Transcript system restoration

Defence plan
and
System restoration
NLDC
1
Introduction
 Inter-Connected
operation
-
widespread
propagation
of
disturbance
 Reliable defense plan essential
 Isolation or Islanding of Faulty portion to save rest of the system
 Load-Generation balance by UFR load-shedding required prior to
islanding
 Consideration of large number of contingencies required for
designing successful islanding schemes
 Common Sequence of events in blackouts
Initiating Events
Load /Generation
Imbalance in islands
Blackout of
Islands
System
Separation
Formation of
Islands
Begin Restoration
Process
Effect on Society
 Production
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Loss of productivity
Loss of product or property
 Health

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Food contamination
Medication problems
Anxiety
 Safety
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Traffic accidents
Accidents due to visibility problems
Civil unrest
Public Scrutiny
 Any widespread electric outage draws a lot of
attention from:
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Politicians
Governmental agencies
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ERCs
Special interest groups
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MOP
Consumer, Advocates, Environmentalists
Large customers
Media
Types of Blackouts
 Localized
 Partial System
 Full System With Outside Help
 Full System Without Outside Help
Restoration strategy may be different
for each type of outage !
Blackouts
 System separations and blackouts are possible at all
loading levels!
 System separations and blackouts are possible at all
times of the day and year!
 Heavily stressed system is more likely to black out!
 Prevention is the key to system restoration!
Causes of Blackouts
 Pre-disturbance conditions that could contribute
to a system blackout:
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Maintenance outages
Heavy/Uncontrolled loop flow through system
Changing generation patterns
Weather
Unexpected events/FAULTS
Relay mal-operation
Circuit breaker failure
Causes of Blackouts
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Cascading Thermal over loads
Voltage Instability
Dynamic Instability
Load Generation Imbalance
Power Flow
Thermal Limit
Stability Limit
Load
Voltage Limit
Total Transfer capability
Time Horizon
Causes of Blackouts
 Voltage Collapse

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KV
Deficit of MVAR Supply
Over the “knee” of the voltage curve
Results in system separations and generation tripping
safe
MW
unsafe
Causes of Blackouts
 Voltage Collapse
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Difficult to predict boundaries of separation
May be detected by looking for areas of voltage decay

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However, use of shunt capacitors can maintain near normal
voltage up to the point where voltage support resources run out
Time Frame: minutes to tens of minutes
Var Support
KV
safe
MW
unsafe
Causes of Blackouts
 Dynamic Instability
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System does not damp out normal oscillations
Groups of generators “swing” against each other resulting in
large oscillations in MW, MVAR.
Could result in:
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Generation tripping
Voltage collapse
Equipment damage
Time Frame: 5 -15 seconds
 Load Generation Imbalance
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Insufficient generation w.r.t connected load
Insufficient spinning reserve
Low frequency leading to low voltage
SYSTEM RESTORATION
Last Blackout In WR:
-
-
Date: 30th July 2002
System Affected: Whole
Region except parts of
Mumbai (21,500 MW)
Time of Disturbance:
20:11 / 30.07.2002
Time of Restoration :
06.00 / 31.07.2002
SYSTEM RESTORATION
Last Blackout In ER:
- Date: 25th July 2000
- System Affected: Whole
Region (7,300 MW)
- Time of Disturbance:
21:10 / 25.07.2000
- Time of Restoration :
07:00 / 26.07.2000
SYSTEM RESTORATION
Last Blackout In NR:
- Date: 2nd January 2001
- System Affected: Whole
Region (19,800 MW)
- Time of Disturbance:
04:44 / 02.01.2001
- Time of Restoration :
13:32 / 02.01.2001
Defence Plan
 Element Protection
 Line Protection
 Generator Protection
 Transformer Protection
 Relays to prevent cascade tripping
 UFR
 dF/ dT
 Under Voltage
 Islanding
 System Islanding
 Power Station Islanding
 System Protection Schemes
Defense Plan Ingredients

Defense plan need to be coordinated with planning, operations, and
maintenance

Not intended to compensate for lack of other investments
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Could help better utilize system margins, but as a last line of defense to
improve system security and prevent disturbance propagation
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Clear understanding of the requirements and consequences
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Coordination with neighboring systems
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High performance equipment
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Emphasis on security vs. dependability
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Real-time measurements and reliable communication
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Planned & designed for future system and technology expansions
Restoration Planning
Objective
 Extending start up/survival power to all the Thermal power plants
and Synchronising at least one unit at all power station
 Restoring normal system operation as early as possible
 Restoring essential loads
 Establishing all interconnections
 Minimizing amount of unserved energy
 Starting contracted and economic despatch
General Guidelines
 FORMATION OF A PLANNING TEAM
 PARTICIPATION OF EXPERIENCED/ KNOWLEDGEABLE PERSONNEL
FROM RESPECTIVE FIELDS LIKE PROTECTION, COMMUNICATION,
OPERATION, SYSTEM ANALYSIS ETC.
 REVIEW
OF SYSTEM CHARACTERISTICS (RELEVANT TO
RESTORATION)
Problems /Constraints
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Impaired communications, limited information.
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Non-availability of SCADA/EMS application system.
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Unfamiliarity with the situation (does not occur regularly)
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Non availability /breakdown of a critical element
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Time constraints re-assembling tie elements of power system.
Common Concerns
Time consuming nature of switching operation
Start-up timings of thermal units
Voltage problems during energisation of
underloaded lines
Cold load inrush, power factors and
coincident demand factors
Behaviour of protection system
System Characteristics
Structural
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System size
Metropolitan or rural
Nature of generation distribution and its mix
Transmission voltage levels
Types and sizes of load blocks
Availability of Interconnection Assistance
Dynamics
 Reactive capabilities of generators
 Generator max and min output under different conditions
 Shunt reactors and capacitor sizes and mode of control
 Charging current and maximum sustainable overvoltage
 Tap changers and modes of control
 Synchronising facilities available other than generating stations
 Fault MVA- during early stages of restoration
Formulation of Assumptions
 Wide variation of constraints under peak and lean condition
 Start up of cycling steam units under lean condition (may not be
necessarily applicable in Indian context)
 Coordination of load pickup with generator response – essential
to arrest dangerous decline of frequency particularly during peak
condition.
 Non-Availability of Black start facility during odd hours or during
week ends.
 Restricted
seasons.
Capacity of
Hydro
units
during non-monsoon
Strategies and Tactics
Restoration Process
 Bottom-up/Build-up Strategy
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Steps involved in the “Bottom-up Strategy”
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1)Select units to black-start.
2)Start and stabilize black-start units.
3)Determine restoration transmission path.
4)Begin expanding island(s) by restoring
transmission and load.
5)Synchronize island(s) when appropriate.
Restoration Process
Top-down / Build down strategy
 Restore backbone transmission system, usually from outside
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assistance.
Restore critical generating station and substation load from
transmission system.
Bring on more generation.
Restore underlying transmission system.
Continue restoring load.
Combination Approach
 Combines the “Build-up” and “Build-down”
approach.
 Steps in this approach include:
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Restoring transmission from an outside source at the
same time as building “islands” of generation.
Interconnecting “islands” with each other or outside
source when able.
Selection of Restoration Strategy
 Restoration method chosen depends on:
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Extent of blackout
Availability of outside assistance
Availability of internal black-start generation
Objectives of restoration
Utility philosophy/procedure
Restoration Tasks
Restoration Tasks
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INITIAL ASSESSMENT
SYSTEM STATUS DETERMINATION
PLANT PREPARATION SERVICES/START-UP
NETWORK PREPARATION
NETWORK ENERGISATION
LOAD RESTORATION
SYSTEM REBUILDING
Initial Assessment
 SCADA/EMS Alarm
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First indication of a problem
Barrage of alarms will appear
SCADA/EMS performance may be slowed due to amount of
alarm processing.
Communication failures
RTU failure or substation battery failure
Data received may be of questionable integrity.
Initial Assessment

Communications
 Functional communications are critical
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Assessment of the extent of a blackout.
Verify communication with
 Control centers
 Other Generating Stations
 Substations
 Verify backup communication systems
 Eliminate non-productive telephone communications.
 Call for help
 Extra manpower
System Status Determination
 Extent of black out and actual requirement
 Identification of boundaries of energised areas
 Ascertaining frequency & voltage of energised area
 Status of generating plants (hot/cold)
 Equipment overloads and troubles
 Loads interrupted by under- frequency relay operation or
direct tripping
Determining Generator Status
Determine
surviving on line
Generation
Stabilize
surviving on line
Generation
Determine status
of off-line
generation
Restore aux
power to off-line
generation
Begin start-up of
off-line blackstart generation
Determine
optimum sequence
of unit start-up
Determining Generator Status
 For generation that is still on-line determine:
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Location
Damage
Stability
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Frequency of island
Can load be added?
 Unloaded capacity
 Connectivity to the rest of the system
 Islanded completely
Determining Generator Status
 For generation off-line determine:
Status prior to blackout
 Black-start capability of unit
Unit type
 individual unit characteristics
Damage assessment
On-site source of power available or is off-site source
(cranking power) required
Availability and location of cranking power
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Determining Generator Status
 Auxiliary power should be restored to generation
stations as soon as possible.
 Short delays in restoring auxiliary power could result
in long delays in restoring generation
Determining Generator Status
 Prioritization of available cranking power to
generation depends on:
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Individual restoration plan
Start-up time of unit
Availability of on-site auxiliary power
Distance of cranking power from generation
 Effective communication with Local Control Center is
essential in this process!
Determining Generator Status
 Generating plant operators take actions to perform a safe
plant shutdown.
 Steam plant operators implement start-up procedures
immediately following a plant shutdown unless instructed
otherwise by the dispatcher.
 Governors must be in service.
 Plant operators must take action on their own
 To control frequency outside the range of 49-50.5 Hz
 To maintain coordination with appropriate load despatch
centre under control
Network Preparation
 Clearing all de-energised buses
 Global opening of all the breakers
 Sectionalising a system into sub-systems to enable parallel
restoration of islands
 Under frequency relays may have to be kept out of service at the
initial stage
 Making provision of cranking power for generating units
 Immediate resumption of power supply to the pumps meant for
high pressure cable
Reactive Power Balance
 Energising EHV circuits or High voltage cable to be avoided
as far as possible
 Shunt reactor at the far end of the cable/EHV O/H line being
energised to be taken into service first
 Radial load to be put first
 Global knowledge about the magnitude and location of
reactive reserves of the system
Load Restoration
 Priority load for restoration
Generating Unit auxiliary power
 Nuclear Station auxiliary power
Substation light and power
Traction Supplies
Supply to Collieries
Natural gas or oil supply facilities
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Start Up Power Supply to Traction
Ready availability of feeding points, transformer
capacities, contract demands, phases used etc.
details
25kV network instead of 132kV system, for
extension of power
Assistance from healthy feeding
neighbouring regions
points
in
Judicious use of power (e.g. only passenger
trains to be hauled to the nearest station)
No new trains to originate
SLDCs to check phase balancing to avoid negative
sequence problems
Load Restoration
 Frequency Control
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Maintain frequency between 49 and 50.5 Hz with an attempt
to regulate toward 50
Increase frequency to 50.00 -50.5Hz prior to restoring a
block of load.
Manual load shedding may need to be done to keep the
frequency above 49.50
Shedding approximately 6-10% of the load to restore the
frequency 1 Hz.
Load Restoration
 Frequency Control
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Restore large blocks of load only if the system frequency can be
maintained at 49.5 or higher
Restore load in small increments to minimize impact on frequency.
Do not restore blocks of load that exceed 5% of the total
synchronized generating capability.
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For example: If you have 1000 MW of generating capacity
synchronized on the system, restore no more than 50 MW of load at
one time.
Island Interconnection
 How do I know if my system is stable?
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Voltage within limits
Small voltage deviations when restoring load or transmission
Frequency within 49.5 and 50
Small frequency deviations when restoring load
Adequate reserves (spinning and dynamic)
Island Interconnection
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Synchronisation
 Frequency and voltage of the smaller island should be adjusted to match the frequency and
voltage of larger island
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Frequency and voltage in a smaller system are able to be moved more easily with smaller generation
shifts.
Failure to match frequency and voltage between the two areas can result in significant
equipment damage and possible shut-down of one or both areas.
Post-synchronism
 If possible, close any other available tie-lines between the two newly connected systems to
strengthen stability
 Larger company has more resources to control frequency
 The larger Utility/Area will control frequency while the other will resort to tie-line control through
appropriate demand/generation management.
Island Interconnection
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Benefits of Island Interconnection
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Provides a more stable combined system.
 More system inertia
 Enables quicker load pickup
Allows for sharing of reserves
 Each area now only required to approximately carry 1/2
as much reserve.
 Allows for supply of energy for load among connected
areas.
Additional control and regulation of Generation
Further opportunity to connect another island
Logistics and communication
•Close and continual co-ordination among power system,
power plant and field operators
•Neighbouring utilities, local governmental authorities to be
informed time to time
about the progress of restoration.
•To depend more on the utilities own communication facilities
Expert / Commando Group
•Commando group to be formed as the system
complexity grows.
•Group should consist of engineers from different fields
and belonging to different utilities.
•Perfect understanding in this core group
•It enchances the moral strength of field officers as well as
reduces restoration time
AUDITS AND UPDATES
A TECHNICAL PERSON OUTSIDE TIHE RESTORATION
TEAM SHOULD AUDIT THE ACTIVITIES.
AUDITED RESTORATION PLAN MUST BE UPDATED.
DOCUMENTS
MUST BE REVISED REGULARLY TO
REFLECT THE LATEST CHARACTERISTICS OF THE
SYSTEM.
CHANGES IN THE SCADA/EMS
INSTALLATION OR
MAJOR PLANT CONTROL, AVAILABLE TOOLS ALSO TO
BE INCORPORATED.
TRAINING
INSTRUCTION MANUALS OR AUDIO –VISUAL TAPES, FOR
INDEPENDENT STUDY
CLASSROOM INSTRUCTIONS
LEARNING FROM PAST EXPERIENCE DURING RESTORATION.
OPERATOR TRAINING ON SIMULATOR.
ROLE PLAY
OPERATOR’ S PROBLEM SOLVING
CAPABILITY COULD ALSO BE, EXPLORED AND DEVELOPED.
ALTERNATIVE SOURCE OF FINDING NEW IDEAS.
DETAILED INTERACTION WITH THE PERSONS INVOLVED IN
RESTORATION
DOCUMENTATION
PURPOSE: TRAINING, REFERENCE, IMPROVEMENT OF
RESTORATION PROCEDURES.
SHOULD BE READILY ACCESSIBLE AND EASILY
UNDERSTOOD.
SHOULD BE STORED IN A CONVENIENT MEDIA FOR
QUICK PROCESSING.
SHOULD BE ILLUSTRATED WITH FAMILIAR DIAGRAMS
AND CHARTS
ACTIONS REJECTED AND INCORPORATED IN THE
PLAN MUST BE RECORDED
Experience
 UFR actuated Automatic Load shedding was not fully
implemented
 Inadequate communication and telemetering arrangement
reduced the logistic to load despatchers
 Communication & Co-ordination problem
 Laid down procedure of restoration was not readily available
to all
 Procedure needs to be reviewed periodically in view of
changing configuration of the system
Few Useful Data/Thumb rules
Permissible Voltage Limits
Voltage Under Normal
Condition
Nominal
(kV)
Max permissible Voltage
at the far end of lines
normally kept energized
from one end
Max
Min
kV
220
139
231
126
209
148
245
400
420
380
420
765
800
728
800
132
Line Charging MVAR
Voltage
MVAR
Conductor used
Line Charging
800kv Class
Four Bersimis
2.91MVAR/km
400kV
Quad Moose
0.73 MVAR/km
400kV
Twin Moose
0.555MVAR/km
220kV
Zebra
0.135MVAR/km
132kV
Panther:
0. 05 MVAR/km
Approximate Voltage rise at Recv end per 100km of
uncompensated line
0.75 kV for 132kV S/C panther
1.5 kV for 220kV S/C Zebra
3.0kV for 400kV Twin Moose
Approx. Fault Contribution at HV Bus of unit transformer by
60 MW
120 MW
210 MW
500 MW
( Hydro)
(Thermal)
(Thermal)
(Thermal)
280
490
735
1800
Approx. Voiltage rise/fall at a bus due to removal/addition
of a reactor of cap. dQ MVAR
= dQ / Fault MVA of the Bus
MVA
MVA
MVA
MVA
Shift
Paradigm
Normal Mode of Operation
Maintain Status quo
Peace time Operation
Restoration
Challenging –Change Status quo
War time Operation