State of the System Studies

1. State of the System Studies Presentation to TRANSAC October 23, 2008 Electric Transmission Planning

2. Purpose • State of the system studies will be used to determine system needs through the 15 year planning horizon. • Ascertain compliance with NERC TPL reliability standards, with emphasis on the bulk electric system (100 kV and above) • Reveal critical outages, weak links, operational constraints, and future planning and budget requirements. Electric Transmission Planning

3. System Models Recap • Base case study models used: Heavy Summer (HS), Heavy Winter (HW), Light Autumn (LA), and Light Spring (LSP) for the present and future over the 15 year planning horizon…2008, 2013, 2018, and 2023….16 cases total. • Existing system configuration (2008) in all cases; 1/10 load forecast. No proposed generation, mitigation, or planned system upgrades included. Electric Transmission Planning

4. Contingencies Studied • System normal (N-0), single segment or element outage (N-1), and selected “credible” double segment or element outage (N-2) and extreme contingencies files have been studied through each base case. Outages include: • Lines • Transformers • Buses • Plants (Generation) Electric Transmission Planning

5. Study Criteria • System Normal and Outage conditions were modeled and studied for adequacy and 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 were “tripped” per standard relaying practice (100% thermal on lines, 175 – 200% thermal on transformers) to check for “cascading” outages. • Voltages outside of NWE planning criteria were noted (93%, 95%, or 97% low voltage limit depending on line voltage; 105% high voltage limit). • 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 2008, 2013, 2018, and 2023 base cases to determine the present “state of the system” as it exists today. • N-0, N-1, and some N-2 and extreme study results have been analyzed. • Dynamic/transient studies are not yet completed. Electric Transmission Planning

7. State of the System Studies Summary Results and Findings: • As expected, heavy summer conditions govern voltage and thermal conditions on nearly all normal and outage scenarios in most areas, especially for the lower voltage system. • Heavy winter conditions governs in some areas (Bozeman/Big Sky and Missoula/Hamilton). • Light autumn conditions govern for the high voltage bulk electric system (230 and 500 kV) in most cases. Electric Transmission Planning

8. State of the System Studies Summary Results and Findings: • No major problems with the higher voltage bulk electric system are observed. Almost all higher voltage bulk system segments and elements meet criteria under all operating conditions and scenarios (500, 230, and 161 kV). Minor thermal problems observed under outage conditions are now addressed with OMS procedures: • Minor overload of 500 kV series capacitors • Minor overloads of 500 kV transformers and lines. • Loss of the Garrison BPA 500/230 kV auto transformer may cause area voltage problems in the future. Electric Transmission Planning

9. State of the System Studies Summary Results and Findings: • Most other problems observed are known or were anticipated, and exist on lower voltage system segments or elements (50, 69, and 100 kV)…older, smaller conductor or transformers, etc. Transformer overloads significantly out number line overloads. In many cases, mitigation is planned or in study. • Several outages can cause low voltage problems or cascade into other system elements, but all are confined to local areas. • Minor occurrences of high voltages are observed along the Hi-Line, and in some cases, the Rainbow and Taft areas. Electric Transmission Planning

10. State of the System Studies Trends: • As expected, without mitigation or enhancements, voltage and thermal problems grow and spread into the future with load growth. More outages become critical. • Thermal loads and problems grow • Voltages decline • Substation related outages (transformers or buses) become more and more critical into the future, surpassing line outages. • Outages that were “survivable” in the past become much more critical into the future. Electric Transmission Planning

11. State of the System Studies Summary Results: Thermal Issues • At present, only one system element illustrates minor thermal overload under system normal conditions (104% on Mill Creek Bank 3 Tertiary winding due to connected reactor load). Mitigation is already planned. • As loads grow into the future, other system elements reach thermal limitations under system normal conditions: • Within 5 years: • Missoula Miller Creek #4 100/69 kV Auto Banks • With 10 years: • Billings Steam 50/69 kV Auto Bank • Jackrabbit 161/50 kV Auto Bank • Within 15 years: • Trident 100/50 kV Auto Bank • Clyde Park – Wilsal 161 kV Line (presently current limited by air break switches and wave traps). Electric Transmission Planning

12. State of the System Studies Summary Results: Thermal Issues • Several system elements can become thermally overloaded under certain outage conditions. In addition to those elements listed under normal system conditions: • Certain 230, 161, 100, and 50 kV line outages in the Columbus area may cause element overloads (Columbus Rapelje 161/100 kV auto transformer), severe low voltages, and cascading outages on the 50 kV system in the Columbus and Stillwater areas. Outages are confined to the local area, and remaining bus voltages are within criteria at present. Mitigation planning has begun. • 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 planning has begun. Electric Transmission Planning

13. State of the System Studies Summary Results: Thermal Issues • Billings Rimrock 161/100 kV auto transformers may overload for loss of the Billings Steamplant 230/100 kV auto transformers. • Garrison – Anaconda BPA 230 kV line may overload up to for loss of the Mill Creek – Garrison 230 kV line. • Mill Creek 230/161 kV 200 MVA auto transformer may overload for the loss of the 400 MVA sister unit. • Many other elements begin to overload under outage conditions in the future. Electric Transmission Planning

14. State of the System Studies Summary Results: Thermal Issues • 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. • Bus outages become much more critical in the future: Steam Plant, East Gallatin, Clyde Park, Mill Creek, Missoula 4, and Garrison bus outages may produce serious voltage and thermal problems now or in the future. Also Ennis and Dillon-Salmon. • In many of these cases, problems had been discovered previously, and mitigation plans are already in process. Electric Transmission Planning

15. State of the System Studies Summary Results: Voltage Issues • Under normal operating conditions, minor high voltage problems are observed in the Butte, Taft, and Hi-Line areas in all study seasons throughout the planning horizon. • It is expected that this condition can be easily corrected with transformer tap changes, generation set point changes, or shunt devices. • Verification of this condition is still required. Electric Transmission Planning

16. State of the System Studies Summary Results: Voltage Issues • Under normal operating conditions at present, no low voltage criteria violations are observed in all study seasons, but voltages are tending low in the Chrome Jct and Townsend areas. • Within 5 years, minor voltage problems are appearing in the Chrome Jct and Townsend/Helena area 100 kV systems under normal conditions. • Within 10 years, the voltage problems in these areas are growing in scope and severity. 100 kV systems (and associated low voltage systems) in the Butte, Hamilton, and Bozeman – Billings areas begin to show problems. • Within 15 years, as previous problems worsen, problems begin to appear on the Laurel – Bridger area 100 kV system, and the Butte – Bozeman area 161 kV systems. Electric Transmission Planning

17. State of the System Studies Summary Results: Voltage Issues • Low voltage may occur after the loss of some system elements, including the following, all worse with time: • Columbus Rapelje – Alkali Creek 161 kV line under all operating conditions. Critical to the Columbus area voltages. Also area auto banks. • Clyde Park 161/50 kV auto transformer, resulting in widespread low voltage across the area 50 kV system. • Columbus Rapelje – Lower Duck Creek 161 kV line. Critical to Big Timber – Livingston area. • Billings Steamplant 230/100 kV auto transformer(s). • Shorey – Wilsal 230 kV line. • Great Falls – Judith Gap 230 kV line • Three Rivers – Wilsal 230 kV line • Rattlesnake 230/161 auto transformer Electric Transmission Planning

18. State of the System Studies Summary Results: Voltage Issues • Dillon – Sheridan – Ennis 161 kV line. • South Butte 161/100 kV auto transformer. • Trident area 100 kV lines. • Canyon Ferry generation under HS conditions. • Three Rivers 161/100 kV auto bank. • 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

19. State of the System Studies Summary Results: Stability Problems • Stability (Dynamics) runs have not yet been performed. • These runs should be completed this fall. • No major problems are anticipated. Electric Transmission Planning

20. Next Steps • Prioritize critical outages for mitigation work. • Complete dynamics/stability runs. • Begin Mitigation Studies Electric Transmission Planning

21. State of the System Studies Questions? Electric Transmission Planning