Fault Ride-Through (FRT)

The LVRT curve defines the minimum voltage magnitude and maximum duration a generator must withstand without tripping. A typical profile requires:

• Zero-voltage ride-through: Withstand 0% voltage at the point of interconnection for 150 milliseconds
• Voltage recovery slope: Remain connected as voltage recovers to 90% within 3 seconds
• No injection of reactive current during the dead zone is permitted; the unit must actively support voltage recovery

These profiles are codified in standards such as IEEE 2800-2022 and the European NC RfG (Network Code on Requirements for Generators).

During a voltage sag, modern inverter-based resources must shift from active power export to reactive current injection to support grid voltage. Key requirements include:

• K-factor compliance: Inject reactive current proportional to voltage deviation, typically 2-6% of rated current per 1% voltage drop
• Response time: Full reactive current response within 20-40 milliseconds of fault detection
• Asymmetrical injection: During unbalanced faults, inject positive-sequence reactive current while managing negative-sequence components to avoid overcurrent tripping

This behavior is fundamentally different from legacy rotating machines, which relied on inherent electromagnetic characteristics.

The phase-locked loop (PLL) is the critical control component that synchronizes an inverter to the grid. During FRT events, PLL performance determines survival:

• Frequency tracking: Must maintain lock during rapid phase angle jumps caused by fault clearance
• Ride-through of unbalanced sags: Advanced PLL designs with dual second-order generalized integrator (DSOGI) or decoupled double synchronous reference frame (DDSRF) architectures extract positive-sequence voltage even during severe unbalance
• Loss of synchronism prevention: Poor PLL tuning causes transient instability, leading to inverter tripping despite adequate voltage support capability

FRT requirements differ significantly across jurisdictions, creating compliance challenges for equipment manufacturers:

• Germany (VDE-AR-N 4120): Type 2 FRT with extended zero-voltage duration for synchronous areas
• ERCOT (Texas): Requires ride-through for high-voltage events up to 120% of nominal, not just low-voltage
• Australia (AEMO): Mandates voltage phase angle step change ride-through of up to 20 degrees in addition to magnitude sags
• Hawaii (Rule 14H): Aggressive frequency ride-through requirements due to island grid characteristics

Manufacturers must certify equipment against the specific grid code of the target market.

After fault clearance, the generator must restore active power output at a controlled rate to avoid destabilizing the grid:

• Ramp rate: Typically 20-50% of rated power per second, as specified in the grid code
• Settling time: Must return to 90% of pre-fault active power within 1-5 seconds
• Oscillation damping: Active power recovery must not excite inter-area oscillations; poorly tuned recovery logic can cause post-fault power swings

This characteristic is especially critical for inverter-based resources that lack the inertial response of synchronous machines.

FRT capability is not verified through field tests on live grids but through hardware-in-the-loop (HIL) simulation:

• Real-time digital simulators: RTDS or OPAL-RT platforms inject fault waveforms into the physical controller
• Full converter testing: The entire power conversion chain, including IGBT switching, is validated against voltage sag generators
• Certification bodies: DNV, UL, and TÜV provide type certification based on HIL test reports

This testing ensures the inverter's control firmware responds correctly to the exact voltage profiles defined in the applicable grid code.