Complex sentence: In 1906 Norsk Hydro started building a huge hydro electrical powerplant at Vemork, it was finished in 1911 and was then the largest hydro electric powerplant in the world.
Split into simpler sentences: In 1906 Norsk Hydro started building a huge hydro electrical powerplant at Vemork. It was finished in 1911 and was then the largest hydro electric powerplant in the world.