Frades II, Portugal

Europe´s most advanced pumped-storage plant

The pumped storage power plant is located in the north-west of Portugal and is installed in an underground cavern. It is not designed as a standalone facility but represents an important addition to the eight plant Cávado-Rabagão-Homem cascade system. The difference in height between upper and lower basins is 420 meters. Voith was awarded the order to equip the Frades II pumped storage power plant in the fall of 2010. It is operated by Energias de Portugal (EDP).

Portugal goes for renewable energy

Portugal relies on wind, solar and hydropower to generate its electricity supply. Since the turn of the millennium, the country has multiplied its capacity for wind-powered electricity generation. In 1998, Portugal had around 51 MW of installed capacity in its wind turbines, 18 years later it was already more than 5,000 MW. The basis for the further development of power generation from renewable energy sources are its powerful pumped storage plants. Almost a third of renewable energy in Portugal comes from hydropower.

Voith has delivered two variable-speed pump turbines, each with 390 MW rated power, two asynchronous motor generators, each offering 440 MVA in rated power, the frequency converter and the control systems, as well as the hydromechanical equipment for the modern pumped storage power plant, Frades II. Since commissioning in April 2017 the machine sets are the most powerful and largest of their type.

Innovative pumped storage technology using variable speed turbines

"The key element of the plant is a special asynchronous motor-generator – the doubly fed asynchronous generator," explains Thomas König, who is responsible for Electrical Balance of Plant at Voith Hydro in Heidenheim, Germany. Unlike a conventional asynchronous generator, with a rotational speed firmly locked to the line frequency of 50 Hz, in the new doubly fed asynchronous generators the mechanical speed is not tied to line frequency and so can vary.
The new system offers two advantages: On the one hand, the plant can respond faster and more flexibly to the active and reactive demand from the power grid. On the other, it offers additional stability in the event of a drop in voltage, reduces the likelihood of a power failure and enables a fast restart if a power outage does occur. "If the grid voltage drops to 5% of its normal value, the Frades II power plant can remain stable for 600 milliseconds, that is four times longer than conventional turbines with fixed speeds. In emergency situations this time difference can be crucial when there is a need to prevent a large-scale power outage," says Thomas König.

Innovation as benchmark

The new Frades II pumped storage power plant called for innovative thinking of Voith's engineers in a large number of areas. For example, the rotor of the motor-generator had to be completely redesigned in order to cope with the high centrifugal forces, the much higher current and the voltage from the frequency converter. The converter for the excitation is larger, heavier and 25 times more powerful than in a comparable synchronous unit and therefore also requires a new electrical protection system.
EDP reaps the benefits of these innovations, as Lars Meier, Head of Technical Sales and Tender Management at Voith Hydro in Heidenheim, explains: "The motor-generator technology with variable speed at Frades II increases the total number of operating hours for the plant. More operating hours and higher availability generate more revenue and ultimately more benefit. This is one of the reasons why comparable power plants can very quickly write off the necessary investments. Another fantastic spin-off of the technology is the increased operational efficiency overall."