Complex sentence: On its completion it became the largest power station in Denmark, a position it kept until 1940, and it had enough capacity to supply all of the Copenhagen area with lighting, reducing the three existing power stations to back-up systems and transformer stations.
Split into simpler sentences: On its completion it became the largest power station in Denmark, a position it kept until 1940. It had enough capacity to supply all of the Copenhagen area with lighting, reducing the three existing power stations to back-up systems and transformer stations.