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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

Remedial Action Schemes:Practical Solutions for Power System Stability Problems

Scott Manson, PE

March, 2011

what dictates power system stability
What Dictates Power System Stability?
  • Frequency Response Characteristic
  • Major Disturbances
  • Volt/MVAr Margins
  • Frequency/MW Margins
  • Economics
  • Undesired Oscillations
frequency depressions
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



Power Out

Power In –





frequency response characteristic
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
frequency response characteristic frc example for large offshore ngl plant
Frequency Response Characteristic (FRC) Example for large offshore NGL plant

Sudden increase of 0.3 pu load

three common frc variants
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

what does frc tell you about a power system
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
solutions for a poor frc
Solutions for a Poor FRC
  • Governor tuning
  • Add Inertia
  • Limit electronic loads
  • More Synchronous Machines
  • BIG Battery Backed Statcom
  • Load Shedding
  • Generation Shedding/Runback
power corridor transport limits
Power Corridor Transport Limits
  • Out of Step (OOS) Behavior Lethal to machines and power systems
  • Thermal limits must be obeyed to prevent conductor damage
sel ras protection required
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
stability studies determine ras timing requirements
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
jb ras also protects against
JB RAS Also protects against…
  • Subsynchronous resonance (SSR) protection – capacitor bypass control
  • Transmission corridor capacity scheduling limits
idaho power system conundrum
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
ras was lowest cost solution
RAS Was Lowest Cost Solution
  • New transmission line: $100s of millions
  • New transmission substation: $10s of millions
  • This project: approximately $2 million
ras functional requirements
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
ras actions based on combinations of factors
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)
gain tables allow operations to adjust ras performance for any system event
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
major disturbances put power systems at risk
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)
generator trip at chevron refinery cause massive financial and environment problems

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

generation tripping remediated by sub cycle load shedding techniques invented at sel


Output Remediation

Trip G1

Trip G2

Trip G3

Trip G4

Bypass C1

Bypass C2


















Generation Tripping Remediated by sub-cycle load shedding Techniques Invented at SEL




Crosspoint Switch


Preloaded and Ready to Go

CB Opens

Tripping Outputs

three main techniques for load shedding
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
contingency based load shed systems for chevron plant
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
contingency based load shed system for chevron
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
typical volt var stability problems
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
how to contain a voltage collapse
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)
frequency mw margins
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
typical steam turbine irm characteristic
Typical Steam Turbine IRM characteristic

Output (%)

100 %

0 %

Time (Seconds)



typical irm values
Typical IRM values
  • Steam Turbines: 20-50%
  • Combustion Turbines
    • Single Shaft Industrials: 5-10%
    • Aero Derivatives: 10 – 50%
  • Hydro Turbines: 1 - 25%
economics affecting stability
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
solution active load balancing and tie flow control for optimal stability
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
unwanted oscillations
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
power system stabilizers
Power System Stabilizers
  • Provide Damping based on two possible input types:
    • Frequency (Hz)/Speed (rpm) – US
    • Power (MW) - Europe