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Remedial Action Schemes: Practical Solutions for Power System Stability Problems

Remedial Action Schemes: Practical Solutions for Power System Stability Problems. Scott Manson, PE March, 2011. What Dictates Power System Stability?. Frequency Response Characteristic Major Disturbances Volt/ MVAr Margins Frequency/MW Margins Economics Undesired Oscillations.

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Remedial Action Schemes: Practical Solutions for Power System Stability Problems

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  1. Remedial Action Schemes:Practical Solutions for Power System Stability Problems Scott Manson, PE March, 2011

  2. What Dictates Power System Stability? • Frequency Response Characteristic • Major Disturbances • Volt/MVAr Margins • Frequency/MW Margins • Economics • Undesired Oscillations

  3. Governors/Turbines Simply Can’t Respond Instantly Red – Electrical Power

  4. Typical Governor Controller WGOV1

  5. Frequency Depressions • Most turbines control packages trip off at ~ 57.5 Hz to protect themselves from damage • Large, Expensive Motors trip for same reason • Will Cascade into uncontrolled blackouts S S Power Out Power In – = Df/dt w J

  6. frequency decay rate proportional to the magnitude of the power deficit

  7. Frequency Response Characteristic • Many different definitions and names throughout the world • R, FRC, dF/dP, etc • Some countries (not US) define generator FRC requirements • Effects Dominated by: • Load composition • System Inertia • Generator Tuning

  8. Frequency Response Characteristic (FRC) Example for large offshore NGL plant Sudden increase of 0.3 pu load

  9. Three common FRC Variants • Point A - ‘Transient’ FRC = 50 (0.3)/ (50-48.7) = 11.5 • Point B – Locked Rotor FRC = Extraction mode FRC = 50 (0.3)/ (50-48) = 7.5 • Point C – ‘System Long Term FRC’ = ‘System Droop Characteristic’ = 50 (0.3)/ (50-49.4) = 25

  10. What does FRC tell you about a Power System? • A quantity of ‘stiffness’ • Example: Long Term FRC • 25*150 MW/50Hz = 75 MW/Hz • 75 MW of load will reduce system frequency by 1 Hz • Extraction Mode FRC = 22.5 MW/Hz • Transient FRC = 34.5 MW/Hz

  11. Solutions for a Poor FRC • Governor tuning • Add Inertia • Limit electronic loads • More Synchronous Machines • BIG Battery Backed Statcom • Load Shedding • Generation Shedding/Runback

  12. SEL Project to improve Power Quality Presidio, TX (By Controlling Some Big Batteries)

  13. Power Corridor Transport Limits • Out of Step (OOS) Behavior Lethal to machines and power systems • Thermal limits must be obeyed to prevent conductor damage

  14. Jim Bridger Power Plant – Long History of Severe Faults and OOS behavior

  15. Power System Overview

  16. SEL RAS Protection Required • Prevent loss of stability caused by • Transmission line loss • Fault types • Jim Bridger Plant output levels • WECC requires Jim Bridger output reduced to 1,300 MW without RAS

  17. Stability Studies Determine RAS Timing Requirements • Total time from event to resulting action must not exceed 5 cycles • 20 ms available for RAS, including inputde-bounce and output contact

  18. JB RAS Also protects against… • Subsynchronous resonance (SSR) protection – capacitor bypass control • Transmission corridor capacity scheduling limits

  19. Dynamic Remedial Action for Idaho Power Co.

  20. Idaho Power System Conundrum • Maintain the stability, reliability, and security • Operate system at maximum efficiency • Prevent permanent damage to equipment • Minimal Capital expenditures • Maximize Revenue • Serve increasing load base

  21. RAS Was Lowest Cost Solution • New transmission line: $100s of millions • New transmission substation: $10s of millions • This project: approximately $2 million

  22. RAS Functional Requirements • Protect lines against thermal damage • Optimize power transfer across critical corridors • Predict power flow scheduling limits dynamically • Follow WECC requirements • Track Changing power system topography • 20 ms response requirement

  23. RAS Actions Based on Combinations of Factors • N events (64) • J states (64) • System states (1,000) • Arming level calculation • Action tables combinations (32) • Crosspoint switch (32x32)

  24. Gain Tables Allow Operations to Adjust RAS Performance for Any System Event • 7 gain entries used in arming level equation • 64 N events • 32 actions • 1,000 system states • 4 seasons • 8,192,000 possible gains per gain entry • 57,344,000 total gains

  25. RAS Gains Configured From HMI

  26. Most Sophisticated RAS in the World exists in South Idaho

  27. Major Disturbances Put Power Systems at Risk • Faults • Critical Clearing Time to prevent OOS • Fault Type • Protection speed • Fast breakers • Load startup or trip (FRC problem) • Generator trip (FRC problem)

  28. Asian Electrical Operating Company (National Grid) 3 x 34.5 MW ea. 4 x 32 MW ea 2 x 105 MW ea. Generation Station No. 1 Generation Station No. 2 & Prod. Plt 2 Load ~ 40MW Generation Station No. 3 & Prod. Plt. No. 3 Load ~ 60MW Production Plant No. 1 Load ~ 120MW Fig. 1 – Simplified One-Line Asian Oil Production Complex Generator Trip at Chevron Refinery Cause Massive Financial and Environment Problems Potential for power system collapse

  29. Contingency Output Remediation Trip G1 Trip G2 Trip G3 Trip G4 Bypass C1 Bypass C2 X X N1 N2 X X N3 X X N4 X X X N5 X X X Generation Tripping Remediated by sub-cycle load shedding Techniques Invented at SEL f Trigger Inputs Crosspoint Switch t Preloaded and Ready to Go CB Opens Tripping Outputs

  30. Generation Tripping Problem Requires a sub-cycle Load Shedding Scheme

  31. Three main techniques for Load Shedding • Contingency-based (aka ‘FLS’) • Tie line • Bus Tie • Generator • Asset Overloads • U/F based • Traditional technique in relays (lots of problems) • Enhanced SEL technique, generally a backup to contingency-based system • U/V based

  32. Contingency Based Load Shed Systems for Chevron Plant • Sub cycle response time prevent frequency sag • Advises operator of every possible future action • Expandable to thousands of sheddable loads with modern protocols • Tight integration to existing protective relays

  33. Contingency Based Load Shed system for Chevron • Must have live knowledge of machine IRMs, Spinning Reserves, Power output • initiating event is the sudden loss (circuit breaker trip) of a generator, bus coupler breaker, or tie breaker. • perform all of their calculations prior to any contingency event • System topology tracking

  34. Typical Volt/VAR Stability problems • Typical problems • Fault induced long term suppressed voltage conditions • Large Motor Starting Risk Plant blackouts • Typical Solutions • Dynamic control of exciters on large synchronous motors • FACTS devices • Misc power quality improvement electronics

  35. Low Cost Solution: Controlling Exciters on 15 MVA SM on a 700 MW GOSP preserves VAR margins

  36. How to contain a Voltage Collapse? • Increase generation – reduce demand, match supply and demand • Increase reactive power support • Reduce power flow on heavily loaded lines (use Flexible AC Transmission Systems) • Reduce OLTC at distribution level, to reduce loads and avoid blackouts (Brownout)

  37. Frequency/MW Margins • Problem1: Long Term Problem. Caused by Insufficient Reserve Margins (RM) of generation. Solution: Add more generators. • Problem2: Short Term Problem. Caused by insufficient Incremental Reserve Margin (IRM) of generators. • Solution1: RAS load/generation shedding • Solution2: Machines with larger IRM

  38. Typical Steam Turbine IRM characteristic Output (%) 100 % 0 % Time (Seconds) 500 0

  39. Typical IRM values • Steam Turbines: 20-50% • Combustion Turbines • Single Shaft Industrials: 5-10% • Aero Derivatives: 10 – 50% • Hydro Turbines: 1 - 25%

  40. Economics Affecting Stability • Danger: Fewer, larger generators • Less expensive, more efficient • More risk upon losing one generator • Economic Dispatch Contradicting Stability Optimization • NIMBY: Local Thermal/ Remote Hydro plants • MW transactions across critical corridors put plants or system islands at risk

  41. Solution: Active Load Balancing and Tie flow control for Optimal Stability • Economic Dispatch (Low Risk Scenarios) • Tie line flows (MW) per contracted schedule • Distributes MW between units per Heat Rate • Tie-line closed (High Risk Scenarios): • Control intertie MW to a user defined low value • Distributes MW between units, equal % criterion • Tie-line open (Islanded Operation – high risk) • Control system frequency to a user defined set-point • Distributes MW between units , equal % criterion

  42. Common PowerMAX Screen:AGC/VCS Interface

  43. Common PowerMAX Screen:ICS Interface

  44. Unwanted Oscillations • Explain Spectrum of a power system • Sub Synchronous Resonance (SSR) • First detected in 1970’s during commissioning of high speed/gain exciters • Mechanical/Electrical Mode Interaction • Shaft oscillation modes • Heavily Series compensated lines • Improperly Reactive Compensation in Exciters

  45. Power System Stabilizers • Provide Damping based on two possible input types: • Frequency (Hz)/Speed (rpm) – US • Power (MW) - Europe

  46. Any Questions?

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