Steady-state analysis

ATC’s steady-state multiple outage assessment started with Commonwealth Associates (CAI) performing more extensive analysis of our transmission system in 2004 to identify NERC Category C type contingencies that potentially could lead to cascading. Since then, ATC has taken this initial screening and enhanced our review in succeeding years.

NERC Category C contingencies are specific sets of multiple outages including lines, transformers and generators. ATC revisited Category C event analysis in 2005, 2006 and 2008. In 2006, we evaluated additional Category C events by screening Zone 3 100-kV and above facilities and ATC 345-kV transmission facilities in addition to the critical contingencies selected in 2005.

For the 2008 work, ATC used the 2009 summer peak model including all projects identified in the 10-Year Assessment for additional steady state multiple outage analysis. Physical Operational Margin (POM)-Optimal Mitigation Measure (OPM) software was used to determine the amount of load that needed to be shed to avoid cascading. Based on the 2006 work, a set of critical outages consisting of 145 multiple contingencies and 4 breaker failure contingencies were selected for restudy in 2008. As part of ATC’s efforts to continue to be aware of multiple outage impacts on our system, in 2008, we have performed additional Category C analysis and assessments by screening Zone 5 (100-kV and above) facilities and the combination of ATC 345-kV facility (including ATC 345/138-kV transformers) and generator (100 MW and above). The total number of events tested in this initial study of the 2009 model was 30842 which included:

• 98 multiple outages selected and tested in 2005 and 2006
• 28 multiple outages associated with Zone 3, which were identified as critical in 2006
• 23 multiple outages associated with ATC 345-kV transmission facilities, which were identified as critical in 2006
• 26429 multiple outages associated with Zone 5, which has one of the major ATC load centers such as Metro Milwaukee area, relatively large number of underground cables, and higher voltage transmission facilities:
• Transmission lines above 100 kV
• Transformers with 100 kV above (both high and low sides).
• Generation with 100 MW and above
• N-2 rules:
• Transmission line + Transmission line
• Transmission line + Transformer
• Transformer + Transformer
• Transmission line + Generator
• Transformer + Generator
• 4264 multiple outages associated with ATC 345-kV facility and generator:
• N-2 rules:
• ATC 345 kV Transmission line + ATC Generation (100 MW and above)
• ATC 345/138 kV Transformer + ATC Generation (100 MW and above)

Of the 30,842 events analyzed, 851 resulted in potential voltage problems or thermal overloads in 2009. The most severe 317 events among 851 were reviewed in more detail to determine if cascading was likely. Among the events of Zone 5 and ATC 345-kV facility/generation, if an event results in more than 100 MW of load curtailment, it is considered as severe events and included in the 317 events. As a result of further review, 36 events would require load shedding, 199 events would require other remedial actions such as generation redispatch and operating guide, and 82 turned out to be invalid outages. As in 2006, we found through the use of specific load shedding and generation redispatch that cascading could be ruled out for Category C contingencies on our 2009 system. Three breaker failure events involving Columbia and a single breaker failure event involving Rocky Run were also tested by POM-OPM using the 2009 model. No system problems were identified for the breaker failures. Ranked based on the amount of load shedding, the top 5 Category C events for the 2009 system are

• North Appleton T1 and T2 transformers
• North Appleton T1 and T3 transformers
• North Appleton T2 and T3 transformers
• Portage Columbia 138-kV line #1 and #2
• Arcadian 345/138-kV #1 and #2 transformers

To reassess the long-term planning horizon, the 36 Category C events resulting in load shedding to mitigate potential voltage or overloads in 2009 were repeated using the 2013 summer peak model including all projects identified for this 10-Year Assessment.

Of the 36 events analyzed, 10 Category C events had no system problems and 26 Category C events had potential thermal overloads or low voltages in 2013. 11 of the 26 events resulted in overloads or low voltage problems that were more severe in 2013 than in 2009.

Following are the 13 Category C contingencies that became worse in 2013 and the additional load that needed to be shed compared to 2009:

Although it is estimated that cascading could be ruled out for Category C contingencies on our 2009 and 2013 systems through the use of specific load shedding and generation redispatch, the study results are subject to further review by ATC Planning and System Operations to develop or confirm appropriate and more specific operating procedures.