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
9
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
15UIUC
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
17UIUC
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
19UIUC
20UIUC
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
25UIUC
26UIUC