Generator Grounding

[joan271273]

I am working at a data center project were we have 4-2MW (480V) gen sets tied to a paralelling gear, these are currently solidly grounded and will be energized in a couple of weeks. Now our customer is stating that they are thinking on 1) grounding these generators via reactors or 2) making the system grounded at the Unit Substations via the reactors. My concern is that lead times for these reactors will basically put the project on the following situation:

a) The data center will become active with IT equipment.
b) We would have to do the grouding changes later with the project on line.

I would like some opinions, will the implementation of this grounding system with a facility that is operational be a source of disruptions. Were should the system be grounded at gen sets or Unit S/S.

Thanks for any help.

 

[joan271273]

a) The paralelling gear and ATS are 3 phase/3 wire type.
b) I knew about the cable insulation dielectric % (100/133/173) classification . This % classification is not applicable to devices such as Gen Sets, UPS,etc as they are governed by other limitations and values which I do not know.

The question would be if we have purchased all of the equipment ( Low Voltage < 600V) for solidly grounded system and then we changed to a reactor grounded system what changes do I need to perform. My first tought would be that the equipment needs to be rated at 1.73. I know as a fact from transmission & distribution systems.

 

[peebee]

Now I'm talking at the very edge of things I know about, but. . .

I think what you need to check is the BIL rating of your generators and other distribution equipment. I think (even less sure) that you'd want a BIL of about 6 or 7 times 480 volts, roughly 3kV. Maybe even higher -- 10kV?

Someone please confirm. I'm not even sure BIL is the important rating here. . . .

If I was you, I'd call your equipment vendors and see what they think of your plan.

 

[peebee]

Just curious -- I don't think you ever clarified why you're using reactors rather than resistors. . . . I've seen a few resistors installed in different places, but never a reactor (not for grounding anyway).

How come you're planning on using the reactor?

 

[joan271273]

Peebee you hit the nail I have not received a straight answer on why! would someone do reactance grounded in lieu of High-Resistance grounding and it seems that everyone is taking pretty lightly in my side.

 

[alehman]

I would guess that a small value reactor is smaller and less expensive than a resistor of equivalent impedance. One reason to prefer a resistor (may be a stretch)- low voltage circuit breaker interrupting ratings are based on standard X/R ratios. Higher values require derating circuit breakers. Adding resistance would bring X/R (L-G only) down a bit.

You may want to confirm the generator X/R ratios you posted. They seem a little high for LV generators.

 

[alehman]

Just curious here, I ran some quick calcs on SKM. Assuming 3 generators, with one 0.05ohm reactor at each generator and zero impedance between the generators and the bus, I get 13,075A L-G (X/R=110.9), and 43,581A L-L.

Changing to one 0.05ohm resistor at each generator, I get 15,877A L-G (X/R=0.3), and 43,581A L-L.

If I solidly ground all generators, I get 61,157A L-G (X/R=23.7), and 43,581A L-L.

 

[joan271273]

alehman:

I checked the printout of the SC study I have and it states 60KAmo for 3 phase fault and 13KA for SLG.

 

[joan271273]

Can someone point to any web reference on the overvoltages present in this system with this type of grounding.

 

[peebee]

No web references -- but have you looked through the IEEE Red Book yet? A few quotes from my dated 1986 copy:

7.2.1 (Ungrounded Systems) ". . .Because of the capacitance coupling to ground, the ungrounded system is subject to dangerous overvoltages (five times normal or more) as a result of an intermittent contact ground fault (arcing ground) or a high inductive reactance connected from one line to ground. . . ." -- Also includes a Reference to the Green Book, Chapter 1, for a "detailed diucussion of the factors influencing a choice of the grounded or ungrounded system" (and by the way, Ch 1 of the Green Book is available online for free in PDF form at the IEEE website, they give away Ch 1 of the color series books as part of their marketing)

7.2.3 "Reactance-grounded systems are not ordinarily employed in industrial powr systems. The permissible reduction in availab le ground-fault current without risk of transitory overvoltages is limited. . . ."

7.2.5 ". . .There may be sound justification for the insertion of a reactor in the neutral connection of a generator that is to be connected to a solidly grounded three-phase four-wire distribution system in order to avoid excessive . . . current. . . ."

Lots of other nuggets in the Red Book, too, its a good read. Hope this helps.

 

[alehman]

I should clarify the results of my calculations above. The 3-phase fault current was the same in each case as L-L.

http://www.gilbertelectrical.com/products/reactors/neutral.reactors.htm

http://geindustrial.com/cwc/products?id=ngr