IEEE Power & Energy Magazine - November/December 2020 - 73

turbines (11.5 MW), four Pelton hydro turbines (11.32 MW),
a head of 655 m, eight pumping systems (6.24 MW), an upper
reservoir (380,000 m3), a downhill reservoir (149,000 m3), and a
pipeline of 2.3 km. This configuration offers approximately
2 days of hydropower autonomy to the island (~230 MWh),
without the withdrawal of fresh water to the sea. The annual
penetration level of renewable energy from Gorona del
Viento reached 56.5% in 2018 [Figure 7(b)]. In July 2019,
El Hierro reached a record 24 consecutive days with energy
exclusively supplied from this power plant.
The efficiency of the system is approximately 50%
because the main task is not to produce as much electricity
as possible but, rather, properly integrate the energy from the
wind power plant into the El Hierro network. In a noninterconnected system and for a small island, the primary concern
for the power grid is integrating the power ramps produced
by wind and solar energy because those sources produce frequency variations that are out of range in the grid. When the
frequency variations are out of range, a substantial part of
the wind energy reaching the wind power plant is curtailed
because of the severe gusts experienced at the location during certain periods. The frequency of the power system in El
Hierro has been out of range (∆f >250 mHz) more often than
that of any of the other Canary Islands (81 h in 2017); thus,
it was necessary to further reduce the average efficiency of
the wind-hydropower system to limit the out-of-range hours
(20.6 h in January-August 2019).
Additional strategies are being tested to further increase
the share of wind in the power mix and reduce the periods
when the frequency of the system is out of range. One strategy
was to act on the primary frequency control of the hydropower
turbine units (speed governors) and pump frequency converters, but this was not enough. A second effort consisted of
reducing the pumped-unit shedding and revising the speed of
the governor logic of the hydropower turbine generator units
(i.e., by using a damper to mitigate the pressure waves in the
turbine penstock, which causes oscillations in both the water
pressure and mechanical power in the hydropower turbine
units because the higher the nozzle rate of the opening, the
larger the oscillation). Also, the procedure for processing the
wind plant's active power settings was revised (each wind turbine receives a specific active power setting value; the minor
power setting is between a 5-min moving average value calculated from a statistical method of regression and a consigned
active power value) for power-frequency control. The next
steps are to transition the hydropower turbines from generator
mode to synchronous compensator mode, review the active
power settings for each wind turbine, and enforce the wind
power plant control capabilities (active power-frequency control and inertia emulation capability; see Marrero at al. in the
"For Further Reading" section) to provide plenty of frequency
response and regulation when operating.
There has been controversy about the cost of electricity generation in the El Hierro power system since Gorona
del Viento began service. The El Hierro power system is the
november/december 2020

smallest in the Canaries, and it usually had the highest cost
per kilowatt hour of all six energy networks. Also, the cost
increases sharply when Gorona del Viento and the conventional power plant are both operating, so to lower expenses, it
is necessary to shut down the conventional power plant (which
includes only diesel engines) when Gorona del Viento has
enough stored energy. Much of the extra cost can be attributed to the fact that the return on investment in Gorona del
Viento was accelerated by the CNMC because of the delays
for the plant's entry into service and the repayment of loans to
the banks financing the venture. All this happened in parallel
with an intense economic crisis. Finally, the loans were repaid
in 2018, and currently the recognized price for the generated
electricity is €15.57/MWh (which is revised every three years).
REE is planning to replicate Gorona del Viento in Gran
Canaria by using the existing reservoirs of Chira (5.6 million m3) and Soria (32.3 million m3) and commissioning a
200-MW/3.82-GWh storage system. The project, which is valued at €391 million, began in 2007, with a scheduled completion in 65 months. There is some public opposition to the project, however, because of the potential environmental impact to
the area. Also, there is a discussion about the ownership of this
power plant, if it eventually enters service, given the simultaneous role of REE as the system operator and a producer. REE
justifies its position by arguing that the pumping system will
serve only as a regulation device for the power grid.
It is obvious that to reach 100% renewable energy in
insular power systems, storing energy in pumping systems,
batteries, and thermal units for tertiary regulation can be a
key strategy. Also, gas-to-power storage systems could play
a central role because the land available in the islands is
scarce and expensive, the time for receiving the administrative permits required for occupying large areas is long, and
the energy density of the hydrogen and methane per unit of
occupied surface is much higher than that of batteries and, of
course, pumping systems. In primary and secondary regulation, however, there is another interesting option to keep the
frequency of the grid in the permitted range: short-term forecasting technology. In this area, satellites and (mostly) allsky imagers can play a fundamental role. The attractiveness
of satellites for island forecasting is limited because of the
spatial resolution and the time range between served images,
but the information that is provided can be helpful.
All-sky imagers give more localized information with a
higher resolution. The accuracy of the sky imagers for forecasting PV power is limited, but the technology can play a role as
a warning system for the utility. Low-cost (fewer than US$500
each) sky imagers have been placed on two of the Canary
Islands in areas where more PV systems are located (Figure 8), and they were installed as pairs to calculate the height
of the cloud base at the lowest cost compared to other technologies. Forecasts are made with images obtained every 10 s, and
the algorithm learns by comparing the PV energy recorded
in the substations where the multimegawatt PV plants inject
into the grid. The goal is to offer warning services instead of
ieee power & energy magazine

IEEE Power & Energy Magazine - November/December 2020

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2020