Q&A: Do portable inverters need FRT or just faster protection?

Portable inverters keep lights on, tools running, and batteries safe. But protection choices can get confusing. Do you need full Fault Ride Through (FRT) like utility grid codes demand, or just fast-acting short-circuit and overcurrent protection? This Q&A cuts through the noise and gives practical steps you can apply to portable and temporary systems.

Quick answer

Most portable inverters do not need FRT. They need fast-acting short-circuit protection, clear overcurrent limits, and sensible ride-through for load steps. FRT applies to grid-connected resources under formal grid codes. Portable or off-grid units should prioritize rapid fault clearing and coordination with cables, battery management, and downstream devices.

What FRT actually requires

FRT asks inverters to stay connected and support voltage during deep sags for a defined time window. Grid codes implement FRT envelopes based on system stability studies and Critical Clearing Time (CCT). As IRENA’s Grid Codes for Renewable Powered Systems explains, operators derive FRT envelopes from high-resolution fault measurements, dynamic stability analysis, and manufacturer capability, then update them as the system evolves. The same report notes that grid codes often coordinate protection to avoid over-protection by customers, and that anti-islanding requirements remain a separate, explicit item.

Grid-forming inverters that can hold voltage during disturbances bring extra costs and design trade-offs. According to IRENA, instantaneous response needs internal energy storage headroom and higher power ratings for short-circuit support, and standards are still emerging. Retrofitting grid-following units to grid-forming is limited, especially for PV inverters.

Portable inverters: protection priorities

Fault current reality in inverter-based sources

Portable inverters and LiFePO4 batteries seldom drive high AC fault currents at the output. The semiconductor stage limits current to protect switches. Field ranges are commonly 1.25–2.0 per-unit for up to 1–2 cycles, then the controller shuts down or folds back. This aligns with analysis in the IEA Integrating Solar and Wind report: converter-based resources are current-limited, which reduces available fault current and forces a rethink of protection settings.

Protection goals differ from grid-tied plants

In a portable system you want fast fault isolation to protect people, cables, and the inverter. You also want brief ride-through for motor inrush and switching events. FRT to support a wider grid is not the target unless you are connecting under a utility interconnection agreement.

Device coordination in a current-limited source

The limited fault current means many thermal-magnetic breakers will not reach the instantaneous region. You rely on the inverter’s electronic current limit for the first milliseconds, plus properly sized branch fuses or DC breakers that trip thermally under sustained overload. Residual current devices (RCD/GFCI) add personnel protection on AC circuits. The IEA notes that protection must adapt under high inverter-based resource levels due to reduced fault currents and bidirectional flows.

Compare: FRT vs fast-acting protection

FRT envelopes stem from system CCT studies, as described by IRENA. Portable systems do not depend on system-wide rotor angle stability. They need acute local safety and uptime for mission loads.

Edge cases that change the answer

Backfeeding a utility service

If a portable inverter connects to a distribution feeder under a formal agreement, expect FRT and anti-islanding. Grid codes may mandate specific voltage and time windows and coordination studies with the operator. IRENA highlights that some codes require protection coordination studies prior to connection.

Multi-inverter portable microgrids

For incident response sites or construction yards, parallel inverters can form a small grid. Ride-through for motor starts and selective clearing become important. The IEA points to the need for visibility and controllability of distributed resources as inverter penetration rises. Use graded settings across sources and branches, and add headroom so current limits do not collapse voltage during large inrush.

Critical loads that must not trip on sags

Medical tents, ICT racks, and pump controls may demand short ride-through for sags caused by internal events. That is not utility FRT, but it mimics the intent. A small undervoltage delay lets contactors pull through a brief dip while hard faults still trip fast.

Practical protection settings for portable inverters

AC side

• Undervoltage trip: 70–80% of nominal with a 100–300 ms delay to ride through motor inrush, then latch trip on sustained sag. Adjust based on load tests.
• Overvoltage trip: 110–120% of nominal with 100–200 ms delay. Protects sensitive electronics from regulator overshoot.
• Short-circuit response: Electronic current limit or desaturation shutdown targeting <2–3 cycles to open. Many inverters detect and cut within tens of microseconds at the switch stage, then command AC zero output by next cycle.
• RCD/GFCI: 30 mA for personnel protection on outlets; 100–300 mA selective upstream device for feeder protection if needed.

DC side (battery and PV)

• Battery OCPD: DC-rated fuse or breaker sized to cable ampacity and BMS max discharge. Verify DC interrupt rating exceeds worst-case battery fault.
• PV input: String fuses or combiner-level OCPD per module Isc and voltage. Add surge protection if cables run outdoors.
• Isolation and bonding: Follow manufacturer guidance on negative-to-chassis bonding and RCD type. Misintegration can create nuisance trips.

Why these numbers? Inverter outputs are current-limited, so external AC breakers may not see a high kA surge. Fast electronic limiting provides the first line of defense. The IEA notes the limited overcurrent capacity of inverters and the need to adapt protection. Grid-forming features can help in larger systems but add cost and energy headroom requirements as IRENA summarizes.

Sizing snapshot for a 2 kVA portable inverter

• Inverter rating: 2 kVA, 120 V AC. Continuous current ≈ 16.7 A. Short surge limit: 1.8× for 2 cycles.
• Branch circuit to outlet: 12 AWG cable, 20 A breaker. Expect thermal trip for overloads; instantaneous may not operate on a 1.8× surge.
• Inverter electronic limit: clamp at ≈ 30 A peak and command shutdown on hard fault within 1–2 cycles.
• Battery side: 24 V LiFePO4 with BMS limit 100 A. Use a 125 A DC fuse with adequate DC voltage rating and interrupt capacity, sized to cable ampacity and BMS peak.

This arrangement relies on the inverter’s fast limit to protect semiconductors and the branch breaker to protect conductors against prolonged overloads. Add RCD/GFCI on outlets for shock protection.

Testing and commissioning

• Functional trip test: Inject a controlled short on a sacrificial test lead with current clamps. Confirm inverter fold-back or shutdown occurs in <3 cycles.
• Voltage sag ride-through: Start the largest motor or compressor while logging voltage. Tune undervoltage delay so contactors hold, but faults still trip fast.
• Selective coordination: Trip downstream device first by grading current limits and time delays. Verify upstream device remains closed during downstream faults.
• Record settings: Keep protection setpoints and firmware versions for traceability.

Standards and policy context

Protection expectations flow from grid codes and DER practice. Key takeaways from public sources:

• IRENA Grid Codes: FRT envelopes derive from CCT and are revised as grids evolve. The functionality brings added cost and storage headroom. Some codes require coordination studies so customers do not overprotect facilities.
• IEA Integrating Solar and Wind: High inverter penetration lowers fault currents and complicates protection; inverter overcurrent capacity is limited; planners adapt settings and maintain headroom.
• U.S. DOE Solar Energy: DOE resources outline PV inverter roles, grid integration priorities, and evolving DER controls. Use agency content to cross-check protection concepts and commissioning good practice.

What this means for your design

Pick fast-acting protection first for portable systems. Add brief ride-through only to keep your own loads stable. Reserve FRT for grid-tied cases under a utility process. If you build a temporary microgrid, consider grid-forming features and tune settings to maintain voltage under inrush while still clearing faults rapidly.

FAQ

Do portable inverters require Fault Ride Through?

Not in typical standalone use. FRT is a grid-code function. Portable systems benefit more from fast-acting short-circuit and overcurrent protection and modest ride-through for internal load steps.

How much fault current should I expect from an inverter?

Often 1.25–2.0 times rated current for up to 1–2 cycles, then fold-back or shutdown. That range reflects the current limits highlighted in the IEA report for converter-based resources.

Should I use fuses or breakers downstream?

Use devices that match cable ampacity and the limited fault current. Breakers may not hit instantaneous trip with a current-limited source. Fast electronic limiting in the inverter handles the first milliseconds; fuses or thermal trips protect conductors against sustained overloads.

What changes if I parallel inverters?

Available fault current increases, and settings must grade across units. Add a small undervoltage delay and coordinate current limits so loads ride through inrush but faults still clear quickly.

Any official references I can cite for my safety file?

Yes. See IRENA Grid Codes for FRT concepts and coordination studies, and IEA Integrating Solar and Wind for protection challenges under high inverter penetration. DOE’s Solar Energy pages provide DER integration resources.

Safety and compliance note

This content supports engineering judgment and training. It is not legal advice. Always follow local codes and standards, manufacturer manuals, and qualified professional review.