Perhaps a high-voltage AC "transmission" line might be practical. Starting with the low-voltage AC from the turbine, step up to a higher voltage for the long distance wire, then step down again before rectifying. It's been discussed before on the forum here, but I don't have details handy about any that were successfully tried. I remember a hydro installation that did this. Less variation in speed in hydro turbines (seasonal changes, not minute by minute). Low frequencies are a problem for generators with low pole counts turning slowly. If the generator has a high number of poles (eg. 32 or more) and was of a size that cut-in happens around 40-50 Hz, then ordinary transformers could be used. The step up-step-down process involves losses, so they would have to be acceptable compared to the heavy copper wire costs for the long distance run.