1. April 15 and May 15, 2003 ERCOT System Disturbances ERCOT TAC Meeting June 4, 2003

2. 2 April 15, 2003 Bryan-College Station Voltage Collapse

3. 3 What Happened? At 11:39 CenterPoint Energy took an ERCOT approved planned outage of the Roans Prairie to Kuykendall 345KV line to accommodate highway department work When breakers at Roans Prairie and Kuykendall opened, clearing the line, breakers controlling the following 345 KV lines also opened: –Gibbons Creek – Roans Prairie –Gibbons Creek – O’Brien –Gibbons Creek – Twin Oak (one line) Leaving Gibbons Creek plant connected through only one breaker to Twin Oak and both buses serving the two Gibbons Creek 345/138KV autotransformers deenergized

4. 4 What Happened? (cont.) With both autotransformers deenergized, there was not enough support of the load serving 138KV system in the Bryan-College Station area Voltage collapsed in the area resulting in a blackout of approximately 212 MW of load and 63,000 people Load in the area was completely restored in about 4 hours around 1600.

5. 5 Why it Happened The deenergization of Roans Prairie to Kuykendall 345KV line caused increased flows on Roans Prairie to Gibbons Creek 345 KV line Faulty control equipment at Gibbons Creek sensed this increased flow as a fault and also incorrectly initiated a breaker failure operation that deenergized both Gibbons Creek busses and the two autotransformers serving the Bryan - College Station area

6. 6 One-Line of B-CS Area

7. 7 What’s Been Done About It? Faulty control equipment at Gibbons Creek has been fixed by TMPA ERCOT is working with Garland to prevent telemetry failures that happened during the event from reoccurring The System Protection Working Group of the Reliability and Operations Subcommittee of TAC is investigating to see if additional follow-up is needed

8. 8 May 15, 2003 Under-Frequency Firm Load Shedding

9. 9 What Happened? At about 2:54 a.m. an insulator flashed on the Comanche Peak – Parker 345 KV line, probably due to lightning Primary protective relaying at Comanche Peak did not respond Back-up protective relaying at Comanche Peak also failed to respond Breaker failure relaying at each substation connected to Comanche Peak tripped all 345 KV lines connected to Comanche Peak

10. 10 What Happened? (cont.) With loss of lines out of the plant, both Comanche Peak units were tripped (total 2275 MW) A number of other units in ERCOT (total 1146 MW) tripped a few seconds afterward and another large unit (775 MW) tripped about 43 seconds later ERCOT frequency got down to 59.26 Hz 471 MW of High-set Underfrequency load providing Responsive Reserve tripped at 59.7 Hz The first stage of underfrequency firm load shedding representing 5% of ERCOT load (~1549 MW) tripped at 59.3 Hz Frequency returned to normal in about 12 minutes All firm load restored by 6:30 am

11. 11 ERCOT Frequency

12. 12 Going Forward Bad News – It happened - It happened at 3:00 am Good News – It happened at 3:00 am - A successful real test of firm underfrequency load shedding Further Action –ERCOT and System Protection Working Group will get details of relay failure and follow up –ERCOT will determine why generation other than Comanche Peak tripped and what might be done to avoid in future