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

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

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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

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SLIDE 1

(c) IRD 2004 1

Power System Restoration - The Graceful Degradation Phase

Mike Adibi, IRD Corporation Bethesda, Maryland, USA madibird@aol.com

(c) IRD 2004 2

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

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SLIDE 2

(c) IRD 2004 3

Major Power System Disturbances Given: that blackouts are likely to

  • ccur,

What can be done to: reduce their impact (i.e., their extent, intensity and duration)?

(c) IRD 2004 4

Major Power System Disturbances Sequence of Events

System === Northeast PJM Year === 1965 1967 Event Initial Islands Numbers 5 3 Formed Seconds 7 5 Blackout Minutes 12 9 Restored Hours 13 8

______________________________ Federal Power Commission Reports

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SLIDE 3

(c) IRD 2004 5

Major Power System Disturbances After the Initial Cause

They may result in islands with:

  • Insufficient generation (load-rich)

experiencing a decay in system frequency,

  • r
  • Insufficient load (generation-rich)

experiencing a rise in system frequency

(c) IRD 2004 6

Turbine Blade Damage

Bucket Limit Calculation

57 58 59 60 61 62 63 1 10 100 1000 Time, Minutes Turbine Frequency, Hertz 60.6 61.2 61.8 62.4 59.4 58.8 58.2 57.6 60.0 60.6 59.4 Safe Operation Safe Operation

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SLIDE 4

(c) IRD 2004 7

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 8

Immediately After the Initiating Cause

Success rate: Over fifty percent ! Challenge: Coordination of control and protective systems between power plants and electrical system.

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SLIDE 5

(c) IRD 2004 9

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 10

Basic System for Load Rejection (Boiler:106 pph, >3,000 psi, >103 °F)

TCV: Turbine Control Valve, IV : Intercept Valve, CV : Check Valve

Reheater Condenser Superheater Steam Generator

Gen

TCV T BY-PASS I V LP BY-PASS C V

HP IP LP

SPRAY SPRAY

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SLIDE 6

(c) IRD 2004 11

Intercept “Fast Valving”

20 40 60 80 100 120 1 2 3 4 5

Time After Fault, Seconds Values in Percent

Transient Reduction of Turbine Power Output Flow Through Intercept Valves During "Fast Valving"

(c) IRD 2004 12

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-rejected2
  • USA: Analysis of 50 BTG trips, 30% due to TG, 42%

due to BT & 28% due to operators3

___________________________

  • 1. Blackout of August 14, 2003
  • 2. Blackout of September 28, 2003
  • 3. Very few operational, primarily due to conservative operating philosophy
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SLIDE 7

(c) IRD 2004 13

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 14 Typical Frequency Decay Rate

58.20 58.40 58.60 58.80 59.00 59.20 59.40 59.60 59.80 60.00 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 Frequency in Hz 1st Step , 10% at 59.1 Hz 2nd Step, 10% at 58.9 3rd Step, 10% at 58.6 Hz Actual Initial Decay: 1.2 Hz/sec. Expected Initial decay: 1.6 Hz/sec.

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SLIDE 8

(c) IRD 2004 15

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 16

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

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SLIDE 9

(c) IRD 2004 17

The Signgnificance of Initial Source (Based on Generation availability)

20 40 60 80 100 2 4 6 8 10 12

Restoration Duration - Hours Generation Restored - %

10% Initial Source MW = 100% MWH = 100% MW = 52% MWH = 42%

(c) IRD 2004 18

Low Frequency Isolation Schemes

Performance

Over 50 US utilities have successfully used LFIS to isolate

  • ne 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
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SLIDE 10

(c) IRD 2004 19

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 20

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.

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SLIDE 11

(c) IRD 2004 21 Figure 2 - Swing Curves Under Heavy-Load Condition 20 60 100 140 180 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Time in Seconds Angular Displacement in Degrees Fault Occurs Breakers Open Breakers Reclose

  • Gen. 142 H
  • Gen. 138 H

Coherent Gen. Grp

  • Gen. 134 CT

(c) IRD 2004 22

Figure 6 - Angle-Impedance relay (Buffer or Blinder)

  • 0.5

0.0 0.5 1.0 1.5 2.0

  • 0.5

0.0 0.5 1.0 1.5 2.0 Del t

i j

R jX

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SLIDE 12

(c) IRD 2004 23

Figure 4 - Apparent Impedance Path Line 136-135 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9

Apparent Resistance in p.u. of Line'sImpedance Apparent Reactance in p.u. of Line's Impedance

Legend (Time in Seconds) N.O. - Normal Operation

  • F. - Fault Occurs

F.C. - Fault Clears B.R. - Breakers Reclose

  • S. - System Settles

0.00 N.O. 0.10 F 0.20 F.C. 0.40 B.R. 1.10 S. 0.30 0.32 0.34 0.36 0.38 0.28 0.26 0.24 0.22

(c) IRD 2004 24

Figure 3 - Swing Curves Under Light-Load Condition 100 200 300 400 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 Time in Seconds Angular Displacement in Degrees Fault Occurs Breakers Open Breakers Reclose

  • Gen. 142 H
  • Gen. 138 H
  • Gen. 134 CT

Coherent Gen. Grp

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SLIDE 13

(c) IRD 2004 25

Figure 5 - Apparent Impedance Path Line136 - 139 0.5 1 1.5 2 2.5

  • 1
  • 0.5

0.5 1 1.5 2

Ap-parent Resistance in p.u. of Line's Impedance Apparent Reactance in p.u. of Line's Impedance

Legend (Time In Seconds) N.O. - Normal Operation

  • F. - Fault Occurs

F.C. - Fault Clears R.T. - Relay Trips B.O. - Breakers Open R.T2. - Relay Trips Again B.O.L. - Breakers Open & Lock

0.00 N.O. 0.10 F 0.20 F.C. 0.32 R.T. 0.40 B.O. 0.62 B.R.

B.R. - Breakers Reclose

0.64 R.T2. 0.72 B.O.L.

(c) IRD 2004 26

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).

Real Time Controlled Separation

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SLIDE 14

(c) IRD 2004 27

The Graceful Degradation Phase

Past Experience: The probability of success in retaining initial sources

  • f 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 28

Restoration Stages

After Subsequent Effect

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SLIDE 15

(c) IRD 2004 29

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
  • rder.

(c) IRD 2004 30

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.

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SLIDE 16

(c) IRD 2004 31

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 32

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
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SLIDE 17

(c) IRD 2004 33

Initial Sources of Power

After Subsequent Effect

Minutes Success Probability Run-of-the-River Hydro 5-10 High Pump-Storage Hydro 5-10 High Combustion Turbine 5-15 Medium (50%) Tie-Line with Adjacent Systems Short Not Relied On * * Policy: Provide Remote Cranking Power

(c) IRD 2004 34

Restoration After a Blackout

Preparation Stage (1 to 2 Hours)

  • Evaluate Pre-Disturbance Condition & the Post-Disturbance Status
  • Define the Target System
  • Restart Generators & Rebuild Transmission Network

System Restoration (3 to 4 Hours)

  • Energize Transmission Path
  • Restore Load to Stabilize Generation and Voltage
  • Synchronize Islands and Reintegrate Bulk Power System

Load Restoration (8 to 10 Hours)

  • Load Restoration is the Governing Control Objective
  • Load Pickup is Scheduled Based on Generation Availability
  • Load Restoration is Effected in Increasingly Larger steps