The Phantom Problem
Sometime in late June, we undertook a troubleshooting job onboard MV Aargau in Sarawak, Malaysia.
It was reported that AE#2 unexpectedly disconnected by itself without any operator action.
The main challenge with this problem is its randomness. It happened 3 times over the past 6 months.
Unfortunately, during this service job, we were unable to replicate nor witness the problem. Therefore, we could only rely on information that was reported by the crew.
The problem happened only to AE#2. The MSB was using DEIF PPUs and ABB PLC for power management system control. It was reported that the unexpected disconnection happened when the MSB was in semi-auto mode. It is not usual for automatic disconnection to take place during semi-auto mode based on our experience.
We found that for this vessel, the disconnection of the breaker follows the following sequences: a. Semi-Auto mode - Operator presses PB 205 (Load shift) -> signal goes to PLC input I2.2 -> PLC sends output signal from Q1.3 -> Signal goes to relay K300.10 -> Signal goes to PPU2 terminal 43 -> PPU2 carries out deloading/disconnection
From the point above, it is clear that if there's a defect that causes auto-disconnection, the possibilities (some likely, some unlikely) are: a. Unreliable relay K300.10 b. Unreliable PPU card that's reponsible for carrying out the deloading (maybe spontaneous deloading) c. Unreliable S205 push button (very unlikely) d. Unreliable PLC I2.2 input channel, PLC Q1.3 output channel
The PPU card was replaced with an unused spare card. DG2 operations were tested and verified to be ok. However , observations need to be made over a period of time to ascertain if this card was indeed the cause of the intermittent occurrence.
The relay should be replaced as well. As there are no spares onboard, and there are many of these relays in the MSB ,we recommend spares be purchased ASAP.
It is unlikely that the PLC's software program is corrupted. However, there is a chance that PLC I2.2 or Q1.3 channels may have some hardware issues. But for practical reasons, it is suggested that we make further observations to AE#2 now that the deloading card has been replaced (and relay's to be replaced as well)
The key takeaway is that intermittent and non-systematic problems are an engineer's nemesis. Shooting in the dark, from the hip is by no means ideal. But it is still possible to tackle the problem in a systematic way . i.e. identifying the variables and changing them one or two at a time to verify the outcome. Let's keep our fingers crossed that the problem is resolved. But even if not, we are at least one step closer to resolving it!!!