Dead-Bus Closing with Synchro-Check Relays: Application Guide

In manual synchronization, synchronizing check is the default method: close only when the voltage, frequency, and phase‑angle differences are within preset limits. In backup or special conditions, one side may be de‑energized; the normal synchronizing criteria cannot be satisfied. In these cases, an operator may use dead‑bus closing detection to proceed safely.

Device Function (WT‑32 example)

On the ODES WT‑32 synchro‑check relay, terminals 7–8 accept a dry, normally open contact (DI) for mode selection:

• DI open → Synchronizing‑check mode (default)

• DI closed → Dead‑bus closing detection mode

Mode A — Synchronizing‑Check (DI open)

The relay evaluates whether all three conditions are met on the grid side vs the incoming side:

• Voltage difference within the set limit

• Frequency difference within the set limit

• Phase‑angle difference within the set limit

When all criteria are satisfied, OUT1/OUT2 change state to issue the closing signal to the breaker permissive logic.

Mode B — Dead‑Bus Closing Detection (DI closed)

The relay checks for no‑voltage on either side (or both). Closing logic is enabled when any of the following is true:

• System‑side voltage < 0.1 × Us (i.e., < 10 V AC)

• Incoming‑side voltage < 0.1 × Ug (i.e., < 10 V AC)

• Both sides simultaneously < 0.1 × rated

When the condition is satisfied, OUT1/OUT2 change state per scheme to allow closing under dead‑bus conditions.

Integration Guidance

• DI wiring: Connect a local selector or interlock contact (dry, NO) to 7–8 to toggle between modes.

• Settings discipline:

• Commissioning checks:

• Operational note: Dead‑bus closing is a special/backup method; return the selector to synchronizing‑check for normal operation.

Conclusion

A single DI on the synchro‑check relay cleanly separates synchronizing‑check from dead‑bus closing. With proper wiring and thresholds, operators obtain deterministic closing permission for both normal and special conditions—reducing wiring complexity and ensuring safe, standards‑aligned interconnection.

Need application guidance on mode selection, thresholds, or OUT1/OUT2 wiring into permissive logic?

Contact: tonyzhang@odes.com.cn

Or visit: https://www.odes-electric.com/

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