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Local Area Planning Update

Local Area Planning Update. Presentation to TRANSAC January 20, 2011. Base Case Status.

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Local Area Planning Update

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  1. Local Area Planning Update Presentation to TRANSAC January 20, 2011 Electric Transmission Planning

  2. Base Case Status • Base case study models representing the base scenarios have been completed for all four seasons: Heavy Summer (HS), Heavy Winter (HW), Light Autumn (LA), and Light Spring (LSP) for the present and future over the 15 year planning horizon…2010, 2015, 2020, and 2025….16 cases total. • Existing system configuration has been modeled in 2010 case; signed projects (GIA’s, EP&C’s, etc) and budgeted/approved system improvements are included in 2015 and later cases. • Existing system configuration (2010) in all cases; 1/10 load forecast. Electric Transmission Planning

  3. Contingencies to Study • System normal (N-0), single segment or element outage (N-1), and selected “credible” double segment or element outage (N-2) contingency files have been developed/updated to facilitate simulation runs of the same system conditions through each base case. Outages include: • Lines • Transformers • Buses • Plants (Generation) Electric Transmission Planning

  4. Study Criteria • System Normal and Outage conditions were modeled and studied for adequacy and system security. • Segment or element thermal loads >80% were noted under these conditions; loads above 100% were noted as overloads (transformers were allowed up to 125% load in winter). • Overloaded segments will be “tripped” per standard relaying practice (100% thermal on lines, 175 – 200% thermal on transformers) to check for “cascading” outages except as noted below per new NERC requirements; • 230 kV lines are not tripped below 150% thermal per requirements of NERC PRC-023. • May apply to path lines or other system lines and elements deemed critical. Electric Transmission Planning

  5. Study Criteria • Voltages outside of NWE planning criteria were noted (93%, 95%, or 97% low voltage limit depending on line voltage; 105% high voltage limit). • Load-serving bus voltages may fall above or below the tabled value if conditions allow a higher/lower voltage without harm to NWE or customer equipment. • Certain equipment ratings may dictate different limits. • Existing OMS or RAS schemes were considered Electric Transmission Planning

  6. State of the System Studies • System Normal and Outage runs have been completed on all seasons of the 2010 base cases to determine the present “state of the system” as it exists today. • N-0, N-1, and N-2 study results have been analyzed for 2010 models. Electric Transmission Planning

  7. State of the System Studies Summary Results and Finding for 2010 Cases: • Heavy summer conditions govern for voltage and thermal conditions on most normal and outage scenarios (but less than 2008). Heavy winter governs in some others (more than 2008); few problems are observed under light load conditions. • Almost all higher voltage bulk system segments and elements meet criteria under all operating conditions and scenarios (500, 230, and 161 kV). • Loss of such elements may increase load on underlying system such as 161 kV auto banks and lower voltage lines, or produce low voltage problems. Electric Transmission Planning

  8. State of the System Studies Summary Results and Finding for 2010 Cases: • Most problems are observed on lower voltage system segments or elements (50, 69, and 100 kV)…older, smaller conductor or transformers, etc. • Fewer system loading and overload problems have been observed compared 2008 studies. • Minor voltage problems are beginning to appear in some new areas compared to 2008. • Great Falls area – high voltage • Missoula Area – high voltage on 100 kV system • Dillon – Sheridan 69 kV system • Lewistown Area 50 kV system Electric Transmission Planning

  9. State of the System Studies Summary Results 2010 Cases: Thermal Issues • No system elements illustrate thermal overload under system normal conditions. • Several system elements can become thermally overloaded under certain outage conditions: • Great Falls Switchyard – Great Falls City 100 kV line overloads for the loss of the Rainbow – Great Falls Northeast 100 kV line under HS conditions. This N-1 condition causes further overloads which cascade as a result. Mitigation will be completed in 2011. • Multiple overloads and widespread low voltage throughout the Billings area are caused for the loss of Billings Steam Plant 230kV bus. Further cascades may result due to this N-2 condition. Initial mitigation is scheduled for 2011, additional mitigation will be completed within 5 years. Electric Transmission Planning

  10. State of the System Studies Summary Results 2010 Cases: Thermal Issues • A bus fault (N-2 condition) during heavy loading at Missoula 4 161 kV bus produces cascading thermal overloads which results in loss of the Bitterroot Valley and three Missoula City substations. Mitigation will be completed in 2011. • A loss of the East Gallatin 161 kV bus puts Bozeman at risk and could result in significant loss of the Bozeman 50 kV system due to another transformer overload, and resulting cascades. This was considered an N-2 condition. • Certain 230, 161, and 100 kV line outages in the Columbus area may cause additional element overloads, severe low voltages, and cascading outages on the 50 kV system in the Columbus and Stillwater areas. Outages are confined to the local area. Mitigation planning continues with new alternatives under consideration. Electric Transmission Planning

  11. State of the System Studies Summary Results 2010 Cases: Thermal Issues • Ennis – Lone Mountain 69 kV line or Bozeman area auto transformers may overload for certain outages in the Bozeman or Ennis areas under heavy winter conditions. Cascades are unlikely, but low voltages may occur, especially in the Big Sky area. Mitigation is in progress. • Several substations have two auto transformers operated in parallel (in service side by side). In some cases, loss of one bank will cause the sister unit to overload. • In many of these cases, problems had been discovered previously, and mitigation plans are already in process. Electric Transmission Planning

  12. State of the System Studies Summary Results 2010 Cases: Voltage Issues • Under normal operating conditions, minor high voltage problems are observed in the Great Falls, and Hi-Line areas in all study seasons. A combination of mitigation alternatives are under consideration. Also, high voltage has been observed in the Missoula area. • Summary of high voltage issues under N-0 conditions: • Great Falls area 100 kV system has high voltage under all season conditions. Mitigation should be completed by 2011. • Missoula area 100 kV and the Bitterroot 69 kV system has high voltage due to loss of a large industrial load. Tap changes on the four 161/100 kV auto transformers in the area should correct the problem in 2011. Electric Transmission Planning

  13. State of the System Studies Summary Results 2010 Cases: Voltage Issues • Under normal operating conditions, minor low voltage are observed under heavy load conditions. • Under N-0 and N-1 conditions the Columbus area 50 and 100 kV system voltage are currently declining and worsen with outages. Heavy loading seasons govern this condition. Partial mitigation is planned for 2011, long term mitigation is still under review. • Low voltage may occur after the loss of some system elements, including the following: • Clyde Park 161/50 kV auto transformer or bus fault, resulting in widespread low voltage across the area 50 kV system under HS or HW conditions. A spare transformer is on-site. Electric Transmission Planning

  14. State of the System Studies Summary Results 2010 Cases: Voltage Issues • For N-2 conditions, a Mill Creek 161 kV bus outage presents low voltage, overloads and widespread outage risks to the Butte/Helena areas. Initial mitigation will start in 2011 and be completed in 2012. Further long term solutions are planned within approximately five years. Electric Transmission Planning

  15. State of the System Studies Summary Results 2010 Cases: Voltage Issues • Columbus Rapelje – Alkali Creek 161 kV line under all operating conditions. Critical to the Columbus area voltages. • Columbus Rapelje – Lower Duck Creek 161 kV line under heavy load conditions. Critical to Big Timber – Livingston area. • Dillon – Sheridan – Ennis 161 kV line under HS conditions. • Three Rivers 161/100 kV auto bank under HS conditions. • Loss of other auto banks around the system may produce low voltage problems (Ennis, Columbus, Harlowton, Glengarry, Sheridan)…the high voltage links to low voltage systems are more critical today. Electric Transmission Planning

  16. State of the System Studies Summary Results 2010 Cases: Stability Problems • Stability (Dynamics) runs have not yet been performed on the 2010 base cases, or any others. • These runs should be completed later this year. • No major problems are anticipated. Electric Transmission Planning

  17. Additional Progress and Next Steps • Analysis of 2015 and some 2020 cases is complete. • Complete the analysis of 2020 and 2025 cases. • Verification of new problems • Prioritize problems using Decision Rules. • Begin or continue Mitigation Studies Electric Transmission Planning

  18. Local Area Planning Update Questions? Electric Transmission Planning

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