Parallel transformer infeeds change both fault magnitude and current direction. A relay that was selective with one transformer in service may see higher through-fault current, reverse contribution or a different source split when the second unit is connected. Coordination should be repeated for normal and N-1 bus configurations, including transformer, feeder, bus and upstream protection operating times.
A feeder and an incomer may see different currents
For a fault on a common-bus feeder, the feeder protection can see the combined contribution of both transformers, while each incomer sees only its own contribution. Comparing every relay curve at the total bus fault current can therefore give a false coordination result. Calculate the current through each CT location for each fault. Use those branch currents with the relevant operating characteristics, not a single short-circuit number copied into every protection row.
Maximum fault duty and minimum sensitivity need different cases
The all-sources-connected case may govern interrupting duty or CT saturation. A single weak source with a remote fault may govern detection sensitivity and clearing time. Include both, together with ground faults affected by the grounding arrangement. Directional elements may be relevant where reverse contribution is possible, but their use and polarization must follow the protection design. The fact that a relay operates quickly for a strong-source fault does not prove it detects the weakest credible fault.
Preserve selectivity after a transformer is removed
If settings or logic change between single and parallel operation, the initiating state information must be reliable and the transition behavior defined. Review the consequences of a stale breaker status, failed communication or unsuccessful setting-group change. Coordination should include bus protection, transformer protection and upstream backup, not only feeder and incomer overcurrent curves. Record the required clearing sequence and the permitted loss of supply for each fault zone without prescribing generic relay settings.
Technical reference framework
[1] IEC 60076-8:1997 — Application guide.
[2] Schneider Electric — Substation including generators and parallel operation of transformers.
[3] IEEE C37.91-2021 — Protecting power transformers.