Case Description
Issue:
After engine start, the No. 3 Generator comes online but fails to parallel with the other generators. The "Gen Out" light extinguishes, but the load meter reads zero, and the bus tie connector chatters.
Investigation:
1. Verified frequency and voltage of No. 3 Gen; Frequency was 400Hz, Voltage 115V (Normal).
2. Checked the Generator Control Unit (GCU) connections; secure.
3. Swapped the GCU with a known good unit; problem persisted (rules out GCU).
4. Inspected the current transformer (CT) wiring; found a loose crimp on the phase B wire at the terminal block.
Root Cause:
A loose connection on the current transformer (CT) phase sensing wire prevented the paralleling circuit from sensing the correct phase relationship, causing the bus tie breaker to cycle (chatter) and preventing load sharing.
Lessons Learned
- Visual inspections of wiring are critical; a loose crimp can mimic a component failure.
- Swapping "black boxes" (GCUs) is a quick troubleshooting step but shouldn't replace basic wiring checks.
- Listen for the distinct sound of a chattering relay; it usually indicates a sensing or logic mismatch.