The Midcontinent ISO recently proposed revising its market tariff to allow energy storage resources to serve as transmission-only assets.
"The proposed tariff revisions reflect a fundamental first step forward for the use of storage resources to maximize the reliability and efficiency of the electric system," MISO said in its Dec. 12 request to FERC. The use of energy storage to serve multiple functions "is of great interest to MISO and its stakeholders [and] responds to the expressed policy interests of the commission."
The proposal includes a comprehensive tariff framework for considering storage as a transmission-only asset, or SATOA, and allowing storage resources to be evaluated in MISO's Transmission Expansion Plan and valued similarly to a wires solution. Any storage being considered as possible transmission assets would not be subject to MISO's generation interconnection procedures.
Under the proposal, storage assets would be reviewed "in the same context and with the same rigor as traditional wires solutions," the filing stated. It added that the proposed SATOA's life cycle costs for a particular period would be compared with those for a wires solution, and a SATOA would not be credited or compensated for excess capacity.
The proposal also aims to ensure that SATOAs are operated in a manner that "preserves [grid operator] independence" and that any revenues from the resource are properly credited back to the transmission function.
MISO would define a SATOA as an electric facility that is connected or will connect to the transmission system only to support that system. As a result, such a storage asset "may not participate in the market for purposes other than receiving and injecting energy needed to perform the required transmission function," the proposal said. The definition would also bar SATOAs from participating in energy and operating reserve markets and planning resource auctions unless the tariff is updated to include such provisions.
"Any future provisions for dual transmission- and market-service operation will be supported [in] a new tariff proposal," the Dec. 12 filing stated. In early 2020, MISO said it would begin working with stakeholders to address issues related to using storage resources as both transmission assets and to provide market services.
The grid operator stressed that its new proposal was complementary to but "expressly separate" from tariff revisions FERC recently approved for allowing energy storage to participate in MISO's wholesale market under Order 841. The Dec. 12 filing would allow storage resources to serve as a transmission-only asset, MISO explained.
MISO asked the Federal Energy Regulatory Commission to accept the proposed tariff changes following a 30-day comment period and with an effective date of March 11, 2020. (FERC docket ER20-588)