Arc Flash, Harmonic and Motor Starting Studies at a Houston Terminal, With Two More Coastal Sites (2018-2021)

A liquid storage terminal near Houston needed more than arc flash labels in 2020. New motor control centers had been added, and the owner also wanted to know how much harmonic distortion the site was sending back to the utility. Two smaller coastal studies, in Galveston and Corpus Christi, round out this write-up.

Two feeds, no relays, six ways to run

The terminal takes two utility feeds at 12.47 kV into its primary switchgear, and several transformers step that down. Protection is by fuses and circuit breakers only, with no protective relays. Because the system can be lined up in different ways, we built six separate scenarios into the model and calculated each one. The one-lines, the utility data and the walkdown data came from the electrical contractor working at the site, and we checked bolted fault currents, both three-phase and single line to ground, at every bus.

Motor starting and harmonics

For the new motor control centers we ran a motor starting study. At the first hydraulic power unit switchgear the largest voltage dip stayed under 5%, which is marginal. At the second it reached roughly 10% on the last motor start, and we passed that result to the engineer responsible for that system to decide whether it was acceptable. The harmonic study looked at both utility connection points and reported the total harmonic distortion and each individual harmonic order at each one.

Labels that read like the old ones

Arcing time was capped at 2 seconds under section 6.9.1 of IEEE 1584-2018, and we found no location where a worker would need longer than that to move away from an arc. Enclosure and bus gap dimensions came from the typical values in the same standard instead of opening energized gear. The energy level categories on the labels followed the levels of the earlier model, so the crews would see a familiar format. No setting changes were needed; the report asks that each device be checked against the settings document before the labels go up.

Galveston, 2021

An industrial site in Galveston takes one utility feed at 12.47 kV. We modeled it in ETAP^® 20.6.0 and ran two configurations, normal operation and the standby generator carrying the load. No setting changes were proposed, and the system alerts produced by the model went to the owner in an appendix. Where incident energy came out above 40 cal/cm², the report treats de-energizing the gear as the first option.

Corpus Christi, 2018

The smallest of the three, an energy company site in Corpus Christi, has one utility connection through a 750 kVA transformer, a main panel, an automatic transfer switch, a load bank and a generator. Coordination was shown separately for the utility feed and the generator feed. A revision changed the trip units and settings on the 2,000 A transfer switch breakers, with no effect on the arc flash results.

Related services

How these studies are put together is described on our page for arc flash study services. Device settings and time-current curves are covered under protection coordination.