1. VRT Study Update José Conto ERCOT System Planning

2. 2 Dynamic Studies System Planning May 2010 Voltage Ride-Through Boundaries For WTG Ref.: ERCOT Op. Guides, Section 3.1.4.6.1

3. 3 Dynamic Studies System Planning May 2010 Type IType IIType IIIType IV NEG Micon NM72 0.7 MW Vestas V47 660 kW GE 1.5 MW Kennetech 33-MVS 400kW Mitsubishi MWT1000A 1 MW Vestas V66 1.75 MW Vestas V90 1.8 /3 MW Siemens 2.3 MW Bonus 1.3 MW Vestas V80 1.8 MW Gamesa G80, G83 & G87 2.0 MW Clipper C93 2.5 MW Vestas V82 1.65 MW Suzlon S88 2.1 MW Mitsubishi MWT-92/95 2.4 MW Vestas NM72 1.65 MW Nordex N80 2.5 MW 1404 MW 16.5 % 463 MW 5.5 % 5042 MW 59.3 % 1598 MW 18.7 % Wind Turbine Generator Types * 2009 Fall case

4. 4 Dynamic Studies System Planning May 2010 The VRT Study Identify system reliability risks associated with existing wind plants that do not have full VRT capabilities. –Phase I Dynamic simulations utilizing existing ERCOT model data set (PSS ® E ver. 30) –Phase II Verify wind plant modeling and develop model improvements as necessary Collect modeling data directly from wind plant owners –Phase III Dynamic simulations utilizing updated models based on model verification (PSS ® E ver. 31)

5. 5 Dynamic Studies System Planning May 2010 Phase II Wind plant modeling outcome – –Collection of actual field data needed for plant modeling of all the wind plants in ERCOT –Methodology to develop a collector system equivalent and wind turbine aggregation Steady State test Active and Reactive Power Loss of wind plant Active and Reactive Power Exchange at POI Short-Circuit test Short-Circuit Current Levels at POI for 3-Phase Faults Dynamic Response test Electrical Power Output Response Terminal Voltage Response System Frequency and Voltage Response

6. 6 Dynamic Studies System Planning May 2010 Phase III Dynamic simulations utilizing updated models. HWHL and HWLL cases rerun.  Scenarios: 1.HWLL case w/no conventional generation in West Texas 2.HWHL case w/dynamic motor load modeled in West Texas 3.HWLL case w/no conventional generation and w/dynamic motor load modeled in West Texas 4.Investigation of the impact of wind generator terminal and/or POI voltage set point on VRT response 5.Investigation of problematic contingencies to develop potential solutions that ensure an operational margin 6.Application of the updated wind models to an ERCOT 2010 base case

7. 7 Dynamic Studies System Planning May 2010 Simulations for HWHL and HWLL cases completed. Simulations for scenarios 1,2 and 3 completed. Results are being finalized. Scenarios 4 and 5 are under investigation. Scenario 6 has not started. Final Report outline has been developed and certain sections are currently under documentation. Presentation scheduled for June ROS meeting. Phase III Status

8. 8 Dynamic Studies System Planning May 2010 Currently, no small signal stability limit on West to North when no conventional generation is on-line in the West; thermal limits only Based on study with updated models, voltage stability limit has potential to bind before thermal limits with no conventional generation on-line The critical contingencies: double circuit with and without fault and line-to-ground fault with breaker failure. Reduction in output at any of the units west of the West-to-North interface eliminate the non-convergence problem. ERCOT Operations will implement this new conditional stability limit ASAP. No Conventional Generation in West Texas – Stability Limit?

9. 9 Dynamic Studies System Planning May 2010 Far West interface composed of three 345 kV lines west of Morgan Creek substation. West-North interface flow kept at ~1976 MW. A stability limit is reached when the Far West interface flow is about 1300 MW (~ 1150 MW Far West wind output.) Thermal overload binding before reaching the stability limit. ERCOT PI data for the period 7/1/2009 to 5/1/2010 indicates that the highest flow on the interface into Morgan Creek was 1140 MW and that the W-N stability limit was constraining most of the time when high transfer into Morgan Creek was observed. A stability limit will not be implemented for the Far West. Far West Texas – Stability Limit?

10. 10 Dynamic Studies System Planning May 2010 José Conto Supervisor, Dynamic Studies System Planning Electric Reliability of Texas, Inc. 512-248-3141 jconto@ercot.com