FERC on Thursday approved Arizona Public Service’s proposal to begin using the flowgate methodology to calculate available transfer capability (ATC) within its transmission system.

In approving the change, the commission rejected a protest by the Southwest Public Power Agency (SPPA), which complained that APS did not sufficiently explain the impact the move might have on transmission customers and other transmission facility owners in the region.

The commission also denied APS’s request to waive a requirement that the utility continue to post total transfer capability (TTC) on its Open Access Same Time Information System (OASIS) after transitioning to the new methodology (ER22-2476).

Provider Discretion

The APS proceeding has its roots in FERC Order 890, which sought to “increase nondiscriminatory access to the grid by eliminating the wide discretion that transmission providers currently have in calculating” ATC. The order required utilities to develop consistent methodologies for performing the calculation and to publish those methodologies for review.

Issued in 2007, Order 890 revised the pro forma Open Access Transmission Tariff (OATT) to require that transmission providers clearly identify the methodology and mathematical algorithms used “to calculate firm and non-firm ATC (and [available flowgate capability] AFC, if applicable) for its scheduling, operating and planning horizons.”

APS sought to revise its OATT by replacing the rated system path methodology with the flowgate methodology to calculate ATC across the three horizons.

According to NERC, the flowgate methodology identifies key transmission facilities as flowgates, a mathematical construct used to analyze the impact of power flows on the bulk electric system.

Under the method, NERC explains, “total flowgate capabilities (TFC) are determined based on facility ratings and voltage and stability limits. The impacts of existing transmission commitments (ETC) are determined by simulation. The impacts of ETC, capacity benefit margin (CBM) and transmission reliability margin (TRM) are subtracted from the total flowgate capability, and postbacks and counterflows are added, to determine the available flowgate capability (AFC) value for that flowgate.”

In Thursday’s order, the commission explained that “In order to have consistent posting of ATC, TTC, capacity benefit margin, and transmission reliability margin values on OASIS, the commission directed public utilities working through NERC to develop the available transmission system capability reliability standard, a rule to convert available flowgate capability values into ATC values.” The commission also affirmed that providers relying on the flowgate methodology are required to convert their AFC values to ATC and post the associated calculations on their OASIS and web sites.

The commission noted that, in response to a deficiency letter, APS had provided the required documentation for its various calculations, including those related to AFC and ATC — as well as its process for converting AFC to ATC.

But the proposed changes prompted a protest by SPPA. While claiming no objection to the use of the flowgate methodology, the power agency said it would be more “sensible” for all entities with component facilities on the APS system to adopt the methodology together.

SPPA also contended that APS’ filing with FERC lacked key details about the utility’s implementation of the methodology and that APS failed to inform the commission, transmission owners and customers how the methodology would affect transmission allocation and scheduling.

SPPA additionally argued that APS failed to explain how the new methodology would affect other transmission facility owners and transmission customers, particularly those in the Palo Verde area. It asked FERC to reject the changes or suspend APS’ filing for five months to either set the issue for settlement judge procedures or a technical conference.

The commission acknowledged SPPA’s concerns about transitioning to the flowgate methodology but agreed with APS that Order 890 gives transmission providers discretion in choosing their ATC calculation methodology.

The commission found that SPPA had “not identified any specific concerns with APS’ proposed OATT revisions,” and that APS had “appropriately revised its OATT to reflect the transition consistent with FERC requirements, finding the revisions to be just and reasonable.”

FERC also rejected SPPA’s requests to suspend the filing or convene settlement judge procedures or a technical conference, saying “there are no issues of material fact that would warrant a hearing.”

‘Industry-wide Consistency’

Thursday’s ruling also denied APS’s request for waiver of the requirement to post TTC values on its OASIS site. The utility had argued that while Order 890 references TTC as a component of ATC, TTC is not actually a component of ATC for providers relying on the flowgate methodology, who instead use TFC in their calculations. APS said the requirement to post TTC on OASIS would be a “burdensome, manual process” with little customer value.

In denying the waver, the commission said Order 890 “addressed the potential for undue discrimination by requiring industry-wide consistency and transparency of all components of the ATC calculation methodology and certain definitions, data and modeling assumptions. The commission [in Order 890] noted its concern that the lack of consistent, industry-wide ATC calculation standards poses a threat to the reliable operation of the bulk-power system, particularly because a transmission provider may not know its neighbors’ system conditions and how that might affect its own ATC values.”

The commission also found that APS erred in citing previous FERC cases, specifically 2009 rulings involving SPP (127 FERC ¶ 61,207) and Midwest ISO (126 FERC ¶ 61,107) to support its argument. It noted that the applicant in SPP requested only a temporary waiver of the requirement to post ATC, TTC, CBM and TRM values on its OASIS site. In Midwest ISO, the commission granted MISO a waiver of the requirement to post certain ATC components on its OASIS site for paths internal to the MISO system, but not for other transmission service requests, the commission added.

“APS has also failed to adequately explain how requiring APS to convert TFC to TTC would impose significant burdens on its staff,” the commission determined.