Short Circuit and Arc Flash Study on a Microgrid Facility

Overview

The study asks how to compute and label short-circuit levels and arc-flash risk in a microgrid with a generator, wind, PV and battery inverter. The testbed is fed by a 12.47 kV utility and includes a 569 kVA diesel generator, wind, PV and a battery inverter system. The workflow is listed in six steps: data collection, operating topologies, three-phase bolted fault currents, arcing current and incident energy, PPE category, and equipment labelling.

Faults were computed in PowerWorld for both grid-connected and islanded (generator-slack) topologies, with inverter fault contributions taken as datasheet maximum short-circuit current multiplied by 1.4. The highest fault level at each bus was then used with IEEE 1584-2018 to compute arcing current, incident energy, arc-flash boundary and PPE category. Assumptions: VCB electrode configuration, 5-cycle arc duration, 32 mm gap, 457.2 mm working distance and 508 mm × 508 mm enclosures.

Results: at the 480 V MCC bus the three-phase fault current was 22,793 A grid-connected but only 1,296 A when islanded on the 569 kVA diesel generator. At 480 V buses, bolted current was about 22.8 kA, arcing current about 16.8 kA, incident energy 3.6 cal/cm² and arc-flash boundary 91 cm; at the 208 V buses, 3.8 cal/cm² and 94 cm. All buses fell in PPE category 1. Bare-skin injury starts at about 1.2 cal/cm², and the boundary is defined at 5 J/cm².

Limits: it is a short study; only the highest fault level was used, protection coordination and clearing-time changes are not its focus, and arc duration was assumed fixed. The paper notes that Doan's maximum power method for DC applies only to linear DC sources, not PV. It still provides a compact template for the same workflow in a data-centre LV system with generator backup.

Key points

1. The arc-flash workflow is listed as six steps: data collection, operating topologies, three-phase bolted fault currents, arcing current and incident energy, PPE category, and equipment labelling.
2. A factor of 1.4 was applied to the datasheet maximum short-circuit current of inverter-based sources as a safety band.
3. At the 480 V MCC bus the three-phase fault current was 22,793 A grid-connected but only 1,296 A when islanded on the 569 kVA diesel generator.
4. Assumptions: VCB electrode configuration, 5-cycle arc duration, 32 mm gap, 457.2 mm working distance, 508 mm × 508 mm enclosures.
5. At 480 V buses, bolted current was about 22.8 kA, arcing current about 16.8 kA, incident energy 3.6 cal/cm² and arc-flash boundary 91 cm; at 208 V buses, 3.8 cal/cm² and 94 cm; all buses PPE category 1.
6. Bare-skin injury starts at about 1.2 cal/cm²; the boundary is defined at 5 J/cm². For DC systems, Doan's maximum power method applies only to linear DC sources, not PV.

Using it in design

• In a data-centre LV system with generator backup, run the arc-flash study for both grid-connected and islanded topologies, and check that the much lower islanded fault current (1,296 A instead of 22,793 A) does not lengthen clearing time.
• Take inverter fault contribution from the datasheet and multiply by a safety margin (here 1.4), and state that assumption in the study report.
• Use the six-step workflow as a checklist and generate labels from the highest fault level across topologies.

Who should read it

Engineers performing short-circuit and arc-flash labelling studies on low-voltage systems with generators, inverters and batteries.

Document contents

1. I. Introduction
2. II. Microgrid Testbed Topology
3. III. Short Circuit Study
4. IV. Arc Flash Study
5. V. Results and Equipment Labeling
6. VI. Final Considerations

Related standards

• IEEE 1584-2018
• NFPA 70E

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Publication details

Authors

K. E. K. Schmitt; C. A. Negri; S. Daneshvardehnavi; S. Bayne; M. Giesselmann

Publisher

arXiv (Texas Tech University)

Year

2021

Document number

arXiv:2105.09927

Document language

English

Source type

Scientific

Access

Open licence — downloadable

DOI

10.48550/arXiv.2105.09927

Licence

CC BY 4.0 — share with attribution

Licence evidence

arxiv.org

Citation