Power System Restoration - The Graceful Degradation Phase Mike - PDF document

Power System Restoration - The Graceful Degradation Phase Mike Adibi, IRD Corporation Bethesda, Maryland, USA madibird@aol.com (c) IRD 2004 1 Major Power System Disturbances M e a s u r e s Preventive A t Frequency Corrective t r Extent Restorative i b u Duration t e Before P s h a During s e s After (c) IRD 2004 2

Major Power System Disturbances Given: that blackouts are likely to occur, What can be done to: reduce their impact (i.e., their extent, intensity and duration)? (c) IRD 2004 3 Major Power System Disturbances Sequence of Events System === � Northeast PJM Year === � 1965 1967 Event Initial 0 0 Islands Numbers 5 3 Formed Seconds 7 5 Blackout Minutes 12 9 Restored Hours 13 8 ______________________________ Federal Power Commission Reports (c) IRD 2004 4

Major Power System Disturbances After the Initial Cause They may result in islands with: • Insufficient generation (load-rich) experiencing a decay in system frequency, or • Insufficient load (generation-rich) experiencing a rise in system frequency (c) IRD 2004 5 Turbine Blade Damage Bucket Limit Calculation 63 62.4 61.8 62 61.2 Turbine Frequency, Hertz 60.6 61 60.6 Safe Operation 60.0 60 Safe Operation 59.4 59.4 59 58.8 58 58.2 57.6 57 1 10 100 1000 Time, Minutes (c) IRD 2004 6

Immediately After the Initiating Cause Frequency rise and decay are automatically arrested by: • Load rejection, • Load shedding, • Low frequency isolation scheme, and • Controlled islanding. (c) IRD 2004 7 Immediately After the Initiating Cause Success rate: Over fifty percent ! Challenge: Coordination of control and protective systems between power plants and electrical system. (c) IRD 2004 8

Load Rejections Generation Rich To match generation with load, load rejections are used Full-load Rejection: • The main generator breaker trips • Loss of synchronization and full-load • Steam generators runback from full-load to no-load (7%). Partial-load Rejection: • The main generator breaker remains closed • Loss of partial load (10 to 30%) • Steam generator usually requires no-runback (c) IRD 2004 9 Basic System for Load Rejection ( Boiler:10 6 pph, >3,000 psi, >10 3 ° F) Superheater T BY-PASS TCV Gen SPRAY I V IP C V HP LP Steam Generator Reheater Condenser LP BY-PASS SPRAY TCV: Turbine Control Valve, IV : Intercept Valve, CV : Check Valve (c) IRD 2004 10

Intercept “Fast Valving” 120 100 Values in Percent 80 60 Transient Reduction of Turbine Power Output 40 Flow Through Intercept Valves During "Fast Valving" 20 0 0 1 2 3 4 5 Time After Fault, Seconds (c) IRD 2004 11 Load Rejection Performance Depends on % of PLR • Germany: One in two (50%) • France: One to four in five (20 to 80%) • Ontario Power: Eight in twelve reactors (66%) 1 • Italy: Few thermal units successfully load-rejected 2 • USA: Analysis of 50 BTG trips, 30% due to TG, 42% due to BT & 28% due to operators 3 ___________________________ 1. Blackout of August 14, 2003 2. Blackout of September 28, 2003 3. Very few operational, primarily due to conservative operating philosophy (c) IRD 2004 12

Under-frequency Load Shedding Load Rich To match load with generation, under- frequency load shedding is used: • Number of frequency step, 3 • Frequency set points, 59.3, 58.9 and 58.5Hz • Load shed per step, 10% • Fixed time delay per step, 5-8 cycles • Correct operation of over 50% (c) IRD 2004 13 Typical Frequency Decay Rate 60.00 59.80 Actual Initial Decay: 1.2 Hz/sec. Expected Initial decay: 1.6 Hz/sec. 59.60 59.40 Frequency in Hz 59.20 1st Step , 10% at 59.1 Hz 59.00 2nd Step, 10% at 58.9 58.80 3rd Step, 10% at 58.6 Hz 58.60 58.40 58.20 0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000 4.500 5.000 Time in Seconds (c) IRD 2004 14

Under-frequency Load Shedding • Shed radial feeders, interrupting many small loads • Restore the interrupted load manually at about 59.3 Hz • Have experienced incorrect operation at high and low temperatures • Have observed difference in frequency as much as 0.2 Hz • Assume the change in load as 0.5% per 0.1 Hz (c) IRD 2004 15 Low Frequency Isolation Scheme After the Initial Event 4 - 100 MW, 13.8 kV Generators 13.8/69 kV Xfmrs 69 kV Bus Cables 69/13.2 kV Xfmrs 13.2 kV Bus Municipal Load 2 - 5,000 HP Motors (c) IRD 2004 16

The Signgnificance of Initial Source ( Based on Generation availability) 100 Generation Restored - % 80 60 MW = 100% MWH = 100% 40 20 10% Initial Source MW = 52% MWH = 42% 0 0 2 4 6 8 10 12 Restoration Duration - Hours (c) IRD 2004 17 Low Frequency Isolation Schemes Performance Over 50 US utilities have successfully used LFIS to isolate one or more generators with matching loads. The majority: • Use automatic under-frequency relay to initiate the action, • Select generators for isolation, • Set the under-frequency relay between 58 and 58.5 Hz., & • Allow time delay of 6 to 8 cycles, and (c) IRD 2004 18

Controlled Islanding After the Initial Event • Initial Events: Faults occur and are cleared in milli-seconds • Subsequent Effects: Systems separate into islands in seconds • Final Results: Load & gen. imbalance causes blackout in minutes (c) IRD 2004 19 Conjectures (Not Fully Verified) • Out-of-Step Location: Depends on the prevailing system configuration and load level, Is independent of initial fault location or fault intensity, and Occurs one operation at a time (cascades) with adequate time interval. • Transfer Tripping Locations: Split the system into two parts (islands), and Each part having minimal load and generation imbalance. (c) IRD 2004 20

Figure 2 - Swing Curves Under Heavy-Load Condition 180 Breakers Reclose Gen. 142 H Gen. 138 H Angular Displacement in Degrees 140 Breakers Open Fault Occurs Gen. 134 CT 100 Coherent Gen. Grp 60 20 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Time in Seconds (c) IRD 2004 21 Figure 6 - Angle-Impedance relay (Buffer or Blinder) 2.0 jX 1.5 j 1.0 Del t 0.5 i 0.0 R -0.5 -0.5 0.0 0.5 1.0 1.5 2.0 (c) IRD 2004 22

Figure 4 - Apparent Impedance Path Line 136-135 9 0.30 0.28 Legend (Time in Seconds) 8 0.32 0.26 0.34 Apparent Reactance in p.u. of Line's Impedance N.O. - Normal Operation 0.24 7 0.36 F. - Fault Occurs 0.22 F.C. - Fault Clears 6 0.38 0.20 F.C. B.R. - Breakers Reclose 0.40 B.R . S. - System Settles 5 4 3 1.10 S. 2 0.10 F 0.00 N.O. 1 0 0 1 2 3 4 5 6 7 8 9 Apparent Resistance in p.u. of Line'sImpedance (c) IRD 2004 23 Figure 3 - Swing Curves Under Light-Load Condition 400 Gen. 142 H Gen. 138 H Breakers Reclose Angular Displacement in Degrees 300 Breakers Open Fault Occurs 200 Coherent Gen. Grp 100 Gen. 134 CT 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Time in Seconds (c) IRD 2004 24

Figure 5 - Apparent Impedance Path Line136 - 139 2.5 Legend (Time In Seconds) N.O. - Normal Operation 0.00 N.O. Apparent Reactance in p.u. of Line's Impedance 2 F. - Fault Occurs F.C. - Fault Clears R.T. - Relay Trips B.O. - Breakers Open 1.5 B.R. - Breakers Reclose 0.20 F.C. R.T2. - Relay Trips Again 0.10 F B.O.L. - Breakers Open & Lock 1 0.40 B.O. 0.64 R.T2. 0.32 R.T. 0.5 0.62 B.R. 0.72 B.O.L . 0 -1 -0.5 0 0.5 1 1.5 2 Ap-parent Resistance in p.u. of Line's Impedance (c) IRD 2004 25 Real Time Controlled Separation Out-of-step blocking relays: • Prevent separation where there is heavy power flow (unbalanced load and generation). Transfer tripping relays: • Allow separation where there are light power flows (balanced load and generation). (c) IRD 2004 26

The Graceful Degradation Phase Past Experience: The probability of success in retaining initial sources of power by: • Full and Partial Load Rejections, • Under-frequency Load Shedding • Low Frequency Isolation Schemes, • Controlled System Separation, and has been greater than 50%. Future Challenge: Need better control & protection coordination between: • Prime mover’s (BTG), and • Electrical systems. (c) IRD 2004 27 Restoration Stages After Subsequent Effect (c) IRD 2004 28

After the Subsequent Effect Partial or Complete Blackout Load and generation are manually balanced by: • Starting with the initial sources of power, and • Supplying the critical loads in the priority order. (c) IRD 2004 29 After the Subsequent Effect Partial or Complete Blackout Success rate: Complete blackouts need improvements. Challenge: Coordination of actions by power plants and electrical system operators. (c) IRD 2004 30

Power System Restoration After Subsequent Effect The tasks are to: • List and rank the critical loads by priority, • List and rank the initial sources of power by availability, and • Determine the most effective ways of bringing the two together. (c) IRD 2004 31 Initial Critical Loads After Subsequent Effect Priorities • Cranking Drum-Type Units High • Pipe-Type Cables Pumping System High • Transmission Stations Medium • Distribution Stations Medium • Industrial Loads Low* • Is Used in the Initial Stage to An Advantage (c) IRD 2004 32