How will the Tranche 2 portfolio complement the Tranche 1 portfolio?
MISO is committed to the reliability of the system which is a significant driver for both the Tranche 1 and Tranche 2 portfolios.  The refreshed Futures (A series) indicate a more rapid shift in timing and magnitude of the resource change and evolution, reinforcing what MISO anticipated in Tranche 1.  The Tranche 2 analysis accounts for Tranche 1 projects and will continue to resolve additional drivers (reliability, economic) shown in the Futures Series A.  As MISO has stated, 765kV is a strong consideration for the Tranche 2 portfolio.  The 345kV solutions that were approved in Tranche 1 will provide and further strengthen the backbone and enable 765kV additions.

Will a Tranche 2 portfolio resolve all the issues in Future 2A?
As with Tranche 1, Tranche 2 does not resolve all identified issues. Instead, the Tranche 2 transmission lines focus on creating a logical next step in the development of a regional backbone (e.g., highway system), balancing needs with benefits and cost.  MISO’s existing processes will support future resource and load additions as they become more certain (e.g., local roads).

MISO’s other planning processes, such as annual MTEP reliability and generator interconnection processes, will identify the transmission needed to address local issues not resolved by Tranche 2. MISO will continue to work with stakeholders in those processes to identify these local transmission needs, which will build-off the regional transmission highway identified in Tranche 2.

Why is 765 kV being considered?  
The needs identified in the LRTP Tranche 2 studies, which are based on the F2A Future, are not incremental, thus 765 kV is a more cost-effective solution.  765 kV is selected since the cost per mile for a double-circuit 345 kV line is less and HVDC is best suited for long distances beyond 300 miles. 

The capacity of a 765 kV line is nearly three times the capacity of a double-circuit 345 kV line. The cost per MW-mile for 765 kV is less than one-third the cost per MW-mile of 345 kV.  It is important to note that LRTP Tranche 2 also includes 345 kV upgrades to facilitate on/off ramps to the 765 kV system, but the bulk regional lines being proposed to establish the transmission superhighway in LRTP Tranche 2 are 765 kV. 

HVDC is best suited for long distance transmission because of the ability to maintain a rating equal to its thermal limit, where the capabilities of the AC transmission line can fall well below their thermal limits for long transmission lines lengths. As a line’s length decreases, the cost advantage of 765 kV over HVDC increases; and as a line length increases, the cost advantage of 765 kV over HVDC decreases. Based on the Transmission Cost Estimating Guide for MTEP 23, the breakeven distance is greater than 300 miles. The average length of the 765 kV lines being considered in Tranche 2 is 130 miles, with the longest line being below 300 miles. 

What Is the impact of a 765 kV contingency?
One issue being considered is the contingency impact of 765 kV to ensure it is mitigated by both existing and proposed 345 and 765 facilities. The impact of a 765 kV contingency could be more severe if there are fewer 765 kV circuits to makeup the 765 kV backbone.  For example, if only two 765 kV circuits were installed, an N-1 contingency would result in loss of half the 765 kV system and an N-2 contingency would result in the loss of the entire 765 kV system.  The anticipated portfolio has multiple 765 kV transmission line circuits and numerous 765-345 kV transformer banks.  Under an N-2 contingency, most of the 765 kV backbone would remain intact.  Care must be taken to consider the local impacts of nearby 765 kV contingencies, and the existence of an underlying 345 kV system to support and provide backup to the 765 kV system helps mitigate the impacts of 765 kV contingencies.  

Does MISO still have an “All Things Considered” philosophy regarding the consideration of legacy voltage levels vs. 765 kV vs. HVDC? 
Yes. The 345 kV supporting facilities that exist today remain needed and valuable for supporting local and contingency operation of the 765 kV. They also ensure strong connections between the 765 kV system and the underlying transmission system. The regional 765 kV backbone will assist in enabling the new resource mix while addressing load growth, dispatch volatility and extreme weather situations. The anticipated transmission plan for Tranche 2 does not eliminate opportunities to continue consideration of HVDC, 345 kV or 765 kV for future needs.  

There are related materials posted for the March 8, 2023, Planning Advisory Committee meeting.  These materials can be accessed via the MISO calendar. 

Did MISO consider the use of Grid Enhancing Technologies for LRTP Tranche 2?
With such a high percentage of facilities overloaded, including legacy EHV 345 kV facilities, Grid Enhancing Technologies would not be sufficient to solve the issues identified in Tranche 2.  A key benefit of Grid Enhancing Technologies is their ability to capitalize on opportunities in the operations time frame. MISO is open to incorporating such technologies into identified solutions at the discretion of the Transmission Owner to provide increased operational flexibility in the operating horizon.

What portions of the final Tranche 2 portfolio will be competitively bid?

Once MISO’s Board of Directors approves a Tranche 2 portfolio, MISO will follow its Tariff defined Competitive Developer Selection Process to determine which transmission and substation facilities are Competitive Transmission Facilities. All identified Competitive Transmission Facilities will be part of one or more Request For Proposal and evaluated by MISO to determine the Selected Developer.
Comments (0)