Local Area Study

Mitigation Plan Update and

Uncertainty Scenarios2012 - 2013

2

Mitigation Plans Updates

East Helena bus fault or transformer outage» Ranked # 7 - Outages result in complete loss of the Helena 69

kV system

» Construct 100 kV loop to radial fed Helena area 100 kV distribution substation

» Tie proposed distribution substation needed for continued growth in the Helena area to new 100 kV facilities

» Construct and tie new 100 MVA 100/69 kV auto substation to new 100 kV facilities

» Add breakers to East Helena 69 kV bus to complete breaker-and-a-half scheme

3

Mitigation Plans Updates

Broadview to Billings Area 230 kV double contingency» Ranked # 8 - Outage results in low to very low voltage in the

Billings Area♦ Problem worsens after loss of Corette and continued Billings area growth

» Build a new 230 kV line from Broadview into Billings♦ Assumes slightly diverse route from existing Broadview to Billings 230 kV

lines

Clyde Park bus fault or transformer outage» Ranked # 13 - Outages result in loss of Livingston and Big

Timber Loads» Upgrade Big Timber Auto to a 50 MVA transformer» Reconductor weak 50 kV line near Big Timber» Add capacitors to Big Timber and Livingston area distribution

substations

4

Uncertainty Scenarios

Scenarios Considered:

Extreme Winter

Low Thermal, Heavy Import

High Wind System Wide

High Generation North of Great Falls (NOGF)» Similar to stakeholder suggestion

Study Assumptions: N-0 and N-1 conditions considered

All existing Special Protection Schemes considered

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Extreme Winter

Scenario Details 2017 Extreme Winter load profile (1:50 forecast)

Hydro plants significantly reduced» Rivers/reservoirs partially frozen

Thermal plants dispatched at max» Corette assumed to be mothballed

Wind dispatched at 5%

Scenario Findings 230 kV and 161 kV systems more heavily relied on to supply the

underlying 100 kV systems

Loss of Butte area 161/100 kV tie overloads remaining 161/100 kV tie

100 kV system in Judith Gap area overloads from loss of major 230 kV line in Great Falls area

6

Low Thermal, Heavy Import

Scenario Details 2017 Heavy Summer load profile

All major thermal plants are offline

Wind dispatched at 5%

Path 8 heavily importing

Scenario Findings Kalispell 115 kV tie line overloads (within emergency limits) for the

loss of Rattlesnake 230/161 kV auto transformer

Loss of Butte area 161/100 kV tie overloads remaining 161/100 kV tie

Steamplant bus outage causes additional overloads on Billings area 100 kV ties to the higher voltage BES

7

High Wind System Wide

Scenario Details 2017 Heavy Summer and Light Autumn load profiles

Existing wind projects dispatched at capacity

All other generation dispatched same as original case

MATL has little to no flow at Great Falls

Scenario Findings No new thermal issues

Minor high voltage under outage conditions identified in Two Dot area

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High Generation NOGF

Scenario Details 2017 Light Spring load profile

Great Falls and NOGF area hydro generation at max

Other Montana hydro dispatched same as original case

Great Falls and NOGF area wind generation at max

Remaining wind generation dispatched at moderate levels

MATL importing at Great Falls near 300 MW

Path 8, 18, 80 transfers similar to original case

9

High Generation NOGF

Scenario Findings Butte area 100 kV line heavily loaded under N-0

conditions

Loss of Great Falls – Ovando 230 kV line overloads 100 kV lines in Helena area

Loss of Great Falls – Judith Gap – Broadview 230 kV line overloads 100 kV lines in Harlowton, Judith Gap, and Broadview areas

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Conclusions

System still robust in most areas, few new potential problems found.

Benefits of widely dispersed generation are demonstrated.

Potential weaknesses in the 100 kV system were identified.» Line or transformer upgrades and/or new facilities could be

required to mitigate problems observed.

This analysis is “Informational” and not the primary driver for mitigation.» Because of the uncertain nature of these scenarios, formal

mitigation plans have not been developed.» In some cases, new generation or TSR requests would drive

mitigation for problems observed.