Arc-fault mitigation in unit substations (white paper)

Overview

The starting point is that a fault on the line side of the LV main breaker, or in the main breaker itself, can only be cleared by the transformer primary device, and a fuse is slow at this. The paper therefore compares maintenance (energy-reducing) switches, arc-sensing relays and replacing the primary fused switch with a vacuum circuit breaker.

The worked example is a typical substation: 2,000 kVA, 13.8 kV primary, 480/277 V secondary, 5.75% impedance, with energies calculated per IEEE 1584 for eight scenarios. A fault cleared by the MV primary fuse takes about 900 ms and gives 38.3 cal/cm2; with the LV main on a 0.4 s short-time delay the figure is 17.0 cal/cm2.

When arc sensing trips the LV main, clearing takes about 35 ms and the energy is 1.5 cal/cm2, but a fault on the line side of the main with a fused primary still sees 38.3 cal/cm2. With a primary vacuum breaker and normal relay settings energy can reach 106.5 cal/cm2 (2,500 ms); arc sensing that trips the primary breaker cuts it to 2.1 cal/cm2 (49 ms), about 2% of that.

Arc-sensing devices can bring LV breaker clearing down to about 50 ms but need electrically operated source breakers and current detection able to see the fault. Maintenance switches sacrifice selectivity while active and depend on staff remembering to switch and reset them.

Key points

1. Worked example: 2,000 kVA unit substation, 13.8 kV primary, 480/277 V secondary, 5.75% impedance, IEEE 1584.
2. MV primary fuse clearing: about 900 ms and 38.3 cal/cm2; LV main with 0.4 s short-time delay: 17.0 cal/cm2.
3. Arc sensing tripping the LV main clears in about 35 ms at 1.5 cal/cm2, but a line-side fault with a fused primary still sees 38.3 cal/cm2.
4. With a primary vacuum breaker and normal settings energy can reach 106.5 cal/cm2 (2,500 ms); arc sensing tripping the primary gives 2.1 cal/cm2 (49 ms), about 2%.
5. Arc sensing can cut LV breaker clearing to about 50 ms but needs electrically operated source breakers and fault-visible current detection.
6. Maintenance switches sacrifice selectivity while active and rely on staff remembering to switch and reset them.

Using it in design

• In the arc-flash module, evaluate incident energy at the LV main line-side terminals using the upstream MV device's clearing time, not the LV main's.
• Warn when a unit substation uses a fused MV switch and line-side incident energy exceeds a threshold.
• Suggest a primary vacuum breaker plus arc-sensing transfer trip as the remedy.
• Add catalogue fields for primary protective device type, arc-sensing relay option and maintenance mode setting.

Who should read it

Electrical designers and operational-safety staff assessing arc-flash risk and protective-device choice in unit substations.

Related standards

• IEEE 1584
• NFPA 70E
• IEEE C37.20.7

Publication details

Authors

Siemens

Publisher

Siemens

Year

2020

Document number

SIDS-T40074-00-4AUS

Document language

English

Source type

Manufacturer

Access

Free to read

Licence

Copyright retained; citation only — the publisher’s terms apply to the original document

Citation