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COMPREHENSIVE APPROACH TO COMPREHENSIVE APPROACH TO POWER SYSTEM SECURITYPOWER SYSTEM SECURITY

Copyright © P. KundurThis material should not be used without the author's consent

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Power System SecurityPower System Security

The Physical System

the integrated generation, transmission and distribution system, and loads

protection and controls

The Business Structures

owning and operating entities

performance and service contracts

The Regulatory Framework

roles and responsibilities of individual entities

well chosen, clearly defined and properly enforced

Security of power systems depends on three factors:

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Challenges to Secure Operation of Challenges to Secure Operation of Today's Power SystemsToday's Power Systems

Large complex power systems

thousands of devices requiring harmonious interplay

Complex modes instability

global problems

different forms of instability: rotor angle, voltage, frequency

"Deregulated" market environment

many independent entities with diverse business interests

lack of integrated and inter-regional planning

power systems can no longer be operated conservatively within pre-established limits

A comprehensive approach to system security is required

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Comprehensive Approach to System Comprehensive Approach to System SecuritySecurity

1. Proper selection, design and application of power system controls and protective relaying

2. Development and deployment of a good “defense plan” against extreme contingencies

3. Development of a well documented and organized plan for rapid and safe restoration of the power system

4. Use of state-of-the-art techniques for on-line dynamic security assessment to determine stability margins and identify any corrective actions

5. Implementation of a Reliability Management System (RMS) for setting, monitoring and enforcing security related standards

6. Development and application of real-time wide area Monitoring and Control

an emerging technology

7. Widespread use of distributed generation

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Power System Controls and Power System Controls and Protective RelayingProtective Relaying

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Normal State ControlsNormal State Controls

Generator controls:

excitation controls: AVR, PSS

prime-mover, energy supply system controls

Transmission controls:

voltage regulators

switched reactors/capacitors, SVCs

HVDC and FACTS controls

Secondary/tertiary voltage control:

used by EDF, ENEL

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Preventive and Emergency ControlsPreventive and Emergency Controls

Preventive Controls

Generation shifting

Increase in VAR reserve

Emergency Controls

Generator tripping

Generation runback/fast valving

Load shedding

Dynamic braking

Transient excitation boosting

HVDC link rapid power ramping

Controlled system separation

Transformer tap-changer blocking

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Power System Controls in the New Power System Controls in the New EnvironmentEnvironment

Efficient utilization of facilities while ensuring security:

greater dependence on controls

Successful energy trading (buying, wheeling and selling of power):

can overwhelm existing controls

need for more sophisticated controls using advanced technologies

New business structure of owning and operating entities impacts:

what controls are used

how they are designed and deployed

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Implications of OwnershipImplications of Ownership

Industry will comprise corporate entities having diverse roles and business interests

Physical functioning of the integrated power system will remain the same

Control of individual equipment should

not to be left to owner’s discretion

be vested with the independent system operator

Specification and design of controls:

part of overall system planning/design

carried out by an independent entity

Otherwise security and overall economy will be sacrificed:

defeats the very purpose of restructuring

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Generator ControlsGenerator Controls

Essential to recognize the critical role of generator controls

Use of fast exciters, AVR, PSS and speed governor should be mandatory

No difficulty in motivating power plant owners to install controls:

needed to meet local plant needs

enhance plant operability and stability

Financial incentives for controls needed to:

meet global system needs

enhance overall system performance

Many of the existing equipment are old and outdated

need for upgrading on a prioritized basis

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Coordinated Design of Robust Coordinated Design of Robust ControlsControls

Increasing use of:

multi-purpose controllers

multiple controllers to solve a common problem

Satisfactory and harmonious performance of different controllers with overlapping spheres of influence requires:

coordination and integration

Controller design must consider performance under all probable conditions:

wide range of conditions encountered during normal operation

severe system upsets: coordination with protective systems

Addressed in a recent report by CIGRE TF38.02.16: “Impact of Interactions among Power System Controls”

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Analytical Techniques for Design of Analytical Techniques for Design of Normal ControlsNormal Controls

Proper design techniques and procedures to ensure:

utilization of full potential of the controller

no adverse interaction with other controls or with protective systems

Key design issues:

selection of devices and input signals

robustness

coordination

impact on overall system performance

Complementary use of small-signal analysis and nonlinear time-domain simulation

cont’d

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Analytical Techniques Analytical Techniques (cont’d)(cont’d)

Small-signal analysis using eigenvalue techniques provides valuable information useful in control design:

transfer function residues, participation factors, frequency response, controllability and observability

examination of interaction with other controls

Nonlinear time-domain (short- and long-term) simulations assist in:

establishing signal limits

assessment of performance during large disturbances

checking adverse interaction with protective systems

designing emergency controls

cont’d

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Analytical TechniquesAnalytical Techniques (cont'd) (cont'd)

Design one controller at a time with all other relevant devices/controls modelled

Robustness to changing system conditions achieved by:

considering different operating conditions

using engineering judgement

Robustness to parameter uncertainty achieved by:

carrying out sensitivity analysis

Alternatively, robust controller design technique may be used:

for example, H-infinity approach

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Improved Protective RelayingImproved Protective Relaying

State-of-the-Art protective relaying for generating units and transmission lines

Adaptive relaying with settings that adapt to the real-time system states

Replacement of zone 3 and other backup relaying on important lines with improved relaying

Improved protection and control at power plants to minimize unit tripping for voltage and frequency excursions

Protective relay improvements to prevent tripping of critical elements on overload

control actions to relieve overload

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Defense Plans Against Extreme Defense Plans Against Extreme ContingenciesContingencies

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Extreme Contingencies (ECs)Extreme Contingencies (ECs)

Major system disturbances: result of contingencies more severe than

normal design contingencies occurrence rare, but impact very high likely to be experienced more often in the

new environment Brought about by a combination of events:

multiple outages caused by severe weather conditions

inadequate design of system and equipment; equipment malfunction

human error Examples of major system upsets:

French system, 1978 and 1987 WSCC system, July and August 1996 Brazilian system, March 1999 NE U.S.A. and Ontario, August 2003 Italian System, September 2003 Sweden and Denmark, September 2003

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Defense Plans to Minimize Impact of Defense Plans to Minimize Impact of Extreme ContingenciesExtreme Contingencies

Judicious choice of several forms of emergency controls will provide protection against different forms of possible disturbances

Key design and implementation issues:

detection

control action

timing

automation and Adaptiveness

side effects on equipment and system

coordination

CIGRE TF 38.02.19 report on "System Protection Schemes in Power Networks" published in 2001 provides a good summary of emergency controls used by utilities worldwide, future trends and suggested design procedures

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Steps in the Development of a Steps in the Development of a Defense PlanDefense Plan

Detailed modeling of power system, including fast and slow processes triggered by EC’s:

includes wide range of protection and controls

Identification of scenarios of ECs:

based on past experience, knowledge of unique characteristics of system

probabilistic approach

Simulation and analysis of contingencies:

extended time-domain simulation

Identification of measures to minimize the causes of ECs:

improved protection/controls; better coordination

Development of a comprehensive set of emergency controls to mitigate consequences of ECs

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Guidelines for Design and Guidelines for Design and Deployment of a Good Defense PlanDeployment of a Good Defense Plan

Should, as far as possible, provide coverage against all possible ECs

Simplicity, reliability,and low cost should be prime considerations

Inadvertent operation of emergency controls must not severely affect system security

Response-based emergency controls should generally be preferred:

as opposed to those based on direct detection of outages

Various emergency controls should be coordinated:

complement each other

act properly in a complex situation triggering several controls

Ensure compatibility of defense plans developed by neighboring utilities

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Power System RestorationPower System Restoration

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Power System RestorationPower System Restoration

Even if power systems are designed and operated in the best possible manner: impossible to prevent all contingencies which could cause widespread blackouts

While the physical extent of the blackout is a concern, the duration is equally important:

detailed restoration plans required

The new competitive environment requires a well documented and organized plan: to ensure that the system, with its numerous independent entities, can be

re-energized safely and quickly

Successful system restoration has been a challenge for traditional monopolistic environment:

will be a greater challenge in the new competitive structure with many owners

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Power System Restoration ProcessPower System Restoration Process

Assessment of the system status and initial cranking sources

Identification and preparation of restoration paths to build subsystems

Resynchronization of subsystems and restoration of loads

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Key Issues for System RestorationKey Issues for System Restoration

Ensuring sufficient black start capability with due regard to: generator startup times and loading rates; governor droop characteristics,

and VAr capability

Maintaining voltages and other key parameters within acceptable bounds avoid tripping of critical elements or equipment damage

Developing a consistent switching strategy throughout the procedure

Coordinating system protection schemes

Organizing the restoration plan with well defined roles for each participant

Training all participants in the restoration procedure

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Analytical Tools for Developing Analytical Tools for Developing Restoration PlansRestoration Plans

Steady-state analysis:

power flow analysis, including examination of sustained overvoltages; fault level calculation; harmonic analysis

Quasi steady-state analysis:

operator training simulator, long-term dynamic simulation

Dynamic analysis:

transient stability (TS) programs for verifying subsystem resynchronization

extended TS programs for verifying startup of auxiliaries of power plants, i.e., large induction motors

ElectroMagnetic Transients Program (EMTP) for analysis of switching transients

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Reliability Management SystemReliability Management System

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Reliability Management System Reliability Management System (RMS)(RMS)

In the monopolistic structure, power systems were owned and operated by a few vertically integrated entities:

planning and operating standards were developed cooperatively and implemented voluntarily

In the competitive environment, with many new players, global management of power system reliability requires a process that is legislated

Roles and responsibilities of individual entities should be well chosen, clearly defined and properly coordinated and enforced

For proper functioning of the overall system

a “shared vision” is necessary among all the entities involved

a good monitoring system for ”standards” violations

The RMS approach provides a contractual method of dealing with the many entities of a single interconnected system:

ensures overall system security through a well defined and enforceable criteria

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Components of Reliability Components of Reliability Management SystemManagement System

A typical Reliability Management System has four components:

1. Reliability criteria applicable to Control Area operators

operating reserves, disturbance control, control performance standards, operating transfer capability

2. Reliability criteria applicable to generators

requirements for AVR and PSS

“grid codes” for new sources of generation

3. Reliability criteria applicable to transmission system users

4. Excuse of performance

excused non-compliance, specific excuses

For each component, the reliability system specifies: participants, criteria, data reporting, compliance standard, non-compliance standard, sanctions

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On-Line Dynamic Security On-Line Dynamic Security AssessmentAssessment

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Dynamic Security Assessment Dynamic Security Assessment (DSA)(DSA)

A challenging task

changing system conditions; complexity and size of power systems

Historically based on off-line studies

system operated conservatively withinpre-established limits

On-line DSA essential in the new competitive environment

evaluation of available transfer capability (ATC)

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Components of DSAComponents of DSA

All forms of system instability must be addressed

Two categories important for on-line assessment

Transient (angle) stability

Voltage stability

Small-signal (angle) stability

control problem addressed in system design

on-line assessment important for some systems

Here we provide a description On-line Voltage Stability and Transient Stability Tools developed at Powertech Labs Inc.

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On-Line Voltage Stability On-Line Voltage Stability Assessment PackageAssessment Package

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Key Elements of VSAKey Elements of VSA

Interface with EMS; Model Initialization

Contingency screening and selection

Determination of secure operating region

using static analysis

Determination of remedial actions

Fast time-domain simulation

validation and checking

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Contingency Selection ModuleContingency Selection Module

Impractical to consider every conceivable contingency

A limited number (typically 20) critical contingencies determined for detailed studies

Performance Indices based on a few power flow solutions and reactive reserve not reliable

A fast screening method used:

based on exact margin to voltage collapse and full power flow solutions

number of power flow solutions 1.2 to 2.0 times number of contingencies

Supplemented with user-specified contingencies

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Fig. 4 Automatic Critical Contingency Selection

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Security Computation ModuleSecurity Computation Module

Engine for voltage stability analysis

static analysis with detailed models

Secure region is defined by a number of Coordinates (SRCs)

key system parameters: MW generation, area load, interface transfers, etc.

Voltage stability determined by

existence of powerflow solution

MVAr reserves of key reactive sources

post-contingency voltage decline

Specialized powerflow dispatcher and solver to quickly search for stability limit

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Modelling: generator capability curves

governor response, economic dispatch, AGC

nonlinear loads

control of ULTCs, switched shunts, etc.

Inputs and Outputs:

Inputs

list of contingencies produced by screening and ranking (+user defined)

base case powerflow from state estimator

definition of SCRs

voltage security criteria and definition of parameter of stress

Output

secure region in secure region space

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Secure Operating RegionSecure Operating Region

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Secure Operating RegionSecure Operating Region

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Remedial Measures ModuleRemedial Measures Module

Determines necessary remedial measures to

ensure sufficient stability margins

expand the secure region

Preventative control actions:

taken prior to a contingency

caps/reactor switching, generation redispatch, voltage rescheduling

Corrective (emergency) control actions:

applied following a contingency

load shedding, generator runback, transformer tap changer blocking

Ranking of each remedial measure using sensitivity analysis

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Ranking and Applying Remedial Ranking and Applying Remedial MeasuresMeasures

Objective is to identify the most effective remedial measures to give the desired stability margin

Obtain solved power flow case for the most severe contingency

gradually introduce the effect of the contingency

bus injection compensation technique

Compute the sensitivities of reactive power (or bus voltage) to different control measures

rank the remedial measures

Apply controls one at a time in order of ranking until power flow solves for the most severe contingency

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Expanding the Secure Region: Expanding the Secure Region: Remedial MeasuresRemedial Measures

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Fast Time-Domain Simulation Fast Time-Domain Simulation ModuleModule

Determines the essential dynamic phenomena without step-by-step numerical integration

when chronology of events significant

for validating the effect of remedial measures

Focuses on the evolution of system dynamic response driven by slow dynamics

transformer tap changers, field current limiters, switched caps

Captures the effects of fast dynamics by solving associated steady state equations

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Fig. 3 VSAT Structure

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Transient Stability Assessment Transient Stability Assessment PackagePackage

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Transient Stability Assessment Transient Stability Assessment (TSA)(TSA)

Time-domain simulations essential

modeling detail and accuracy

Sole dependence on time-domain simulations has severe limitations

high computational burden

no stability margin/sensitivity information

requires considerable human interaction

Supplementary techniques for speeding up and automating overall process

Methods available for deriving useful indices

Transient Energy Function (TEF)

Signal Energy Analysis

Extended Equal Area Criterion (EEAC)

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Key Elements of TSAKey Elements of TSA

Interface with EMS; Model Initialization

Contingency screening and selection

Simulation engine

detailed modeling

time-domain simulation

speed enhancement

Post-processing of detailed simulation

stability margin index using EEAC

power transfer limit search

remedial measures

damping calculation using PRONY

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A Practical Tool for TSAA Practical Tool for TSA

Overall architecture similar to that of VSA

Time-domain program, with detailed models and efficient solution techniques, forms simulation engine

EEAC used for screening contingencies, computing stability margin, stability limit search, and early termination of simulation

“Prony analysis” for calculation of damping of critical modes of oscillation

A powerflow dispatcher and solver for finding the stability limit

a fully automated process

No modeling compromises; can handle multi-swing instability

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EEACEEAC

Integrates the dynamic response in the multimachine space, and maps the resultant trajectory into a set of one-machine-infinite-bus planes

by applying complementary cluster center of inertia (CCCI) transformations

keeps all dynamic information in the multimachine space

stability analysis can be quantitatively performed for the image OMIB systems

has the same accuracy and modeling flexibility

fast, quantitative

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EEACEEAC

Loss of transient stability in a power system always starts in a binary splitting of generators:

critical cluster of generators

rest of the system

At any given point in the time-domain trajectory of the system, the system can be visualized as a one-machine-infinite-bus (OMIB) system

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EEACEEAC

The classical equal area criterion can be extended to the visual OMIB system

Stability margin of the system is defined as

da

a

ad

add

ad

AAunstable is system the ifA

AAx100

AAstable is system the ifA

AAx100

Thus, -100 , and

if the system is stable if the system is unstable

can be used as a stability index

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Use of EEAC TheoryUse of EEAC Theory

Contingency screening

stability margin gives an indication of the relative severity

Corrective measures for maintaining secure system operation

critical cluster of generators (CCG) provides valuable information

Power transfer limit search

stability limit can be determined in four iterations using stability margin

each iteration involves a detailed simulation and computation of stability index

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Results - Test SystemResults - Test System

System description

BC Hydro system 1430 buses 186 generators 4 HVDC links

Interface

GMS and PCN output Base case transfer = 3158

MW

Contingency

Three phase fault at GMS 500 kV bus

Tripping of one of two 500 kV lines from GMS to WSN

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Limit Search ResultsLimit Search Results

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Speed Enhancement: Parallel Speed Enhancement: Parallel ProcessingProcessing

Code parallelization differential equations easily parallelized, but not network equations

speed-ups limited by serial slowdown effect

up to 7 times speed-up can be achieved with 20-30 processors

not an effective way

Conventional serial computers offer much faster computational per-CPU

Best approach is to use multiple processors

Perform TS analysis and VS analysis in parallel

For multiple contingencies

perform initialization only once

run contingencies on multiple processors

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TSAT StructureTSAT Structure

PowerflowDispatcher

Time-DomainSimulation

StabilityIndices

IncreaseTransfer

RemedialMeasures

Must RunContingencies

TransactionDefinitions

Security Limit?

SufficientMargin?

STOP

Yes

Yes

No

FullContingency List

Solved Powerflow+

Dynamic Data

Contingency Screening & Ranking

(EEAC)

No

Fig. 8 TSAT Structure

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Computational Performance of DSAComputational Performance of DSA

Target cycle time from capture of state estimation to completion of security assessment for all specified transactions:

20 minutes

TSA and VSA functions performed in parallel

distributed processing on separate CPUs

This can be readily achieved with low cost PCs

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Example of Computational Example of Computational Performance of VSATPerformance of VSAT

Screening 300 contingencies to select 20 critical contingencies: 20 secs

Detailed security analysis of base case with 20 critical contingencies: 1.2 secs

One transaction limit search with 20 critical contingencies: 12 secs

Computation times for a 4655 bus, 156 generator system on a 1.7 GHz Pentium 4 PC with 256 MB memory:

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Example of Computational Example of Computational Performance of TSATPerformance of TSAT

Screening 100 contingencies for ranking 10 critical contingencies: 75 secs

Detailed security analysis of 10 contingencies including 3 second time domain simulations and stability index calculation: 75 secs

A four-iteration power transfer limit search for one contingency: 120 secs

Total time for complete power transfer limit calculation, including screening of 100 contingencies, stability limit search with an optimal order of 10 contingencies: 5 mins

NOTE: Both TSAT and VSAT have distributed processing capability, allowing each contingency or each transfer limit search to be processed in parallel on separate CPUs

Computation times for a 4655 bus, 156 generator system on a 1.7 GHz Pentium 4 PC:

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SummarySummary

On-line DSA is a complex problem

It is a challenge to provide comprehensive analysis with the required

accuracy, speed, and robustness

A practical tool for use with large complex systems has been built by

drawing on techniques developed over many years;

enhancement and integration of these techniques;

use of specialized software designs and distributed hardware architectures

May be used for real time application, or previous day to post ATC

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New and Emerging TechnologiesNew and Emerging Technologies

Real-Time Monitoring and Control

Risk-Based Security Assessment

Intelligent Control

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Real-Time Monitoring and Real-Time Monitoring and ControlControl

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Real-Time Wide Area Monitoring Real-Time Wide Area Monitoring

Advances in communications technology have made it possible to: monitor power system over a wide area

remotely control many functions

Wide Area Monitoring: phasor measurement units (PMUs) provide time synchronized measurements with an accuracy of 1

microsecond, utilizing Global Position System (GPS)

PMUs send measured voltage and current phasors to a Centralized Monitoring System, typically at 100 millisecond intervals

Data stored and processed for various applications

Results displayed on a Graphical User Interface

Examples of Wide Area Monitoring Systems: North American Western Interconnected System's Wide Area Measurement System (WAMS) project;

BPA, EPRI, DOE as participants

ETRANS Wide Area Monitoring (WAM) project for the Swiss Power Grid; developed by ABB

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Wide Area Monitoring Current Wide Area Monitoring Current ApplicationsApplications

On-line monitoring of transmission corridors for loading

Fast detection on critical situations

voltage stability

power system oscillations

transmission overloading

Additional input values of system variables for state estimator

Disturbance recording

for calibration of power system model

validation of stability analysis software

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WAM Potential Future Applications: WAM Potential Future Applications: Wide Area Emergency ControlWide Area Emergency Control

Prevention of partial or total blackout of power systems

trigger emergency controls based on system response and measurements

Research into the application of "Multisensor Data Fusion" technology

process data from different monitors and integrate information

determine nature of impending emergency

make intelligent control decisions in real time

A fast and effective way to predict onset of emergency conditions and take remedial control actions

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Risk-BasedRisk-BasedDynamic System AssessmentDynamic System Assessment

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Dynamic Security AssessmentDynamic Security AssessmentCurrent PracticeCurrent Practice

The utility practice has been to use deterministic approach

build strong systems and operate with large security margins

overly conservative, but cost could be passed on to captive customers

The deterministic approach has served the industry well

high security levels

study effort minimized

In the new environment, with a diversity of new participants, the deterministic approach not readily acceptable

need to account for the probabilistic nature of conditions and events

need to quantify and manage risk

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Risk-Based Dynamic Security Risk-Based Dynamic Security AssessmentAssessment

Examines the probability of power system becoming unstable and its consequence

Computes indices that measure security level or degree of exposure to failure

capture all cost consequences

Notion of security posed in a language and form understood by marketers and financial analysts

Possible with today’s computing and analysis tools

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Intelligent ControlIntelligent Control

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Power System ControlPower System Control

Overall control functions highly distributed

several levels of control

involve complex array of devices

Human operators provide important links at various levels

acquire and organize information

make decisions requiring a combination of deductive, inductive, and intuitive reasoning

“Intuitive reasoning” allows quick analysis of unforeseen and difficult situations and make corrective decisions

most important skill of an operator

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Off-Line

Security Limits

System ModelsLoad Forecast

Contingency ListsSecurity Criteria

State Estimator

Build Model for Current System State

Look-up tables ofSecurity Limits

Generation

Transmission and Distribution

Customers

OPERATIONSPLANNING

Transaction Requests

CONTROL

DECISIONS

SYSTEMCONTROL CENTER

MONITOREDQUANTITIES

CONTROLACTIONS

Utilities

Energy Providers

Power Marketers

Human Controls

Human Controls

Human

Controls

AutomaticLocal

Controls

AutomaticLocal

Controls

AutomaticLocal

Controls

Human Controls

Other ControlCenters

Interconnected

Power Systems

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Intelligent Control of Power Intelligent Control of Power SystemsSystems

Future power systems more complex to operate

less structured environment

Current controls do not have

“human-like” intelligence

Add intelligent components to conventional controls

learn to make decisions quickly

process imprecise information

provide high level of adaptation

Overall control of power systems

utilize both conventional methods and decision making symbolic methods

intelligent components form higher level of control

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Distributed GenerationDistributed Generation

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Distributed Generation (DG)Distributed Generation (DG)

Offer significant economic, environmental and security benefits

Microturbines

small, high speed power plants

operation on natural gas or gas from landfills

Fuel Cells

combines hydrogen with oxygen from air to generate electricity with water

hydrogen may be supplied from an external source or generated inside fuel cell by reforming a hydrocarbon fuel

Not vulnerable to power grid failure due to system instability or natural calamities !

Protection and controls for DG should be designed so that units continue to operate when isolated from the power grid