Ameren to Complete a $250 Million Transmission Line That Will Make Congestion Worse

power transmission lines
Blog December 17, 2019

The transmission grid in the U.S. is facing the unprecedented challenge of a shifting generation portfolio from massive additions of intermittent renewable energy. The continued advance of renewables will require grid operations and infrastructure to advance in tandem. However, there is a serious issue regarding transmission expansion plans: the process of building new generation happens much faster than building new transmission. The new 100-mile, 345kV MISO transmission line, dubbed the Mark Twain Transmission Project, is the latest example of the challenges Regional Transmission Organizations (RTOs) face when it comes to the shifting grid in the Midwest.

MISO Transmission Upgrades and Generation Retirements
Figure 1: MISO Transmission Upgrades and Generation Retirements. Source: EIA

In 2011, MISO approved the Mark Twain line in order to carry power from Palmyra, Missouri to the border of Iowa for “energy-grid reliability, increased transmission capacity, and greater access to renewable sources such as wind”. During this time, nameplate wind generation capacity in MISO was just under 10 GW; by the end of 2018 it reached 18.2 GW. The rapid addition of wind generation led to immense impacts on energy prices in the region by injecting zero marginal cost power to the grid. It also altered the way power flows through the grid driving new patterns of transmission congestion. Congestion is the result of when the power flow over a transmission element exceeds its available capacity. This makes balancing the grid on high wind generation days difficult, often leading to either negative or very expensive energy prices depending on the location of the congestion. Ameren began construction on the line in May 2018 and is scheduled to officially go into service on December 20, 2019.

What impact will the new line have on congestion?

This new line is going to bypass some localized constraints in MISO; however, at the same time it is going to inject power directly into one of the top constraints in MISO and PJM, at the Marblehead 161kV transformer. Localized constraints have a smaller impact near its region, whereas the Marblehead transformer has much more widespread impacts in both MISO and PJM. The Marblehead transformer first appeared in 2016 due to the continuous addition of new wind generation in MISO North and SPP. Other transmission upgrades in 2017 and 2018 caused the Marblehead transformer to become more impactful and eventually reach PJM. The connection with PJM occurred just months before construction started on the Mark Twain lines. The increased congestion will result in large price discrepancies between locations, which disincentives new wind farm development and causes a less efficient dispatch of generation.

The addition of the line also happens to coincide with the retirements of four Illinois coal plants totaling over 2 GW of capacity: Coffeen, Havana, Hennepin, and Duck Creek. The announcement of these retirements happened in August 2019, and the final plant, Duck Creek, retired on December 13 – just days before the new line is scheduled to going into service. The latter three of those plants provided reactive power to the Mark Twain lines; with their retirement, additional power flows over those lines and directly through the Marblehead transformer. To better understand and forecast the extent of this new congestion, our Power Market analysts use proprietary wind generation data and keep our customers updated on a daily basis.


Complications in transmission are not necessarily a result of poor planning; in fact, upgrades similar to this are necessary to allow new wind power in rural areas to efficiently reach large cities where it is needed. Instead, it sheds light on the difficulties of maintaining an efficient and reliable grid in the face of rapid transition in the generating fleet. RTOs operate a competitive market and as new low-cost wind, solar, and gas generation continue to price out older more expensive coal generation, flow patterns and congestion will continue to change. Additionally, storage and smart grid technology continue to advance, which may alleviate the need to build out expensive and difficult transmission expansions. These are just some of the many considerations as the regions continue to push for a more renewable and clean energy future. As changes in the market develop, stay informed and ahead of competition with access to Genscape and Wood Mackenzie power solutions. To learn more or request a demo, please click here.