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 ~ 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)/ ( ) = 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)/ ( ) = 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 input de-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)
Generator Trip at Chevron Refinery Cause Massive Financial and Environment Problems
Generation Station
No. 1
Production Plant No. 1
Load ~ 120MW
No. 2 & Prod. Plt 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

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

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

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?