1. Local Area Planning Update to TRANSAC – September 26, 2018
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Local Area Planning Update to TRANSAC – September 26, 2018

2. Base Case Status
Base case study models representing the near-term scenarios have been completed:
Heavy Winter (Winter Peak)
2019 Light Spring (Off-peak)
2019 Heavy Summer (Summer Peak)
Future scenario cases over the 15-year planning horizon will be completed shortly
Summer Peak: HS, 2028 HS, 2033 HS
Winter Peak: HW, HW, HW
Off-Peak: LSP
10 total scenario base cases will be developed.
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3. Base Case Status
Existing system configuration has been modeled in near-term base cases ( ).
Projects under construction, as well as budgeted and approved system improvements are included in the future scenarios.
All peak scenario cases were based on a 1-in-10 load forecast.
All off-peak scenario cases represent 65% of peak load.
Wind dispatched at 50%.
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4. Contingencies List
Near-term Scenario Events Studied – single point of failure:
P0 – System normal
P1 – Single segment or element outage
Generators
Transmission Lines
Transformers
Capacitors and Reactors
P2 – Bus Faults and Breaker Failure

5. Contingencies List
Future Scenario Events to be studied (Steady-State):
P0 – System normal
P1 – Single segment or element outage
Generators
Transmission Lines
Transformers
Capacitors and Reactors
P2 – Bus Faults and Breaker Failure
P3 – Loss of a Generator followed by:
Loss of a Transmission Line
Loss of a Transformer
P6 – Loss of a Transmission Line followed by:
P7 – Loss of two Transmission Lines on a common structure

6. Study Criteria
System normal and contingencies were modeled and studied for adequacy and system security.
System protection and automatic system adjustments were simulated:
Normal Fault Clearing
Generator Automatic Voltage Regulation (AVR)
Thermal Protection
Capacitor and Reactor Switching
LTC adjustments
Special Protection Schemes

7. Study Criteria Transmission Line and Transformer thermal loading:
Loadings >90% were noted.
Loadings above 100% were noted as overloads.
Winter ratings were applied allowing lines and transformers up to 125% continuous load.
Voltages outside of NWE planning criteria were noted:
Voltage deviation >8% for P1 contingencies (per WECC-100 Criteria)
Low voltage: <90% to 95% depending on nominal voltage and contingency (per FERC Form 715 Criteria)
High voltage: >105% (per FERC Form 715 Criteria)
Non-load serving bus voltages may fall above or below criteria if conditions allow a higher/lower voltage without harm to NWE or customer equipment.
Certain equipment ratings may dictate different limits.

8. State-of-the-System Studies
System Normal and Contingencies have been simulated on all near-term scenarios to determine the “State-of-the-System” as it exists today.
P0, P1, and P2 study results have been analyzed. These contingencies represent a single point of failure.
P3 and P6 contingencies were not studied.
Similar studies are completed as NWE conducts outage studies for routine maintenance work or loss of a P1/P2 system element.
Any “State-of-the-System” issues regarding these N-1-1 events would be observed by Peak RC, and real-time mitigation orders would be issued.
Study results show previously known issues have been resolved from recently completed mitigation.
Newly studied P2 contingencies (non-bus-tie and bus-tie breaker failures) indicated additional system problems not identified in previous study cycles.

9. State-of-the-System Studies
Near-Term Study Results – P0 Events (System Normal)
Thermal Loading:
Only Generator Step-up Transformers had loading over 90%
No overloads observed
Voltage Issues:
No low voltage issues observed
Minor high voltage observed
Off-peak loading in the following areas:
Livingston/Big Timber 50 kV Area
Bitterroot Valley 69 kV
Hi-Line 69 kV system

10. State-of-the-System Studies
Summary Results for Near-term Scenario P1 Events:
Peak conditions govern for low voltage and thermal violations on most contingency scenarios.
High voltage was observed under light load conditions.
Almost all higher voltage bulk system segments and elements meet criteria under all contingency scenarios (500, 230, and 161 kV).
Loss of such elements may increase loading on underlying system such as autotransformers and lower voltage lines, or produce low voltage problems.

11. P1 Event – Thermal Violations
State-of-the-System Studies
P1 Event – Thermal Violations
Loss of the Great Falls Swyd to MT Refining 100 kV line
Causes cascading overloads on the Great Falls 100 kV system.
Loss of Great Falls area 100 kV load.
Noted in the 5-year scenario in the planning cycle.
Peak summer scenario.
Mitigation plan underway.
Loss of Glengarry 100/50 kV autotransformer.
Peak scenarios
Removes two additional 100 kV lines for normal clearing.
Causes multiple 50 kV overloads between Stanford and Harlowton.
Loss of the 50 kV load in the Stanford, Lewistown, Judith Gap areas.

12. P1 Event – Thermal Violations
State-of-the-System Studies
P1 Event – Thermal Violations
Loss of Steamplant 230/100 kV autotransformer – Bank #2
Peak scenarios
Removes two additional 230 kV lines for normal clearing
Causes an overload on the Broadview – Alkali Creek 230 kV line
Does not trip per PRC-023 requirements.
30-minute emergency summer rating established in 2016, allows time to isolate transformer and restore other lost 230 kV facilities to alleviate overload
Mitigation plan underway.
Loss of either Missoula #4 100/69 kV autotransformers.
Removes all Missoula #4 100 kV and 69 kV facilities for normal clearing
Loss of the 69 kV load between Missoula and Hamilton

13. P1 Event – Voltage Violations – High Voltage
State-of-the-System Studies
P1 Event – Voltage Violations – High Voltage
Loss of Two Dot area 100 kV lines
Winter peak and off-peak scenarios
Minor high voltage on the Martinsdale area 100 kV system
Loss of Hardin – Crossover 230 kV line
Winter and off-peak scenario
Minor high voltage on the Hardin 69 kV system

14. P1 Event – Voltage Violations – Low Voltage
State-of-the-System Studies
P1 Event – Voltage Violations – Low Voltage
Loss of Dillon-Salmon – Ennis Auto 161 kV line
Summer Peak Scenario: voltages well below 90% on the 69 kV between Dillon and Sheridan
Winter Peak Scenario: voltages just below criteria
Mitigation plan in place.
Loss of Stanford 100/50 kV autotransformer
Peak scenarios
Low voltage below 93% on the Stanford 50 kV system
Loss of Clyde Park 161/50 kV autotransformer
Winter peak scenario
Minor low voltage just below criteria near Pine Creek REA 69 kV

15. P1 Event – Voltage Violations – Low Voltage
State-of-the-System Studies
P1 Event – Voltage Violations – Low Voltage
Loss of either Broadview 230/100 kV autotransformers
Summer peak scenario
Low voltage below 90% on the Roundup 50 kV system
Loss of Judith Gap 230/100 kV Autotransformer
Peak scenarios
Removes three additional 100 kV lines for normal clearing
Widespread low voltage near 90% on the Stanford, Lewistown, Harlowton 100 kV and 50 kV systems
Mitigation plan in place.

16. State-of-the-System Studies
Summary Results for Near-term Scenario P2 Events:
Peak conditions mostly govern for low voltage and thermal violations on most contingency scenarios.
High voltage and minimal thermal violations are mostly observed under light load conditions.
Events with only high or low voltage violations (no thermal violations) are mostly consistent with P1 Events.
Many problematic bus fault events similar to P13 (loss of a transformer) events.
Breaker failure events very similar to bus fault events.
Bus tie-breaker failure most problematic breaker failure events.

17. P2 Event – Thermal Violations
State-of-the-System Studies
P2 Event – Thermal Violations
Steamplant PCB
Summer peak scenario
Opens two local 230 kV lines, one local 230/100 kV autotransformer, and one remote 230/161 kV autotransformer
Causes cascading overloads in the Billings/Laurel area
Loss of the 100 kV load in Billings/Laurel area
Mitigation plan in place
Steamplant PCB or bus tie-breakers
Peak scenarios
Complete loss of the Steamplant 230 kV bus

18. P2 Event – Thermal Violations
State-of-the-System Studies
P2 Event – Thermal Violations
Laurel Auto 100 kV south bus fault
Peak scenarios
Causes system overloads and low voltages on the 50 kV system in the Red Lodge area
Opens four 100 kV lines including both 100 kV lines to Bridger Auto
Loss of the Red Lodge and Bridger area 50 kV load
Preliminary mitigation plan in place
Colstrip PCB bus tie-breaker
Both peak and off-peak scenarios
Opens one 500/230 kV autotransformer and one 230 kV line
Causes an overload on the remaining 500/230 kV autotransformer within emergency ratings
Requires curtailment of Colstrip Generation to relieve overload

19. P2 Event – Thermal Violations
State-of-the-System Studies
P2 Event – Thermal Violations
Alkali Creek PCB or
Summer peak scenario
Opens two of three 230 kV lines and both 230/161 kV autotransformers
Causes an overload on both Steamplant 230/100 kV autotransformers within emergency ratings.
Mitigation plan in place
Great Falls PCB bus tie-breaker
Off-peak scenario
Opens two critical south of Great Falls 230 kV lines
Causes multiple 100 kV overloads from Great Falls to:
Raynesford Pump
Helena
Drummond
Butte

20. P2 Event – Thermal Violations
State-of-the-System Studies
P2 Event – Thermal Violations
Great Falls Switchyard PCB
Peak scenarios
Opens two critical 100 kV lines out of the Great Falls Switchyard
Causes multiple/cascading overloads on the 100 kV system in Great Falls
Loss of the Great Falls City 100 kV load
Mitigation plan in place
Great Falls Switchyard PCB or PCB
Off-peak scenario
Opens two of three 100 kV lines between the Great Falls and Crooked Falls switchyards
Causes cascading overloads out of the Crooked Falls Switchyard
Loss of the Crooked Falls Switchyard and over 200 MW of hydro generation

21. P2 Event – Thermal Violations
State-of-the-System Studies
P2 Event – Thermal Violations
Missoula Miller Creek #4 PCB Bus Tie Breaker
Peak and off-peak scenarios
Complete loss of the Missoula #4 161 kV bus
Causes complete loss of the Bitterroot 69 kV system
Missoula Miller Creek #4 PCB
Complete loss of the Missoula #4 69 kV, 100 kV, and 161 kV West buses leaving a single 161 kV line from Bonner to Hamilton Heights
Causes cascading overloads and/or low voltage on the Bitterroot 69 kV system
Loss of the Bitterroot 69 kV system

22. P2 Event – Voltage Violations – Low Voltage
State-of-the-System Studies
P2 Event – Voltage Violations – Low Voltage
Harlowton 100 kV Bus Fault
Peak scenarios
Low voltage well below 90% along the Harlowton – Glengarry 50 kV line
Judith Gap 100 kV Bus Fault
Widespread low voltage near 90% on the Stanford, Lewistown, Harlowton 100 kV and 50 kV systems
Mitigation plan in place
Bridger Auto Tie-breaker
Low voltage below 90% in the Bridger and Red Lodge 50 kV systems.

23. P2 Event – Voltage Violations – Low Voltage
State-of-the-System Studies
P2 Event – Voltage Violations – Low Voltage
Havre WAPA 161 kV Breaker 262
Summer peak scenario
Low voltage just below 90% east of Havre

24. Additional progress and next steps
Complete the analysis of future scenarios.
Verification of new problems
Prioritize problems using Decision Rules.
Begin or continue Mitigation Studies.

25. Questions?
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