Such factors as outages, extreme weather and power transfers have led to a transmission system in the American Electric Power (AEP)-Southwest Power Pool (SPP) zone that is constrained when generation is dispatched in a manner inconsistent with the original design of using local generation to serve local load, according to Southwestern Electric Power Company (SWEPCO).
The portion of the AEP transmission system operating in SPP (AEP-SPP zone) consists of about 1,290 miles of 345-kV, about 3,490 miles of 138-kV, and 2,320 miles of 69-kV lines, SWEPCO, a unit of AEP, said in its November integrated resource plan (IRP).
A lack of seams agreements between SPP and its neighbors has slowed down the process of developing new interconnections.
SWEPCO also noted that the 2012 annual SPP transmission expansion plan (STEP) identified 492 transmission network upgrades with a total cost of about $7.1bn.
The process of truly integrating generation and transmission planning is still developing in SPP, and AEP continues to stand ready to engage in that process, SWEPCO said.
At this time, however, AEP affiliate, Public Service Company of Oklahoma (PSO) and SWEPCO can do little to import capacity from outside of its control area. Both companies have been open to such imports as shown by the issuing of recent request for proposals (RFPs) for non-site specific generation types. These RFPs, SWEPCO added, allow bidding entities to offer generation coupled with transmission solutions, which would be subject to SPP approvals.
Major transmission enhancements
Over the past several years, several major transmission enhancements have begun to reinforce the AEP-SPP transmission system, including in northwest Arkansas.
The AEP transmission system serves about 1,300 MW of load in that area, about 49% of which is Arkansas Electric Cooperative Corporation (AECC) load.
This load, SWEPCO added, is supplied mainly by the SWEPCO and AECC jointly owned Flint Creek generating plant, the SWEPCO Mattison generating plant, the Grand River Dam Authority-Flint Creek 345-kV line and the Clarksville-Chamber Springs 345-kV line.
A new 345-kV line is under construction from Flint Creek to the new Shipe Road 345/161-kV substation, with a 161-kV line connecting the Shipe Road substation to the East Centerton substation.
SWEPCO also said another enhancement is the Turk Generation Interconnection. In order to connect the 600-MW coal-fired Turk power plant in southwestern Arkansas, near McNab, to the transmission system, the Turk 345/138/115-kV substation was built and several new transmission lines were built or upgraded.
“This expansion provides the interconnection of the Turk power plant, transmission service, improved reliability for the city of Hope and southwestern Arkansas, and improved reliability to Texarkana by completing a 138-kV loop around the city,” SWEPCO added.
Transmission upgrades may be needed
Among other things, SWEPCO said that integration of additional generation capacity within the AEP-SPP zone will likely require significant transmission upgrades, and at most locations, any additional generation resources will aggravate existing transmission constraints.
For instance, while there are few extra high voltage (EHV) transmission lines in western Oklahoma/Texas Panhandle, the area is one of the highest wind density areas within the SPP RTO footprint. SWEPCO also said that the potential wind farm capacity for this area has been estimated to exceed 4,000 MW.
SWEPCO noted that there are only five east-west EHV lines into the SPP region, which stretches from the Gulf of Mexico – east of Houston – north to Des Moines, Iowa.
This limitation constrains the amount of imports and exports along the eastern interface of SPP with neighboring regions, SWEPCO said.
“Significant generation additions near or along the SPP eastern interface would likely require significant transmission enhancements, including EHV line and station construction, to address thermal and stability constraints should such generation additions adversely impact existing transactions along the interface,” SWEPCO said.
Some of the needed transmission upgrades could be reduced or increased in scope if existing generating capacity is retired concurrent with the addition of new capacity.
SWEPCO also said that currently, the transmission system cannot accommodate large incremental firm imports to the AEP-SPP area. Generally, SWEPCO added, the transfers are limited by the facilities of neighboring systems instead of by transmission lines or equipment owned by AEP.
SWEPCO also said that increasing the import capabilities with AEP-SPP’s neighboring companies could require a large capital investment for new transmission facilities by the neighboring systems or through sponsored upgrades by SPP transmission owners, adding that an analysis of the cost of the upgrades cannot be done until the capacity resources are determined.