tony I can give our references of the variable speed to hydro application: we manufacture both generators and static converters. I understood that your moulin is rated near 200 kW ok we did application with similar application, 300kW, 600kW with permanent magnet generator water cooled, driven by inverter and coupled to the national grid by an IGBT active front end converter. the “hill” curve is Q(m3/s) vs H(m), water flow versus head; you can try to find the best efficiency by variating one of them. The algoritm should be fixed by the turbine manufacturer, who knows the “hill” curve of its runner profile. You supplied a feedback of speed (or torque) to the inverter. The inverter “brakes” the generator in order to match the speed. The active front end convert to the grid the power produced at the nominal voltage (400V, 690V, MV…) The applications are with Pelton, with variable head (500-600 rpm), S-Kaplan variable pitch with variable head (180-230 rpm),. .. I remain agostino