SPP describes market and operational design to optimize DC ties across expanded RTO footprint

Seth Mayfield, manager of the market design team at Southwest Power Pool, described how SPP plans to operate its expanded regional transmission organization beginning in 2026 when it will cover two asynchronous interconnects linked by a small set of DC ties.

Mayfield said SPP will run a single-market solution that nonetheless preserves separate balancing-authority-area obligations and operating reserves for the Western and Eastern interconnects. "We're going to be a single balancing authority with two balancing authority areas, that will be loosely connected with DC ties in between," he said.

To accommodate asynchronous operation, SPP will maintain two reference buses and, consequently, two marginal energy costs (MECs). Mayfield said that could produce price separation that should be interpreted as congestion between the two footprints rather than purely local congestion. To manage the ties, SPP's day-ahead and real-time processes will use the security-constrained unit commitment (SCUC) to select a nonbinding direction for each DC tie (locking in an intended direction hour-before-operation) and security-constrained economic dispatch (SCED) to optimize the magnitude and dispatch while considering ramp rates and operational bounds.

Mayfield explained that DC-tie owners will provide parameters needed for accurate modeling (normal and emergency ramp rates, max/min flows, allowable direction switches and loss curves), and SPP will run multiple contingency-related solves (East-only, West-only, and simultaneous cases) frequently to ensure feasible outcomes. "We're solving an East case, a West case, and a simultaneous case every single five minutes," he said, adding that the number of cases and mitigation processes increases computational complexity but is required for robustness.

Mayfield used an example where an eastern resource trip reduces east-to-west transferability and forces the SCED to re-dispatch the ties toward minimum flow, illustrating how the day-ahead direction choice can change under real-time conditions. He acknowledged operational and pricing complexities that come with two MECs, noting the mechanics of congestion and price separation will deserve careful stakeholder communication.

In audience Q&A, Mayfield noted that many design choices were driven by reliability concerns: having separate solves ensures one interconnect can operate independently if ties or transfers are unavailable. He also said some optimization elements could inform work in other RTOs but that those cases differ if a single RTO does not control both sides of a tie.

The approach is planned to go live in early 2026 for SPP's integrated market; Mayfield invited follow-up with SPP staff for implementation detail.