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Generators as load banks
2

Generators as load banks

Generators as load banks

(OP)
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 ?

RE: Generators as load banks

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

RE: Generators as load banks

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.  

RE: Generators as load banks

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.

RE: Generators as load banks

(OP)
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.

RE: Generators as load banks

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.  

RE: Generators as load banks

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.

RE: Generators as load banks

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.

RE: Generators as load banks

(OP)
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 ?

RE: Generators as load banks

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.

RE: Generators as load banks

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

Quote:

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

RE: Generators as load banks

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.

RE: Generators as load banks

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

RE: Generators as load banks

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.




























'

RE: Generators as load banks

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

Quote:

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...

RE: Generators as load banks

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

RE: Generators as load banks

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.....

RE: Generators as load banks

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

RE: Generators as load banks

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!

RE: Generators as load banks

(OP)
pebee

In our case, the driven "generator" is also a synchronous machine, its speed is dependent only on the supply frequency.
In the case mentioned by you, it is probably an induction (asynchronous) machine, which, by being forced to rotate above its synchronous speed, becomes a generator (if it is connected to the power grid).
But, with regard to the fact that that generator cannot accept over 0.2 of its rated load, if it is a regular 3-phase motor, its characteristics I-n (current - rotating speed) should be similar to its motor one, so it should be able to accept up to its rated load, with a slip equal to its full load slip, but in the opposite direction (with minus); thus causing an overspeed.

Hoxton

Yes I was talking about generating sets, but the driving machine is a Diesel engine, the generator is a synchronous generator.


rmw

The question arose as a general idea, there is no specific Diesel engine involved.
But by the way, how do you increase the speed of a Diesel engine, do you admit more fuel ?



RE: Generators as load banks

Its not the generator that will have the problem.  The generator could function as a motor up to its full nameplate rating and it will rotate in the same direction.  No slip for synchronous motors or generators, both rotate at exactly synchronous speed.  It is the diesel engine that can't accept more than 20% of rated power as motoring torque on its shaft.  Load bank or grid tie are the best way to do what you want.

There is one other option that might work if you can connect more load to the pair of generators than one of them could provide but less than the maximum output of both together.  Run the generator to be tested in base load mode at what ever output level you desire and then let the other generator operate in load following mode to make up the difference.  Generator system controls will not have been installed with that option so it will take some custom control work but at least it won't ruin your diesel engines.

RE: Generators as load banks

Tomad,

To answer your last question, if your diesel/gen set is tied to the grid, no, adding fuel only adds torque to the generator shaft.  The grid controls the speed.  If the generator is separated from the grid (or driving another gen set as a load unsynchronized,) then yes, you will spin the other gen set up to the frequency determined by the capability (governor, power limits, etc) of the driving gen set's diesel.

rmw

RE: Generators as load banks

But! While the alternator can motor at its rated power there will be no mechanical load to use that power.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: Generators as load banks

I would change the verb in DavidBeach's sentence from "...can't accept more than 20% of rated power..." to "can't dissipate more than 20%..."

The work of compression is being returned to the shaft by expansion during what should have been the power stroke so the only power being dissipated is the purely friction losses of spinning the crank and dragging the auxiliaries.

rmw

RE: Generators as load banks

Compression stroke; hot.
Water jacket cools the compressed air charge, removing energy.
There is less energy to return to the shaft on the expansion stroke due to the cooling of the charge.
The amount of energy loss varies greatly depending on several factors.
David has an excellent suggestion. It may not be usable in all circumstances. The loading may be done manually by adjusting throttle settings if you are running in droop.
If you don't have enough load available to load one set to 100% then then bite the bullet and find a load bank.
Grid loading is technically feasible but may not be possible because of utility policy or may be very expensive.
With electric motor driven sets the loading drops to the windage loading which will be much less than with a diesel engine.
Beware of tinkerers. It is possible to play with governor and exciter settings and get circulating currents equal to full load current. Despite what your ammeter is telling you, the engine is not being loaded. A power meter (watt meter) will confirm this. This condition is not a valid load test.
respectfully

RE: Generators as load banks

David Beach wrote,

"Its not the generator that will have the problem.  The generator could function as a motor up to its full nameplate rating and it will rotate in the same direction."

When I first started as a service technician working on "smaller" generator sets between 100 and 3000 ekW, on several occasions we would go to a customer site where an engine had failed and "motored" all night due to some kind of control or component problem.  The usual cleanup of cylinders, exhaust, oil mess, etc as discussed followed these events.  Rarely we found the surge suppresor on the rotating rectifier assembly blown, and once in a great while a diode failed, but in general the generators did make pretty good motors.

In recent times, as generators have gotten "better engineered", meaning smaller, lighter, higher temp rises and more power/pound, we are seeing failures to gen ends after motoring events, primarily in exciter components and in rotors.  When we do report failures in the sizes mentioned above to the manufacturers we use, one of the first questions is always "was the unit motored?".  

So one of the current challenges we face today is a rather large population of very experienced engineers, who are not recognizing the changes made by manufacturers and their operational effects.  And we are having to re-evaluate many of the protection features we use and the settings applied.

And please Mr Beach, don't take this as any sort of negative feedback, as in you former job I was a participant in many of the seminars and training sessions you were present at, and miss the your expertise in your former company.

RE: Generators as load banks

Dear Catserveng.
Thank you for your infromation ( news from site).
We always used 32 function as mover protection and not as generator protection.
Now, I see it's important also for generator.
OK, I take this in account and also we will request additional information from manuf.
BTW, I think it's big problem for excit system with rotated diodes and not for static excit. system.

Additional Q: in rotary UPS you used motor/generator option.
Why it's not problem?
Regards
Slava

RE: Generators as load banks

Slava,

If you are talking about systems that use a lineup with a diesel engine, a generator, and an electric motor in a single drive line, then the systems I am famliar with use some type of clutch at each coupling.  So when the electric motor is driving the generator the engine is uncoupled and when the engine is driving the motor is uncoupled.  I know there may be some other types out there, but this is what I am familiar with.

As a general comment on this subject

Load testing of standby generators is becoming, at least in our area, an increasingly discussed topic.  The usual options have been to either install a permanent load bank, from partial to full load rating, or the regularly contract a load bank test with a portable unit.  Some facilites are designed where there is opportunity to use facility loads to load test without disrupting operations, but these are pretty rare.

The single biggest factor driving these changes are emissions reduction requirments for the prme movers.  To meet the upcoming tier levels for allowable emissions there have been significant changes to the diesel engines.  And one of the primary operational "paybacks" for these changes is that newer engines from most all manufacturers don't like running lightly loaded.  Add aftertreatment for PM and NOx reduction to this, and yes, even on standby units it will be required, and the ability to run unloaded or lightly loaded for any length of time is dramatically reduced.

So customers are looking for options, and load banks are expensive and hard to finacially justify in a lot of cases.  And portable load bank testing usually introduces system outages and interference with operations as when installed no one hardly ever makes accomadations for future testing.

One recent development in our area, and several California utilites are doing similar programs, is Demand Response programs where the utility is providing incentives for customers to make their standby units parallel capable so that in power emergencies the utility can dispatch these standby units to parallel with the grid and export power, or the customer can choose to seperate and remove his load from the system in a closed transition.  A great side benefit of these programs is that the required regular testing by the utility allows the customer to load test his unit against the grid.

I personally think these types of programs are of great benefit to both customers and utilities, the downside is that it does reduce some of our service business, but it is hard to watch thousands of BTU's go do nothing when the power produced could have some beneift somewhere.  And the installed generators in these programs actually have fewer problems because there are being run loaded and regularly.  It also better tests the generator ends and the controls as we have the opportunity to go into a lagging power factor and also supply VARs into the system.

Great topic, thanks for starting it.

Mike LeClair
Service Engineer
Hawthorne Power Systems

RE: Generators as load banks

Dear Mike.
Thanks a lot.
I'm also recommend to our customers (my side is protection
and station control syatem)close some agreement with utilities about this type of test.
Best Regards.
Slava.
PS.
Maybe you have some form of this program? it's help

RE: Generators as load banks

Hi.
Mike, sorry, I mean form of agreement with utilities, not of test procedure.
Regards.
Slava

RE: Generators as load banks

catserveng, my guess it that the excitation controls are not responding properly to the motoring events you describe.  For a synchronous machine, rotating at synchronous speed (starting issues ignored), the only difference between a generator and a motor is whether the shaft is delivering or receiving torque from whatever is connect.  The typical generator with brushless excitation might have a difficult time starting, but once it is going it could certainly operate as a motor.

What is probably happening is that while motoring the terminal voltage is lower than the AVR set point and the excitation system goes wild trying to bring the voltage up.  Obviously there are missing protective elements, or a reverse power relay would have kicked things off early on, so an excitation system blowing itself out might not be too surprising.

In general, the multiple levels of safety factors have been worked out of equipment compared to many years ago and there is a lot less room for error before something goes seriously wrong.

All in all, it is best to not allow a generator to be motored unless the whole system was designed to use the generator as a starting motor as is done with some gas turbine system.

RE: Generators as load banks

David Beach,  Thank you for the explanation.  What is making this a little hard is that with new digital AVR's and supposedly improved protection from the on-board controls, you would have thought we would see fewer, not more failures related to motoring events.  Although I would have to admit we are seeing more failures in general with generators in the last few years than in previous years, at least in our typical size ranges.  

Slava,  I don't get directly involved with these programs except when the customer or in some cases a third party get's involved and we do the system modifications to allow for the peak demand response to function per the utility.  I know all the California utilities are actively doing this, and there is a third party handling a number of the projects we have been associated with called ENERNOC.  The Utilities in Hawaii are also starting to look at this option as well.  In most cases I believe the utility is contacting likely candidates and then looking at the ablility of the installed equipment to be made parallel and air permit compliant.

RE: Generators as load banks

New York State also allows facilities to participate in EDRPs (energy demand response programs).  Many state facilities participate in these programs and receive a kick back dependant upon the amount of power that can be "removed" from the grid.  Enernoc and Energy Analytics are among a few third party companies that assist facilities with these programs.

RE: Generators as load banks

I still don't see why it wouldn't be possible use one generator to load the other to 10-20% of nameplate.

Bear in mind there's a difference between what's a good idea, and what is possible.

If I can use engine braking in my car to coast down to a stop sign, then why can't I use one generator to load another, even if it's only to a very small percentage of its nameplate rating?

And if I only need a 10-20% load on my generator during excercise, and it's possible to use one gen to load another to 10-20%, then why wouldn't it be possible to avoid the expense of a load bank?

I personally would not recommend it.  But there's lots of things that are possible but not recommended.  Some of the posts above seem to be confusing the two.  Will an input power of 10-20% nameplate damage a generator?  Or would it just be difficult to control?

RE: Generators as load banks

If the voltage regulator worked properly in motoring configuration, the generator should be able to deal with the motoring.  The problems will come from the engine.  Probably 10% to 20% load for short periods of time might be theoretically possible.  

RE: Generators as load banks

peebee:

10-20% load is not a issue for the "motoring" unit, but its virtually useless as the "load test" for the unit under test. For a diesel unit you need at least 30% of rated load to avoid "wet stacking" or "slobbering" that is to burn the fuel completely. Therefore, it is not worth the effort. Bad idea is a bad idea, whether or not it is possible.

RE: Generators as load banks

I have once discussed this issue. I guess it depends how much you rely on this emergency generator when the time comes. In other words, you may want to test the generator(s) with the real load as a complete package (transfer switch, regulator, etc)

RE: Generators as load banks

Hmm... so maybe it would work if you stuck some Jake Brakes on your gens - or if you had 4 gens and used 3 as a load for the other 4....

Neither would be very advisable....

I just realized that we all got very far off the original question, which was specific to code issues.
I agree with itsmoked that there should be no code issue -- the only possible issue I could imagine is that using a generator as a load bank might be inconsistent with it's UL listing.  

PS -- the cheapest load bank I've ever heard of is a drum of salt water with two electrodes, just keep dumping salt in until resistance is low enough (and load is high enough) for your purposes.  I've never seen that done, just read about it here on engtips.  Motoring the generator sounds safer to me than this somehow.....

RE: Generators as load banks

As far as load testing and codes;
If a load test is specified by any codes, it will most probably specify a test loading. To be realistic, such loading should be related to the full capacity of the generator or related to the normally supplied load. A realistic load test may be from 75% to 110% of the set rating, depending on the duty classification and/or the normally connected load.
You are not going to get that by loading a similar gen-set.
Re; the salt water trick.
Instance #1; This was a new set of about 15 KW, 120/240V. The set normally ran lightly loaded and had a severe oil pumping problem from day one. We used a 60 US gallon plastic barrel for 2 or three days and were able to seat the rings. After the loading the set ran happily at light loads with no further problems. (Why not use a smaller set? Motor starting, specifically A/Cs. After a few years the set was changed out for a 28 KW, which ran at a slightly higher load  because it was able to start a few more A/Cs. The load as a percentage of set capacity was even lower than the load on the small set.)
Instance #2; This was a new set that did not have any problems but the vendor was threatening to void the warranty on the grounds of light loading. Vendor agreed to honor tyhe warranty if we did a 5 day load test. The set was 280 KW 120/208 V. We used six 60 US gallon barrels. I was on site, up on the roof (for safety concerns) filling the barrels with water, four times a day. We used a lot of fuel and boiled a lot of water. The set still worked well and the warranty was not voided.
This is cheap and quick for a one time test, but if regular load tests are a code requirement the safety concerns and the monthly labor costs would probably rule this method out.    And remember the water vapor produced. It doesn't take even a moderately sized set long to put several hundred pounds of water vapor into the air.
Speaking as one with hands on experience with the salt water method, I would may use the salt water for a one time acceptance test but not for monthly load tests.
An exception may be a purpose designed liquid rheostat where the make-up water, water vapor venting, control and safety issues are properly engineered.
respectfully

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