Author

# Christoforos N. Hadjicostis

Other affiliations: University of Illinois at Urbana–Champaign, National Renewable Energy Laboratory, Massachusetts Institute of Technology ...read more

Bio: Christoforos N. Hadjicostis is an academic researcher from University of Cyprus. The author has contributed to research in topics: Distributed algorithm & Finite-state machine. The author has an hindex of 41, co-authored 307 publications receiving 6605 citations. Previous affiliations of Christoforos N. Hadjicostis include University of Illinois at Urbana–Champaign & National Renewable Energy Laboratory.

##### Papers published on a yearly basis

##### Papers

More filters

••

TL;DR: The topology of the network completely characterizes the resilience of linear iterative strategies to this kind of malicious behavior and it is shown that node xi is guaranteed to be able to calculate any arbitrary function of all node values when the number of malicious nodes is f or less.

Abstract: Given a network of interconnected nodes, each with its own value (such as a measurement, position, vote, or other data), we develop a distributed strategy that enables some or all of the nodes to calculate any arbitrary function of the node values, despite the actions of malicious nodes in the network. Our scheme assumes a broadcast model of communication (where all nodes transmit the same value to all of their neighbors) and utilizes a linear iteration where, at each time-step, each node updates its value to be a weighted average of its own previous value and those of its neighbors. We consider a node to be malicious or faulty if, instead of following the predefined linear strategy, it updates its value arbitrarily at each time-step (perhaps conspiring with other malicious nodes in the process). We show that the topology of the network completely characterizes the resilience of linear iterative strategies to this kind of malicious behavior. First, when the network contains 2f or fewer vertex-disjoint paths from some node xj to another node xi , we provide an explicit strategy for f malicious nodes to follow in order to prevent node xi from receiving any information about xj's value. Next, if node xi has at least 2f+1 vertex-disjoint paths from every other (non-neighboring) node, we show that xi is guaranteed to be able to calculate any arbitrary function of all node values when the number of malicious nodes is f or less. Furthermore, we show that this function can be calculated after running the linear iteration for a finite number of time-steps (upper bounded by the number of nodes in the network) with almost any set of weights (i.e., for all weights except for a set of measure zero).

500 citations

••

TL;DR: In this article, the authors propose an architecture for voltage regulation in distribution networks that relies on controlling reactive power injections provided by distributed energy resources (DERs), where a local controller on each bus monitors the bus voltage and, whenever there is a voltage violation, it uses locally available information to estimate the amount of reactive power that needs to be injected into the bus in order to correct the violation.

Abstract: In this paper, we propose an architecture for voltage regulation in distribution networks that relies on controlling reactive power injections provided by distributed energy resources (DERs). A local controller on each bus of the network monitors the bus voltage and, whenever there is a voltage violation, it uses locally available information to estimate the amount of reactive power that needs to be injected into the bus in order to correct the violation. If the DERs connected to the bus can collectively provide the reactive power estimated by the local controller, they are instructed to do so. Otherwise, the local controller initiates a request for additional reactive power support from other controllers at neighboring buses through a distributed algorithm that relies on a local exchange of information among neighboring controllers. We show that the proposed architecture helps prevent voltage violations and shapes the voltage profile in radial distribution networks, even in the presence of considerable penetration of variable generation and loads. We present several case studies involving 8-, 13-, and 123-bus distribution systems to illustrate the operation of the architecture.

273 citations

••

09 Jul 2007TL;DR: It is shown that each node can immediately calculate the consensus value as a linear combination of its own past values over at most D time-steps, which has the potential to significantly reduce the time and communication required to reach consensus in distributed systems.

Abstract: We present a method for achieving consensus in distributed systems in a finite number of time-steps. Our scheme involves a linear iteration where, at each time-step, each node updates its value to be a weighted average of its own previous value and those of its neighbors. If D denotes the degree of the minimal polynomial of the weight matrix associated with the linear iteration, we show that each node can immediately calculate the consensus value as a linear combination of its own past values over at most D time-steps. We also show that each node can determine the coefficients for this linear combination in a decentralized manner. The proposed scheme has the potential to significantly reduce the time and communication required to reach consensus in distributed systems.

227 citations

••

01 Dec 2007TL;DR: It is shown that the state-based definition of opacity enables the use of observer constructions for verification purposes and the verification of (S,P, K)-opacity is accomplished via an observer with K-delay which is constructed to capture state estimates withK-delay.

Abstract: In this paper, we follow a state-based approach to extend the notion of opacity in computer security to discrete event systems. A system is (S, P)-opaque if the evolution of its true state through a set of secret states S remains opaque to an observer who is observing activity in the system through the projection map P. In other words, based on observations through the mapping P, the observer is never certain that the current state of the system is within the set of secret states S. We also introduce the stronger notion of (S,P, K)-opacity which requires opacity to remain true for K observations following the departure of the system's state from the set S. We show that the state-based definition of opacity enables the use of observer constructions for verification purposes. In particular, the verification of (S,P, K)-opacity is accomplished via an observer with K-delay which is constructed to capture state estimates with K-delay. These are the estimates of the state of the system K observations ago and are consistent with all observations (including the last K observations). We also analyze the properties and complexity of the observer with K- delay.

205 citations

••

01 Dec 2012TL;DR: This paper proposes a low-complexity iterative algorithm for DER optimal dispatch that relies, at each iteration, on simple computations using local information acquired through exchange of information with neighboring DERs.

Abstract: In this paper, we address the problem of optimally dispatching a set of distributed energy resources (DERs) without relying on a centralized decision maker. We consider a scenario where each DER can provide a certain resource (e.g., active or reactive power) at some cost (namely, quadratic in the amount of resource), with the additional constraint that the amount of resource that each DER provides is upper and lower bounded by its capacity limits. We propose a low-complexity iterative algorithm for DER optimal dispatch that relies, at each iteration, on simple computations using local information acquired through exchange of information with neighboring DERs. We show convergence of the proposed algorithm to the (unique) optimal solution of the DER dispatch problem. We also describe a wireless testbed we developed for testing the performance of the algorithms.

185 citations

##### Cited by

More filters

••

[...]

TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.
Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

33,785 citations

••

[...]

TL;DR: Some of the major results in random graphs and some of the more challenging open problems are reviewed, including those related to the WWW.

Abstract: We will review some of the major results in random graphs and some of the more challenging open problems. We will cover algorithmic and structural questions. We will touch on newer models, including those related to the WWW.

7,116 citations

••

05 Mar 2007TL;DR: This work reviews several recent results on estimation, analysis, and controller synthesis for NCSs, and addresses channel limitations in terms of packet-rates, sampling, network delay, and packet dropouts.

Abstract: Networked control systems (NCSs) are spatially distributed systems for which the communication between sensors, actuators, and controllers is supported by a shared communication network. We review several recent results on estimation, analysis, and controller synthesis for NCSs. The results surveyed address channel limitations in terms of packet-rates, sampling, network delay, and packet dropouts. The results are presented in a tutorial fashion, comparing alternative methodologies

3,748 citations

01 Jan 2003

2,810 citations