IEEE Power & Energy Magazine - September/October 2020 - 37

By Daniel Flores, Andrew Bridgeman,
Francois Welt, Juan Jose Aveiro, Delfín Benítez,
and José Vallejos

HYDROPOWER IN PARAGUAY MAKES UP NEARLY 100% OF THE COUNTRY'S ENERGY PORTfolio. Ninety percent of its generated energy is exported, with a majority going to neighboring Argentina and
Brazil. Paraguay is one of the world's largest electricity net exporters, and virtually 100% of its hydropower
is produced by three stations across the country:
✔ Itaipú: a Brazilian-Paraguayan binational station with an installed capacity of 14,000 MW and one of
the world's largest energy producers
✔ Yacyreta: an Argentine-Paraguayan binational station with an installed capacity of 3,200 MW
✔ Acaray: the only hydropower station completely owned by the Administracion Nacional de Electricidad
(ANDE) with an installed capacity of 210 MW.
The Acaray generating station is composed of two powerhouses, each with two vertical Francis generating units. Acaray I was commissioned in 1967, and each unit has a nominal output of 49 MW. Acaray II was
commissioned in 1971, and each unit is designed for a nominal output of 65 MW. Both powerhouses are
connected by an independent water-conveyance system (an underground tunnel, penstock, and surge tank)
to a dam structure with hydraulic intake gates and stop logs for each powerhouse, and a common 60-ton
gantry crane. The dam also has seven spillway gates with upstream stop logs and one low-level outlet gate
with a total spill capacity of 7,000 m3/s, enough to pass the 1,000-years Probable Maximum Flood (PMF).
The Yguazú dam was also evaluated as part of the study. It is located 50 km upstream of the Acaray dam
and is used for flood control. Two radial gates, two low-level outlet gates, and a free surface spillway provide
a total spill capacity of 2,700 m3/s, enough to pass the 1,000-years PMF.
In 2015, ANDE, with the financial support of the Inter-American Development Bank, initiated a tender
process to evaluate the existing conditions and life-extension options of the Acaray generating station and
Yguazú dam; the contract was awarded to Manitoba Hydro International Ltd. and Hatch Energy Projects in
2016. The project aimed to plan and execute a comprehensive diagnostic of ANDE's existing generation and
water-retention assets (Acaray and Yguazú, respectively). This involved performing multiple site visits, including mechanical and electrical inspections and absolute turbine performance testing.
After completing the condition assessment, the study focused on all options to increase power and efficiency.
Then an evaluation was conducted based on current and future energy market conditions. Simulating for future
load demand under representative operating conditions was the basis for evaluating all the scenarios.
The best option in terms of cost-benefit ratio was selected to develop a capital investment plan that would
improve the reliability and capacity of the Acaray generating stations. The plan included life extension of the
existing facilities as well as the expansion of new facilities to operate reliably over the next 30 years.

Objectives and Methodolog y

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Comparative Analysis Approach
A comparative analysis of possible development options was conducted to derive an economically optimal,
feasible, and sustainable long-term development plan. This analysis used the following optimization models:
✔ power generation planning model software (Vista DSS, see Glabau et al.), which accurately calculates the
future revenues of each development option (the software was developed and is owned by Hatch)
✔ asset management model software (HydroVantage, see Bhan et al. and de Meel), which calculates the
costs of operating existing and upgraded facilities (the software was developed and is owned by Hatch)
✔ various cost-estimation techniques, which calculate the cost of developing new sites for power generation on the Acaray River.
Each model was set up to consider future uncertainty, either through scenario analysis (e.g., hydrologic and
load uncertainty) or through probabilistic modeling of the uncertainty variables (e.g., failure risks on existing
equipment). The study period extended over 50 future years, starting in 2017. The relationship of the various
models in developing a cost-benefit analysis is illustrated in Figure 1.

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IEEE Power & Energy Magazine - September/October 2020

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