation in 2020, and indices such as the average lost load, and maximum lost load will be given.

Wireless sensor and actuator networks (WSANs) suffer from interference making them unfeasible for actuators that require reliability. This asks for an IEEE 802.15.4 behavior analysis for building automation actuators and a robust WSAN solution. This thesis uses the IEEE 802.15.4 based JenNet communication stack for experiments to define, measure, and create robustness. Failures are classified between soft and hard to identify the impact on the system. Equations are introduced to show the failure probabilities based on packet arrival probabilities. Experiments show the impact of interference on the failure rate with an increased failure rate during office hours, and a ratio between hard and soft failures ranging from 1:5 to 1:25 for single hops depending on the link quality. A setpoint based heartbeat solution is proposed that solves hard failures and copes with soft failures. Equations show the impact of different heartbeat properties on the performance of the heartbeat solution. The solution is implemented and experiments show that it meets the robust properties required by WSANs. To make a WSAN predictable and adaptive to its environment, future implementations could monitor the environment and reconsider timing properties, based on gathered data and hopcount.

This article assesses the risk to life for the Natomas Basin, a low-lying, rapidly urbanizing region in the Sacramento-San Joaquin Delta in California. Using an empirical method, the loss of life is determined for a flood (high water), seismic, and sunny-day levee breach scenario. The analysis indicated that more than 1000 fatalities may occur in the flood scenario and that there is a high flood risk compared to similar systems (such as dams and flood-prone areas in the Netherlands). Findings show that risk to life highly depends on evacuation effectiveness. The evacuation and emergency management system (EEM) was further analyzed through interviews with regional emergency managers and training exercise evaluation reports. Using an analytic framework, critical factors that affect EEM performance and reliability were identified. Results indicate a need to assess EEM performance to improve preparedness and reduce the risk to life. Findings from the investigation contribute to more integrated risk analyses of both the technical and management components for engineered systems.

We present an integrated analysis of bank erosion in a high-curvature bend of the gravel bed Cecina River (central Italy). Our analysis combines a model of fluvial bank erosion with groundwater flow and bank stability analyses to account for the influence of hydraulic erosion on mass failure processes, the key novel aspect being that the fluvial erosion model is parameterized using outputs from detailed hydrodynamic simulations. The results identify two mechanisms that explain how most bank retreat usually occurs after, rather than during, flood peaks. First, in the high curvature bend investigated here the maximum flow velocity core migrates away from the outer bank as flow discharge increases, reducing sidewall boundary shear stress and fluvial erosion at peak flow stages. Second, bank failure episodes are triggered by combinations of pore water and hydrostatic confining pressures induced in the period between the drawdown and rising phases of multipeaked flow events.

Piping is an important failure mechanism of flood defense structures. A dike fails due to piping when a head difference causes first the uplift of an inland blanket layer, and subsequently soil erosion due to a ground water flow. Spatial variability of subsoil parameters causes the probability of piping failure to increase, often to unacceptable levels. The general research question is: How can we incorporate spatial variability in a flood defense system design dealing with the piping failure mechanism? The question in solved in three steps: first by quantifying the spatial variability in subsoil parameters, second by assessing the influence of this spatial variability on the piping mechanism and third by analyzing optimal decisions to deal with unacceptable situations. There are two new models presented in this thesis. The first model, is a simple design model that uses historical failures to assess the piping safety. The second model describes the formation of piping erosion paths in spatial variable soils. Especially the second model might potentially lead to improvements in piping modeling and potentially in cost reductions. The main conclusions from this thesis is that the piping mechanism and influence of spatial variability in the subsoil are a very significant threat to flood defenses. However, performing local soil measurements in combination with local dike improvements can be a cost-effective method to deal with unacceptable piping failure probabilities.

With the increasing adoption of distributed systems in both academia and industry, and with the increasing computational and storage requirements of distributed applications, users inevitably demand more from these systems. Moreover, users also depend on these systems for latency and throughput sensitive applications, such as interactive perception applications and MapReduce applications, which make the performance of these systems even more important. Therefore, for the users it is very important that distributed systems provide consistent performance, that is, the system provides a similar level of performance at all times. In this thesis we address the problem of understanding and improving the performance consistency of state-of-the-art distributed computing systems. Towards this end, we take an empirical approach and we investigate various resource management, scheduling, and statistical modeling techniques with real system experiments in diverse distributed systems, such as clusters, multi-cluster grids, and clouds, using various types of workloads, such as Bags-of-tasks (BoTs), interactive perception applications, and scientific workloads. In addition, as failures are known to be an important source of significant performance inconsistency, we also provide fundamental insights into the characteristics of failures in distributed systems, which is required to design systems that can mitigate the impact of failures on performance consistency.