mitigating cascading failures in interdependent power grids and communication networks 1 eytan...

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Mitigating Cascading Failures in Interdependent Power Grids and

Communication Networks

Eytan Modiano

Joint work with Marzieh Parandehgheibi

David Hay

IEEE SmartGridComm November 4, 2014

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Motivation

Risk of Large blackouts such as 2003 blackout in North-East America

Risk of blackout increases due to nature of Renewable Energies

(fluctuations stress the grid more)

Communication Network

Power Grid

What if we lose part of communication network in the presence of large disturbances in the power grid?

Extra Failures in Power Grid

Extra Failures in Communication

Network

Concerns Requires

Strong Interdependency

Challenge

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Abstract Interdependency Model First Model on Interdependency: “Catastrophic cascade of failures in interdependent networks”, Buldyrev, et al, 2010

Many Follow-ups on this model

Two Networks A and B Node i in network A operates if

1) it is connected to a node in network B2) It is part of the largest component in network A

One-to-one interdependencypicture from “Catastrophic cascade of failures in interdependent networks”

Interdependent Networks are more vulnerable than Single Networks

Erdos-Renyi Graph with 500 nodes and expected degree of 4

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Is this a good model for a power grid?

Power Flow Equations:

When a Failure in power grid occurs1) Power redistributes according to power flow equations2) Some lines may be overloaded and fail3) Steps 1 and 2 continue until no more lines fail

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Very different behavior in Power Grid!

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

Metric in Power Grid: Fraction of Served Load; i.e. Yield

Metric: Ave size of largest component (fraction of remaining nodes)

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Model: Dependence of Communication on Power

Pc1

P1

Pc2

Pc3

P2

TransmissionPower Grid

DistributionPower Grid

C1

C2

C3

CommunicationNetwork

Network ECP

C1

P1

C2

C3

P2

TransmissionPower Grid

CommunicationNetwork

Dependency of communication on 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

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Dependence of Power on Communication

• Each Power node depends on at least one communication node

• What happens if communication is lost?

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

Generators may fail due to Frequency drop

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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

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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

Mitigation Policy inside Power Grid:

Intelligent Load Shedding/redispatch

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Accounting for Failures in the Communication Network

1) Mitigate Failures inside Power Grid using load shedding

2) Remove all the communication nodes that receive less than required power

3) Remove all power nodes that lose their correspondent communication node

4) go back to step 1 until no failure occurs

Simple Mitigation Policy for Interdependent Networks:

• What if the communication nodes are vulnerable to power failures?

• Failures will cascade between the communication network and power grid

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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

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Unavoidable Failures Due to Loss of Connectivity

• Phase 1) Find the Set of all unavoidable failures

Power node Generator Control node Control center

Power line Communication line Communication NetworkPower Grid

CG

Unavoidable Failures – Without Considering the Power FlowsA Power node fails if it loses its connection to 1) Communication Network OR 2) GeneratorA Communication node fails if it loses its connection to 1) Power Grid OR 2) Control Center

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CG


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CG


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CG


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CG


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Load Control Mitigation Policy

• Phase 1) Find the Set of all unavoidable failures• Phase 2) Re-dispatch the generators and loads

Minimum Load Shedding

Communication Nodes receive enough PowerConnecting

communication and power grid

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Load Control Mitigation Policy

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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

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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.

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Sensitivity Analysis: Interdependent Degree

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

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

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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

mitigating cascading failures in interdependent power grids and communication networks 1 eytan...

Documents

power node

dependence of power

power grid extra failures

random power grid erdos

interdependent power

power p req

communication networks

communication node slide