Complex sentence: After 7 days on December 5 the second unit No.17 started to operate and on December 12,1963, units No.16 and No.15 were included into the Unified Energy System of Siberia.
Split into simpler sentences: After 7 days on December 5 the second unit no.17 started to operate. On December 12,1963 units no.16 and no.15 were included into the Unified Energy System of Siberia.