Fault Ride-Through (FRT) — three-phase + unbalanced events
FRT is the specific case of ride-through during system fault events. Four fault types by frequency: SLG (single line-to-ground) 70-80% · LL (line-to-line) 15-20% · LLG (two line-to-ground) 5-10% · 3φ (three-phase) <5%. Clearing 3-6 cycles (50-100 ms @ 60 Hz). IEEE 2800-2022 defines the Mandatory Operation Region (MOR): IBR must remain in piecewise linear V-vs-t envelope — 0.0 pu for up to 4 cycles, recovery to 0.7 pu by 9 cycles, recovery to 0.9 pu by 1 s, continuous 0.9-1.1 pu after that. HVRT to 1.2 pu @ 0.2 s, 1.15 pu @ 5 s. MOR applies to positive-sequence V in unbalanced faults; negative-sequence current limited. Reactive current injection during fault: K-factor (typically K=2) proportional to V deviation, detection-to-injection < 40 ms, takes priority over P within total current capacity. Post-fault recovery: P returns to pre-fault in 5-10 s with ramp limit ~10 %/s. Reconnection after permitted trip: hold-off (e.g. 5 min) + soft-start. Hyperscale integration: BESS + UPS + STS aligned, GFM vs GFL different FRT characteristics, hybrid plant supervisory control. Type tests certified per IEEE P2800.2. Failures during real faults reported under NERC Event Analysis Program → MOD-026 model updates.
Step 1 - FRT: ride-through during system fault events (SLG, LL, LLG, 3-phase)
Reference notes
Fault Ride-Through (FRT) is the specific case of voltage ride-through during system fault events. IEEE 2800-2022 defines the binding Mandatory Operation Region (MOR) for inverter-based resources, plus reactive-current-injection requirements during the fault and ramp-limited recovery after fault clearing. Hyperscale facilities with co-located generation (especially battery storage) require integrated FRT design across the BESS inverter, plant-level supervisory control, and the load-side UPS + STS chain covered in the VRT lesson. Use Next → to walk through the four fault types, the MOR, reactive current injection, recovery, and hyperscale integration.
Four fault types (most → least common)
| Type | Frequency | V signature |
|---|---|---|
| SLG (single line-to-ground) | 70-80 % of faults | Faulted phase V → 0; healthy phases mildly affected |
| LL (line-to-line) | 15-20 % | Two phases V drops at fault; healthy phase largely unaffected |
| LLG (two line-to-ground) | 5-10 % | Two phases V drops; healthy phase V can rise above nominal |
| 3φ (three-phase balanced) | < 5 % | All three phases V drop together — most severe |
Fault clearing: 3-6 cycles (50-100 ms @ 60 Hz) for transmission. Some single-pole reclosing schemes leave faulted phase open longer.
Mandatory Operation Region (MOR) per IEEE 2800-2022
- Definition: piecewise linear V-vs-t envelope that IBR must remain in
- Fault window (3-5 cycles): V may reach 0.0 pu
- Recovery 5-9 cycles: V must recover to 0.7 pu
- 9 cycles - 1 s: V must recover to 0.9 pu
- Continuous (after 1 s): 0.9 - 1.1 pu
- HVRT: Up to 1.2 pu @ 0.2 s · 1.15 pu @ 5 s
- Unbalanced faults: MOR applies to positive-sequence voltage
- Negative-sequence current contribution is limited to avoid overstressing equipment
- Type testing: IEEE P2800.2-202x specifies prescribed waveforms, sequences, pass criteria · certified by independent labs
Reactive current injection during fault
- Trigger: V < 0.85 pu → inject Q · V > 1.15 pu → absorb Q
- K-factor: typically K = 2 — 0.1 pu V dip → 0.2 pu injected Q current
- Response time: detection-to-injection < 40 ms typical
- Priority: reactive current takes priority over active power during fault, within total current capacity
- Recovery: Q returns to zero as V recovers — smooth recovery required, no overshoot
- Hyperscale: co-located BESS + solar inverters must implement reactive-current injection per IEEE 2800
Post-fault recovery
- P restoration: active power returns to pre-fault level within 5-10 s
- Ramp limit: typically 10 % of rated capacity per second
- Frequency response: primary frequency droop active during recovery if freq has excursed
- Stability: avoid overshoot that creates secondary disturbances at neighbors
- Reconnection (after permitted trip): hold-off time (e.g. 5 min) + soft-start ramp limits
- Testing: inject defined V profile (e.g. 5-cycle balanced fault + recovery) · measure P/Q/freq/V at terminals
Hyperscale + co-located BESS integrated FRT
- BESS inverters: most hyperscale-relevant designs meet IEEE 2800-2022 MOR + reactive injection + recovery · vendor type-test data is the foundation
- Grid-forming (GFM) vs grid-following (GFL): GFM establishes V/freq without external reference · GFL synchronizes to grid · FRT behavior differs — GFM typically more robust ride-through but different current-limiting challenges
- Hybrid plant control: coordinated supervisory control (solar + BESS + gas peakers) · all assets respond per individual MOR; plant-level coordinates post-fault recovery
- Load-side FRT: UPS + STS must align with source-side FRT — UPS that trips during 5-cycle fault undermines integrated compliance
- Studies: inverter type tests + plant-level PSCAD MOR simulation + on-site commissioning tests
- Event reporting: FRT performance failures during real grid faults must be reported under NERC Event Analysis Program · drives root-cause + corrective action + MOD-026 updates
Why it matters for the AWS Grid Code Compliance Manager role
FRT is the most operationally consequential ride-through domain. Each fault is a real-world test of compliance. Failures are reported, analyzed, and may produce financial penalties via the CMEP process (up to $1 M / day). Compliance teams must engage with operations engineering, equipment vendors, and the Transmission Owner continuously to maintain FRT compliance across the asset life.