1. Small Signal Stability Analysis Study: study prepared by Powertech Labs Inc. for ERCOT
Vance Beauregard
AEP
9 April 2002

2. Classification Of Power System Stability
Angle Stability
Frequency Stability
Voltage Stability
Angle Stability
Angle Stability
Frequency Stability
Frequency Stability
Voltage Stability
Voltage Stability
Small Signal Stability
Transient Stability
Large Disturbance
Voltage Stability
Small Disturbance
Voltage Stability
Small Signal
Stability
Large Disturbance
Large Disturbance
Small Disturbance
Small Disturbance
Small Signal Stability
Transient Stability
Transient Stability
Voltage Stability
Voltage Stability
Voltage Stability
Voltage Stability
Short Term
Long Term
Long Term
Short Term
Short Term
Short Term
Long Term
Long Term
Long Term
Long Term
Short Term
Short Term
9 April 2002

3. Eigenvalue And Stability
Stability of the linearized system is described by the eigenvalues of the state matrix
A real eigenvalue, or a pair of complex eigenvalues, is usually referred to as a mode
For a complex mode =j, two quantities are of main interest:
Frequency (in Hz)
Damping ratio (in %)
The system is unstable if  is negative
To ensure the acceptable performance, a damping margin in the range of 3%-5% is normally required
9 April 2002

4. Modal Characteristics
While an eigenvalue indicates the stability, its right and left eigenvectors give much more information on the characteristics of the mode
The right eigenvector shows the mode shape, i.e., the observability of the mode
A mode should be observable from generator rotor oscillations if the generator is high in its mode shape
A weighted left eigenvector shows the participation factors, i.e., the controllability of the mode
A mode should be controllable from generator if the generator is high in its participation factors
A generator which is high in the mode shape of a mode is not necessarily high in the participation factor of the same mode
9 April 2002

5. Powertech’s Approach
To perform a comprehensive small signal stability study on the ERCOT system
Overall system performance assessment from a modal perspective
Optimal tuning for new or existing PSS to provide the best damping
Results fully verified by time-domain simulations
To assess multiple scenarios to ensure the robustness of the recommended control settings
Load levels
Generation dispatches
Transmission configurations
To incorporate appropriate study criteria
Contingencies
Damping criteria
9 April 2002

6. Task 6 – Tuning Of Controls
Only generator controls (mainly AVR and PSS) will be considered for tuning
Tuning will be performed for the worst cases identified in Task 5
Generators to be tuned will be determined by the participation factors
Parameters of the existing controls will be tuned for best damping
New PSS will be recommended to provide additional damping if necessary
Tuning will be based on the damping criteria set in Task 2
A combination of the following methods will be used to derive the optimal setting of the control parameters:
Phase compensation
Sensitivity analysis
Eigenvalue analysis
The performance of the tuned controls will be verified by time-domain simulations
9 April 2002

7. Project Objectives
Examination of the small signal stability of the ERCOT interconnected system, particularly the sensitivities of the critical system modes with respect to different loading, transfer levels, system configurations, and contingencies.
Tuning of the power system stabilizers (PSS) in the ERCOT area so as to provide optimal damping for the critical modes
This project is focused only on small signal (oscillatory) stability; other types of system security concerns (such as transient stability or voltage stability) are not examined
9 April 2002

8. Data Assembly Four base models were provided by ERCOT
2001 light load
2001 peak load
2004 light load
2004 peak load
All at market transfer
Major data modifications/updates
Generator parameter updates to fix limit violation problems during initialization of the models
Addition of 4 Sand Hill PSS
Addition of 18 AEP PSS
18 AEP PSS were not included in most studies as they were provided very late in the project
Some benchmarking was made to show the impact of these PSS
These PSS have been tuned in a previous project by Powertech
9 April 2002

9. Creation of Study Scenarios
High transfer scenarios
7 high transfer scenarios for each loading condition
Total 32 scenarios - including the marketing (base) transfer scenarios
West-east
East-West
North-south
Target transaction amount for each transfer may not be reached in the final scenario because of generation availability and powerflow solution
South-North
From-Houston
South-Rio Grande valley
Rio Grande valley-south
9 April 2002

10. Creation of Study Scenarios (Cont’d)
Configuration change scenarios
Used only to examine the sensitivities of the critical modes to some transmission enhancement projects expected to complete by 2004
Constructed based on the 2004 peak load high south-north transfer scenario
The following projects were removed from the base model individually
Morgan Creek-Twin Buttes-Red Creek-Comanche Switch 345 kV Line
Graham-Jacksboro 345 kV Line
San Miguel-Pawnee-Coleto Creek 345 kV Line
Monticello-Farmersville-Valley Junction-Anna Switch 345 kV Line (powerflow does not converge)
Venus-Liggett 345 kV Line
Anna Switch-Collin-NW Carrollton 345 kV Line Upgrade
5 scenarios were created
9 April 2002

11. North-South Mode
Summary of the mode
Mode shape
9 April 2002

12. East-West Mode
Summary of the mode
Mode shape
9 April 2002

13. Center-North/South Mode
Summary of the mode
Mode shape
9 April 2002

14. Critical Contingencies
Selected from the worst N-1 and N-2 contingencies
Applied to all 32 scenarios for PSS tuning verification
9 April 2002

15. Analysis of the Configuration Change Scenarios
Two main inter-area modes were compared
The transmission project in southern ERCOT has noticeable positive impact on the north-south mode
North-south mode
East-west mode
9 April 2002

16. Impact of the PSS in the AEP Area
Addition of 18 PSS in the AEP area significantly improves the damping of the inter-area modes (all results are with tuned PSS and without contingencies)
In order to maintain a minimum level of damping for the inter-area modes, the following minimum number of PSS must be kelp in service in the AEP area
4 for a 3% minimum damping
7 for a 5% minimum damping
9 April 2002

17. Details of the north-south mode: 2004 Peak Load
High south-north transfer
No contingency was applied for the eigenvalue analysis
Time-domain simulation plot is for the contingency shown
9 April 2002

18. 9 April 2002

19. 9 April 2002