NLDC1 Defence plan and System restoration

Introduction Inter-Connected operation - widespread propagation of disturbance Inter-Connected operation - widespread propagation of disturbance Reliable defense plan essential Reliable defense plan essential Isolation or Islanding of Faulty portion to save rest of the system Isolation or Islanding of Faulty portion to save rest of the system Load-Generation balance by UFR load-shedding required prior to islanding Load-Generation balance by UFR load-shedding required prior to islanding Consideration of large number of contingencies required for designing successful islanding schemes Consideration of large number of contingencies required for designing successful islanding schemes

Common Sequence of events in blackouts Initiating Events Formation of Islands System Separation Load /Generation Imbalance in islands Blackout of Islands Begin Restoration Process

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 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 KV MW safe unsafe

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 KV MW safe unsafe Var 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: 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: 30 th July System Affected: Whole Region except parts of Mumbai (21,500 MW) - Time of Disturbance: 20:11 / Time of Restoration : /

SYSTEM RESTORATION Last Blackout In ER: - Date: 25 th July System Affected: Whole Region (7,300 MW) - Time of Disturbance: 21:10 / Time of Restoration : 07:00 /

SYSTEM RESTORATION Last Blackout In NR: - Date: 2 nd January System Affected: Whole Region (19,800 MW) - Time of Disturbance: 04:44 / Time of Restoration : 13:32 /

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 TEAM FORMATION OF A PLANNING TEAM PARTICIPATION OF EXPERIENCED/ KNOWLEDGEABLE PERSONNEL FROM RESPECTIVE FIELDS LIKE PROTECTION, COMMUNICATION, OPERATION, SYSTEM ANALYSIS ETC. PARTICIPATION OF EXPERIENCED/ KNOWLEDGEABLE PERSONNEL FROM RESPECTIVE FIELDS LIKE PROTECTION, COMMUNICATION, OPERATION, SYSTEM ANALYSIS ETC. REVIEW OF SYSTEM CHARACTERISTICS (RELEVANT TO RESTORATION) REVIEW OF SYSTEM CHARACTERISTICS (RELEVANT TO RESTORATION)

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 Strategy Bottom-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 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

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 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 black- start generation Determine optimum sequence of 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 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 regions  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 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 Hz prior to restoring a block of load.  Manual load shedding may need to be done to keep the frequency above  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

 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 Normal Condition Max permissible Voltage at the far end of lines normally kept energized from one end Nominal (kV) MaxMin kV

Line Charging MVAR VoltageConductor usedLine Charging MVAR 800kv ClassFour Bersimis2.91MVAR/km 400kV Quad Moose 0.73 MVAR/km 400kVTwin Moose0.555MVAR/km 220kVZebra0.135MVAR/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 ( Hydro)280MVA 120 MW(Thermal)490MVA 210 MW (Thermal)735MVA 500 MW(Thermal)1800MVA 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 Operation Maintain Status quo Peace time Operation Restoration Challenging –Change Status quo War time Operation