Genset: Immediate disconnection risky for the engine ? 1

[Electro2Mech]

Hi forum !

Is the immediate disconnection from the grid under full load condition critical for the combustion engine of a genset ? (diesel or natural gas)
(E.g. the generator circuit breaker disconnects the load due to fault.)

Could this lead to damaging overheat or mechanical overstress of the engine ?

I would assume that the ECU is limiting the motors speed automatically because in automotive applications rpm limiting is state of the art for a long time.
Is it the same in genset applications ?
Or are there perhaps other risky side effects ? (E.g. Transient overvoltage of the generator)

I would appreciate any input.

Thanks in advance and best regards !
Thomas

 

[davidbeach]

It happens to (nearly) every generator running in parallel with a larger system at some point in the life of the generator. Not a great thing to have happen, but not so horrible either, often included in the commissioning testing.

 

[X49]

I would not expect anything to be damaged, but I suppose there is a small possibility that the genset could trip on overspeed with a rapid reduction in load. If this happens, governor settings should be changed so that it does not happen. It is not good for a turbocharged engine to shut off immediately after running at full load, as this prevents oil from circulating through the turbo.

 

[waross]

I used to service a couple of dozen standby sets.
Normal operation when the grid returned was to open the generator circuit and reconnect to the grid. We occasionally would have over speed trips on one or two machines. Usually some mechanic had fiddled with the governor setting. Tweaking the governor setting would cure the over speed tripping when the load dumped.
The motor would not stop immediately. A cool down timer would run the engine without load for several minutes to give the turbo time to spool down.
A power failure every Sunday was common, so these sets ran at least once a week. They were installed about 15 or 20 years ago and most of them are still in service.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Electro2Mech]

Hi all,

thanks you for your replies.

So a cooldown phase seems to be very important to provide lubrication and cooling for the engine. Ok.

Are the observed overspeed events critical to the engine ?
Or do you know about an engines damage due to overspeed trip ?
Of course as you mentioned, if the governor is configured correctly this should never occur.
(Like I have assumed from the automotive powertrain)

Thomas

 

[catserveng]

A lot will depend on the overspeed settings, speed at which the protection responds, engine and generator design, coupling type and how often it occurs.

Engine damage from overspeed usually shows up in the valve mechanism, especially in high speed diesel engines. Overspeed can result in "valve float" and may lead to valves contacting piston crowns. On some CAT engine families we recommended hand barring the engine over at least two full turns and inspecting/checking valves prior to retart after an overspeed event, their are other manufacturers who recommend this as well, depending on engine type.

Some engines, newer designs, in an effort to improve performance and reduce weight/cost have increased water and oil pump speeds, in those engines we sometimes see damage to pumps showing up before other typical signs of overspeed. Newer, smaller higher speed turbochargers and also see problems associated with overspeed and large load reductions, all that air that was flowing into the engine has to go somewhere. Recently on a site where we had to demonstrate and document voltage and frequency response for a 100% load dump, genset responded nicely but the turbos made a terrible noise, engine rep said it was normal and they advised not dumping full load off the engine too many times.

Sometimes in larger generator drivelines the most overspeed critical component is the torsional coupling, it maximum rated speed in some cases may be less than the maximum allowable speed for the prime mover or the generator. In those cases the overspeed protection may be driven by something other than the prime movers maximum speed requirement.

The generator end also has a maximum speed, and in some cases repeated overspeeding can result in rotor wedge loosening, rotor winding movement, etc. Recently had a case where an overspeed event on an older unit that had recent had work done on the exciter damaged some wiring that shifted (not as well tied down after the repair as it was before) from the forces imposed due to the higher speed.

Hope that helps, Mike L.

 

[waross]

For large sets, be guided by Mikes comments. My sets were mostly under 100 KVA. The over speed events were not runaways, they were just slightly over the trip setting. The worst effect in this case would be the loss of cool down time on the turbo. Many of the sets were naturally aspirated. The small engines were all rated for a maximum speed well above the trip speed. The smallest sets ran at 1854 RPM at no load and had a maximum speed rating of 4000 RPM.
Transient over voltage may have been present but we had few very few electrical failures. I suspect that high humidity and possible condensation may have been more of an issue than over voltage transients.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Electro2Mech]

Hi Mike,

thank you very much for your contribution !
I think that I know you from the "Yellow - Black - colored" forum, correct ? (regarding also your nick...)

Well, I am more interested in larger engines. (lets say ~1MW and higher)
You wrote that overspeed events with all associated consequences occurs also on -newer- sets.
Even for "state of the art" techology sets ?

Would it be correct to say, that these effects are even particularly critical for large ones ?

Thanks, also to Bill

Thomas

 

[catserveng]

There is a big dose of "it depends" in all of this. From my "yellow" experience, before I left the dealership we had three engine models that were rated at 1500 ekW, a V12 5.7" bore engine based on a machine/trucking design with a huge turbo and fuel system at 1800 RPM, a V12 6.7 inch bore engine rated at 1800 RPM, and an inline 11" bore medium speed engine rated at 900 RPM.

Now the V12 5.4 bore engine at 1800 would only make 1500 kW for a SHORT period of time, was a special application, but the base iron came in ratings up to 2400 RPM, so overspeed tolerance for that engine was very high, and at that rating a 100% load dump always resulted in an overspeed trip, but never any indication I ever saw of engine issues due to the high speed until the overspeed protection shut it down, which was set 120% or 2160 RPM, well below that 2400 RPM rating for other applications.

The 6.7" bore engine would make 1500ekW for quite a bit longer, but had a top design rating of 1900 RPM, so it design maximum was a bit tighter than the smaller engine, on those engine we also set the overspeed at 120%, but on these engines they frequently could withstand a 100% load dump with a speed overshoot that was below the overspeed trip point, they only time we had issues with these engines when someone "tweaked" the governing and dummied them up a bit. But in no case can I ever remember an overspeed event due to a load ump resulting in any engine or unit damage. And in this size range this was by far the most popular model.

The 11" bore engine had a 1500ekW rating, continuous. The maximum design speed at the time for this engine family was 1000 RPM, this engine at the time also did not come as an electronic engine and governing was determined by the customer. These units typically had an overspeed settings of 115%. Now because on these units so many variables could come into play, like customer specified couplings, high inertia generators and governing systems that may not be very responsive in all instances, we did rarely see some indications of overspeed damage following full load trips, usually polishing of the valve springs and on one occasion I can recall of valve to piston contact, in that unit we also found the governing VERY poorly adjusted and after digging into it deeper found it has suffered a fairly large number of trips at high load.

I would say most current "modern designs" should withstand a reasonable number of full load rejections without problem, in most cases if you can do a "cooldown" Some faults result in a hard stop, and those should be avoided when possible and an SOP in place to assure unit is ok before going back on line.

In my current job I get around all colors of engines these days, and find in general same type concerns are shared by pretty much all the manufacturers.

Hope that helps, Mike L.

 

[davidbeach]

Realistically, there is nothing electrical that can happen beyond the terminal end CTs that should result in a hard shutdown of the prime mover. 100% load rejection should always result in the governor slamming the fuel intake to minimum operating fuel level, but the prime mover should stay running to allow it a proper cool down. Now, for an electrical fault in the generator, or for mechanical issues with the prime mover, then we need a complete fuel stop. At least for the electrical part of that, the average unit will experience well less than one such event in its life time. So, worst case that the average prime mover/generator combination should ever see is a 100% load rejection. That's a well known condition; any prime mover/generator integrator should take that into consideration and have sufficient margin around the edges.

Coming at this from the protection side of things, I will trip the unit breaker for any of a verity of reasons (though mostly I deal with much larger machines then a MW or two). The expectation is that the machine design deals with it. Hydro machines are designed for indefinite operation at run away speed, no harm there even if everybody in the plant has to go change their underwear. No recips in our fleet quite yet, soon though in the 12MW range. Combustion turbines risk flame out if they try to clamp down of the fuel supply too hard. Far and away the worst are the steam turbine machines where a load rejection results in an immediate steam dump as the turbine can't deal well with much overspeed.