NLDC 1

Defence plan and

System restoration

Introduction

Inter-Connected operation - widespread propagation of Inter-Connected operation - widespread propagation of disturbancedisturbance

Reliable defense plan essentialReliable defense plan essential

Isolation or Islanding of Faulty portion to save rest of the systemIsolation or Islanding of Faulty portion to save rest of the system

Load-Generation balance by UFR load-shedding required prior to Load-Generation balance by UFR load-shedding required prior to islandingislanding

Consideration of large number of contingencies required for Consideration of large number of contingencies required for designing successful islanding schemesdesigning successful islanding schemes

Common Sequence of events in blackouts

Initiating EventsInitiating Events

Formation of Formation of IslandsIslands

System System SeparationSeparation

Load /Generation Load /Generation Imbalance in islandsImbalance in islands

Blackout of Blackout of IslandsIslands

Begin Restoration Begin Restoration ProcessProcess

Effect on Society

Production Loss of productivity Loss of product or property

Health Food contamination Medication problems Anxiety

Safety Traffic accidents Accidents due to visibility problems Civil unrest

Public Scrutiny

Any widespread electric outage draws a lot of attention from: Politicians Governmental agencies

MOP ERCs Special interest groups

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 Restoration strategy may be different for each type of outage !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: Maintenance outages Heavy/Uncontrolled loop flow through system Changing generation patterns Weather Unexpected events/FAULTS Relay mal-operation Circuit breaker failure

Causes of Blackouts

Cascading Thermal over loads Voltage Instability Dynamic Instability Load Generation Imbalance

Time Horizon

Power Flow

Thermal Limit

Voltage Limit

Stability Limit

Total Transfer capability

Load

Causes of Blackouts

Voltage Collapse Deficit of MVAR Supply Over the “knee” of the voltage curve Results in system separations and generation tripping

KVKV

MWMW

safesafeunsafeunsafe

Causes of Blackouts

Voltage Collapse Difficult to predict boundaries of separation May be detected by looking for areas of voltage decay

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

KVKV

MWMW

safesafeunsafeunsafe

Var SupportVar Support

Causes of Blackouts

Dynamic Instability System does not damp out normal oscillations Groups of generators “swing” against each other resulting in

large oscillations in MW, MVAR. Could result in:

Generation tripping Voltage collapse Equipment damage

Time Frame: 5 -15 seconds

Load Generation Imbalance 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 ofMumbai (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

Could help better utilize system margins, but as a last line of defense to improve system security and prevent disturbance propagation

Clear understanding of the requirements and consequences

Coordination with neighboring systems

High performance equipment

Emphasis on security vs. dependability

Real-time measurements and reliable communication

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 TEAMFORMATION OF A PLANNING TEAM PARTICIPATION OF EXPERIENCED/ KNOWLEDGEABLE PERSONNEL PARTICIPATION OF EXPERIENCED/ KNOWLEDGEABLE PERSONNEL

FROM RESPECTIVE FIELDS LIKE PROTECTION, COMMUNICATION, FROM RESPECTIVE FIELDS LIKE PROTECTION, COMMUNICATION, OPERATION, SYSTEM ANALYSIS ETC. OPERATION, SYSTEM ANALYSIS ETC.

REVIEW OF SYSTEM CHARACTERISTICS (RELEVANT TO REVIEW OF SYSTEM CHARACTERISTICS (RELEVANT TO RESTORATIONRESTORATION))

Problems /Constraints

Impaired communications, limited information.

Non-availability of SCADA/EMS application system.

Unfamiliarity with the situation (does not occur regularly)

Non availability /breakdown of a critical element

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

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 Capacity of Hydro units during non-monsoon seasons.

Strategies and Tactics

Restoration Process

Bottom-up/Build-up StrategyBottom-up/Build-up Strategy Steps involved in the “Bottom-up Strategy”

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

Restore backbone transmission system, usually from outside assistance.

Restore critical generating station and substation load from transmission system.

Bring on more generation. Restore underlying transmission system. Continue restoring load.

Top-down / Build down strategyTop-down / Build down strategy

Combination Approach

Combines the “Build-up” and “Build-down” approach.

Steps in this approach include: 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: Extent of blackout Availability of outside assistance Availability of internal black-start generation Objectives of restoration Utility philosophy/procedure

Restoration Tasks

Restoration Tasks

INITIAL ASSESSMENT SYSTEM STATUS DETERMINATION PLANT PREPARATION SERVICES/START-UP NETWORK PREPARATION NETWORK ENERGISATION LOAD RESTORATION SYSTEM REBUILDING

Initial Assessment

SCADA/EMS Alarm 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.

Communications Functional communications are critical

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

Initial Assessment

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 Determine surviving on line surviving on line

GenerationGeneration

Determine Determine surviving on line surviving on line

GenerationGeneration

Stabilize Stabilize surviving on line surviving on line

GenerationGeneration

Stabilize Stabilize surviving on line surviving on line

GenerationGeneration

Determine status Determine status of off-line of off-line generationgeneration

Determine status Determine status of off-line of off-line generationgeneration

Restore aux Restore aux power to off-line power to off-line

generationgeneration

Restore aux Restore aux power to off-line power to off-line

generationgeneration

Begin start-up of Begin start-up of off-line black-off-line black-

start generationstart generation

Begin start-up of Begin start-up of off-line black-off-line black-

start generationstart generation

Determine Determine optimum sequence optimum sequence

of unit start-upof unit start-up

Determine Determine optimum sequence optimum sequence

of unit start-upof unit start-up

For generation that is still on-line determine: Location Damage Stability

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

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: 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

Determining Generator Status

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

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 points in neighbouring regionsJudicious 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

Start Up Power Supply to Traction

Load Restoration

Frequency Control 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 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. 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? 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)

Synchronisation Frequency and voltage of the smaller island should be adjusted to match the frequency and

voltage of larger island 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

Island Interconnection

Benefits of Island Interconnection 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

•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

Logistics and communication

•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

Expert / Commando Group

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.

AUDITS AND UPDATES

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

TRAINING

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 NormalCondition

Max permissible Voltageat the far end of lines

normally kept energizedfrom one end

Nominal(kV)

Max Min kV

132 139 126 148

220

400

765

231

420

800

209

380

728

245

420

800

Line Charging MVAR

Voltage Conductor used Line Charging MVAR

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 panther1.5 kV for 220kV S/C Zebra3.0kV for 400kV Twin Moose

Approx. Fault Contribution at HV Bus of unit transformer by

60 MW ( Hydro) 280 MVA120 MW (Thermal) 490 MVA210 MW (Thermal) 735 MVA500 MW (Thermal) 1800 MVA

Approx. Voiltage rise/fall at a bus due to removal/addition of a reactor of cap. Q MVAR

= Q / Fault MVA of the Bus

Paradigm Shift

Normal Mode of OperationNormal Mode of Operation

Maintain Status quo

Peace time Operation

RestorationRestoration

Challenging –Change Status quo

War time Operation