The Chaira pumped storage hydro power plant (Chaira PSHPP) was built in Rila mountains, about 100 km southeast of capital city, Sofia. Chaira has generating capacity of 864 MW and a pumping capacity of 788 MW, and is thus the largest pumped-storage plant in southeast Europe. The power plant is equipped with four reversible Francis pump-turbines, each rated at 216 MW in the generating mode, and 197 MW in pumping mode.
Units 1 and 2 have been in operation since 1995, and that time Chaira was still first in the world as regards the highest head for a single-stage pump turbine ( 690 m generating and 701 pumping ). Units 3 and 4 came online in 1999. The pump-turbines and motor - generators were supplied by Toshiba, and three of them were manufactured under Japanese supervision in Bulgaria.
The upper compensating basin for Chaira is the Belmeken reservoir that is connected to the Chaira pumped storage hydro power plant by two headrace tunnels with a diameter of 4.20 m and two penstocks with diameter 4.40 m, reducing to 4.20 m.