Argentina`s largest hydroelectric plant is controlled by one operator

A single operator control system provides the edge needed to compete with other hydro plants in the region while balancing power needs with water availability

Brad A. Hoffman

Elsag Bailey Process Automation

Exploiting the benefits of low-cost hydroelectric power, Argentina is constructing a series of plants that will provide more than 70 percent of the nation`s electricity by the year 2000. The largest of those now in operation is the 4-MW by 350-MW Piedra del Aguila station in the southern province of Neuquen. Designed and constructed by the state, the plant has since been privatized, providing some unusual challenges to increase operating efficiency. State-of-the-art distributed controls are helping meet the challenges.

Competitive hydro grid

Situated approximately 1,100 km southwest of Buenos Aires, Piedra del Aguila is one of five hydro plants completed or under construction along the Limay River in Neuquen Province (Figure 1). The plant`s name, which translates to “Stone of the Eagle,” comes from some distinctive nearby rock formations which mark the otherwise featureless Patagonia Desert. A deceptive term, this “desert” and its winding rivers have become a critical resource to meet the nation`s growing power needs. Approximately 1,800 km of high-voltage transmission lines link the five Rio Limay hydro stations with metropolitan Buenos Aires and its population of 13 million.

Constructed by the former state agency, Hidronor S.A., the agency transferred the Piedra del Aguila plant to private owner/operators in December 1993. Duke Energy Corp. of the United States, Trans Alta of Canada and the Chilean utility Chilgener are the consortium`s primary investors. Duke also acquired the 500 kV system which supports Piedra del Aguila and its sister plants.

Newly privatized ownership of the region`s multiple hydro plants poses a scenario for power pricing very different from that administered in most locales by public utility commissions. A long-term rate “tariff” is unavailable to Hidroelectrica Piedra del Aguila and its fellow investor consortia. Rather, the five plant operators compete against one another, bidding almost constantly on the rates for power supplied to the southwestern Argentine grid. Compounding the need for operating efficiency and detailed plant information are the dictates of a regional basin authority which regulates water flows along the Rio Limay. They must constantly balance the needs of the power producers with water availability for consumer, recreational and agricultural uses.

Construction challenges

Development of the (US)$1.4 billion Piedra del Aguila plant began in 1988, spurred by the largest individual construction loan ever made to Argentina`s electric power sector. The dam`s vast proportions–820 meters wide at its crown and 170 meters tall–and the plant`s remote location combined to create the need for a temporary city of more than 9,000 workers and dependents at the site during peak construction. Workers excavated more than 1.3 million cubic meters of soil and rock, replacing it with nearly 3.5 million cubic meters of poured concrete.

An unusual challenge discovered after construction had begun was the existence of a “paleocauce” at the Piedra del Aguila site. This ancient, former channel of the Rio Limay left what appeared to be solid bluffs filled with porous voids overlooking the dam. To control erosion and ensure integrity of the reservoir, Piedra del Aguila engineers constructed an impermeable barrier more than 1 km long. In addition, they constructed more than 3 km of controlled drainage and inspection tunnels at the dam`s western end. They excavated nearly 1 million cubic meters of soil and rock and injected more than 67,000 cubic meters of grout just to tame the ancient paleocauce.

Plant-wide controls

The massive effort at Piedra del Aguila has paid off in a state-of-the art facility which provides more than 10 percent of Argentina`s hydroelectric power. With four units commissioned, the plant is now operating at its design capacity of 1,400 MW. Engineers designed and constructed the plant with space for possible fifth and sixth units.

A 292-square km reservoir fed by melting snows from the nearby southern Andes Mountains provides the power for Piedra del Aguila. Spiral penstocks, 9 meters in diameter, supply four 356-MW Francis vertical-axis turbines. Nominal water flow through each penstock is 350 cubic meters per second. Approximately 70 operations and maintenance personnel will staff the site at full production.

To control Piedra del Aguila`s power generation and ancillary operations, the designers selected a Bailey INFI 90 distributed control system (DCS) from Elsag Bailey Process Automation. Elsag Bailey`s Genoa, Italy, unit supplied the (US)$6 million control system, with installation, commissioning and training support provided by the group`s representatives, Electronica S.A. of Buenos Aires.

Comprising approximately 4,000 input/output (I/O) sites, the DCS design places control components close to the controlled activities. Modular process control unit (PCU) cabinets are installed near each generating unit. The PCU cabinets house plug-in modules which monitor plant operations, calculate the desired values, and transmit required changes to the appropriate equipment. Typical of the control devices is the Multi-Function Processor (MFP) module, which incorporates more than 200 integral control and processing algorithms for analog, sequential, data acquisition and advanced control functions. Redundant MFP configuration provides “hot backup” security for key functions.

Operator interface to the DCS is through four computer-based consoles housed in a main control room overlooking the turbine house. Communications are via asynchronous loop. Among the system`s unique features is the first application in Argentina of a communications loop comprised mainly of redundant fiber optic. Engineers selected fiber optic technology to reduce the effect of electrical noise from nearby transformers and other equipment.

The DCS provides total monitoring and control of the Piedra del Aguila station, including load scheduling, turbine start/stop and diagnostics, gate and pump control, backup diesel monitoring, fire alarms and building automation. The DCS also provides the setpoints which govern operation of the turbines and generators. Information from the plant`s main electrical dispatch system furnishes the basis for these setpoints.

Diagnostic I/O is an important element of the control system`s job. Water pressure against the Piedra del Aguila dam is reported to the system via more than 100 analog sensors. Although the dam design provides for natural seepage to relieve pressures, operators monitor sump water content constantly and adjust it to specified levels. Provided with a comprehensive “window” to plant operations through the control system, under normal conditions, one person staffs the control room.

According to Ruben Vallejos, electromechanical coordinator for Hidronor S.A. during design, plant designers selected the DCS on the basis of economics, distributed control philosophy and maintenance ease. Although state economic factors prompted a brief delay near the end of site construction, Vallejos reports that they commissioned the DCS well within the revised timetable and budget.

Charles McSwain, a Duke Energy veteran now on-site as Piedra del Aguila production manager, summarizes the key challenges: “The complexity of competitive bidding among our region`s multiple plants leaves little margin for error in operating efficiency. Our remote location, nearly a day`s travel from Buenos Aires, requires plenty of self sufficiency.” Responding to this, the Piedra del Aguila staff now handles maintenance of the DCS and other key systems on-site, and has played an active role in configuring the controls as new units come on line.

Bright future for hydro power

Hydroelectric plants now provide approximately 60 percent of Argentina`s electric generating capacity. Nearly one-third of this capacity comes from the Rio Limay facilities. By the turn of the century, analysts expect hydro to meet more than 70 percent of the nation`s demand–estimated at 80 million GWhr annually.

The fifth Rio Limay facility, approximately 20 km downstream from Piedra del Aguila, is now under construction. Scheduled to begin operation in late 1996, the Pichi Picun Leufu station will add 270 MW of generating capacity to the southwestern Argentine grid.

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Shown during installation of its second unit, Piedra del Aguila is powered by four 356-MW vertical-axis turbines.

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Piedra del Aguila station helps harness the Limay River via a dam that is 820 meters wide at its crown and 170 meters tall.

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Distributed controls provide a CRT-based “window” to plant-wide operations. Processing hardware is distributed throughout the site, linked by fiber optic communications loop.

AUTHOR

Brad A. Hoffman is director of marketing communications for Elsag Bailey Process Automation. He holds a degree in business management from Youngs-town State Uni-versity, Youngstown, Ohio.