Three phase model of single phase system in SKM
Three phase model of single phase system in SKM
(OP)
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.
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.






RE: Three phase model of single phase system in SKM
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.
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM
Thanks.
RE: Three phase model of single phase system in SKM
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.
RE: Three phase model of single phase system in SKM
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.
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM
I think if the OP goes to www.arcflashforum.com he will find plenty on this subject basically supporting no analysis in this situation.
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM
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.
RE: Three phase model of single phase system in SKM
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.
RE: Three phase model of single phase system in SKM
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.
RE: Three phase model of single phase system in SKM
Thanks though.
RE: Three phase model of single phase system in SKM
RE: Three phase model of single phase system in SKM