A September 2005 power outage that affected two million people in the California Southland was initiated when workers cut live electrical wires after consulting erroneous design drawings, but it was exacerbated by a number of extant problems with local generation and protection configurations.
The outage occurred while utility workers were installing new protective relay equipment for a transformer at a local receiving station, RS-E. The project was part of a systemwide effort to upgrade relays at 179 substations. Acting on a faulty drawing that had cable numbering errors, a utility worker cut a bundle of three wires that were supposed to be left intact, triggering a short circuit that led to the shutdown of other transmission and generation stations. Because the specific facilities are secondary to the sequence of incidents, the generation and transmission facility names have been made generic.
Sequence of Events Leading to the Voltage Collapse and Recovery
About 12:35 p.m. local time, Sept. 12, 2005, both 230-kV busses at RS-E relayed simultaneously due to a worker inadvertently activating breaker failure protection on both RS-E busses. This immediately de-energized 390 MW of load at RS-E and RS-S. All RS-E transmission circuits opened at RS-E. This resulted in 2,447 MW of load being supplied by 635 net MW of H Generation Station and S Generation Station and by four transmission circuits: T-O Lines 1 and 3, and V-C Lines 1 and 2. These circuits were loaded above their normal ratings. System voltage fell to around 90% of nominal, but sub-transmission dispatchers stabilized and restored the voltage by immediately energizing the available 34.5-kV capacitors.
All generation at H and S tripped off within 7 minutes of the disturbance. Voltage fell to 72% of nominal, but sub-transmission dispatchers again stabilized and restored it by manually shedding 395 MW of load and by energizing additional 34.5-kV capacitors.
Attempts to restore transmission at RS-E failed due to the large power angle, later estimated by system simulations to be around 40 degrees. Thirty minutes into the disturbance, T-O Line 3 relayed off line after sagging into a tree. The remaining three circuits became extremely overloaded, voltage fell to 77% of nominal, and sub-transmission dispatchers shed 195 MW of load in order to stabilize the voltage.
Five minutes later T-O Line 1 relayed off line due to relay misoperation. V-C Lines 1 and 2 then loaded so high that relay protection interpreted the heavy loading as a fault and tripped both circuits. The system blacked out due to loss of supply, de-energizing the remaining 1,584 MW of load not already manually shed. During the several cycles it took for the blackout to completely occur, underfrequency load-shedding (UFLS) relays automatically disconnected 1,040 MW of load.
T-O Line 3 was restored 25 seconds after the system blackout, picking up 544 MW of system load not already disconnected by UFLS action. After T-O Line 1 and V-C Lines 1 and 2 were restored, system load was low enough that the resulting small power angle at RS-E allowed RS-E transmission restoration to begin. Dispatchers fully restored all transmission and load within 51 minutes of the system blackout without further incident.
During the 84-minute event, eight generators and seven condensers either tripped off or became isolated, 23 bulk power transmission circuits either relayed or became open-ended, 39 bulk power transformers became either open-ended or de-energized, and 15 receiving stations blacked out. A major sewage treatment facility, a major international airport, four major oil refineries, and 897,992 customers experienced a service interruption.
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