Balancing arc flash reduction and selectivity in DER-heavy stations

Recent substation upgrade in Pittsburgh: replaced a 25/33 MVA transformer, added caps, refreshed relays. The arc flash study jumped at the 480 V MCCs because source Xd fell and feeders are short. ETAP said we could knock it down with faster clearing, but that broke coordination with downstream fuses, especially with two 5 MW PV sites backfeeding.

Commissioning added a twist. The old bushing CTs on the transformer saturate sooner than nameplate suggests, so 50/51s were slower in reality. We ended up lowering pickups, adding light+current arc-flash detection on 15 kV gear, and enabling high-speed bus differential on the low-side bus. We also enabled maintenance mode on SEL-751s for energized work. Tradeoff: better PPE levels, higher chance of a wider trip on internal faults.

For those modernizing similar 1970s stations: how are you managing arc flash targets without sacrificing selectivity when DER can feed from the load side? Are you comfortable relying on light-based trips, or do you prefer fast bus/zone interlocking and live with a bigger outage footprint? What’s worked in the field versus in the study?