1. Fardin Abdi, Brett Robins, Marco Caccamo University of Illinois at Urbana-Champaign Urbana-Champaign, USA {abditag2, robbins3, mcaccamo}@ILLINOIS.EDU 1UIUC

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

3.  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 : http://geospatial.blogs.com/geospatial/2009/07/alternative-energy-green-nonemitting-clean-renewable-or-low-carbon-.html http://www.thewatertreatments.com/water/distribution-system/ 3UIUC

4.  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

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

6.  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

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

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

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

12.  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

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

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

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16.  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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18.  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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21.  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

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

23.  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

24.  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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