TransAmerica Interconnections

Throughout the last few years, electric utilities around the world have started implementing important changes in their organizations to promote efficiency and improve their financial situation. The majority of the changes are related to the introduction of competition and disaggregation of electric services. This unbundling of transmission, distribution and generation has altered the nature of traditional planning and operation methods used previously by the vertically integrated utilities.

In Mexico, the electricity sector has been undergoing a process of deregulation since 1993, with its power industry changing to a competitive integrated model. One area of particular interest is the potential for increased cross-border trade of electricity between Mexico and the United States, and, more recently, between Mexico and Central America. In addition to financial and efficiency benefits, cross-border trade could generate substantial environmental benefits in terms of reduced emissions of global pollutants such as carbon dioxide and regional pollutants such as sulfur oxides and nitrogen oxides, depending on the composition of the generating capacity.

The level of exchanges of electric energy between the United States and Mexico has been influenced by a variety of geographic, economic and political factors. Throughout the middle of the 20^th century, demographic and economic conditions led to a significant increase in electric energy transactions between Mexico and utilities in states near the U.S.-Mexico border.

The United States established the Western System Coordinating Council, now called the Western Electricity Coordinating Council (WECC), to promote bulk power system reliability through coordinated planning and operation. The establishment of this regional council, along with the founding of the Electric Reliability Council of Texas (ERCOT), has facilitated the regional coordination of electric energy exchanges between utilities in the United States and the Comisión Federal de Electricidad (CFE) in Mexico. It should be noted that the United States and Mexico share a common border of about 1,310 km (814 miles) within the WECC region, and ERCOT and CFE share a common border of about 1,200 km (746 miles).

Existing U.S.-Mexico Interconnections

CFE has several high-voltage interconnections with the transmission systems of various utilities along the U.S.-Mexico border. Some of these interconnections are used for permanent interchanges of energy, while others are used only for emergency purposes.

The most substantive interconnections between WECC member systems and CFE exist between the Southern California Edison (SCE) and CFE's Baja California region. These interconnections are as follows:

Two 230-kV transmission lines between San Diego Gas and Electric (SDG&E) and CFE
Double-circuit 230-kV transmission line between SDG&E's Miguel and CFE's Tijuana substations
Single-circuit 230-kV tie between SDG&E's Imperial Valley and CFE's La Rosita substations.

These are synchronous interconnections with a transfer capability of up to 800 MW.

The only additional interconnection of significance between WECC and CFE exists between El Paso Electric Co. and CFE's Norte region. This consists of two 115-kV transmission lines providing a combined transfer capability of 200 MW. However, the CFE load must be electrically isolated from the Mexican system to be synchronized with and supplied by EPECO.

In 2001, American Electric Power Texas commissioned its first asynchronous interconnection between ERCOT and CFE in Eagle Pass, Texas, U.S., installing a 36-MW voltage source converter to supply reactive power support in addition to real power injection. Eagle Pass is located in the western part of the Central Power and Light system in ERCOT and is supplied by two 138-kV transmission lines.

In Laredo, Texas, reliability was the basis for expanding interconnections between ERCOT and CFE, when the second asynchronous interconnection was endorsed by ERCOT. To allow time for the construction of a new 345-kV line from the San Antonio area to Laredo, a 100-MW variable frequency transformer (VFT) commissioned in 2007 was selected as the most appropriate device for this asynchronous interconnection.

The third asynchronous interconnection also was installed in 2007 — at Sharyland Utilities LP's Railroad substation near Mission, Texas — and links the ERCOT grid with CFE, where it has been used to exchange up to 150 MW of power between the asynchronous power grids.

At the southern border of Mexico, energy is exported to Belize at 115 kV in accordance with a contract to purchase 25 MW of firm capacity from CFE. This power flow is achieved through a 115-kV transmission line that has a load-transfer capacity of 40 MW, which is routed from the Xul-Ha substation to CFE's generating station in the Merida area.

U.S. Interconnection

CFE and ERCOT have a long history of emergency assistance across the Mexico-U.S. border. However, in 2003, they acknowledged mutual benefits could arise from undertaking a contemporary study of possible interconnections between their respective grids. The study to determine opportunities for increased interconnection was conducted on a short-term (Phase 1) and long-term (Phase 2) basis.

Phase 1 considered the need to support the existing transmission system along the Texas border where older inefficient generation was no longer economical to operate. Recently, the reliability of both transmission systems has been improved — through two new interconnections using the existing transmission lines and by incorporating new technologies in the form of VFT and the back-to-back high-voltage direct current (HVDC) — allowing improved control of reactive power and voltage along the border.

Phase 2 will evaluate opportunities for long-term interconnections that can support additional economic transactions and emergency assistance between CFE and ERCOT. The study, which will not be constrained by infrastructure limitations, is likely to involve new transmission improvements for higher transfer capabilities.

In both phases of this study, high-voltage synchronous and asynchronous transmission interconnections are being considered, but the primary effort is focused on asynchronous interconnections that will use flexible alternating current transmission systems (FACTS) technology to allow the scheduling of power transfer between the transmission systems.

Central America Interconnections

The Mexico-Guatemala interconnection project consists of a 103-km (64-mile), 400-kV transmission line, with 32 km (20 miles) in Mexico and 71 km (44 miles) in Guatemala. This requires expansion of the substations at each end of the circuit, Tapachula (Mexico) and Los Brillantes (Guatemala). The interconnection also includes shunt reactive power compensation with two main purposes: voltage control and a single-pole tripping scheme implementation, respectively, to enhance the stability, power-transfer capability, reliability and availability of the transmission corridor during and after a ground fault.

The benefit of having a single pole of the transmission line breakers open is the two ends of the transmission line remain metallically connected by the other two phases, allowing power transfer and reducing the possibility the two ends will lose synchronism. Therefore, when a single phase-to-ground fault occurs, the protective system should detect the ground fault and identify the faulted phase, tripping a single pole of the breaker to clear the primary arc current. The open-pole period should be long enough to ensure the secondary arc current is extinguished.

On the basis of the contract signed, Guatemala's electric utility, the National Institute of Electrification (INDE), will acquire 120 MW of power from CFE, increasing to 200 MW when CFE has surplus energy to make this possible. Simultaneously, the contract will enable CFE to purchase energy on the Guatemalan electricity market in the event of a contingency. This interconnection has helped to establish the Mexican transmission system with the Central American market, facilitating energy transactions between both countries through medium- and long-term bilateral contracts. Additionally, the transmission line will help to control power flows in Guatemala, reducing losses and improving power reactive margins.

Grid Interconnections in Central America

Currently, six Central American countries are interconnected:

Honduras-Nicaragua (1976)
Costa Rica-Nicaragua (1982)
Costa Rica-Panama (1986)
El Salvador-Guatemala (1986)
El Salvador-Honduras (2002).

All of these interconnections are 230-kV circuits for mutual support in emergency situations and for the interchange of surplus hydro power. They each have a power-transfer capability level of approximately 50 MW.

The Electricity Interconnection System for the Central American countries (SIEPAC) is one of the main projects underway to foster regional integration and cooperation between six Central American countries: Panama, Costa Rica, Nicaragua, Honduras, El Salvador and Guatemala. This project involves the construction of a 1,830-km (1,137-mile), 230-kV transmission line linking Guatemala, El Salvador, Honduras, Costa Rica, Nicaragua and Panama. The new transmission system interconnector will alleviate periodic power shortages and decrease operating costs in the regions, thereby reducing the cost of electricity to consumers. Empresa Propietaria de la Red administers the project.

Construction of this single-circuit 230-kV transmission line, with a rated capacity of 300 MW, began in 2006 and is scheduled to begin operation by the end of 2011. A second circuit can be added in the future to double the circuit capacity to 600 MW. The SIEPAC project not only consists of the construction of this transmission line, it also will establish a regional electricity market, a regional regulator and a regional operator to reduce the cost of energy and rapidly expand energy capacity by attracting private sector investment.

Increasing Reliability

The reliability of CFE and ERCOT systems has improved recently as a result of the commissioning of two new interconnections, using available transmission lines and the advantages of new transmission technologies such as VFT and back-to-back HVDC. These technologies allow improved control of the interchange of power as well as control of reactive power and voltage along the border.

Reliability and operational agreements have been signed for the operation of these interconnections and are expected to continue growing along with additional joint studies and coordination groups.

The Mexico-Guatemala interconnection project will facilitate support during emergencies and increase operation reliability. This interconnection has helped initiate the integration of the Mexican transmission system with the Central American market and will allow economic energy transactions between both countries in accordance with medium- and long-term bilateral contracts.

Miguel Angel Avila Rosales ([email protected]) was awarded BSEE and MSEE degrees from the Instituto Politecnico Nacional, Mexico, as well as a MSEE degree from the University of Toronto in Ontario, Canada. He retired from Comisión Federal de Electricidad, where he was area manager of transmission planning, with primary responsibilities in transmission planning studies for the main CFE bulk electric system. Rosales is a senior member of IEEE, a member of the CIGRÉ SC C1 Power System Planning and Development group and chairman of the CIGRÉ National Committee for Mexico from 1999 to 2007.

Hector Gerardo Sarmiento ([email protected]) graduated with a BSEE degree from Universidad Iberoamericana in 1976, a master's degree in engineering from Rensselaer Polytechnic Institute in New York and a Ph.D. degree in electrical engineering from Concordia University in Montreal, Canada. Since 1978, he has worked as a project manager at the Instituto de Investigaciones Electricas of Mexico, and his main interests are the analysis and design of transmission systems.

Daniel Leon Rodriguez ([email protected]) received BSEE and MSEE degrees from the National Polytechnic Institute in Mexico in 1993 and 2001, respectively. He worked for the Comisión Federal de Electricidad, the government electric utility in Mexico, from 1993 to 2007. Since 2008, Rodriguez has been a senior engineer in the system studies group at Powertech Labs Inc. in Vancouver, Canada. His main interests are the stability and security assessment of electrical power systems.