Wiring Generator in Parallel

[jropert]

I am tasked with designing a slab on grade for a new generator and am wondering about the effects of wiring this generator in parallel to the utility. One manufacturer that I have talked to mentioned that the generator could potentially "jump" off the actual foundation if out of phase enough with the utility at the time of initiation. Has anyone encountered this and what magnitude of force would result? Could a physical "jump" of significant magnitude result? I did a quick internet search about this and basically found that electrical/mechanical engineers appear to be aware of this phenomenon and there are "safe" ways of initiating this wired configuration when an out of phase occurrence is likely.

Thanks,
Josh R

 

[davidbeach]

Sufficient force to do all sorts of damage. Secure it all to the mounting well enough and something else will break instead. That's why only an idiot would not have a synch check relay supervising the paralleling breaker closing circuit.

 

[edison123]

Josh - There is no 'safe' way for out of phase synchronizing. You need to get professional guidance for this critical job. You can't do it over nets.

Muthu

http://www.edison.co.in

 

[waross]

As I understand it, your query is the forces that may be exerted on your slab as a result of an out of phase connection.
On flange driven 350 KW generators (about 470 HP) I have seen drive keys in the flanges sheared off. As a suggestion, if you design your slab based on withstanding the maximum torque of a motor of about three times the rating of the generator you should be safe. If the operators succeed in damaging your slab, the cost of electrical and mechanical damages will probably be much greater than the cost of a replacement slab.

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

 

[rbulsara]

Most generator manufacturers would have anchoring guidelines, more important than just the strength of the slab itself.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

I meant more important with regard to limiting generator dislocation in case of a fault. Foundation strength and mass is always important just for the normal duty to handle dynamic forces.

Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

David got it - if you make the foundation big enough and heavy enough then something else will break, a con-rod or a crankshaft on a diesel set, the shaft or blading on a turbo set. Winding movement might well take place.

The forces are caused by the sudden acceleration or deceleration of the rotating mass as it is pulled in to synchronism. Just how much force is involved would need to be calculated from the rotating mass and the period over which it decelerated. Thinking aloud, if the grid were able to absorb or deliver enough energy then the machine would drop into sync within a quarter of one cycle, or <5ms, during which time it might make virtually half of one revolution.

The key message is make sure an out-of-step sync can't happen. Back up the synchroniser with a check-sync relay. Machines aren't normally designed to survive this kind of fault: in the worst case the forces will greatly exceed those during a full three-phase fault at the main terminals which is normally the worst case catered for.


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

 

[jropert]

From what I understand so far, the electrical/mechanical engineers that would be tasked with wiring the generator in parallel should very well be aware of the danger of an improper synch. My electricians agree any competent person should know about this.

Just a bit of background to maybe clarify: This is a government job in which the scope of work for my company is to supply a diesel generator and a slab-on-grade to support it at ground level (among other electrical items such as switchgear, transformers, etc). The RFP that we bid from stated under the structural section that "the generator slab must be designed to be at least 3x the operating weight of the generator to mitigate vibration effects as recommended by most generator manufacturers". The generator will come supplied with vibration isolators (seismically rated and in which I assume would help mitigate normal operational vibration effects), and be mounted on an enclosure frame (which is then mounted to the slab). I do find it somewhat amusing that even though it states that most generator manufacturers know about this, ours doesn't really (along with another 3rd party we contacted)!

My main question at this point is with regards to liability on our behalf. The RFP mentioned that we are to design with the known intention that the government MIGHT in the future plan to parallel the generator to the utility. We won't be doing the paralleling during this project (maybe if we get a future bid) and therefore are wondering if we don't design the slab for someone in the future improperly synching the generator, would we be liable since we were informed that future plans for paralleling was known? I would argue that it wouldn't be our fault for someone else being ignorant about wiring the generator improperly, but wanted your take on this. I know we are all engineers here and not lawyers, but in essence we should be thinking of "safe" and ethical design!

Thanks for your input,
Josh R

 

[rbulsara]

What is amusing about that? It is far less amusing than asking strangers on internet. When I say generator manufacturer, it is not their salesman! Talking to right person of the manufacturer is important. Not all salesmen or dealers are created equal. Ultimately, they do have the information, it is up to you as to how to go about getting it.

Tech support is most important to me when selecting or recommend any products. It may vary by regions.

Rafiq Bulsara

http://www.srengineersct.com

 

[edison123]

If the generator is going to sitting on anti-vibration mounts, does one need such 3 x weight slab ?

Muthu

http://www.edison.co.in

 

[waross]

Hi Edison;
My approach has been to use twice the weight of the genset, and I have no issues with three times the set weight for the mass of the base.
The reason for the extra weight is to force the anti-vibration weights to do their job. I have been told that if the base is too light that the base may start to vibrate in harmony with the generator This supposedly may cause more than design stresses to be imposed on the anti-vibration mounts. The set may go east as the base goes west and vice-versa. Designing the base with two to three times the mass of the set avoids this issue.

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

 

[7anoter4]

First of all see thread237-186654.
In "Handbook of machine foundations" –See:

http://books.google.com/books?id=BK...esult&ct=result&resnum=8&ved=0CDQQ6AEwBw#v=on

pp.150 ch. b "dynamic loads"
3 ways to appreciate the Short-circuit Moment [Torque] are indicated:
1) Tsc=4*MW [tf.m]
2) Tsc=R*D*N/3000 R= Rotor weight[tf] ; D= Rotor diameter [m] ; N= rated velocity [rpm]
3) Tsc=7*Trated [Russian practice]
Since the torque depends on MW but on velocity also first equation will give from 2 to 12 times the rated torques [from 750 to 3000 rpm]. From the second Tsc could be from 4 to 10 times Trated. The 3 rd could be more realistic.