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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

9 April 2002

9 April 2002