Generator Overload Parallel

The first thing you should do is to check that each genset can go full load by itself.

It looks like one of the genset is power limited, the diesel engine is unable to go high power. When it is heavily loaded, it slows down , the generator gets out of sync with the other generator, triggering overload.

Thanks bravo, I appreciate your interest.

The situation continues, but we have a few new discoveries.

First the gen sets seem to work ok when running under a heavy load singularly( alone ).

I took them to 550 Kws tonight and all seemed good.

Each one passed this test.

When in parallel the situation is much different.

When in parallel, heavy loading of one unit ( unequal load ) will cause the # 1 to rise in current but not in Kws.

We only have Volts, Amps, Hz and Kw meters , no VArs

When doing the same test by heavy loading # 1 generator, # 2 runs along just fine.

To throw another curve into the problem, these two gensets are running in parallel with the main engine shaft generator, following the main engine Hz.

They both can run with the main engine and take the load off the shaft gen no problem.

The only time we have a problem ( over current ) is when both run together and # 2 is heavy loaded, then # 1 goes full scale current and trips out.

Did you check the droop setting of each? The unequal load sharing is usually associaetd with this.

It looks like the genset are not sync in phase, resulting in one absorbing a lot of VAR, the other has to generate it , therefore you see rise in current but no kW. That's why by itself it can go full load, but not in parallel.

I meant the 2 genset are out of phase.

Can you turn-off the AVR and switch it to manual mode? It might help.

Since thge generators are small, voltage regulation by AVR mode may not be needed.

Sounds to me like you have high circulating or "wattless" current! What I would do in the same situation is as follows :-

I am assuming that the fuel filters on both generators are clean?

Run each generator up, and measure the speed at no load and full load. Use a digital tacho, not the engine RPM counter, which will not be accurate enough. Both generators should have roughly the same no load and full load speeds, if not you need to adjust the governor settings (speed droop) to ensure same settings on both generator engines.

Next stage, run both generators up, and measure the off load voltage, and full load voltage when loaded on their own! Again, you need to use an accurate voltmeter for this - the switchboard voltmeter may not be accurate enough. Remember, the off load voltage is the generated voltage at the generator, with the circuit breaker open, and also check you have voltage at each phase.

I suspect that you will find the full load voltages quite different on each of the machines, and this will be the cause of you circulating currents. Set up your off load voltages to be roughly the same as the switchboard voltage, using the AVR voltage setting pot. Ensure that the full load voltages are equal, by carefully adjusting the voltage droop setting pot. You will need to run each generator in turn up to full load on it's own, to set the voltage droop, and ensure it is set up the same for each machine, i.e. the terminal voltage at, say 550kw, is the same for both machines. This should cure your problems.

Good stuff guys, I certainly appreciate the help.

I have photos taken just this morning while running them in parallel , along with teh main engine.

These photos are taken while the ship s very quiet, the load is steady, photos taken 3 seconds apart.

You can see that the alternators are very different in current, but nearly the same in Kws.

In manual mode, I can take the load on one no problem, but not while in parallel, in parallel , heavy loading # 2 causes the amps to rise on # 1.

Halibut, I measured the off line voltage last night,its 440.2 and 339.8 , # 1 and # 2 respectfully.

I will do as you say and report back.

Are you sure of the values as mentioned in your statement

"Halibut, I measured the off line voltage last night,its 440.2 and 339.8 , # 1 and # 2 respectfully."

440.2 and 339.8 ???

This is a classic case of circulating current due to one generator being over excited and the other one is under excited.

Hi Barnroof, From the info and pictures you have posted, you need to adjust the avr of No.2 Gen, the one at 338v, up to 440v. The value you are running at is quite low. If you are unable to achieve 440v using the avr adjustment, then I would suspect either the avr, or that one of the exciter rotating diodes is faulty. Also, looking at your photos, the speed of your No.2 appears to be low. Since you are running 60hz, I would assume your gens are running at 1800RPM. Check your speed at no and full load - both gens should have roughly the same speed at no and full load in order to keep the load balanced. For example, you should have 1800 RPM at no load, and, say, 1740 RPM at full load. The speed will drop at full load, (if you are running with speed droop), but the secret is to get both gens with the same speed drop. Please keep us informed how you progress.

Halibuta - Chief Engineer of a UK Marine Patrol Vessel

Halibut, we are slowly working at the issue.

For your information we are a 2500 ton factory ship working of Labrador , Canada.

I am only the new Chief here and must admit I have never see too much of this sort of trouble.

I appreciate your help.

Today we split the board and loaded the auxiliaries to 400Kw ( they are 630 rated ).

We measured the unloaded voltage at very near 440, with 438 and 440 to begin.

We adjusted the volatge to be even.

We loaded each individually, and recorded voltage, found it to be about the same at 438, both machines.

We tested in parallel , they seem to work , but uneven in current by much, 300 amps.

We brought back in the bus tie and tried with the shaft gen with each unit, all went well.

Then with all three, shaft gen and two aux , we came to the same issue.

The current one # 1 rose quickly to overload, 1100 amps.

We finally kept all three coupled and adjusted down the voltage pot for # 1 ( the over current guy ), this lowered the current enough to stay on the board.

When we take one of the auxiliaries, run only one parallel with the shaft , we can take all the load we want, all works well.

The problem seems to be limited to only when both aux are on the bus with a heavy load.

I pasted in the Aux 2 AVR, a digital unit. I really have never adjusted this thing, but I am willing to try if it makes sense.

More later, after testing again.

The problem exsist

Hi Barnroof,

I am looking at the results you have posted. The voltages sound OK at no load and full load. With all 3 gens on load, you say that #1 takes about 300A more for the same KW, and you manually adjusted the AVR voltage down to balance this? Can I ask that, with this adjustment made, you then measure the off load and full load voltage of #1, without any other adjustments, with it the only gen on the board? I am leaning to the following fault area :-

The system you have is designed only to parallel the shaft gen and the stand alone Cats for a short period, so the control gear will not balance the loads very well between the shaft and the Cats. I know you don't have a problem here, but the systems I work on use the Woodward 721 electronic controller. These not only control the speed of the Gens, but also balance the load, or as it is called "load share". When the gens are paralleled, a relay closes and the Woodward 721 units are linked via a load share line. I am thinking aloud that maybe your Cats have a similar electronic governor system. Have a look at the manual, and see if there is any info on the governor and load sharing. Since the AVR's sound OK (voltages are OK), I wonder if you have lost the load share (may be called the paralleling link) between the electronic governors??

I am on leave from my ship at the moment, and don't have access to the Woodward manual for a better description, but will try and help you out.

Halibut,

I so much thank you for your support, today we have had some success.

I will try to explain simply:

The two Aux engines are on different decks, one is near the switchboard room the other is on the main deck in a completely separate compartment.

On the switch board there is mounted a voltage trimmer, a typical potentiometer for each Aux section of the board.

Aux 1 has the original alternator and old analog AVR , a Leroy Somer R448 , old stuff, circa 2004.

The Aux 2 has a new alternator and a digital AVR of the Marathon type , looks very sophisticated.

The Aux 2 and its new alternator are far away on the next deck.

The potentiometer for the Aux 2 looks just like the pot for # 1, and everyone accepts that this is the voltage trim for # 2, except me.

Since this problem started, my first days back of course, I have never been satisfied that I could control the voltage of # 2 Aux.

Therefor when you gave instruction about how to test the units, I was unsuccessful.

This bothered me, until I finally took away the covers to expose the connections of the # 2 AVR.

There I confirmed my suspicions, the remote trim was not connected.

The wires for the potentiometer where taped and tied back, not connected.

The new AVR uses a digital remote signal, simple push button for up, and one for down with a common terminal , three wires only, how simple.

I foud a lovely up and down push button we use on our VFD drives for conveyors, this fit n teh same place as the old trim pot.

We did a real nice job at makng the connections and mounting teh switch.

Ay first try we could easily control the voltage of # 2 remotely at the switchboard.

This allowed us to experiment and eventually we got everything working good.

It took a few attempts but by lowering the voltage on # 2 we improved things, then we lowered some on # 1 and things got even better.

In the end we have very even current, even Kws, and all stay very even with a moderate test load.

We will give them a real load test tonight, and maybe tweak the voltages a little more.

This afternoon when I left the control room everyone though we had found the solution, all are pleased.

We were slow, methodical and persistent, is that not the definition of a good marine engineer?

For me the entire experience has lead me into a step learning curve of this ship, keep in mind I am a new chief here, this is my seventh week working here.

I will let you know how the next haul back goes when we have all three pulling together with about 2000 amps load on the bus.

Also, yes we do run parallel with the main engine, it a modern DIEF control that keeps the Aux's in sync with the main engine, follows the main engine cycles perfectly, even in rough weather.

There is also a Magacon load management system that starts and stops, switches in and out the auxiliary engines dependent on the main engine rack load.

In this way the auxiliaries come online to unload the shaft generator and give more propeller power to the bus driver ( skipper ).

The system works well, difficult to understand at first, but I am fast becoming and expert, with my head stuck in the control every day.

I will offer you my thanks agin for your support, great website!

Regards

Barnroof - well done! Sounds like you have the problem cracked! I find these days that most of my shipboard problems are electronic/control, the worst being those that appear, then disappear again! Sounds like you have a good set up if you can run the shaft alternator with the auxies! Our ships are diesel electric, with a power management system - trying to get the different manufacturers systems to talk to each other correctly all the time, can be a major headache. Deiff and Megacon is good gear. Have a good trip - hopefully the rest of it won't be as trying, and the weather is OK! safe sailing HalibutA