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Defence plan and System restoration

Defence plan and System restoration. 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

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Defence plan and System restoration

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  1. Defence plan and System restoration NLDC

  2. 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

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

  4. 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

  5. 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

  6. 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 !

  7. 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!

  8. 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

  9. Causes of Blackouts • Cascading Thermal over loads • Voltage Instability • Dynamic Instability • Load Generation Imbalance Power Flow Thermal Limit Stability Limit Voltage Limit Load Total Transfer capability Time Horizon

  10. KV safe unsafe MW Causes of Blackouts • Voltage Collapse • Deficit of MVAR Supply • Over the “knee” of the voltage curve • Results in system separations and generation tripping

  11. KV safe unsafe MW 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 Var Support

  12. 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

  13. 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

  14. 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

  15. 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

  16. 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

  17. 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

  18. Restoration Planning

  19. Objective • Extending start up/survival power to all the Thermal power plants and Synchronisingat 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

  20. 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)

  21. 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.

  22. 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

  23. System Characteristics

  24. 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  

  25. 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

  26. 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.

  27. Strategies and Tactics

  28. Restoration Process • 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.

  29. 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

  30. 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.

  31. 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

  32. Restoration Tasks

  33. Restoration Tasks • INITIAL ASSESSMENT • SYSTEM STATUS DETERMINATION • PLANT PREPARATION SERVICES/START-UP • NETWORK PREPARATION • NETWORK ENERGISATION • LOAD RESTORATION • SYSTEM REBUILDING

  34. 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.

  35. Initial Assessment • 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

  36. 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

  37. 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

  38. Determining Generator Status • 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

  39. 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

  40. 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

  41. 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!

  42. 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

  43. 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

  44. 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

  45. 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

  46. 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 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

  47. 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.

  48. 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.

  49. 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)

  50. Island Interconnection • 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.

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