Inverter-Based Resource Ride-Through and Recloser Coordination
A 69 kV substation feeds a 12.47 kV feeder with 6 MW of rooftop PV and a 20 MW solar farm connected through a dedicated 12.47 kV collector. The feeder recloser uses fuse-saving: instantaneous trip on phase faults, with two reclose attempts. Since the solar farm was energized, the recloser has locked out three times on cloudy days when a neighboring feeder fault caused a 0.4 s voltage dip to 0.65 pu. Inverter records show momentary cessation, then ramping. The utility's legacy anti-islanding settings are set to trip at 59.3 Hz and 0.5 s. Fault studies show the solar farm's fault current contribution is only 1.2 times rated and the recloser's instantaneous setting no longer coordinates with the feeder fuses for downstream faults. The commission's interconnection rule requires IEEE 1547-2018 ride-through. Operations wants to disable the recloser's instantaneous element and require the solar farm to trip on any voltage dip below 0.7 pu.
1. Diagnose the repeated feeder lockouts. Is disabling the recloser instantaneous element an acceptable fix?
Read the worked answer
The lockouts are consistent with a voltage-dip ride-through problem, not a fault on the faulted feeder. A remote fault causes a 0.65 pu dip; the inverter-based resource momentarily ceases to energize and then ramps. If the recloser trips and recloses into a feeder with DER, the resource may be offline or contributing little fault current, and the fuse-saving instantaneous element may not coordinate. Disabling instantaneous reclosing is not a complete fix: it slows clearing of downstream faults, increases voltage-sag duration, and can defeat fuse saving. It may be a temporary operating measure only after a fault study shows that backup settings still clear faults and that conductor burn-down risk is acceptable. A better fix is to correct ride-through and protection coordination, not to require tripping at 0.7 pu; tripping all DER on a remote dip violates the interconnection ride-through requirement and worsens reliability.
2. What studies and setting changes would you require before returning the feeder to normal reclosing?
Read the worked answer
Perform an EMT and RMS short-circuit and protection coordination study with the resource's fault-current limits of 1.2 times rated, including the neighboring feeder contingency. Verify the applicable IEEE 1547-2018 voltage and frequency ride-through category and whether momentary cessation is permitted; set the plant controller to ride through the 0.65 pu dip and recover without tripping. Re-coordinate fuses and recloser: consider one reclose rather than two, longer dead time, a sectionalizer, or a communicated blocking scheme; do not rely on instantaneous fuse saving if the resource cannot supply enough fault current. Test anti-islanding and reclose-blocking. Stage the change with a temporary hold on automatic reclosing and a fault-clearing time check.
3. Operations proposes a rule that any DER must trip below 0.7 pu. How should you respond?
Read the worked answer
Reject it as a blanket rule. It conflicts with ride-through requirements, causes unnecessary loss of renewable output, and can create a larger disturbance if many inverters trip together. Instead, define ride-through curves by voltage and duration, allow momentary cessation only where the standard and the plant capability permit, and require stable recovery. For genuine islanding or equipment protection, use the approved anti-islanding settings. If the concern is sympathy tripping, fix the cause through settings, coordination, or communication, and document the technical basis. A blanket trip rule could also be discriminatory among interconnection customers.