Cascading Failures in Power Grids - Analysis and Algorithms

This paper focuses on cascading line failures in the transmission system of the power grid. Recent large-scale power outages demonstrated the limitations of percolation- and epid- emic-based tools in modeling cascades. Hence, we study cascades by using computational tools and a linearized power flow model. We first obtain results regarding the Moore-Penrose pseudo-inverse of the power grid admittance matrix. Based on these results, we study the impact of a single line failure on the flows on other lines. We also illustrate via simulation the impact of the distance and resistance distance on the flow increase following a failure, and discuss the difference from the epidemic models. We then study the cascade properties, considering metrics such as the distance between failures and the fraction of demand (load) satisfied after the cascade (yield). We use the pseudo-inverse of admittance matrix to develop an efficient algorithm to identify the cascading failure evolution, which can be a building block for cascade mitigation. Finally, we show that finding the set of lines whose removal has the most significant impact (under various metrics) is NP-Hard and introduce a simple heuristic for the minimum yield problem. Overall, the results demonstrate that using the resistance distance and the pseudo-inverse of admittance matrix provides important insights and can support the development of efficient algorithms

Quest for Knowledge and Pursuit of Grades: Information, Course Selection, and Grade Inflation at an Ivy League School

This paper exploits a unique natural experiment — Cornell University’s 1996 decision to publish course median grades online - to examine the effect of grade information on course selection and grade inflation. We model students’ course selection as dependent on their tastes, abilities, and expected grades. The model yields three testable hypotheses: (1) students will tend to be drawn to leniently graded courses once exposed to grade information; (2) the most talented students will be less drawn to leniently graded courses than their peers; (3) the change in students’ behavior will contribute to grade inflation. Examining a large dataset that covers the period 1990-2004 our study provides evidence consistent with these predictions

Employment Restrictions and Political Violence in the Israeli-Palestinian Conflict

Following the outbreak of the Second Intifada in 2000, Israel imposed severe restrictions on the employment of Palestinians within its borders. We study the effect of this policy change on the involvement of West Bank Palestinians in fatal confrontations during the first phase of the Intifada. Identification relies on the fact that variation in the pre-Intifada employment rate in Israel across Palestinian localities was not only considerable but also unrelated to prior levels of involvement in the conflict. We find robust evidence that localities that suffered from a sharper drop in employment opportunities were more heavily involved in the conflict

Experimental Evaluation of Large Scale WiFi Multicast Rate Control

WiFi multicast to very large groups has gained attention as a solution for multimedia delivery in crowded areas. Yet, most recently proposed schemes do not provide performance guarantees and none have been tested at scale. To address the issue of providing high multicast throughput with performance guarantees, we present the design and experimental evaluation of the Multicast Dynamic Rate Adaptation (MuDRA) algorithm. MuDRA balances fast adaptation to channel conditions and stability, which is essential for multimedia applications. MuDRA relies on feedback from some nodes collected via a light-weight protocol and dynamically adjusts the rate adaptation response time. Our experimental evaluation of MuDRA on the ORBIT testbed with over 150 nodes shows that MuDRA outperforms other schemes and supports high throughput multicast flows to hundreds of receivers while meeting quality requirements. MuDRA can support multiple high quality video streams, where 90% of the nodes report excellent or very good video quality

Supporting Vulnerable Communities: Strengthening Nonprofits Before and Immediately after a Catastrophic Disaster

The Foundation recognizes that nonprofits play a key role in disaster relief and recovery for vulnerable communities and that many of these organizations will serve as "first responders" because they are already trusted resources in these communities through their daily provision of safety net services. To enable the Foundation to help meet the immediate relief needs of vulnerable communities in the aftermath of a disaster, it developed agreements with key social service grantees for rapid, almost automatic, grantmaking during the initial post-disaster period when communication systems are compromised and needs assessments have not yet been conducted. Additionally, to increase the likelihood that these organizations would be in a position to deliver services and utilize these funds, the Foundation sought their commitment to disaster planning and offered technical assistance to support them in their efforts