Fardin Abdi, Brett Robins, Marco Caccamo University of Illinois at Urbana-Champaign Urbana-Champaign, USA {abditag2, robbins3, 1UIUC

 Introduction to problem  Preliminary  Architecture description ◦ Fault detection ◦ Fault handling  Implementation in electric grid  evaluation 2UIUC

 Interconnected physical plants that physically affect each other!  State of each node is a function of control inputs of other nodes based on system connection graph Images : 3UIUC

 Distributed controllers coordinate with other nodes in order to: ◦ Reach to the desired state for the entire system ◦ Maintain functionality and stability of the system  System relies on Communication ◦ North American Electric Reliability Council report: information system failure is a major reason of cascade failures! 4UIUC

 Unpredictable latency in communication  Possible failures in communication channels ◦ Physical disconnection ◦ Improper functioning of communication unit 5UIUC

 Replacing the old infrastructure with new infrastructure is expensive therefore the old communication infrastructure is unlikely to be replaced any time soon.  Therefore: ◦ Techniques need to be developed for detecting and handling faults using existing communication technology. 6UIUC

Replacing cyber data with physical data to detect and handle faults 7UIUC

 In CPS, in addition to cyber channels, there are also physical channels that can be used as a source of data. ◦ Control commands result in a physical change in the state of a system  Red light and street example ◦ Data should match with physical state  Water pipe and sensors  We exploit the estimated states of remote nodes to detect communication faults and maintain the overall stability of the CPS. 8UIUC

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 Connected nodes {1,2,4,5}  Partially Connected nodes {3}  Totally Disconnected nodes {6,7} 10UIUC

 Estimation Unit  Communication Unit  Switching module  Distributed controller  Hybrid Controller  Local Controller 11UIUC

 Designed for normal operation mode when reliable data is being received from all the neighbors  For most of the existing distributed cyber-physical systems, their existing controller can be used without any modifications.  Only Access to communication unit 12UIUC

 Operates only based on estimated state variables of remote nodes and locally measured variables  Only access to estimation unit 13UIUC

 When there is both connected and disconnected neighbors.  Has access to both communication and estimation unit 14UIUC

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 Packet dist : ◦ Information required by controllers in order to take system to desired final state  Packet meas : ◦ For verification purpose ◦ Estimatable for the neighbors 16UIUC

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 No data received ◦ Communication unit buffer is not updated in a while. There would be a deviation between real data and data on communication buffer.  Incorrect data ◦ Gap between the estimated and received value  Based on the number of disconnected neighbors, a switch is triggered to hybrid or local controllers. 18UIUC

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 When the communication is broken, each node can only use its own reactive power capacity for voltage correction.  Over/under voltages will occur in the nodes with higher needs than their capacity. 21UIUC

 Estimation unit:  Fault declaration:  A fault triggers a switch to Hybrid or Local controllers based on the number of disconnected neighbors. 22UIUC

 Distributed Controller: ◦ Nodes exchange information via communication channels and come up with value of reactive power production.  Hybrid Controller: ◦ For disconnected neighbors, their value of reactive voltage requirement is estimated based on estimation of their voltage.  Local Controller: ◦ All the reactive power requirements of the neighbors are estimated. Finally, in order to satisfy requirements of all the neighbors, maximum estimated power is generated by the node. 23UIUC

 Perfect Communication:  All the nodes in the network can generate power for the node.  Broken Communication: ◦ Original DVC algorithm: only the node itself can provide required power ◦ Fault Resilient DVC algorithm: Immediate neighbors can also provide the reactive power. 24UIUC

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