Minimum Generator Load 1

[cuky2000]

There any concern running a generator at no load or light load?
We are trying to determine if there is a minimum load on a 7MW engine generator that may impact the performance of the unit such as stability or another unwanted issue. On the other hand, we have a concern with an uneconomical operation mode at a low load with very low efficiency in the light load range.

 

[dpc]

Definitely could be an issue for the engine. I don't think there would be any issue for the generator. I'd consult with engine manufacturer.

 

[thermionic1]

I know on the GT we just upgraded relays on, the EPA had VERY strict rules on what the minimum load could be (51% full load) and for how long.

 

[TugboatEng]

Wet stacking is the problem. This is a buildup of oils and soot in the exhaust stack that can eventually lead to a stack fire.

 

[RRaghunath]

Low load operation of Diesel engine is not recommended as it is known to cause problems in the Engine bore, sealing rings, Turbo charger etc. The following extract explains briefly the issue:
Effects of Diesel Generator Light Load Running
27th, February 2011
Ideally, diesel engines should be run at least 60-75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.
Problems of running at low loads include :-
# Internal glazing and carbon build
# Low cylinder pressures and consequent poor piston ring sealing
This poor combustion leads to soot formation and un-burnt fuel residues which clogs and gums piston rings, which causes a further drop in sealing efficiency and exacerbates the initial low pressure.
Complete Article is available at

https://www.workspace-technology.com/effects-of-diesel-generator-light-load-running/

Man Diesel guidance on the subject also may be of interest:
chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/

https://www.man-es.com/docs/default-source/service-letters/sl2011-544.pdf?sfvrsn=9cfe5deb_4

 

[TugboatEng]

My experience is on variable speed engines but I believe the 60-75% load number is outdated. Diesel engines produce tremendous horsepower from small displacements today which gives them the equivalent of a very high turndown ratio.

I do suggest maintaining an operating condition that produces exhaust gas temperatures from 650-750°F periodically.

We operate a fleet of 2000-3000 horsepower diesel engines with a 15% lifetime load factor while greatly exceeding expected overhaul interval of the engine.

 

[crshears]

What type of engine?

Diesel? High-compression natural gas with spark ignition? Other?

I submit that most of the time this is a prime mover loading problem, not a generator loading problem.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[LittleInch]

For engine driven anything below 30% will give you long term problems.

Also what load can it see suddenly?

You can run into issue with pick up, brown out, low voltage drop out if you go fro a few hundred Watts to say 6 kW with the flick off a switch.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[cranky108]

I don't know of a generator problem with low loading, but could be a problem if the field is not stiff enough.

 

[cuky2000]

Hi Crshears, the prime mover in consideration is a reciprocating engine with 16V cylinder, four-stroke, dual-fuel (gas with light fuel oil. The generator is rated for 9,400 kVA, 0.8 PF with a Nominal voltage 6.9 kV

 

[crshears]

Is there a minimum light fuel oil usage for pilot ignition? Or is spark ignition available so that if desired no light oil at all need be used?

In my view the first type would be much more subject to wet stacking at low loads than the second.

Addition via edit, more out of curiosity than anything: any particular reason this wasn't posted in the electric motors & generators engineering forum?

 

[cuky2000]

Following up Crshears questions:

1)Is there a minimum light fuel oil usage for pilot ignition? Yes, The engine is designed for operation either on gas with light fuel oil as pilot fuel, Gas operation, or on Light fuel oil only, backup fuel operation. The engine can be started in gas mode, backup fuel mode, or diesel mode.

2) Any particular reason this wasn't posted in the electric motors & generators engineering forum? No, I did not think hard enough on other forums. So far, the answer obtained here is more than satisfactory.

 

[waross]

The reciprocating engine issue has to do with the compression rings and the oil control rings.
The pressure of combustion is ported to behind the compression ring and increases the pressure of the ring on the cylinder wall.
With the rings well seated, there is no problem.
If the rings are not well seated, the engine may pump oil with resulting wet stacking.
My experience with new sets and light loads has been that most new sets have well seated rings and will run at light loading with no oil pumping.
There have been a few exceptions.
I have installed several new sets with poorly seated rings. They pumped all of the lube oil into the exhaust and shut down on low oil pressure in a few hours.
Several days on a load bank seated the rings and there were no further problems.
By the way, a standby generator has a hard life.
It may go from standstill to full load in under 10 seconds.
With a lot of air conditioning it may run at less than 10% load on cool days.

The other issue is age. After many hours of service with wear and increased clearances, a set may start to wet stack under light loads.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!