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April 10, 2023 GEC Thrissur 1
Power System Power System SecuritySecurity
April 10, 2023 GEC Thrissur 2
Electrical Power SystemElectrical Power SystemSystem which
Generates ………………..…generators Controls ……………….…transformers Transmits …………..transmission lines Consumes Electric Energy ……….load
Power system should be • Economic• Reliable • Secure
April 10, 2023 GEC Thrissur 3
Power System SecurityPower System Security
Degree of risk in power system’s ability to survive imminent disturbances (contingencies) without interruption to customer service
Depends on • System operating condition – reserve
capacity available (unused generators & no. of parallel lines)
• Contingent probability of disturbances
April 10, 2023 GEC Thrissur 4
Necessity for securityInsufficient degree of security leads to
• severe system failures• enormous economic cost• loss of life
To ensure system reliability1. Proper design with security as primary
consideration2. Monitored during operation to ensure
sufficient security margin exists at all times
April 10, 2023 GEC Thrissur 5
In small power system, security assessment can be conducted in an off line operating –planning environment in which performances were determined aso Steady state performance……..power flowo Dynamic performance………..time domain
simulation
System security involves:Practices designed to keep the system operating when component fails
April 10, 2023 GEC Thrissur 6
Generator taken off-line because of auxiliary equipment failure
Remaining units on the system can make up the deficito By maintaining proper amounts of spinning
reserveo without too low frequency dropo without load shedding
ExampleExample
April 10, 2023 GEC Thrissur 7
ExampleExample
Transmission line damaged by storm taken out by automatic relaying
Committing & dispatching generationo Transmission flows is maintainedo Remaining transmission lines can take the increased
loading o Lines still remain within limits
April 10, 2023 GEC Thrissur 8
BlackoutBlackout
• Typical feature of PS Security• Cascading failure of equipments leads
to entire system collapse
Power system equipment is designed to be operated within certain limits, most of them are protected by automatic devices & switched out of the system if limits are violated, may lead to cascading failures.
April 10, 2023 GEC Thrissur 9
ExampleExampleNew York City July
13,1977
2000 hr : Electrical load peaking around 6000 MW (half is being imported via overhead tie-lines & underground cables), system in Normal state.
2031 hr : Severe lightning hits transmission tower carrying two 345 kV lines causing permanent tripping of both.Network losses 1000 MW, remaining lines loaded, system in Alert state.
2055 hr : City generation raised by 550 MW to strain off tie-lines which are still below their thermal limits, still in Alert state.
April 10, 2023 GEC Thrissur 10
2056 hr : Second lightning stroke cripples a third 345 kV line, within fraction of a second a fourth line trips due to power transients, remaining lines are pushed above their thermal limits, system in Emergency state, every serviceable generator is running.
2119 hr : Thermal expansion of one 345 kV line, sag deep to cause short circuit via small tree, the line trips further overload to few remaining ties which one by one break open, system in Extremis state.
April 10, 2023 GEC Thrissur 11
2129 hr : Last tie trips, system with deficiency of 1700 MW resulting in rapid loss of frequency, under frequency relays automatically initiate preset load shedding of section upon section of the city, loss of frequency cannot be halted & generators are tripped automatically & manually to avoid catastrophic machine damage.
2136 hr : New York City goes totally back
April 10, 2023 GEC Thrissur 12
Examples of Major System Examples of Major System Disturbances/BlackoutsDisturbances/Blackouts
• July 2, 1996 disturbance of WSCC (Western North American Interconnected) System
• August 10, 1996 disturbance of WSCC system• 1998 power failure of Auckland business
districts, New Zealand• March 11, 1999 Brazil blackout• July 29, 1999 Taiwan disturbance• August 14, 2003 blackout of Northeast U.S.
and Ontario
April 10, 2023 GEC Thrissur 13
POWER SYSTEM OPERATING STATES
E,I
Normal
E,I
Restorative
E,I
Alert
E,I
Extremis
E,I
Emergency
RestartsLoad Pickup Preventive
control
Emergency control Resynchroni-
zation
April 10, 2023 GEC Thrissur 14
Preventive control:Start-up of stand-by units/ generation
rescheduling/change in interchange schedule
Emergency control:Lowering bus voltage, starting quick start
unitsLoad shedding, network rearrangement
Restorative state:Rescheduling of active & reactive power unit
restraintsResynchronization & gradual load pick-up
April 10, 2023 GEC Thrissur 15
Normal operation of power Normal operation of power systemsystem
Conditions to be fulfilled:1. Load demands are met & load flow equations are
satisfied2. Frequency is constant3. Bus voltage magnitude are within prescribed
limits (+6%)Power equipment are supposed to operate at a given rated voltage
4. No power system element is overloadedOverloading results in higher temperatures & component is likely to be damaged.
April 10, 2023 GEC Thrissur 16
If tried to transmit more power than static power limit synchronism is lost & transmission system will collapse.
For short lines (300 kms) thermal limit fixes the loading of the line.
For long lines static power limit becomes the limiting factor
April 10, 2023 GEC Thrissur 17
DEFINITIONSDEFINITIONS
Security: An instantaneous time-varying condition reflecting the robustness of the system relative to imminent disturbances.
Security Monitoring:The on-line measurement of system and environment variables that affect system security; provides base case conditions for analysis of the effects of contingencies.
April 10, 2023 GEC Thrissur 18
DEFINITIONSDEFINITIONS
Security Assessment:The evaluation of data, provided by security monitoring, to estimate the relative robustness (security level) of the system in its present state. (determination of whether the system is in the normal or Alert operating state).
Security Enhancement:Specific operations taken on-line to improve system robustness. Includes security control, corrective rescheduling, preventive actions etc.
April 10, 2023 GEC Thrissur 19
DEFINITIONSDEFINITIONSSecurity Control:It is an umbrella term comprising security monitoring, assessment and enhancement.
Emergency control:Control taken after one or more operating constraints have been violated in order to return the system to the normal state. May include or be referred to as remedial action, determined in advance relative to one or more possible contingencies for which security enhancement is not feasible.
April 10, 2023 GEC Thrissur 20
DEFINITIONSDEFINITIONS
Reliability :National Electric Reliability Council (NERC) defines reliability as comprising of two components, adequacy and security.
Adequacy is the ability to supply energy to satisfy load demand.
Security is the ability to withstand sudden disturbances.
April 10, 2023 GEC Thrissur 21
Factors affecting power system Factors affecting power system securitysecurity
Characteristics• Aging transmission
infrastructures
• Lack of new transmission facilities
Impact• Increased probability of
component failures and malfunction leading to system disturbances
• Overloading of transmission facilities leading to protection operation or contributing to phenomena such as voltage collapse
April 10, 2023 GEC Thrissur 22
Characteristics• Cutbacks in system
maintenance
• Increased dependence on controls and special protection systems
• Large numbers of small and distributed generators
Impact• Component failures and
disturbances such as flash overs to trees
• Increased probability of inadvertent/incorrect operation of protections
• Increased unpredictability of cascading events
• Increased difficulty in adequate system design due to uncertainty in generation plans
April 10, 2023 GEC Thrissur 23
Characteristics• Market-driven
transactions
• Increased dependence on communications and computer systems
Impact• Unpredictable power
flows and system usage leading to congestion and/or poor dynamic behavior, New forms of stability problems such as voltage and small signal stability
• Software/hardware failures may leave large portion of the system unobservable to operators, leading to inappropriate, or lack of, control actions during disturbances
April 10, 2023 GEC Thrissur 24
Characteristics• Limited integrated
system planning
• Trend toward interconnection
Impact• Insufficient/improper
generation and transmission resources
• Exposure to cascading disturbances brought on by events in neighboring systems
• New forms of stability problems such as small signal stability
April 10, 2023 GEC Thrissur 25
Characteristics• New technologies
such as advanced control systems, wind power, biomass, fuel cells, etc
• Aging and downsized workforces
Impact• Lack of operating
experience with technologies which may have unique dynamic characteristics
• Unpredictable behaviors during disturbances
• Lack of experienced personnel that may lead to the inability to deal appropriately with emergency conditions
April 10, 2023 GEC Thrissur 26
SECURITY ASSESSMENT SECURITY ASSESSMENT APPROACHAPPROACH
Two approaches• Direct - probabilistic view point
- analysis of a fixed set of contingencies
- classify the system as insecure if any member of the set would
result in transition to the emergency state
• Indirect - deterministic view point - tracks a variety of reserve
margins (MW,VAR reserves)- determined by off-line studies
April 10, 2023 GEC Thrissur 27
Important & Probable Contingencies considered:
o Outage of a circuito Outage of a unit o 3-phase or L-G fault
Security monitoring: (continuous activity)o Energy control centre/Load despatch centreo On-line identification of actual operating
condition of power system
April 10, 2023 GEC Thrissur 28
• Static state estimation (computer based technique) provides database for analysis – gives bus voltages at any instant from telemetered data & breaker status.
• Security analysis by security analyzer then finds out impact of contingencies using fast load flow method (FDLF)
• If system is found insecure, system engineer determines preventive control.
• If in emergency state due to cascading events corrective emergency controls are to be applied
April 10, 2023 GEC Thrissur 29
Methods of Enhancing SecurityMethods of Enhancing Security Impractical to achieve complete immunity to blackouts
– need to strike a balance between economy and security
Good design and operating practices could significantly minimize the occurrence and impact of widespread outages– Reliability criteria– On-line security assessment– Robust stability controls– Coordinated emergency controls– Real-time system system monitoring and control– Wide-spread use of distributed generation
April 10, 2023 GEC Thrissur 30
ReferencesReferences1. Allen J.Wood and Bruce F.Woolenberg, “Power
generation, operation and control”, John Wiley & sons Inc.
2. A. K.Mahalanabis, D.P.Kothari, S. I. Ahson,”Computer aided power system analysis and control”, Tata McGraw-Hill publishing company limited, New Delhi.
3. Olle l. Elgerd,”Electric Energy Systems theory An Introduction”, T M H Edition.
4. Kip Morison, Lei Wang, Prabha Kundur,” Power System Security Assessment”,IEEE power & energy magazine, sep/oct 2004, pp30-39.
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