On 17 - 25 May 2022, measurements of four tubular Kaplan turbines were carried out for VEZ Svoge EOOD at the Bulgarian hydroelectric power plants HPP Lakatnik, HPP Svrajen, HPP Opletnya and HPP Tserovo. These power plants (each installed with one unit of about 3 MW) form a cascade on the Iskar River, north of Sofia city.

The goal of tests was to assess the correctness of the existing ON-CAM relations and, if necessary, to optimize them for maximum machine efficiency and acceptable cavitation emissions. The flow rate was determined by index method from the pressure differential values at the turbine, either directly measured or evaluated from the difference between the values of two pressure sensors.  However, at HPP Opletnya an alternative method of flow determination had to be used due to unreliable results.

The results at all HPPs showed small deviations of existing ON-CAMS, and by optimizing them a local increase in the efficiency of the units up to 2% can be achieved.