Complex sentence: Ground work for these 3 units--known at the time as' Drax First Half--started in 1967, with two being synchonised to the grid in 1973, and the third in 1974.
Split into simpler sentences: Ground work for these 3 units, known as Drax First Half "'," started in 1967. Two units were synchronised to the grid in 1973, and the third in 1974.