Event of 10/22/09

1. Event of 10/22/09 A single line to ground fault on an adjacent line caused a condition that placed two wind farms on a radial connection to a set of series capacitors

2. Partial System Diagram Lon Hill PS Nelson Sharpe Ajo Rio Hondo

3. Future Facilities Open Breaker 10/1/2009 345 kV System The series capacitors are at Rio Hondo They provide 50% line compensation of the Z of the Rio Hondo to Lon Hill line There are two stages of capacitors (17% and 33%) Both stages are usually in service AJO 67 miles To Rio Hondo To Nelson Sharpe Zorillo ERCOT Metering Gulf Wind 100 Mvar 100 Mvar 37 miles Fault occurred on this line 2-559 AAAC OPGW 7 miles Double Circuit Sarita ERCOT Metering 96 MW 93.6 MW Penascal 345 kV Gulf Wind South 138 kV 69 kV 12.47 – 34.5 kV

4. Synopsis of the event • A single line to ground fault occurred on the Ajo to Nelson Sharpe line due to a downed static wire. • The initial fault was cleared in 2.5 cycles. • The wind farms became radially connected to the to the Rio Hondo series compensated line. • The system experienced overvoltages (up to about 195%) which resulted in the trip of the Rio Hondo to Ajo line and the shunt reactors at Ajo. • The series capacitors bypassed approximately 1.5 seconds into the event. • The series capacitor controls indicated subsynchronous currents during the event. • This event caused numerous failures of crow bar circuits at the two windfarms.

5. Iline A I subsynch A Iline B I subsynch B Rio Hondo Series Capacitors

6. Rio Hondo SEL421 at Ajo

7. Ajo 311L at Zorillo

8. Conclusions and Follow Up • Ongoing studies by AEP and Electranix • Subsynchronous interactions between wind generators and the series capacitors are a major concern. • This scenario could be a precursor to what may occur in the CREZ if complacency becomes the order of the day.