Entergy’s (NYSE:ETR) Entergy Mississippi on Feb. 2 filed with the Mississippi Public Service Commission a petition for a certificate of public convenience and necessity to build, acquire, expand, operate, own and maintain two 750 mega Volt-Amp (MVA) auto-transformer banks that would replace two existing 500/230-kV auto-transformer banks at its McAdams substation in Attala County, Miss.
The proposed electrical facilities are needed for essential reliability purposes in the company’s service territory, according to the petition. The company also noted that the proposed electrical facilities would increase the transfer capabilities of Entergy Mississippi’s interconnection with TVA, and that the proposed incremental transformer capacity would allow for increased bulk power delivery from the 500-kV network, meaning that distant and more efficiently generated electricity could be available to Entergy Mississippi customers.
The company noted that the Tennessee Valley Authority (TVA) in 2014 requested yearly network service on its own transmission system for 874 MW for the period Jan. 1, 2015, until Jan. 1, 2025. The network resource for the transmission service request (TSR) is TVA’s Ackerman generation plant, Entergy Mississippi said, adding that the receipt and delivery points are located on the TVA transmission system. The TSR was granted and TVA notified the Midcontinent ISO (MISO), as an affected system, of the TSR.
MISO subsequently performed a MISO facilities study that identified transmission constraints on Entergy Mississippi’s portion of the MISO transmission system as a result of the TSR from TVA, the company added.
The transmission constraints would occur when three generators – TVA’s Ackerman, NRG Wholesale Generation LP’s Choctaw, and Entergy Mississippi’s Attala – ran simultaneously at or near their full output. The company also said that the MISO facilities study and a subsequent Entergy Mississippi facilities study concluded that upgrades to two 500/230-kV transformers at the McAdams substation were required to alleviate the transmission constraints caused by implementation of the TSR from TVA and to continue to meet mandatory NERC reliability standards.
The company said that its proposed project involves:
- Removing both existing banks of auto-transformers (AT1 and AT2) at the McAdams substation, and replacing them with new auto-transformers with a minimum rating of at least 750 MVA per bank. The new transformer banks would be comprised of three grounded-wye 500/230-kV single-phase units, each rated at 150/200/250 MVA @65ºC. Those units would attach to the existing tertiary and would be installed in the same locations as the existing units
- Installing a new reactor bay in the vacant area to the east of the existing reactors on AT2 tertiary
- Adding new lightning arresters to the existing AT1 bank reactor bays
- Replacing the existing spare single phase auto-transformer, and reconfiguring the bus so that it is usable for either bank
- Adding one 35 mega Volt-Amp reactive (MVAR) reactor to auto-transformer bank #2, so that there would be four reactors for each auto-transformer bank
One of the existing auto-transformers that is being replaced would be moved to the Lakeover 500/230/115-kV substation and installed as part of the Lakeover 500/230-kV auto-transformer installation project, the company said.
The total estimated cost of the proposed electrical facilities – which does not include the Lakeover 500/230-kV auto-transformer installation project – is about $28.1m, of which Entergy Mississippi would be responsible for less than half under a construction and payment (C&P) agreement, the company said.
Entergy Mississippi said that its cost to be reflected in future rates is estimated to be about $11.8m.
Entergy Mississippi said that it has entered into the C&P agreement with TVA, NRG, and MISO, effective Aug. 12, 2016, that provides for financial contributions from TVA and NRG for the cost of the proposed electrical facilities. Since the affected generation resources of TVA and NRG would also be made firm by the proposed electrical facilities, the cost of the facilities would be shared by Entergy Mississippi, TVA and NRG. The company also noted that TVA would contribute $6.5m to Entergy Mississippi, and NRG would contribute $9.8m to Entergy Mississippi toward the cost of the facilities.
The fixed contributions were determined after discussions among Entergy Mississippi, TVA, NRG, and MISO, and are based on the relative impact that the output from the generation resources of Entergy Mississippi, NRG, and TVA would have on the proposed electrical facilities, the company said.
TVA and NRG would also pay additional amounts to Entergy Mississippi for contributions to taxes, the amount of which would depend on when the proposed electrical facilities are placed in service, the company said.
Among other things, the company said that subject to the receipt of necessary permits and required approvals, the proposed electrical facilities are expected to be placed in service by December 2019, with the exception of the spare auto-transformer, which is expected to be installed by June 2020.