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Intermittant loads on electrical generator

Intermittant loads on electrical generator

Intermittant loads on electrical generator

(OP)
Can anyone give any advice regarding concerns with exposing a continuous duty diesel electric generator (about 1500 HP) to loads that fluctuate?  Without building the system I can't guess at what rate the continuous loads fluctuate but every 3 minutes a 300 amp load (a batch process) kicks in.  In a similar installation I saw a 480V delta x 480V delta transformer installed between generator and load to act as a buffer for the generator but the loads were fluctuating continuously and significantly (300 to 600 amps every couple seconds continously).  Any merit to this?

RE: Intermittant loads on electrical generator

As long as the loads are within the capacity of the generator/engine and the block load change (step loads, on and off) are small, say less than 30% of the rating of machines, usually there should not be an issue. Larger block load changes will require more scrutiny.

Throwing a transformer in the middle does not do anything to the actual load seen by the generator.

It is not clear from your post, if this is a real issue or you are anticipating something. Engage an experienced electrical engineer to look at the issue/design.

Rafiq Bulsara
http://www.srengineersct.com

RE: Intermittant loads on electrical generator

The transformer has no influence on power fluctuations. It may help if you have lots of third and ninth harmonics, but zil influence on load variations. As long as the total load doesn't get higher than what the diesel can deliver, you should be fine.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Intermittant loads on electrical generator

The transformer does, though, provide other benefits.  As Rafiq points out, you really need local engineering help.

RE: Intermittant loads on electrical generator

On the other hand, one adverse effect of the load increase may be a voltage dip which would be made worse with the transformer.
 

RE: Intermittant loads on electrical generator

What is your load? It does make a difference. Some type of soft start may be a good idea. Just because a generator will accept block loading it may not be a good idea to hit it with a heavy block load thousands of times a day. The transformer may have been intentionally selected with a relatively high impedance to let the resulting voltage drop take the edge off the block loading. Or, as jghrist suggests, it may make things worse.
We need a little more information.

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

RE: Intermittant loads on electrical generator

(OP)
In the system we're designing, we have about (5) 15 to 50 HP inductive 480V motors and (4) 200HP inductive 480V motors that should be relatively steady state.  However, we have a connected system that is a batch process that will expose the generator to a varying 50 to 250amp inductive load for about 1 minute (varies during the one minute cycle time) every 3 minutes, then drop down to about 50 amp in standby.  Total peak load for the system is about 1000 KW.

RE: Intermittant loads on electrical generator

You did not mention the size of the generator. You may get opinions on this site, but do not base your final design on them.

Assuming it is larger than 1000kW, 250 amp load fluctuations should not be an issue. I would worry more about accommodating 200 HP motor starts. You will do well to hire a professional help.

Rafiq Bulsara
http://www.srengineersct.com

RE: Intermittant loads on electrical generator

(OP)
200 HP motors are on VFD's, as are some of the other smaller loads.  Anyway, i just need to be concerned if I'm missing any thing large at this point.  Details are a little ways down the road.

We have capable EE's on staff...just not available  presently.

RE: Intermittant loads on electrical generator

What is your inductive load? What is the power factor of the inductive load?
The generator response to real power block loading is not the same as the response to inductive block loading.

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

RE: Intermittant loads on electrical generator

The VFD's can trash the gen voltage as well depending again on the gen size and VFD size.

Alan

RE: Intermittant loads on electrical generator

(OP)
This is just developing a budgetary proposal.  At this point, I just want to be sure that we're not missing something large.  We need to be careful so that the specification for the generator will address the types of loads that it will see along with the a one-line.

Thanks for your helpful comments.

RE: Intermittant loads on electrical generator

Bear in mind that the transformer can allow you to apply specific earthing techniques to advantage. The generator would be easiest to protect if it had a high resistance earthing scheme. The VFDs may not be happy on a high impedance earthed scheme and will almost certainly cause problems on a delta system. Generally you are prohibited from having loads connected line-neutral on anything other than a solidly earthed system. A Dyn transformer with the generator on the delta side and a solidly earthed secondary would give you many advantages, cost unfortunately not being one of them. wink

I doubt many suppliers would offer a cheap AVR on a 1 MVA set but make sure it is a 3-phase averaging type otherwise the rectifier loads of the VFDs will upset things. Are the VFDs equipped with active front ends for PF correction? Active front ends can do some really weird things if the voltage is heavily distorted, like trying force the current to follow a voltage with a triple zero crossing. Long story, but resulted in lots of smoke and some unhappy people. smile A dumb front end is more predictable even if you have to oversize the set a little to cope.
  

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If we learn from our mistakes I'm getting a great education!
 

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