1. 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

2. 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

3. 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”

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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:

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. Load Control Mitigation Policy

19. 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

20. 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.

21. 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

22. 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