1000 kw generator hunting under load after 20-30 min


Thread Starter


This is my first post so excuse any wrong info or not enough info.

We have 8 Cummins diesel generators (KTA50-D 1000 kw, 690V, 50Hz) on a our derrick barge. We usually run just 3 gens to handle the load (900-1500 kw). We have one problem gen that will share the load with the other 3 gens for about 20-30 min. with no issues, then it starts taking the load and dropping load. But not really enough to go into over speed or reverse power.

For instance all 4 gens had a balanced load of 250 kw. #1 gen the problem gen will jump up to 500 kw and then drop down to 90 kw. and back up and back down. So I took it offline. The power factor also goes up and down. Voltage pretty much stays the same (690v-689v). The COS needle acts weird it starts shaking in one place

I have changed the gov control (p/n cummins 3098693)(droop and gain same as other gens 10 droop, 40 gain) the mag pick up, and checked all connections to the actuator (p/n 3408324) and fuel shut down. Even had mechanics install new actuator and fuel shut down.

This gen is a replacement for the original gen that smoked not sure what caused it we believe condensation build up, heater had gone out.

Any help or ideas will be greatly appreciated.
The fact that the Power Factor swings around kind of points to excitation. Can you check the field voltage + current when the swinging happens. Have you done any work on the excitation? Do you have a spare excitation control system that you can try?
Thank you for your response. We do not have a spare excitation control system and will check voltage and current.

I have noticed in normal operation the power factor on these gens to change as the kws go up the power factor goes up and as the kws go down the PF also comes down. Same with voltage and Hz.

These are the readings on PPU display after about 20 min of being on line with 3 other gens. I have not left it on for very long (30 sec to a min.) When it starts acting up since we are on a job and do not want to go it the dark.

50.0 690 0.78 280 sharing load for about 20-30 min
50.2 690 0.53 198 start of problem
50.0 688 0.53 201
50.3 690 0.43 190
50.3 691 0.73 428
50.2 689 0.47 247
50.3 691 0.89 548

One time it shared the load for over 1 hr but as soon as I took off #3 gen so it would run with #4 and #5 gen it did it again. I did notice this time after I took it offline (no load) it was surging for a few sec and leveled off.

I can change the AVR MX321 and set the voltage and stability all other settings I am not familiar with.

Hope this info can determine if it is frequency hunting or fuel hunting or some other problem?
Thanks for the reply. Your problem is fuel hunting will usually give you Kw swings whereas excitation swings will give you PF hunting. As you seem to have a bit of both, you need to try and separate the data from both and identify what is swinging first, if possible
Not sure if I can determine which type hunting it is has, but my guess is fuel but can not pinpoint it.

went to start it up today and it would not run. had mechanics check fuel and they said it should be good. so I opened up the local console and could smell something had burnt. Found the #1 (+24vdc) terminal on the governor control melted and a small burnt out hole in the gov cntl right next to term.1, also the control fuse burnt, not sure what could cause that terminal to melt like that. It most looks like over voltage burn out. I've seen loose connections on terminals melt due to being loose, but it was tight.

I traced the wires from the Gov Cntrl, it goes to a NC contact on the local E-stop than to a NC contact on relay KA25 (stop relay) and to the fuse. There are no other voltages in this console than the batteries and the battery charger.

I am going to install a new Gov Cntl after I check for proper voltage from batteries and charger.

any ides or suggestions will be greatly appreciated

I will keep you all posted
Ok I found that the battery charger is putting out 29.8 vdc seems to be a little high so I disconnected it.

I replaced the gov cntl, set the idle speed and the run speed and cranked it up.

Before all that I had swapped generator main breakers with #2 gen just to see if it wasn't a bad breaker causing the hunting issue. I had noticed that the kws and the amps on the breaker did not match. 500kw usually about 500 amps or more it only had 380a. Well depending on the PF, the higher the PF the lower the Amps.

Anyway I ramped it up to run speed in the ECR 50 hz, 693v was about to syc it when I noticed there was amps on C phase. (The breaker is not even closed yet!) They shot up over the Ir (1,040a) on the breaker.

So I put back into idle the amps went down to about 150a just on C phase. Can a generator produce amperage without a load? It could be the Micro Logic's on that breaker but that breaker came off one of my best Gens. I am at a loss but will not give up.

This Gen really has some high tech gremlins in it. lol
Do you have current clamps to check if that values are real?

Can you check insulation of the generator windings and cables going from generator connection points to breaker?