Marzieh Parandehgheibi Mitigating Cascading Failures in Interdependent Power Grids and Communication Networks Eytan Modiano Joint work with Marzieh Parandehgheibi David Hay IEEE SmartGridComm November 4, 2014

Communication Network Motivation Concerns Risk of Large blackouts such as 2003 blackout in North-East America Requires Communication Network Power Grid Risk of blackout increases due to nature of Renewable Energies (fluctuations stress the grid more) Challenge What if we lose part of communication network in the presence of large disturbances in the power grid? Extra Failures in Communication Network Extra Failures in Power Grid Strong Interdependency

Abstract Interdependency Model First Model on Interdependency: “Catastrophic cascade of failures in interdependent networks”, Buldyrev, et al, 2010 Many Follow-ups on this model Erdos-Renyi Graph with 500 nodes and expected degree of 4 Two Networks A and B Node i in network A operates if 1) it is connected to a node in network B 2) It is part of the largest component in network A The size of largest component is close to the size of network for small sizes of failure, and there is a drop at a failure rate of about 50%. Interdependent Networks are more vulnerable than Single Networks One-to-one interdependency picture from “Catastrophic cascade of failures in interdependent networks”

Is this a good model for a power grid? Power Flow Equations: When a Failure in power grid occurs Power redistributes according to power flow equations Some lines may be overloaded and fail Steps 1 and 2 continue until no more lines fail

Very different behavior in Power Grid! Metric in Power Grid: Fraction of Served Load; i.e. Yield Metric: Ave size of largest component (fraction of remaining nodes) Random Power Grid - Erdos-Renyi with 500 Nodes and average degree of 4; 1/5th of the nodes are generators and 1/5th are loads with random value in range [1000,2000]; unit reactance Power Grids are More Vulnerable to Failures due to Cascading Failures

Model: Dependence of Communication on Power Dependency of communication on power grid Pc1 P1 Pc2 Pc3 P2 Transmission Power Grid Distribution C1 C2 C3 Communication Network Network ECP C1 P1 C2 P2 C3 We assume that the communication and control nodes have the highest priority in the distribution system; thus, they will receive power as long as the power nodes have sufficient power to meet their demand. Slide 17 Communication Network Transmission Power Grid Every communication node requires power Preq for operation If Pci > Pireq for operation, then Ci continues operating Simple model allows us to associate a load with every communication node

Dependence of Power on Communication Each Power node depends on at least one communication node What happens if communication is lost? Generators may fail due to Frequency drop Loads may fail due to voltage drop If a power node loses its correspondent communication node, it cannot be controlled and fails (Deterministic Model) Extendable to a probabilistic model where the power node fails randomly with some probability Clearly, this is not what happens today, as the present grid does not depend on communications for its control in a critical way

Interdependent Power Grid Metric in Power Grid: Fraction of Served Load; i.e. Yield The purpose of designing a communication network intertwined with the power grid is to provide real-time monitoring and control for the grid. a proper analysis of interdependent networks should account for the availability of control schemes that can mitigate cascading failures. Wrong Conclusion: power grid is vulnerable to communication failures, without taking advantage of communications for intelligent control and failure mitigation

Interdependent Power Grid Pessimistic Scenario: Vulnerable Communication Network, but communication nodes do not control the cascading failures inside power grid Optimistic Scenario: Robust Communication Network (e.g. all communication nodes are backed-up with batteries), and communication nodes control the cascading failures: i.e., using centralized load shedding and generator redispatch Intelligent Load Shedding/redispatch Mitigation Policy inside Power Grid:

Accounting for Failures in the Communication Network What if the communication nodes are vulnerable to power failures? Failures will cascade between the communication network and power grid Simple Mitigation Policy for Interdependent Networks: Mitigate Failures inside Power Grid using load shedding Remove all the communication nodes that receive less than required power Remove all power nodes that lose their correspondent communication node go back to step 1 until no failure occurs Mitigate Failures inside Power Grid : re-dispatch generators and loads so that no line is overloaded and yield is maximized

Intelligent Mitigation Policy The previous mitigation strategy did simple load shedding, and as a result cause communication nodes to fail A more intelligent policy will shed load “intelligently” to avoid the failure of critical communication nodes Load Control Policy Phase 1) Find the Set of all unavoidable failures (i.e., disconnected nodes) Phase 2) Re-dispatch the generators and loads so that All remaining communication nodes can operate (receive enough power) Minimum amount of load is shed; i.e. Maximize Yield

Unavoidable Failures Due to Loss of Connectivity Phase 1) Find the Set of all unavoidable failures C G Power node Generator Control node Control center Power line Communication line Power Grid Communication Network Unavoidable Failures – Without Considering the Power Flows A Power node fails if it loses its connection to 1) Communication Network OR 2) Generator A Communication node fails if it loses its connection to 1) Power Grid OR 2) Control Center

Unavoidable Failures Due to Loss of Connectivity Phase 1) Find the Set of all unavoidable failures C G Power node Generator Control node Control center Power line Communication line Power Grid Communication Network Unavoidable Failures – Without Considering the Power Flows A Power node fails if it loses its connection to 1) Communication Network OR 2) Generator A Communication node fails if it loses its connection to 1) Power Grid OR 2) Control Center

Unavoidable Failures Due to Loss of Connectivity Phase 1) Find the Set of all unavoidable failures C G Power node Generator Control node Control center Power line Communication line Power Grid Communication Network Unavoidable Failures – Without Considering the Power Flows A Power node fails if it loses its connection to 1) Communication Network OR 2) Generator A Communication node fails if it loses its connection to 1) Power Grid OR 2) Control Center

Unavoidable Failures Due to Loss of Connectivity Phase 1) Find the Set of all unavoidable failures C G Power node Generator Control node Control center Power line Communication line Power Grid Communication Network Unavoidable Failures – Without Considering the Power Flows A Power node fails if it loses its connection to 1) Communication Network OR 2) Generator A Communication node fails if it loses its connection to 1) Power Grid OR 2) Control Center

Unavoidable Failures Due to Loss of Connectivity Phase 1) Find the Set of all unavoidable failures C G Power node Generator Control node Control center Power line Communication line Power Grid Communication Network Unavoidable Failures – Without Considering the Power Flows A Power node fails if it loses its connection to 1) Communication Network OR 2) Generator A Communication node fails if it loses its connection to 1) Power Grid OR 2) Control Center

Load Control Mitigation Policy Phase 1) Find the Set of all unavoidable failures Phase 2) Re-dispatch the generators and loads Minimum Load Shedding Slide 6 Communication Nodes receive enough Power Connecting communication and power grid

Load Control Mitigation Policy

Improvement due to Interdependency Even with the strong assumption that failure in one network can lead to the immediate failure in the other network, interdependency can improve the power grid

Sensitivity Analysis: Load Factor Load Factor: the ratio of power required by the communication network to the total load in the power grid Note that IT infrastructure uses an increasing fraction of total power in grid.

Sensitivity Analysis: Interdependent Degree Communication Interdependent Degree: average number of power nodes that support every communication node Power Interdependent Degree: average number of communication nodes that support every power node

Summary Highlight importance of power flow in the analysis Results from abstract connectivity model don’t hold Proposed a new model for interdependent power grid and communication network Power nodes depend on communication Communication nodes depend on power Interdependency could benefit the power grid instead of making it more vulnerable Intelligent load shedding scheme attempts to keep critical communication nodes operating Most residual failures are due to loss of connectivity If communication node is connected to the grid, it receives sufficient power Good justification for using the abstract connectivity model Model can be generalized to “partial dependence” I.e., account for available back-up power Allow for probabilistic failure in the event of loss of connectivity