Safety Threshold Framework and Reliability Quantification of Mobile Low-Voltage Loop Closing with Synchronism-Check Supervision

Commercial synchronism-check relays are routinely used for generator synchronization and medium-voltage paralleling, but they are not specifically intended for low-voltage transformer loop-closing, where loop impedances are typically as low as 50–150 mΩ. This paper presents a safety and reliability assessment of a mobile low-voltage load-transfer device applied to the Cam Le distribution network of Da Nang Power Company. A phasor-domain analytical model shows that the minimum 6° relay setting can produce circulating currents of 311–414 A for typical 250–400 kVA transformer pairs, equivalent to as much as 2.76 times the 150 A thermal limit of the prototype transfer chain. Consequently, safe short-distance operation must be characterized by a coupled ΔV–Δφ–Zloop–Imax constraint envelope with post-close current interlocking, rather than by phase-angle permissive alone. The analysis further shows that increasing cable cross-section may reduce loop impedance and increase loop-closing current, although it improves voltage drop during load transfer. For reliability quantification, 19 actual outage events from the Cam Le network are evaluated using Monte Carlo simulation and deterministic sensitivity analysis. For one device, the estimated benefit is ΔSAIDI=0.054 min/customer/year and ΔENS=996 kWh/year, with a P5–P95 interval of 0.027–0.096. Because the assumed deployment time is approximately 20 min, the device primarily reduces interruption duration and energy not supplied, rather than directly reducing IEEE SAIFI or MAIFI. Extended-distance cases for L=450–500 m reveal a transition from circulating-current constraints to voltage-drop constraints.