IEEE Power & Energy Magazine - September/October 2020 - 45

capability upstream. To quantify its impact, the total generation for option 4 (upgrade of Acaray I and II) and option 5
(construction of Acaray III) with ecological flows was compared against those previously calculated without ecological flows.
Minimum flows would generally result in a loss of overall energy generated from the river system over any given
year, as the additional constraint would limit the amount
of flexibility in the use of the water that would otherwise
be available to the hydro operation. The projected results
for options 4 and 5, which assume upgraded Acaray I and
II power plants and powerhouses as well as an Acaray III
powerhouse with an optimal capacity of 90 MW, respectively, are shown in Table 5. Although the loss of generation for the overall river system was found to be on the
order of a few percent, the impact was found to be very
similar between the different options. Therefore, minimum
flows imposed on the future operation of the Acaray plants
are not expected to affect the selection of an optimal development strategy.
A similar investigation was repeated for minimal flows
imposed downstream of the Yguazú dam. Values of 5, 10,
and 15 m 3/s were considered, and their impact on the generation and revenues for option 6 (the new power plant at
Yguazú with an optimal capacity of 70 MW, as listed in
Table 4) was investigated. The analysis was conducted for
various plant sizes to determine whether such minimum
flows would affect the selection of the optimal solution for
the new power plant. The conclusions were similar, i.e., minimum flows did have a significant effect on the optimal size
of the future plant.

Results Evaluation
Rehabilitation/Upgrade Analysis
Asset management software was used to analyze the various
refurbishment and upgrade scenarios as part of development
options 2-4 and to determine the optimum intervention
strategy for the existing facilities. This analysis considered
all of the following cost streams associated with the plant
before and after each intervention:
✔ capital costs of intervention
✔ O&M costs
✔ risk costs of failure
✔ performance benefits.
These costs (rehabilitation or replacement) were first estimated using in-house cost curves and published benchmarking cost data. All of the turbine/generator components were
validated against the following previous contract costs at the
Acaray II plants:
✔ new generators in 2007
✔ new runners in 2001
✔ new transformers in 2007.
Performance benefits for the runner upgrade options were
calculated as part of the long-term system analysis (Vista
september/october 2020

table 5. The effect of minimum flow on reducing
energy generation (yearly average).
Minimum Flow

Option 4 (%)

Option 5 (%)

No minimum flow

5 m /s

2.77

2.86

5.73

5.86

10 m /s

table 6. The performance benefits of runner
upgrade options.

Plant

Relative Reduction
in Energy
Purchases (p.u.)

Acaray I: same capacity, higher efficiency

0.068

Acaray I: higher capacity, higher efficiency

0.136

Acaray II: same capacity, higher efficiency

0.034

DSS). They were the result of detailed simulations that compared future revenues with and without the planned rehabilitations or upgrades, as displayed in Table 6.
These results are expressed as a reduction in total energy
purchases over the period of study. Because the generation
from Acaray is only a small proportion of the total system
load, the numbers expressed in per unit (p.u.) are small in
relative terms; however, they are significant with respect to
Acaray's total energy generation.

Acaray I Runner Analysis
The Acaray I runner analysis considered three different options:
✔ turbine rehabilitation
✔ replacement runner, same capacity, and higher efficiency
✔ replacement runner, higher capacity, and higher
efficiency.
It is noted that for a higher capacity runner, the costs for
a completely new generator were included to accommodate
the higher loads on the thrust bearing and rotor and to allow
for the higher power output.
All of the options were compared against a do-nothing
scenario, which assumes only ongoing O&M costs. Our
calculation included a yearly discount factor during the
study period. For this scenario, the risk costs would consistently increase. The percent reduction for each option
compared against the do-nothing scenario is presented in
Table 7, where higher percentages represent the lowest lifecycle cost.
The results of the analysis indicated that the optimum solution was to increase the capacity with a new runner and full
generator replacement as soon as possible. This solution minimized the overall lifecycle costs, primarily due to the revenue
increase achieved with the runner replacement, and, in addition, lowered both risk during operation and maintenance
ieee power & energy magazine

IEEE Power & Energy Magazine - September/October 2020

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2020