DC generator for cell site

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rojay

Senior Member
Location
Chicago,IL USA
Does anyone have any experience with the use of DC generators? Our company is bidding a cell site job where a DC generator is proposed to provide back up for all -48 volt DC equipment loads.

There is no transfer switch. The DC generator is to backfeed a breaker mounted to a DC power plant busbar inside an outdoor equipment cabinet. The DC power plant is derived from AC supplied rectifiers with back up batteries inside the cabinet.

I see this this as an Article 702 system. This is not a portable generator. Isn’t some sort of transfer equipment required by 702.5?

If utility power was supplied to the rectifiers and the DC generator was supplying power at the same time, would this result in a backfeed or would the rectifier diodes block a backfeed?

Any thoughts?
 

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
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EC
My question is why would they want to use a DC generator in the first place? Why not a usual AC generator and transfer switch that is readily available? That way all loads would be powered, like the A/C.

To answer your question, yes, some sort of device would have to exist to keep battery from feeding back to the generator. I suppose that could be as simple as a diode in series with one side of the generator leads. Perhaps the DC gen already has that accounted for? You are correct in assuming that power from the generator will be blocked by the rectifier diodes, just as battery is blocked if POCO power dies.

-Hal
 

rojay

Senior Member
Location
Chicago,IL USA
My question is why would they want to use a DC generator in the first place? Why not a usual AC generator and transfer switch that is readily available? That way all loads would be powered, like the A/C.

-Hal

Not too sure on the whole DC genset vs AC logic. Manufacturers cut sheet uses “smaller footprint than AC genset, no transfer equipment required, and greater ease in obtaining permits” as part of the sales pitch.

So does the fact that this is a DC generator being used with diodes to prevent backfeed exempt the installation from what seems to be a code requirement for transfer equipment?
 

GoldDigger

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Placerville, CA, USA
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Retired PV System Designer
do a normal ac genset with xfer switch util-genset
shed loads you do not want on gen if gen sizing is a concern
Many cell sites use DC to the equipment with small exceptions powered by inverters. Standard TELCO design uses the battery bank as the primary power at all times, with constant trickle charging from PV or grid AC when available. Using an AC generator could actually increase the complexity and losses when on backup.
 
And the so-called DC generator is likely to be an AC generator with a built-in rectifier. Without seeing a spec sheet, I suspect the main thing is that it doesn't have to run at synchronous speed. Kind of like the newer inverter generators :happyyes:.

I'm somewhat dubious about the smaller-footprint part since the engine usually takes up the lions share of the space anyway. Does save on the ATS, though.

What's the make/model? I'm curious.
 

ActionDave

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Many cell sites use DC to the equipment with small exceptions powered by inverters. Standard TELCO design uses the battery bank as the primary power at all times, with constant trickle charging from PV or grid AC when available. Using an AC generator could actually increase the complexity and losses when on backup.

They all run on batteries. I have never seen or hooked up anything other than a standard AC generator for any of the cell sites I have worked on.
 

ActionDave

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Location
Durango, CO, 10 h 20 min from the winged horses.
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Not too sure on the whole DC genset vs AC logic. Manufacturers cut sheet uses “smaller footprint than AC genset, no transfer equipment required, and greater ease in obtaining permits” as part of the sales pitch.

Sounds like salesman mumbo jumbo to me.

So does the fact that this is a DC generator being used with diodes to prevent backfeed exempt the installation from what seems to be a code requirement for transfer equipment?
Maybe. IME everything after the rectifiers isn't part of the NEC. I think the whole idea is nuts though.
 

brian john

Senior Member
Location
Leesburg, VA
I have seen DC generators utilized to support battery systems, just another design that some engineers believe is a better approach, smaller batteries, longer back, the systems were designed to operate without an ATS. The sites also had an AC generator.
 
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