Three phase model of single phase system in SKM 1

[deltawhy]

Hi there,
I have a single phase system in which I require an arc flash analysis for. However, I do not have the single phase / unbalanced load package for SKM. I will thus attempt to model this as a three phase system to attain the single phase fault currents. I have looked through any threads in relation to this and have drawn the following conclusion. I would just appreciate it if the industry experts were to chime in to with anything wrong in my assumptions:
Info:
-Single phase 25kVA transformer 1.5%Z, connected L-N on a 25kV three phase line - therefore 14.4kV:120/240V
-Utility information gives Z1 and Z0 for the three phase line

Assumptions:
To acquire the proper single phase bolted fault currents at the transformer secondary, I have done the following.
-model a 75kVA YNyn three phase transformer, 1.5%Z, 25kV:415Y/208V (same full load currents as 1P transformer)
-use the same utility data as given

-calculate the L-G fault current at the secondary of the 75kVA transformer to acquire the accurate single phase fault current.
From here, use this current to calculate the clearing times of the single phase protective devices.


Thanks.

 

[magoo2]

Should be 416Y/240 V.

From what you described, you should be able to get the proper results for a 240 V fault this way.

There is also a 120 V fault to consider. You should be able to do this by hand. I'll try to round up an old paper on this subject and post it.

 

[DistCoop]

I have modeled a single phase system in SKM without an unbalanced module as you described. How large is this system? The NFPA 70E notes that the three phase calculations can be applied to a single phase system and will produce conservative results. If it is a tap or two off a large three phase system, you may just call it three phase. Of course if it's predominantly single phase, you may rethink that approach.

 

[magoo2]

See if this helps. You should be able to substitute your actual impedance values and get the resulting fault currents.

 

[deltawhy]

Thanks for the responses. Yes, to acquire the 120V fault currents I'll have to change the impedance of the transformer by some multiplicative factor due to the center tap. No problem there. For arc flash, I've been using the three phase equations as set out in IEEE 1584 as they are a conservative estimate of a single phase arc flash scenario. The system is actually very small (<10 bus). I would do everything by hand, but I just want something to check my values to.
Thanks.

 

[dpc]

I guess you are aware that IEEE 1584-2002 states specifically that it only applies to three-phase systems.

Also, calculations of 120 V short circuit currents on a 120/240 single phase transformer secondary is not nearly as straightforward as you might think. As a reference, I recommend Conrad St. Pierre's book on short circuit calculations. He goes through it in a fair amount of detail.

 

[deltawhy]

dpc:
Yes, I am going through the sequence domain calculations by hand right now and it is definitely not simple. Have been doing it for a couple hours right now and making basic headway. The difficulty I am running into is the fact my utility impedances are VERY large in comparison the rest of my system. For example:

Utility information:
Sbase = 100MVA
Vbase = 25kV
Zbase = 6.25
Z1 = 6 + j8 (rounded up)
Z0 = 13 + j15 (rounded up)

Now when I change Zbase from utility to my system base I get this:

Zpunew = Zpuold(Zbaseold/Zbasenew)
where Zbasenew = (240)^2 / 25000

Makes the utility impedances unreasonably large. I must be making some very simple mistake that I just cannot see right now.

 

[dpc]

Probably a typo, but you have to square the ratio of the two voltages when changing bases.

 

[wbd]

Can I ask why you are required to do an arc flash study on a single phase system that sounds like it is equivalent to a panelboard in a house (25kVA feeding 120/240V load)? IEEE 1584 is specific about arc flash hazards for 3 phase system. Are you trying to determine if you need arc rated PPE to operate a breaker or test voltage in your home panelboard? The 25kVA transformer outside my house feeds me and my neighbor. I am not worried about an arc flash at my panelboard when I turn off a breaker to replace an outlet. Let's think about that.

 

[deltawhy]

wbd, yes it does sound absurd having a category 3 or 4 sticker on a panel of that size. However, there are IEEE papers outlining the voltages and currents at which an arc is still sustainable. This falls in that category.

 

[wbd]

IEEE 1584 says "Equipment below 240 V need not be considered unless it involves at least 125 kVA or larger low impedance transformer in its immediate power supply" I would say that it falls in this category and would be labeled as a Category 0 (<1.2 cal/cm^2) which would be non flammable clothing and proper ppe for shock.
I think if the OP goes to

http://www.arcflashforum.com

he will find plenty on this subject basically supporting no analysis in this situation.

 

[deltawhy]

Correct, below 240V need not be considered. I am not analyzing the 208V equipment. However, there is evidence that suggests the IEEE 1584 be revised to include equipment down to 208V. I will check out that forum though.

 

[wbd]

but you are trying to analyze single phase equipment which historically has not been included in arc flash analysis. So why are you trying to analyze essentially a household service?

 

[dpc]

Based on past history, I'm sure you won't be dissuaded from your mission, but I agree with wbd that there is no reason to be doing arc-flash calculations for small (25 kVA) 120/240 V single-phase services. Also, I don't believe there is reason to worry about 120 V faults at all for arc-flash.

 

[deltawhy]

Thanks for the input everyone. While I agree from a realistic, common sense perspective, there should be no reason to analyze this type of system. However, I find no clause in the NFPA 70E, IEEE 1584, or the CSA Z462 (which is the standard I am following) that alleviates me from doing this. If someone finds said clause, by all means please share!

 

[dpc]

Well we wouldn't want common sense to get in the way of a good study.

 

[wbd]

Well I am sure other people will chime in but IEEE 1584 in Section 1.2 Purpose, paragraph 2 states "Single-phase ac systems and dc systems are not included in this guide."
Section 5. Arcing Current, "The predicted three-phase arcing current must be found..."
NFPA 70E: Look at Annex D, D.2 which says about a bolted 3 phase fault being calculated. So why do all the equations in the documents only refer to 3 phase if single phase was also intended?

I don't why you need to do the 1ph arc flash when all the industry experience says that single phase arc flash can not sustain themselves. Think about the arc extinguishing at zero crossing. How much time is that arcing? Ever short a screwdriver in your house doing an electrical project. There's your arc flash.

So everyone is given advice basically saying it is a not a major safety issue but you still persist.

 

[deltawhy]

I originally asked the question to acquire some expert advice. I got the advice, and I appreciate that. I will now draw my own conclusions. By no means did I intend to start a pissing contest.
But just some food for thought regarding arc sustainability at these low voltages, the 2012 IEEE paper "Investigation of factors affecting the sustainability of arcs below 250V" shows detailed test results supporting the contrary.

 

[magoo2]

eltawye,

dThe math is all worked out in Fig 2 of the paper I posted. I'm guessing you didn't see it.

Regardless of whether you need to do an actual arc flash calc for a 120 V case, it should be noted that the 120 V faults start out with higher fault currents at the transformer terminals. As you add in secondary conductor, you'll get a crossover beyond which the 240 V fault currents become the higher values.

 

[deltawhy]

magoo2, yes I looked through your paper, however to get a proper fault calculation given my information I need to do it in the sequence domain.
Thanks though.