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September 2011 Pacific Southwest Disturbance. Presented by George V. Noller John Page. Course Objectives. By the end of this session, you should be able to: Describe the primary cause of the September 8 2011 Southwest Blackout as stated in the NERC report .

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September 2011 pacific southwest disturbance

September 2011Pacific SouthwestDisturbance

Presented by

George V. Noller

John Page

Course objectives
Course Objectives

  • By the end of this session, you should be able to:

    • Describe the primary cause of the September 8 2011 Southwest Blackout as stated in the NERC report.

    • List applicable NERC Reliability Standards that were referenced in the report on the September 8 2011 event

    • Sequentially list the major events of the disturbance

    • Give a brief description of the restoration process used after the blackout


  • Review the findings of the NERC report regarding the September 8 2011 Southwest Outage

    • This is not intended to critique operator actions


Nerc standards
NERC Standards

  • COM-002-2, R2

    • Issue directives in a clear and concise manner

    • Three-part communication

  • EOP-001-2.1b

    • Developing, maintaining and implementing emergency plans

  • EOP-003-2

    • Shed load rather than risking uncontrolled failure or cascade

  • EOP-005-2

    • Returning system to normal following a disturbance

  • EOP-006-2

    • Coordination with Reliability Coordinator

  • TOP-004-2

    • Operate so that instability, uncontrolled separation, or cascading outages will not occur as a result of the most severe single contingency


  • A regional disturbance occurred on the afternoon of September 8, 2011 which led to cascading outages and loss of load

  • SDG&E, IID and CFE had complete system outages

  • APS and WALC had customers interrupted in the lower Colorado River area


  • This disturbance occurred on a heavily loaded summer day

  • Load interrupted:

    • SDG&E 4293 MW

    • CFE 2150 MW

    • IID 929 MW

    • APS 389 MW

    • WALC 74 MW


  • Report findings: The system was not being operated in a secure state for an N-1 outage due to:

    • Lack of information sharing between entities

    • Lack of adequate studies

    • Sub -100 kV facilities not adequately considered in next-day studies

  • Initiating event: Loss of APS Hassayampa-North Gila 500 kV Line due to an operating error

  • All load was restored in approximately 12 hours

  • Restoration proceeded rather smoothly

State of the system
State of the System

  • A very warm day in the Pacific Southwest with typical high system loads and heavy east-to-west transfers into Southern California

  • Paths leading into the disturbance area were loaded at greater than 50% of path ratings with the highest path at 78%

  • SDG&E imports were at 89% of limit

57 MW

1620 MW

70 MW

1285 MW

1391 MW

Sequence of events event 1
Sequence of Events – Event 1

  • At 1527, APS Hassayampa-North Gila 500 kV Line relayed

  • APS technician was conducting switching to externally bypass the line series capacitor at North Gila Substation

  • The technician performed the switching out of sequence and opened the capacitor line disconnect prior to closing the bypass disconnect

  • This action caused the disconnect to flashover (phase to phase) as it was incapable of interrupting load current

Sequence of events event 11
Sequence of Events – Event 1

  • The loss of this 500 kV line caused the underbuilt 230, 161 and 92 kV systems in the area to load up

  • At IID’s Coachella Valley Substation, the No.1 and No.2 Banks 230/92 kV exceeded their normal ratings by just over 25%, which was within the minimum trip for the bank overcurrent relays

  • CFE lost 156 MW of generation in the Mexicali area and requested assistance from CAISO, which was provided

  • APS told WECC RC that they would be placing the line back in service momentarily

  • Post-disturbance studies showed that the phase angle difference would have been too high for a successful synchronization

Sequence of events event 2
Sequence of Events – Event 2

  • 15:28:16 IID Coachella Valley No.1 and No.2 Banks 230/92 kV relayed on overcurrent .

  • This caused an extreme overload on IID Ramon Sub No.1 Bank 230/92 kV – greater than 207% of full load rating (225 MVA)

  • Severe voltage depression occurred in the WALC 161 kV system near Blythe Substation as the flows from Arizona tried to find ways into the SDGE/IID/CFE area

Sequence of events event 21
Sequence of Events – Event 2

  • Path 44 (South of SONGS) increased to 6600 amperes following this event. The intertie overcurrent relays were set at 8000 amperes

  • Low voltages were observed throughout Southern California and the Colorado River Valley

  • Manual and automated capacitor switching commenced

105 MW


2030 MW


150 MW


2407 MW


North Gila-Imperial Valley

500 kV line flow reverses

to feed Yuma Area Load.

60 MW

CFE Unit

160 MW

Sequence of events event 3
Sequence of Events – Event 3

  • 15:32:10: Less than five minutes after the trip of H-NG, Ramon Sub No.1 Bank 230/92 kV relayed on overcurrent

  • IID northern transmission system entered a voltage collapse condition, with 92 kV voltages at 45% of normal momentarily

  • IID UVLS operated as well as 161 kV and 92 kV lines relaying, causing a loss of nearly 50% of IID’s load

  • WALC’s 161 kV system south of Blythe continued to overload and voltages continued to drop

Sequence of events event 31
Sequence of Events – Event 3

  • CAISO initiated unit starts through exceptional dispatch

  • APS continued steps to return the 500 kV line to service

  • South of SONGS flows increased to 7800 amperes momentarily and settled out at 7200 amperes

  • Voltages continued to decline in the area

105 MW

2120 MW


IID Lost 400 MW

2607 MW


1532: IID Blythe-Niland

and Coachella Valley-

Niland 161 kV Lines

Relay and Ramon 230/92

Banks Relay

60 MW

CFE Unit

160 MW

Sequence of events event 4
Sequence of Events – Event 4

  • 15:35 APS Yuma load pocket separates from the WALC/IID161 kV system when WALC Gila Sub No.1 and 2 Banks 161/69 kV, APS Yucca Sub No.1 and No.2 Banks 161/69 kV and IID Pilot Knob No.1 and 2 Banks 161/92 kV relayed on overcurrent

  • Cogeneration in the Yuma area relayed while carrying 52 MW

  • APS load was now radially fed from the SDG&E system through Imperial Valley to North Gila

  • South of SONGS loading increased to 7800 amperes

Sequence of events event 41
Sequence of Events – Event 4

  • CAISO increased the amount of exceptional dispatch in an attempt to reduce South of SONGS loading. Speed limited by existing technology

  • The additional units requested had a 10 minute start time

  • WALC ordered MWD to shed 80 MW pump load to assist with voltage recovery in the Parker Dam area

Sequence of events event 5
Sequence of Events – Event 5

  • For approximately 25 seconds, a high speed cascade occurs, which separates SDG&E, IID, CFE and portions of APS and WALC from the interconnection

  • IID El Centro-Pilot Knob 161 kV Line relays on zone 3 distance protection

  • SCE Blythe Energy RAS operated, tripping 128 MW of generation

  • IID ‘S’ Line RAS operated, opening Imperial Valley-El Centro 230 kV Line ‘S’ separating IID from the interconnection

105 MW

2120 MW



2890 MW


1537: IID RAS Trips La

Rosita Units and relays

IV-El Centro 230 kV

Line. IID Southern

System Blacked out.



330 MW


La Rosita

420 MW

APS Unit

51 MW

CFE Unit

160 MW

105 MW

2120 MW



2890 MW


1534: IID and APS

Units Relay

1537: WALC & IID

Separate at Pilot Knob

IID Unit

44 MW

118 MW


APS Unit

51 MW

CFE Unit

160 MW

Sequence of events event 6
Sequence of Events – Event 6

  • South of SONGS flows increased to 9500 amperes momentarily and dropped to 8700 amperes, above the minimum trip setting of 8000 amperes

  • Approximately 20 seconds later, the SONGS System Separation Scheme operated, separating SDG&E, CFE and portions of APS & WALC from the interconnection

  • There were no alarms at SDG&E, SCE or CAISO that indicated the flows were above minimum trip

Sequence of events event 7
Sequence of Events – Event 7

  • 15:38 After separation, the islanded area frequency decayed rapidly

  • UFLS occurred throughout the island, but subsequent generator trips prevented the island from balancing

  • UF relays also operated to separate SDG&E, CFE and APS from each other

  • At approximately 7 seconds after separation, the islands collapsed

  • Within several seconds of separation, both San Onofre units relayed when rapid acceleration occurred momentarily (due to change in the impedance of the system) and reactor protection operated

  • 11 minutes from loss of 500 kV Line to collapse

1538: San Onofre System Separation

initiates on overload and Units 2 and

3 subsequently trip during disturbance.



400 MW



4300 MW



550 MW

IID Unit




390 MW



2200 MW


  • According to the NERC report, none of the affected entities utilized their black start plans due to availability of ties with neighboring utilities

    • South of SONGS and Hassayampa – North Gila

  • Outside in restoration

  • Once started, restoration proceeded rather smoothly


  • Delays were encountered at San Onofre in resetting the intertie overcurrent lockout relay due to plant conditions not meeting the requirements to reset

  • This delay caused the dispatcher at SCE to order the operator at San Onofre to reset the relay


  • After resetting the lockout relay, restoration in the SDG&E system commenced

  • The Hassayampa-North Gila 500 kV Line was restored to service and provided SDG&E with two sources for restoration – one from SCE and the other from APS


  • The two sides of the system were synchronized at 2258, seven hours and 20 minutes after the collapse

  • This was delayed due to discussions occurring between the CAISO and the TOPs regarding closing angle limitations.


  • IID system load restored at 2140

  • CFE system load restored at 0137 9/9/11

  • SDGE system load restored at 0323 9/9/11


  • Challenges encountered during the restoration

    • Communication with San Onofre operators

    • Voltage control with lightly loaded lines and San Onofre units off

    • Phase angle difference between CA and AZ with no prior guidelines on limits

    • Maintaining adequate limits on paths

    • Having resources available for rapid restoration


  • Items that worked well

    • Communication between CAISO and TOPs went relatively smooth – Possibly due to CETAC?

    • Coordination between SCE and SDG&E went very well due to annual regional drills.

    • System performed as expected


  • Are there any sub-100 kV areas in your system that could affect the reliability of the BES?

    • SCE 66 kV system in Antelope area

    • SCE 55 kV system to Nevada

    • SDG&E 69 kV underbuilt

    • PGAE 60/70 kV underbuilt

    • CFE 69 kV underbuilt

Report findings
Report Findings

Some of the findings of the report:

  • Failure to Conduct and Share Next-Day Studies

  • Sub-100 kV Facilities Not Adequately Considered in Next-Day Studies

  • Lack of Real-Time External Visibility

Report findings1
Report Findings

  • Inadequate Real-Time Tools

  • Reliance on Post-Contingency Mitigation Plans

  • WECC RC Staffing Concerns

  • WECC RC and affected TOPs and BAs do not consistently recognize the adverse impact sub-100 kV facilities can have on BPS reliability

  • Lack of Review and Studies on Impact of SPSs

Some of the findings of the report:


  • NERC/FERC took over the analysis of the disturbance and issued their report without input from the entities affected

  • The report only covers the causes of the disturbance and their recommendations

  • Any possible violations of NERC standards and any potential sanctions are still being investigated