A utility distribution substation stepping a 69kV subtransmission source down to 13.8kV across a four-breaker ring bus and twelve feeders — a full tier up from a campus-scale substation, with a worked fault-current study, relay settings basis, and IEEE 80 grounding numbers to match.
The project brief: a utility-owned 69/13.8kV distribution substation serving roughly 56 MVA of connected load across twelve distribution feeders, fed by a looped 69kV subtransmission source through a four-breaker ring bus and two 24/32/40 MVA power transformers, with a sectionalized 13.8kV bus and normally open tie. Unlike a self-contained campus system, this substation leans on adjacent substations through distribution feeder ties for part of its N-1 contingency plan — a genuinely utility-scale distinction.
This module walks the full project: why ring bus is the right topology here (and main-tie-main and single bus aren't), a worked transformer sizing and per-unit short-circuit fault current calculation that drives the 40kA switchgear selection, the relay settings basis behind the 87T/50/51/67/50BF protection scheme, SCADA/RTU integration, a worked IEEE 80 touch and step voltage calculation, and a full commissioning procedure summary. The complete numbers, bill of materials, and quiz are part of the unlocked module below.