1. Grid Operations Update
Kent Saathoff
ERCOT BOD Meeting
May 20, 2003

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

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. 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
Load in the area was completely restored in about 4 hours around 1600.

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 relays 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. One-Line of B-CS Area

7. What’s Been Done About It?
Faulty relays at Gibbons Creek have 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. May 15, Under-Frequency
Firm Load Shedding

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. What Happened? (cont.)
With loss of lines out of the plant, both Comanche Peak units were tripped
A number of other units in ERCOT tripped a few seconds afterward and another large unit tripped about 43 seconds later
ERCOT frequency got down to 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. ERCOT Frequency

12. Going Forward Bad News – It happened
Good News – It happened at 3:00 am
- A successful real test of underfrequency load shedding
Further Action
ERCOT and System Protection Working Group will get details of relay failure and follow up
ERCOT will examine relay maintenance and calibration procedures of TSPs