Variable-speed generation units have widely been used in pumped storage hydropower plants. Nevertheless, conventional hydropower plants can also benefit from variable speed operation to compensate for efficiency loss when the hydro turbines work at a water head different from the nominal. When the units are connected to the grid through a DC link, some flexibility in the speed of operation is expected, as the generated AC power will be converted into DC power. This work presents the initial considerations for this operation, including estimating the efficiency achieved for revenue evaluation purposes. The case of the Itaipu Power Plant's 50 Hz units connected to a 60 Hz power system through a DC link will be presented as an example of an application.