Kazuhiro Minami

A first step towards resilient graph partitioning for electrical grids

We study a graph partitioning problem for electrical grids such that a given grid is partitioned into multiple ones that are self-contained concerning electricity balance. Our goal is to find a resilient partition against time-changing power demand and supply over the year. In this paper, we investigate two graph partitioning algorithms applying them to a synthesized dataset based on realistic assumptions about Yokohama, Japan. Our initial results show that a simple algorithm, which only considers horizontal or vertical partitions, possibly produces more resilient partitions than a more general algorithm whose partitions divide a graph into subgraphs of any topology.

Pseudonym exchange for privacy-preserving publishing of trajectory data set

Anonymization is a common technique for publishing a location data set in a privacy-preserving way. However, such an anonymized data set lacks trajectory information of users, which could be beneficial to many location-based analytic services. In this paper, we present a dynamic pseudonym scheme for constructing alternate possible paths of mobile users to protect their location privacy. We introduce a formal definition of location privacy for pseudonym-based location data sets and develop a polynomial-time verification algorithm for determining whether each user in a given location data set has sufficient number of possible paths to disguise the users true movements. We also provide the correctness proof of the algorithm.

Privacy-Preserving Publishing of Pseudonym-Based Trajectory Location Data Set

Anonymization is a common technique for publishing a location data set in a privacy-preserving way. However, such an anonymized data set lacks trajectory information of users, which could be beneficial to many location-based analytic services. In this paper, we present a dynamic pseudonym scheme for constructing alternate possible paths of mobile users to protect their location privacy. We introduce a formal definition of location privacy for pseudonym-based location data sets and develop a polynomial-time verification algorithm for determining whether each user in a given location data set has sufficient number of possible paths to disguise the users true movements. We also provide the correctness proof of the algorithm.

Preventing denial-of-request inference attacks in location-sharing services

Location-sharing services (LSSs), such as Google Latitude, have been popular recently. However, location information is sensitive and access to it must be controlled carefully. We previously study an inference problem against an adversary who performs inference based on a Markov model that represents a users mobility patterns. However, the Markov model does not capture the fact that a denial of a request enforced by the LSS itself implies that a target user is visiting some private location. In this paper, we develop an algorithmic model for representing this new class of inference attacks and conduct experiments with a real location dataset to show that threats posed by the denial-of-request inference attacks are significantly real.

Flexible graph partitioning of power grids with peer-to-peer electricity exchange

We study a clustering problem for electrical grids. Our goal is to find an optimal partition that minimizes the cost of constructing a set of self-sufficient microgrids. To obtain a better solution accommodating smaller microgrids, we develop a verification algorithm that determines whether microgrids can balance their electricity surplus through electricity exchange with each other. Our preliminary results show that our proposed method can effectively reduce the construction cost of decentralized microgrids.

Resilient Community Clustering: A Graph Theoretical Approach

Many complex systems can be modeled as a graph consisting of nodes and connecting edges. Such a graph-based model is useful to study the resilience of decentralized systems that handle a system failure by isolating a subsystem with failed components. In this chapter, we study a graph clustering problem for electrical grids where a given grid is partitioned into multiple microgrids that are self-contained in terms of electricity balance. Our goal is to find an optimal partition that minimizes the cost of constructing a set of self-sufficient microgrids. To obtain a better solution accommodating smaller microgrids, we develop an efficient verification algorithm that determines whether microgrids can balance their electricity surplus through electricity exchange among them. Our experimental results with a dataset about Yokohama city in Japan show that our proposed method can effectively reduce the construction cost of decentralized microgrids.

Computational Framework of Resilience

Many researchers have been studying the resilience in urban cities. However, due to the complexity of the system involving human activities, it is difficult to define the resilience of an urban area quantitatively. We introduce an abstract model that represents an urban system through a set of variables and a utility function (or dually, a cost function) evaluating the “quality” of the states of the variables. This cost function depends on the criterion of interest for evaluating the resilience of the system, and can be easily defined in a succinct way. Then, our contribution is mainly twofold. First, we propose several performance metrics that evaluate how resilient a given system has been in some specific scenario, that is, in the past. Second, assuming we are given some knowledge about the dynamics of the system, we model its possible evolutions by embedding it into a discrete state transition machine, and show how we can adapt the performance metrics to this framework to predict the resilience of the system in the future. Such an adaptation of a performance metric to our dynamic model is called here a performance-based competency metric. This new kind of metric is useful to validate existing competency metrics (Ilmola in Competency metric of economic resilience. Urban resilience: a transformative approach. Springer, 2016) by aligning these competency metrics with our performance-based ones.