Complex sentence: Rotary converter plants existed before the invention of mercury arc rectifiers in the 1920s at each facility where DC gained from an AC power grid was fed into an overhead line or a third rail of a railway.
Split into simpler sentences: Rotary converter plants were commonplace in railway electrification before the invention of mercury arc rectifiers in the 1920s. At each facility, power from an AC power grid was converted to DC to feed into an overhead line or a third rail of a railway.