125VDC Station Battery Fault Current 2

[edienberg]

I'm in the process of designing a 125VDC battery for a power plant. This battery will be the DC supply for the 120VAC inverter as well as the switchgear, 2 emergency oil pumps, and other misc. users throughout the plant. I've sized the battery per the load profile I developed and determined that I will need at least 2 if not 3 (depending on the battery selected) strings of 60 cells to support the load profile. The problem that this presents is that as the number of strings increase the amount of fault current that is available goes through the roof, currently looking at 50kA +. I can't seem to find a panelboard or switchboard that is rated for this kind of fault current. Does anybody out there have any ideas how to handle this?

Erik

 

[ScottyUK]

In my experience it is quite rare to get a DC fault level over about 15kA from a single battery string, even when using cells in the region of 2000AH capacity. Are you seriously looking at a battery of about 7000AH? I would strongly consider going to 220V DC and halving the fault level, or splitting the loads into something more manageable, or both. One string per pump, and another for the inverter would seem a reasonable starting point.


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

 

[Jebb]

50kA seems very high! A typical rule of thumb is 7.5 X battery Ah rating (at the 8h rate). Are you adding the available fault current for each battery in the string by mistake?

 

[dpc]

You may be able to find fuses with higher interrupting ratings than circuit breakers. Back in the day, we typically used a panelboard with fused switches rather than circuit breakers for large battery systems. Fuses will coordinate much better than circuit breakers as well, which is another argument in their favor for critical dc systems.

As Scotty says, if you can go 250 V, it is definitely worth considering.

 

[edienberg]

I agree that 50kA seems high, but that is the best info I have to go with. Right now it looks like I'll be using 3 strings of Enersys DX-27B. Each string will be 60 cells. I have not been able to find this in their literature but when I call their support number Enersys told me the DX-27B has an short circuit current of 25,400A and an internal resistance of 87 micro Ohms. According to the data sheet, the 8hr rating of this battery is 218A, so there is a big differance between 25k and 218*7.5 = 1635. Any help would be great.

The data sheet for the DX battery is here.

http://www.enersysreservepower.com/documents/US-DSG-PS-001_0811.pdf

 

[BJC]

The best artical on DC short circuits IMO is from the General Electric Industrial Power Systems Data Book. I have a copy and it dated Dec 3, 1956. The only thing that has changed is the internal impedance of some inverters.

 

[ScottyUK]

bjc - why not just upload it to the engineering.com website?

Factor in the resistance of the inter-cell links and the cabling to the switchgear. You won't have anything close to 25kA at the board. Not unless you're connecting it up with superconductors.


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

 

[edienberg]

Any idea what reasonable values for resistance and inductance would be for interconnect links and to panelboard cabling?

 

[ScottyUK]

Interconnects, assuming they are lead bars rather than copper, are probably in the order of 50[μ][Ω] or so per link, but difficult to tell without dimensions. Inductance seems a little irrelevant on a DC system.

Cables you can do yourself, you know how long they are!


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

 

[DTR2011]

I would check with the battery mfg on the inter-cell link resistance (often sold as a package). This is typically a commissioning and maintenance test performed. For commissioning, there may be a spec used. Of course this depends on the size of the links, etc.

 

[BJC]

Here's the GE artical.

 

[itsmoked]

Thanks BJC!!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[slavag]

Thanks!

from my point of view, 50kA DC its crazy, what Scotty said.

 

[sibeen]

According to the data sheet, the 8hr rating of this battery is 218A, so there is a big differance between 25k and 218*7.5 = 1635. Any help would be great.

edienberg, you've forgotten to multiply by the 8 hours in the above so it should be 218*7.8*8 = 13020, which is getting closer to the mark.

Another 'rule of thumb' used is to multiply the 1 (one) minute rate by 10 to get an estimate of the short circuit value. The one minute rate for this battery is 3127 amps so a SCC of about 30 kA which is getting in the range of what Enersys has quoted to you.