Generator load sharing issue

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The Dieselduck
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Generator load sharing issue

Postby The Dieselduck » Thu Oct 20, 2011 10:11 am

Got this Question today, can anyone else provide insight?

Martin

------------------------

Dear Sir

I write you anticipating a solution that I am badly in need. So, let me thank you at the beginning. My problem is as under:

There are 2 Gensets (440 V, 60 Hz, 3 Phase, 350 KVA each), they are OK when run individually. But, problem comes when in parallel particularly at high inductive load (Capstan Motor, etc). They share load (say 30%) equally at the beginning, but not the current. Gradually when load is increased, they start shifting/oscilating load and curent difference also increases. Please help. Gensets have, AVR and Load Sharing Governor.

Thanks once again

Kazi

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The Dieselduck
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Re: Generator load sharing issue

Postby The Dieselduck » Thu Oct 20, 2011 10:33 am

Hello Kazi,

I would like to know the history of the problem - is this new, or has it been a gradual issue? What is the engine make?

This is not my forte, but I would first check (just checks) the setting on your governor controller, the thing for this is see how they are individually set up vis a vis droop. You will need to have identical set up for droop or synchronous setting in order for the machine to share the load equally.

If the boat has been in service for some time, I would suspect these will be fine and require no change. For a little more on this topic see http://www.dieselduck.ca/library/02%20e ... pslab7.htm

So if the controller is fine, I would start looking at the mechanical issue with the engine itself. Start with the simple stuff, fuel and air supplies. When was the last time the fuel filter(s) (primary and secondaries) changed? Is the air filter good - not plugged? Is there plenty of fuel with good positive head, ER fan running? Can the engine maintain a load when on the network by itself. Compare the running parameters between the two engines from the logbook, perhaps something obvious will show up.

If the simple stuff is good, start looking at maintenance on the machine, is it up to date, is the machine hours excessive?

Start looking at the governor, look for mechanical linkage connections are they tight. Some governors have limiting screws that prevent further increases in fuel for demand... if these are not set up properly you may have a physical limitations on the load or response. Some engine - like Cat - have fuel shut off levers that sometimes restrict the fuel when at top end of demand... Governors also don't last forever; how long as it been in service, is the oil supply clean?

These are some things I can think of, I am sure other visitors will have more input.

Hope that helps,
Martin Leduc
Certified Marine Engineer and Webmaster
Martin's Marine Engineering Page
http://www.dieselduck.net

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JK
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Re: Generator load sharing issue

Postby JK » Thu Oct 20, 2011 11:56 am

check for black market injectors. I have heard of this causing similar problems with Cat gensets/

We just changed a set of Cats out that developed an governor oscillation that caused the engines to hunt slowly under certain circumstances. We did a lot of investigation and never did discover the reason. We were changing the sets out anyway, but this was the kick to the curb.

Good luck

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Sébastien
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Re: Generator load sharing issue

Postby Sébastien » Fri Oct 21, 2011 1:56 pm

Hello Martin,
I think I have an answer for Kazi, a similar situation has occurred to me several times before. The problem in each case turned out to be a defective AVR card or maladjusted excitation setting on the AVR. If the excitation setting is not the same on both alternators, the reactive load sharing is imbalanced even though the real load is shared evenly between engine governors. The current supplied by each alternator varies in proportion to their share of reactive load, the same way that governor fuel rack setting varies in proportion with real load. Continuing with the governor comparison, when real load comes on line the engine with the highest 'no load' speed will take on a greater share of the load; when reactive load is applied, the alternator with the highest 'no load' voltage will take on a greater share of the current.
I recommend that you check the 'no load' voltage supplied by each alternator. Run each alternator at proper speed with no load and disconnected from the main switchboard then check the voltage supplied at the inlet terminals of the generator breaker. If the difference is slight (within 10 Volts) then the problem is usually a setting problem, find the potentiometer on the AVR card that regulates excitation level and adjust it until you have 440 Volts on each individual alternator. If the difference is large, the problem is in the excitation system and you will have to find the source of the problem, defective diodes in the rectifier, shorted out component in the AVR card, etc. Very often there is a small potentiometer somwewhere on the switchboard where fine adjustments to the excitation level can be made and are usually called 'manual trim'. It would also be very beneficial to calibrate your switchboard ampmeter readings, simply use an amperage clamp with a multimeter to confirm that your switchboard is displaying the proper values of current.

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Big Pete
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Re: Generator load sharing issue

Postby Big Pete » Sat Oct 22, 2011 10:48 pm

AVRs have a droop setting and it sounds to me as if the droop setting is wrong, but a similar effect could be caused by "dry joints" or loose connections so check these before playing with the voltage trimmer or droop controls.
Both gen sets should produce the same Volts at no load, as the kw increases the voltage should droop equally on both gen sets.

The governors control the kw load sharing and the AVR will control the amperes sharing in a similar way (but only, of course if they are set up correctly).

If the voltage (EMF) being generated inside the alternators is different, large reactive currents will circulate between the alternators, such that the extra EMF being produced by the DA running at higher volts is all lost in driving the circulating current through the resistance of the 2 alternator windings in series, which is very small hence the large circulating reactive or "Wattless" currents you are experiencing.
To check this hypothesis, run the two gensets indepently and check the voltage output over a range of different power outputs they should both have the same voltage at the same kwn output.
Personally I think if you hadf a problem with the engine then the KWs would not be shared equally and would vary, if the amps are changing that is a function of voltage not engine speed.
BP
It is always better to ask a stupid question than to do a stupid thing.

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D Winsor
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Re: Generator load sharing issue

Postby D Winsor » Fri Nov 18, 2011 3:42 pm

Mechanical Governors such as the Woodard UG8 with dirty or different viscosity oils, having too little or mismatched droop settings, or the load compensation incorrectly set to compensate for engine operating condition and matched between the governors will cause similar issues. This issues will cause one of the governors to become sluggish or over react during sudden load changes causing the load to hunt back and forth between the units and become unstable.
Troubleshooting 101 "Don't over think it - K.I.S.S. it"


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