Start with permitted operating states
A dual-incomer lineup with a bus coupler is often drawn as two closed mains and one open tie, but the protection must cover every allowed state: one source out with tie closed, maintenance with a bus section dead, manual transfer, automatic return, generator supply and—only when designed for it—momentary or continuous parallel operation.
Write a state table before programming logic. If three breakers can close simultaneously, calculate the combined short-circuit level and confirm synchronism, transformer paralleling, circulating current and switchgear ratings. A software interlock is not a substitute for equipment duty.
1. Typical normal arrangement
On loss of Source 1, a common open-transition sequence is: verify Bus A undervoltage and transfer permissive, trip M1, prove M1 open, prove Source 2/Bus B healthy, then close the tie. Return may be manual or automatic after Source 1 remains healthy for a time. The logic must distinguish source loss from a bus fault; transferring a faulted bus to the healthy source is dangerous.
2. Transfer initiation
| Input/condition | Purpose | Design caution |
|---|---|---|
| 27 undervoltage on affected source/bus | Detect loss or unacceptable depression. | Set below normal motor-start/voltage-dip level but above damaging sustained voltage; use phase logic and delay. |
| Source-side VT healthy and bus-side VT state | Distinguish source loss, VT fuse failure and bus voltage. | VT failure logic must block false transfer and alarm. |
| Alternate source healthy | Voltage/frequency/phase sequence within limits. | “Healthy” must persist for a defined time and include generator readiness where applicable. |
| No protection lockout/bus fault | Prevent transfer onto a faulted section. | Latch and require authorized reset after bus/transformer/arc protection. |
| Mode and breaker availability | Automatic enabled, breaker in service, DC and trip/close circuits healthy. | Do not force operation around a failed interlock or withdrawn breaker. |
3. Open-transition versus closed-transition
Open transition proves the normal source breaker open before closing the alternate path. It avoids paralleling sources but creates an interruption. Closed transition momentarily parallels sources and requires synchronism-check, compatible transformer vector groups/ratios/impedances, utility permission, directional protection review and equipment rated for the combined fault current.
Motor buses may use fast, in-phase or residual-voltage transfer. The decaying motor-bus voltage has magnitude, frequency and phase angle that evolve after source loss. A simple 27 element and fixed delay is not a substitute for a motor-bus transfer study.
4. Protection functions by breaker
| Breaker | Typical protection/control |
|---|---|
| M1/M2 incomer | 50/51, 50N/51N or 67/67N as required; 27/59; 81; 25 for permitted paralleling; 50BF; TCS; source/bus VT supervision. |
| Bus tie/coupler | 50/51 and earth-fault backup, directionality if current can flow either way, 25 for live-live close, 50BF, TCS and bus-zone logic. |
| Outgoing feeders | 50/51, earth fault, directionality for closed-ring/DER conditions, block signals for ZSI and load-shedding inputs where required. |
| Bus protection | 87B, arc-flash or zone-selective/reverse interlocking; trips all sources feeding the faulted section and blocks ATS. |
When the tie closes, an incomer or feeder that was radial can become bidirectional. Verify overcurrent coordination and directional settings in both states. Multiple setting groups may be selected by tie and source status, but state transitions must be deterministic and supervised.
5. Interlocking hierarchy
- Mechanical interlocks: prevent unsafe racking/earthing operations independent of control power.
- Hardwired electrical interlocks: breaker positions, earthing switch, truck position and close-circuit permissives.
- Protection-grade logic/GOOSE: source health, bus protection block, transfer sequence and remote status.
- SCADA commands: supervisory control, never the sole safety interlock.
For an open-transition three-breaker scheme, enforce “not all three closed” by independent logic where practical. Define behavior when 52a and 52b disagree: block automatic closing, alarm and require investigation.
6. Bus-fault discrimination
Undervoltage alone cannot tell a dead source from a bus fault. Use protection starts/trips, arc sensors, bus differential/ZSI and current direction. A bus or incomer lockout should block automatic re-energization. If the source breaker trips for upstream protection but the bus is healthy, transfer may be permitted after the alternate source check; if it trips for bus protection, transfer must be blocked.
7. Load and source capability
Before closing the tie, confirm the surviving transformer/source can carry both bus sections, including motor restart and transformer emergency rating. The ATS may need priority load shedding before or immediately after transfer. Generator transfer needs start/ready sequence, voltage/frequency stabilization, load acceptance steps and protection coordination for lower generator fault current.
8. Example automatic sequence
- Bus A voltage below threshold on required phases for 0.5 s; VT failure absent.
- M1/source current and status logic confirms source loss; no bus/arc/lockout start.
- Source 2 and Bus B voltage/frequency healthy for 1.0 s; tie and M1 available.
- Trip M1; start M1 breaker-failure supervision.
- After M1 open and current zero are proven, check Bus A dead or apply the approved synchronism transfer method.
- Close tie; verify close within time, Bus A restored and no protection start.
- If close fails, alarm/lockout; do not repeatedly close.
- On Source 1 return, wait the stabilization time and perform the approved manual or automatic retransfer.
Values are illustrative; voltage thresholds and delays must be derived from load ride-through, source protection and breaker operating times.
9. FAT/SAT scenario matrix
| Scenario | Expected action |
|---|---|
| True loss of Source 1 | M1 opens, tie closes after permissives; no parallel. |
| Bus A fault | All feeding paths trip; ATS blocked and lockout indicated. |
| M1 fails to open | 50BF isolates upstream/other sources as designed; tie must not close into parallel/fault. |
| Source 2 unhealthy | No transfer; alarm and load strategy applied. |
| Bus A VT fuse fails | VT failure alarm; no false ATS initiation. |
| Tie close fails | Single controlled attempt, alarm/lockout; no hunting. |
| Both sources return together | Deterministic priority and timed retransfer. |
| Manual command conflicts with auto sequence | Defined mode ownership; unsafe close blocked. |
Related protection guides
Engineering limitation
This guide explains a defensible engineering workflow; it is not a project setting calculation. Final protection functions, settings, wiring and trip logic must be based on the approved single-line diagram, short-circuit and coordination studies, equipment data, grid code, relay manual, and verified commissioning results. Changes require formal protection-management control.
References and further reading
- SEL — Practical Guide to Reliable Automatic Transfer Schemes (2024) — design, protection and testing considerations up to 34.5 kV
- SEL — Enhanced Load Transfer Schemes — main-tie-main and motor-bus transfer concepts
- ABB Bus Transfer Solution — MV synchronized and high-speed transfer
- IEC 62271-200:2021 — MV metal-enclosed switchgear
- IEC 61936-1:2021 — design and erection of installations above 1 kV AC
Standards must be applied using the edition required by the project, utility and local law. Standards summaries on public pages are not substitutes for the controlled documents.