Blackout During Bow Thruster Start with Reverse-Power Trip
Two 1,200 kW, 450 V diesel generators are paralleled at 800 kW total load on a 9,000 dwt container feeder. The vessel is manoeuvring in port and the bow thruster is about to start. Both generators are on load-sharing lines; No.1 carries 52% and No.2 48%. When the 600 kW bow thruster starts, frequency dips to 58.2 Hz. No.1 generator reverse-power relay, set at -8% with a 2 s delay, trips and opens No.1 breaker. No.2 then trips on overload after 6 s, causing a blackout. The emergency generator starts and supplies the emergency switchboard after 45 s. The bridge asks why the blackout occurred and whether the bow thruster can be used again. The alarm log shows No.1 governor fuel-rack position did not change during the transient; No.2 load reached 1,350 kW before its breaker opened.
1. Reconstruct the likely sequence and explain why No.1 reverse-power relay operated instead of No.1 simply taking more load.
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At bow-thruster start the load demand jumps by about 600 kW. In a healthy parallel system both governors should increase fuel and share the transient according to their droop and load-sharing settings. Here the alarm log shows No.1 fuel-rack position did not change. No.1 therefore did not accept its share of the new load; No.2 governor increased fuel and picked up most of the demand. No.1 then became a synchronous motor driven by No.2, so real power flowed into No.1 instead of out of it. After the 2 s delay, the -8% reverse-power relay correctly tripped No.1.
That left No.2 alone carrying about 1,350 kW on a 1,200 kW machine, so its overload protection tripped after 6 s and the main switchboard blacked out. The root cause is not simply 'the bow thruster is too big': it is that No.1 did not respond, no adequate spinning reserve was established, and the load-sharing arrangement did not compensate. Contributing causes could be a stuck fuel rack, governor actuator fault, failed load-sharing line, fuel-filter blockage, or No.1 already being at its limit.
2. State your immediate actions to restore power and prevent a second blackout.
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First confirm the emergency generator is running and the emergency switchboard is energised; check steering, emergency lighting, navigation lights, radio and alarms. Inform the bridge and port authority as required. Do not blindly reset both main breakers. Investigate No.1 before reconnecting it: if the fuel rack or governor is suspect, keep No.1 offline electrically and mechanically isolated as appropriate. Check No.2 for overload damage and reset only after confirming it is sound.
Restore load in sequence: start one main generator, take essential loads, then parallel a second machine only when it has been checked. Bring loads on gradually and keep total load well below the available capacity. Do not start the bow thruster again until two reliable generators are online with sufficient spinning reserve, or obtain tug assistance. If only one generator is available, the vessel should not attempt the same manoeuvre. Record the event, alarm targets and actions for the chief engineer and the investigation.
3. Recommend procedural and protection changes, and state what you would check on No.1 before it returns to service.
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Before any bow-thruster start the load calculation must include base load plus thruster demand plus margin. With 800 kW base load and a 600 kW thruster, one 1,200 kW generator cannot carry the transient; two generators should be online, and non-essential load should be shed if necessary. The operating procedure should require a spinning-reserve check and a load-sharing test before manoeuvring. Reverse-power and overload relay settings should be coordinated so that one faulty machine is tripped without collapsing the system, and the time delays should be verified against the prime movers' response.
On No.1 I would check fuel-rack freedom, governor actuator calibration, fuel filters and supply pressure, load-sharing line communication, breaker and relay targets, and governor droop. I would test the machine on load in parallel, not just at no load, and confirm it accepts and sheds load smoothly. The PMS and SMS should be updated, the crew briefed, and the event investigated as a blackout near-miss.