Generators as load banks 2

[tomad]

For facilities served by 2 or more emergency generators, these generators could be load-tested in pairs, one as "generator" and the other one of the pair as "motor".
This way the requirement of providing a load bank could be eliminated.
Is there any code stipulation that prohibits the use of this testing method ?

 

[itsmoked]

I can not imagine any code stipulation about that what-so-ever.

I could certainly imagine a warranty stipulation that might have hit men searching for you.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[dpc]

The engine connected to the generator will not care for being driven as a load. If the engine is running, this creates one set of problems, and if the engine is cranked while not running, I suspect there will be other problems.

The generator will have to be excited to function as a synchronous motor - there are normally reverse power relays to protect the engine that would have to be defeated.

Also, I doubt if you will be able to load it to much over 25%, if that.

Not to mention the warranty issues Keith mentioned.

Sounds like an idea whose time has not come.

 

[BJC]

I don't know the exact parameters but a motoring generator will cool the air flowing through the engine. Frosty exhasut pipes is an indicator of motoring.
So if you get the right piping setup you can take up some of the chiller load during you test.

 

[tomad]

Clarification.The steps for testing would be the following:
1.Start driving engine 1 up to rated speed.
2.Raise excitation voltage of generator 1 to get rated output voltage (no-load running)
3.Start driving engine 2.
4.Raise excitation voltage of generator 2 to get rated output voltage
5.Vary the speed of the driving motor 2 to get the output of generator 2 in synchronism with generator 1.
6.Connect generator 2 to the same bus as generator 1, paralleling generators 1 and 2.
7.Thus generator 1 imposes the voltage and frequency and generator 2 acts as a grid-connected generator.
8.Imposing a speed lowering tendency to the driving engine 2 brings generator 2 in a (synchronous) motor state, acting as load for generator 1.
9.Coordinated control of the two drive engines (for frequency) and excitation of the two generators(for voltage) allows running the two generators at an adjustable rate ( 100%, 75%, etc), one of them as generator while the other as motor.

 

[dpc]

Yes, but all those kW that one generator is pumping out have to go somewhere. Where does all this power go? The only place it can go is into the other generator and engine. A motor consumes kW because of the driven equipment. You have no driven equipment other than the engine. I don't think you will get anywhere close to 100% generator output with no load other than the other gen-set.

Eventually every kW you generate will turn into heat somewhere. Check with the engine supplier and ask for their recommendations on motoring their engine.

 

[BJC]

All these years I've put anti-motoring relays (32) on generators and didn't need them.
tomad you really need to have lunch with your local generator rep.

 

[rbulsara]

The most important issue as identified above is the "load" itself. A typical diesel engine barely takes 15-20% of its rated power for "motoring". So this "load bank" is only worth 15-20% its rated KW power, not enough even to avoid "wet stacking" (slobbering).

32R protection is other issue as mentioned.

 

[tomad]

The drive engine 2 acts in this case as a "brake", since its speed is dictated by the frequency supplied to the synchronous "motor" (generator 2) (the shafts of the engine and motor are rigidly coupled). This would result in a reversed (with regard to the torque of the motor) torque applied to the shaft of the "motor" (generator 2);
So why would the engine develop only 25% of its rated power ?

 

[davidbeach]

tomad, drive engine #2 would be absorbing all of the power out of drive engine #1 less the electrical losses. It isn't great to apply any motoring torque to the engine, but it can withstand up to about 25% of rated HP for short periods. These aren't designed for compression braking as would be done in a vehicle. For one thing when a vehicle compression brakes the engine is turning at a much higher rate than normal operation, but in this case it would not be, so minimal compression braking. Then you have issues of incomplete combustion (slobbering and wet stacking) plus the fuel system (injectors, etc.) uses the fuel as both a lubricant and a coolant (that's why the fuel flow is several times consumption and a full size fuel return line is necessary and day tanks have to be large enough not to boil the fuel). In your motoring case there wouldn't be enough fuel flow and you'd rapidly destroy your fuel system.

Don't do it. It won't work, but it could make for an interesting video at the end.

 

[waross]

I have worked "hands on" on a few generators over the past several decades.

For facilities served by 2 or more emergency generators, these generators could be load-tested in pairs, one as "generator" and the other one of the pair as "motor".
This way the requirement of providing a load bank could be eliminated.

NO

respectfully

 

[catserveng]

I think Waross hit it right on the head.

Modern generators are lousy motors, i have a number with cracked and burned amortissor bars on the rotor to prove it.

Modern engines have pretty low parasitic horsepower, now days it is actually around 8% where a reverse power relay should be set for newer high speed high output engines. So you are not going to have much ability to absorb load. Motoring a newer turbocharged engine will almost always cause the oil seals in the cartridge to leak, not to mention loading up the cylinders with oil.

My company would consider purposely motoring an engine generator as abuse, and I'm quite sure our manufacturer would back us up, as well as just about all of our competitors.

As said above, an idea whose time has not come.

 

[itsmoked]

tomad.. Look at it this way. The power available and put out by a generator is provided via the thermal energy derived from the fuel it consumes. There is no equivalent "power sink" mechanism that comes close to the fuel's energy.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BJC]

I can't find the aritcal but have been several cases where a diesel generator ran out of fuel or had a fuel pump failure etc and "motored" for long periods.
I don't know the details of the Carnot (or Whatever it is)but the compression and exhaust cycles cool the air passing through the engine enough to frost the exhaust stack.

As davidbeach and waross said the wet stacking means no lubrication to the cylinder walls and engines ruin fast.
Newer engines with the electronic contros should be "idiot proof" but they are perfected every year.
Like I said talk to your local CAT rep, he could probably use the laugh.




























'

 

[itsmoked]

Little free with those line feeds BJC? LOL

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[LionelHutz]

I believe catserveng is the local Cat rep here... and

My company would consider purposely motoring an engine generator as abuse, and I'm quite sure our manufacturer would back us up, as well as just about all of our competitor

For a motor to produce it's rated output power it needs a load on it's shaft that uses the power. Otherwise, it will just spin freely. If you use the generator as a motor you need some place for the power to go. It can't go into the prime mover because the prime mover can't use it. As catserveng just stated, the prime mover only requires 8% of its rated power to keep it turning which means the prime mover could only supply 8% of rated load. Oh wait, read the quote again...

 

[rmw]

Look guys and girls, as a ME who reads this forum regularly I have the following comments on this thread.

I'll leave the generator effects to the experts, however, as regards the engine, there are fora on this site over in the automotive area that have had similar discussions of this topic except that it wasn't a motor/generator that was driving the engine.

Tractor trailer engines are used as "motored" devices regularly in down hill and deceleration applications where the engines are doing some portion of the retardation. Toward that end, compression relief devices the most famous of which is indisputably the "Jake Brake" (go Google) which alters the valve timing and is called a compression brake.

Tomad, find out what your engine is capable of before trying this.

I concur with the 15-20-25% of rated engine HP for a 'motored' condition without the aid of a compression brake. Find out what your full speed/no load fuel consumption is and convert that to HP and you get a good approximation of what it takes to spin that engine no leaded at rated speed. And, I guarantee it won't be anywhere near what your full speed, full load fueling rate will be.

So all automotive engines are "motored" in some fashion in a downhill or a deceleration state. That said, not all engines are made to be 'motored' in the "switch off" mode. Some models continue to pass a minimum amount of fuel through the system that provides for the lubrication of the injectors and cooling of the fuel pumps. If you live near the mountains, you probably have seen the light blue exhaust from diesels; that is unburned or poorly burned fuel. Others have the type of fuel rack that goes to a "no fuel" condition and they are designed to operate down long downhills (several miles) in that state of no fuel through the system.

Anyway, that and a couple of bucks will pass a few minutes at Starbucks.

rmw

 

[peebee]

I'm tempted to toss this idea too.

But just for kicks, I'm going to further rmw's discussion....

Another common installation where generators are used as loads is in elevator applications with regenerating controllers. I recall a rule of thumb (sorry, no reference available) that in this situation, a generator cannot accept more than 10-20% of its rated load without overspeeding -- you either need to oversize the generator or ensure that there's always some other load on the system.

Not sure if you could stretch that a bit further, and state that some overspeeding would be acceptable. Let's say hypothetically that you could, and therefore extend that to max 10-30% input kW.

That would not be sufficient for one generator to 100% load test the other, but it would certainly be enough to exercise the generator and avoid wet-stacking (is that still an issue or did electronic engine controls take care of that? Maybe no-load runs are all you need for exercise anymore).

Better check with your vendor before going much furhter with this though.....

 

[waross]

Is any-one else out there old enough to remember the old
220 HP Cummins engines as used in freight trucks?
They were in the group that always injected some fuel even during coasting operation. They had so little "Hold-back" effect and lost rev.s so slowly that double clutching to synchronize gear changes was difficult. On a down hill grade you were dependent only on your brakes. No braking effect from the engine at all. These engines would be hard pressed to accept 5% regenerative power with fuel on the injectors, and removing the fuel from the injector rack, while it would rise the drag up to the levels noted in other posts would expose the injectors to probable damage from running without coolant or lubricant.
If you do any research on remote industrial installations you will see that it is common in diesel generator supplied
plants to add a resistance bank permanently connected to any motors subject to overhauling such as long downhill conveyors. his is because the generator is not capable of absorbing the regenerated power from the conveyor motor.
Wet stacking,- In a large engine the "fire ring" is sealed against the cylinder walls by ported combustion pressure. With no or very little combustion pressure the fire rings don't seal very well and large amounts of lube oil may be pumped past the piston and out the exhaust. This oil may accumulate in low spots in the exhaust system or be thrown out of the exhaust. When the load increases the oil pockets will be heated and/or thrown out of the exhaust pipe. In the worst case, the oil may ignite in the exhaust piping and develop a large flame thrower out the exhaust pipe. This may ignite oil previously thrown out.
There is also a possibility of an explosion in the exhaust piping system if (when) hot oil vapor forms an explosive mixture inside the piping and is then ignited.

If your driven generator losses excitation you will have the kind of damage that catserveng describes.
My short answer was NO.
My long answer is still NO!
respectfully

 

[Hoxton]

Just to throw my half pennoth in (as we say in UK).

Tomad - are you talking about motor - generator sets? That is an ac motor driving an ac generator (alternator)?

Often used in "IEC land" at naval dockyards to provide 60Hz from a 50Hz supply. Alternatively 50Hz: 50Hz for buffer supply.

If so, I could see that you could try to circulate power round a pair of these, if there were other power sources connected. Do not know if this would actually work, since what would drive the power (kW) round?

Otherwise, my "friends" in my service department have twice run reciprocating gas engines on reverse power all night. In both cases, there was no trip circuit supervision relay. Trip battery discharged because somehow the charger got switched off. So the set tried to stop but the barker did not open. The fuel was cut off, so the alternator drove the engine. No use relying on reverse power relays BJC if the breaker will not open. Due to the low load running, lube oil was pumped past the piston rings and into the inlet manifolds. Quick - clean it up before the boss comes!