0. COMPREHENSIVE APPROACH TO POWER SYSTEM SECURITY
Copyright © P. Kundur This material should not be used without the author's consent
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1. Power System Security
Security of power systems depends on three factors:
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
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2. Challenges to Secure Operation of Today'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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3. Comprehensive Approach to System Security
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
Widespread use of distributed generation
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4. Power System Controls and Protective Relaying
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5. Normal State Controls Generator controls: Transmission 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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6. Preventive 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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7. Power System Controls in the New Environment
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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8. Implications 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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9. Generator 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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10. Coordinated Design of Robust Controls
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 TF : “Impact of Interactions among Power System Controls”
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11. Analytical Techniques for Design of Normal 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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12. Analytical Techniques (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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13. Analytical Techniques (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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14. Improved 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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15. Defense Plans Against Extreme Contingencies
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16. 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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17. Defense Plans to Minimize Impact of Extreme 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 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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18. Steps in the Development of a Defense 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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19. Guidelines for Design and Deployment 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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20. Power System Restoration
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21. Power 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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22. Power 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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23. Key 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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24. Analytical Tools for Developing Restoration 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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25. Reliability Management System
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26. Reliability Management System (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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27. Components of Reliability Management System
A typical Reliability Management System has four components:
Reliability criteria applicable to Control Area operators
operating reserves, disturbance control, control performance standards, operating transfer capability
Reliability criteria applicable to generators
requirements for AVR and PSS
“grid codes” for new sources of generation
Reliability criteria applicable to transmission system users
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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28. On-Line Dynamic Security Assessment
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29. Dynamic Security Assessment (DSA)
A challenging task
changing system conditions; complexity and size of power systems
Historically based on off-line studies
system operated conservatively within pre-established limits
On-line DSA essential in the new competitive environment
evaluation of available transfer capability (ATC)
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30. Components 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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31. On-Line Voltage Stability Assessment Package
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32. Key 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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33. Contingency 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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34. Fig. 4 Automatic Critical Contingency Selection
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35. Security 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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36. Modelling: Inputs and Outputs: Inputs Output
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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37. Secure Operating Region
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38. Secure Operating Region
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39. Remedial 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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40. Ranking and Applying Remedial Measures
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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41. Expanding the Secure Region: Remedial Measures
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42. Fast Time-Domain Simulation Module
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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43. Fig. 3 VSAT Structure
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44. Transient Stability Assessment Package
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45. Transient Stability Assessment (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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46. Key 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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47. A 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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48. EEAC
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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49. EEAC
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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50. EEAC
The classical equal area criterion can be extended to the visual OMIB system Stability margin of the system is defined as
Thus, £ h £ 100, and h> 0 if the system is stable h £ 0 if the system is unstable h can be used as a stability index
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51. Use 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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52. Results - Test System System description Interface Contingency
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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53. Limit Search Results
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54. Speed Enhancement: Parallel Processing
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 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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55. Contingency Screening & Ranking (EEAC)
TSAT Structure
Powerflow
Dispatcher
Time-Domain
Simulation
Stability
Indices
Increase
Transfer
Remedial
Measures
Must Run
Contingencies
Transaction
Definitions
Security Limit?
Sufficient
Margin?
STOP
Yes No
Full
Contingency List
Solved Powerflow +
Dynamic Data
Contingency Screening & Ranking (EEAC)
Fig. 8 TSAT Structure
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56. Computational 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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57. Example of Computational Performance of VSAT
Computation times for a 4655 bus, 156 generator system on a 1.7 GHz Pentium 4 PC with 256 MB memory:
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
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58. Example of Computational Performance of TSAT
Computation times for a 4655 bus, 156 generator system on a 1.7 GHz Pentium 4 PC:
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
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59. Summary 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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60. New and Emerging Technologies
Real-Time Monitoring and Control
Risk-Based Security Assessment
Intelligent Control
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61. Real-Time Monitoring and Control
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62. 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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63. Wide Area Monitoring Current Applications
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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64. WAM Potential Future Applications: Wide 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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65. Risk-Based Dynamic System Assessment
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66. Dynamic Security Assessment Current 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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67. Risk-Based Dynamic Security Assessment
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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68. Intelligent Control
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69. Power 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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70. Transmission and Distribution
Off-Line
Security Limits
System Models
Load Forecast
Contingency Lists
Security Criteria
State Estimator
Build Model for
Current System State
Look-up tables of
Generation
Transmission and Distribution
Customers
OPERATIONS
PLANNING
Transaction Requests
CONTROL
DECISIONS
SYSTEM
CENTER
MONITORED
QUANTITIES
ACTIONS
Utilities
Energy Providers
Power Marketers
Human Controls
Human
Controls
Automatic
Local
Other Control
Centers
Interconnected
Power Systems
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71. Intelligent Control of Power Systems
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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72. Distributed Generation
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73. 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
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