Remedial Action Schemes: Practical Solutions for Power System Stability Problems
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Transcript of Remedial Action Schemes: Practical Solutions for Power System Stability Problems
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Copyright © SEL 2010
Remedial Action Schemes:Practical Solutions for Power System
Stability Problems
Scott Manson, PE
March, 2011
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What Dictates Power System Stability?
• Frequency Response Characteristic
• Major Disturbances
• Volt/MVAr Margins
• Frequency/MW Margins
• Economics
• Undesired Oscillations
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Governors/Turbines Simply Can’t Respond Instantly
0.000 2.000 4.000 6.000 8.000 10.000 12.000 14.000 16.000 18.00091.000
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Time (Secs)
Pow
er (M
W)
Blue – Mechanical Power
Black –Speed
Note lag in response
Red – Electrical Power
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Typical Governor Controller
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+ –
s1
1.0
vmin
–
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Cf
+
Cp
Generator Power
Pmwset
Nset
Turbine Speed
Nref
Kimws
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Rate Limited Tracking
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s
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11 sTth
1 sTsa1 sTsb
s7 s8Plim
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ue S
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Speed**Dm
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Frequency Depressions
J=
Power In –w
S Power OutSDf/dt
• 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
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frequency decay rate proportional to the magnitude of the power deficit
300002500020000150001000050000
Time (ms)
454647484950
Mag
nitu
de (M
ag)
Case 1 Case 2
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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
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Frequency Response Characteristic (FRC) Example for large offshore NGL plant
Sudden increase of 0.3 pu load
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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
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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
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Solutions for a Poor FRC
• Governor tuning
• Add Inertia
• Limit electronic loads
• More Synchronous Machines
• BIG Battery Backed Statcom
• Load Shedding
• Generation Shedding/Runback
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SEL Project to improve Power Quality Presidio, TX (By Controlling Some Big
Batteries)
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Power Corridor Transport Limits
• Out of Step (OOS) Behavior Lethal to machines and power systems
• Thermal limits must be obeyed to prevent conductor damage
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Jim Bridger Power Plant – Long History of Severe Faults and OOS
behavior
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Power System OverviewBoise
Midpoint
Portland
GoshenKinport
Borah
Adel
Hunt
Salt Lake
Jim Bridger
500 kV345 kV230 kV138 kV
Legend:
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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
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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
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JB RAS Also protects against…• Subsynchronous resonance (SSR)
protection – capacitor bypass control
• Transmission corridor capacity scheduling limits
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Dynamic Remedial Action for Idaho Power Co.
Oregon
Idaho
Wyoming
Utah
Nevada
Montana
California
Portland
BoisePath 17
J
Substation A
B CD
EG
Salt Lake City
Washington
380-mile drive between Substation A and Substation J
138 kV230 kV345 kV500 kV
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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
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RAS Was Lowest Cost Solution
• New transmission line: $100s of millions
• New transmission substation: $10s of millions
• This project: approximately $2 million
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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
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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)
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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
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RAS Gains Configured From HMI
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Most Sophisticated RAS in the World exists in South Idaho
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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)
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Generator Trip at Chevron Refinery Cause Massive Financial and
Environment Problems
Generation Station No. 1
Production Plant No. 1 Load ~ 120MW
Generation Station No. 2 & Prod. Plt 2 Load ~ 40MW
Generation Station No. 3 & Prod. Plt. No. 3 Load ~ 60MW
Fig. 1 – Simplified One-Line Asian Oil Production Complex
Asian Electrical Operating Company (National Grid)
4 x 32 MW ea 3 x 34.5 MW ea.
2 x 105 MW ea.
Potential for power system collapse
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Generation Tripping Remediated by sub-cycle load shedding Techniques
Invented at SEL
Crosspoint Switch
f
tCB
Opens
Tripping Outputs
TriggerInputs
X
Trip G2
N5
N4
XN3
XN2
N1
Trip G1
Output RemediationContingency
Trip G3
X
Trip G4
X
Bypass C1
X
Bypass C2
X
X
X XX
X
Preloaded and Ready to Go
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Generation Tripping Problem Requires a sub-cycle Load Shedding Scheme
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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
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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
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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
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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
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Low Cost Solution: Controlling Exciters on 15 MVA SM on a 700 MW GOSP preserves VAR margins
11
12
13
14
15
16
0 100000 200000 300000 400000
13.8kV Motor Bus Voltage (Starting Motor Bus Only)
Electrotek Concepts® TOP, The Output Processor®
Magn
itude
(Mag
)
Time (ms)
MBUS2V - VAR Control MBUS2V - Voltage Control Only
MBUS2V - Voltage Control plus Gen
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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)
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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
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Typical Steam Turbine IRM characteristic
Output (%)
Time (Seconds)
100 %
500 0
0 %
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Typical IRM values
• Steam Turbines: 20-50%
• Combustion Turbines ♦ Single Shaft Industrials: 5-10%♦ Aero Derivatives: 10 – 50%
• Hydro Turbines: 1 - 25%
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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
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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
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Common PowerMAX Screen:AGC/VCS Interface
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Common PowerMAX Screen:ICS Interface
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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
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Power System Stabilizers• Provide Damping based on two possible
input types:♦ Frequency (Hz)/Speed (rpm) – US♦ Power (MW) - Europe
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Any Questions?