Ship Generators in Parallel Problem


Thread Starter


I am having problem on on board ship Generators. When 2 generators are in parallel on busbar, they show same KW, Voltage but one of the Generators shows high Amps. Suspect AVR but failing to understand how.

Basic power generation fundamentals:

1) Power is almost always related to amps, and fuel is proportional to amps. Fuel is controlled by the governor.

2) Voltage is controlled by the AVR. And voltage is usually constant when generating power.
Check and compare the kVAR output of the machines, and you may find the source of amp discrepancy there. Ie, different reactive current due to different levels of excitation.

If one of the AVRs is faulty or comes out of automatic voltage control mode to a constant field current mode for any reason, the other AVR will compensate for it (to maintain bus voltage). This will result in unequal sharing of kVAR, and hence unequal reactive current and therefore different total current (although having same kW or real current).

Another thing which could be happening--with the governor--is that whatever kind of load-sharing scheme is being used for the two generators is misadjusted or needs tuning.

If both units are being operated in Droop Speed Control mode, it's pretty likely there is some kind of load-sharing scheme that is attempting to "balance" the load between the two units (unless the human operator is supposed to be doing that--not likely in this day and age).

You just haven't provided enough information to really be of much more help. We don't know how the units are being operated, when the problem started (after a maintenance outage; after one or both of the units tripped; etc.), and what kind of load-sharing scheme might be in place (it could be the governors of each unit have some form of Isochronous Load Sharing scheme; or there is some kind of external, third-party load-sharing controller sending signals to both generators, or maybe just one?). We don't know what the loads (real and reactive) are on the two units. We don't know if the problem only occurs when the real loads are high (such as when shipboard cranes are operating), or when reactive loads are high (such as when lots of induction motors are running). There's just too much we don't know.

We do know that fuel equals load (watts), and fuel is controlled by the governor. The governor(s) of one or both diesels may need adjusting or tuning. But, if the problem is just that the real load (watts; kW) are too high for one machine than the other, and in the past (something we also don't know--how the machines operated before the problem started!) the loads were always pretty well balanced then the problem really points to the diesel governor, or the system sending load-balancing signals to the diesel governors. It could even be that the governor of the diesel which has the lowest load has some issue--including plugged (choked) fuel filters, preventing sufficient fuel from getting to the cylinders.

Without knowing even a little bit more about the situation there's just not much more can be said.
This may due to:

1. The no load voltage of each generator is different from each other.

2. The droop setting character of the generators AVR are not identical.
Hi, CSA,

If both generators are identical, the generator with higher voltage on no load will have higher excitation and thus contributes higher reactive current on paralleling or even creates circulating current.
Hello All....

I've not read all responses,but have two immediate questions.

1) Are both units identical and at max KW?
2) what is the excitation voltage on both?


Check the proportional relationship between ammeter and power meter. Because the power meter shows the active power, and the ammeter shows the sum of the active and reactive current. If the ammeter value is larger than expected, it shows that the reactive power is on the high side. The deviation value can not be ignored. But if it is significantly larger or increasing, it is necessary to adjust the voltage regulator artificially, because the voltage regulator can not be corrected in parallel operation.

For more:
I have seen that issue a number of times, you need to do the follow:

-Complete a verification of the rotating diodes. If a diode or a fuse of one rotative diode installed in the generator rotor is broken or burned, the excitation system will increase the field current in order to maintain the generator voltage reference.