Texas Secretary of State Roger Williams gave the keynote remarks at the formal dedication of a new state-of-the-art Variable Frequency Transformer (VFT), a key component in a 138-kV electric interconnection between Laredo and Mexico, on May 18. The US$74 million unit and related facilities are proposed assets of Electric Transmission Texas (ETT), LLC, a joint venture between American Electric Power (Columbus, Ohio, U.S.) and MidAmerican Energy (Des Moines, Iowa, U.S.).

The VFT will improve reliability in the Laredo area and substantially decrease the need to operate expensive Laredo-area power generation, which has been designated RMR (reliability-must-run) by agreement between the Electric Reliability Council of Texas (ERCOT) and the generation owner (see T&D World, August 2006 issue for more on this project).

Combined with a new 345-kV transmission line, approved by ERCOT to be completed in 2010, these facilities will allow the shutdown of the Laredo generation and support the 3% to 4% annual load growth in the Laredo area.

From now until the 345-kV line is completed, the Laredo area needs an additional 138-kV source of power. AEP and Mexico's Commisión Federal de Electricidad (CFE) have an existing 138-kV transmission line and have just completed a new 230-kV double-circuit transmission line. The VFT technology provides a controlled transmission path between the ERCOT and the CFE electrical grids, permitting power exchange that was previously impractical due to asynchronous boundaries.

“The VFT enhances grid stability for both the Texas and Mexican power systems and will be used to provide emergency support in addition to the transactional transfer of power,” said Calvin Crowder, ETT executive director. “It uses the latest technology to relieve transmission constraints and support the significant growth in an important economic region.”

Compared to the other options considered, the VFT will provide the highest degree of flexibility in scheduling power transfers in high load periods prior to completion of the 345-kV line, while maintaining a reliable power supply for the Laredo area.

As part of this project to interconnect the VFT to the CFE system, AEP and CFE are replacing the existing 138-kV transmission line with a new 230/138-kV double-circuit transmission line.