Arc fault calculations from single phase to 3 phase

Status
Not open for further replies.
Hello Gents,

I am designing a commercial system which is 3 phase and it will tie into a single phase panel. Needless to say I need a step down transformer which is fine.

I am however having a hard time with the arc fault calculations. I am at the AC disco to Transformer portion and my available fault current @ fault is 6955 Amps. I cant seem to find the formula to source the transformer working backwards from 240(single phase) to 480(three phase).

Is there a formula in which I can take my current AFC @ Fault (6,955A) and convert it into three phase fault current?
 
Location
California
Is this for an arc flash study? Is the single phase (6955A) from the utility, inverters, both? If you have a single phase source connected to a 3-phase system, it would contribute fault current to the phases connected. I do not know of any formula that would 'convert' the current to 3-phase. At 240V to 480V, the current on the 240V side would be twice that of the 480V side.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Assuming the 480 is the supply, then you just need to work the secondary available fault current based on the kVA, impedance and supply side available fault current. You don't work anything backwards.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Solar PV in 3 phase goes to Transformer

Transformer -open delta - converts 3 phase to single phase



My understanding is, this is possible. Am missing something? Yes this is for a fault study.
You cannot "convert" three phase to single phase with a transformer. You cannot interconnect a three phase inverter with a single phase service, transformer or not.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Also, when doing fault calculations, you do not need to consider the PV as a source, not even in the same way that you would consider a motor a potential source of high fault current.
When you short the output of a grid tie inverter, nothing comes out at all. (At least after a small fraction of one cycle, and even at the beginning of that time span the current is strictly limited.)
It is also possible to convert a three phase source to a single phase source where the single phase load power is derived equally from all three of the source phases.
But it is actually a network of transformers, not a single transformer.
The only problem with that design is that the transformer network does not operate in reverse to create a three phase output from a single phase source. The consequence of that is that there will be no three phase reference voltage (virtual grid) to let the grid tie inverter start up. So it would be useless for PV purposes. It could be used to supply single phase at full power from a three phase generator, but even then you would have to weigh the cost of the transformer bank against the cost of getting the right generator in the first place.
 
Last edited:
Location
California
Also, when doing fault calculations, you do not need to consider the PV as a source, not even in the same way that you would consider a motor a potential source of high fault current.
When you short the output of a grid tie inverter, nothing comes out at all. (At least after a small fraction of one cycle, and even at the beginning of that time span the current is strictly limited.)

That's not what I've been told by inverter manufacturers. A lot of them will discharge for a few microseconds at a high current then put out their nameplate current for 0.16 seconds until they shut down. It's always a very small contribution compared to the utility, but still nonzero.
 
HMMM


From what I hear by my electrical engineer on the job this is possible mathematically. Using open delta. I will try to get more clarification however before i post again. Thank you for the information gents!
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
HMMM


From what I hear by my electrical engineer on the job this is possible mathematically. Using open delta. I will try to get more clarification however before i post again. Thank you for the information gents!
Arc fault studies - maybe. Connecting a three phase inverter to a single phase service - no way, at least not with transformers. But I don't know everything; if your engineer says it's possible I'd love to talk with him about it. A necessary skill for an engineer is to be able to switch sides in a discussion when the data warrants it, and I have that skill. :D

But why would you want to do this? There are tons of inverters available which can interconnect with single phase services.
 
Last edited:
But why would you want to do this? There are tons of inverters available which can interconnect with single phase services.

I was cogitating on that too. Without spending a few minutes on google, what if he wanted 1000v strings and had a 240 service....are there inverters currently on the market for that? All the 1000 systems i have done are 277/480 3 ph.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I was cogitating on that too. Without spending a few minutes on google, what if he wanted 1000v strings and had a 240 service....are there inverters currently on the market for that? All the 1000 systems i have done are 277/480 3 ph.
There may indeed not be any 1000V DC inverters that can interconnect with 240/120, but changing from 1000V to 600V strings is a far easier design challenge than interconnecting a 480/277V inverter with a single phase service, which I still maintain is not possible via transformers (Disclaimer - AFAIK).
 

SolarPro

Senior Member
Location
Austin, TX
Plus, adding a transformer would just bake additional inefficiencies (iron losses) into the system. Unless you isolate the transformer at night, those losses incur 24/7 and are significant. You can minimize these with a high efficiency transformer, but that a special order item that is quite expensive.

It seems like you'd be better off using single-phase inverters (or interconnecting at a different panel if that is an option).
 
Solar PV in 3 phase goes to Transformer

Transformer -open delta - converts 3 phase to single phase

I hope this stuff below helps. I'm working on the same sort of thing.
The open delta thing won't work with the inverters I'm using- are your 480V inverter(s) delta? And then you're going to single phase..240V? Or do you mean 120/240V split phase?


The open delta transformer connection can be made with only two instead of three transformers as shown in figure-8. This connection is often used when the amount of three phase power needed is not excessive, such as a small business. It should be noted that the output power of an open delta connection is only 87% of the rated power of the two transformers. For example, assume two transformers, each having a capacity of 25 kVA, are connected in an open delta connection. The total output power of this connection is 43.5 kVA (50 kVA x 0.87 = 43.5 kVA).
•Another figure given for this calculation is 58%. This percentage assumes a closed delta bank containing 3 transformers. If three 25 kVA transformers were connected to form a closed delta connection, the total output would be 75 kVA (3 x 25 = 75 kVA). If one of these transformers were removed and the transformer bank operated as an open delta connection, the output power would be reduced to 58% of its original capacity of 75 kVA. The output capacity of the open delta bank is 43.5 kVA (75 kVA x .58% = 43.5 kVA).

The voltage and current values of an open delta connection are computed in the same manner as a standard delta-delta connection when three transformers are employed. The voltage and current rules for a delta connection must be used when determining line and phase values of voltage current.
http://www.purduecal.edu/cpmi/NSF Courses/ECET-212/CLASSPRESENTATION/Three-phase-Transformers.pdf
---
The 3 phase Open Delta configuration would serve the same purpose as the open Wye configuration discussed
above and like the open wye would also require using single phase transformers. In this configuration the phase
voltage equals the line voltage and the load current equals the phase A and the phase B current. The power
available from this configuration is 2/3 of a 3 phase Delta configuration. This configuration is sometimes called V
configurations.
http://www.programmablepower.com/support/FAQs/DF_AC_Distribution.pdf
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
HMMM


From what I hear by my electrical engineer on the job this is possible mathematically. Using open delta. I will try to get more clarification however before i post again. Thank you for the information gents!

Perhaps his math makes some kind of sense if one isn't knowledgeable about UL standards for utility interactive inverters. But it can't make sense if one has that knowledge.

Now would be the time to avoid a big mistake. i.e. before equipment is installed.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
Perhaps his math makes some kind of sense if one isn't knowledgeable about UL standards for utility interactive inverters. But it can't make sense if one has that knowledge.

Now would be the time to avoid a big mistake. i.e. before equipment is installed.
:thumbs up:

It is possible to connect a three phase generator (with synchronization) to a single phase service for distributed generation or (without synchronization) to a single phase load. But you cannot do either with a grid interactive inverter.
 
it will tie into a single phase panel. .. the formula to source the transformer working backwards from 240(single phase) to 480(three phase).

This is a little confusing. Is the grid voltage 480, and you're just trying to run a bit of loads at 240V 1ph, or is the grid 240V 1ph?

If the grid is 240V single phase meaning L1=120V, L2=120V and neutral wires, I'd say the grid people wouldn't dig the 480V 3ph at all, and it would be less than ideal efficiency-wise even if there was some way it would "work".

The grid tied inverters want to be connected at their nominal voltage. So grid, PV output, and the PV side of load transformers should all be the same.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The grid tied inverters want to be connected at their nominal voltage. So grid, PV output, and the PV side of load transformers should all be the same.
Who are you, and what have you done with PVFarmer? :D
 
Who are you, and what have you done with PVFarmer? :D

Ha! I knew either you or jben would say something like that. Don't worry, I haven't given up completely on the Leyton 3-2 transformer thing!
I still think the main problem with it would be getting something inspected which nobody has ever seen before...if there's a place for it somewhere, I'll find it.

So this makes sense!- if an inverter (or combined output of inverters) is three Ls and an N for instance, that's how it wants to be connected. It could be a variety of voltages and delta OR wye on the MV/grid side of 1 or 3 interconnecting xfmr(s), as well as delta or wye at a variety of voltages on the LV/load side of the load xfmr(s)...(or 240D/120 centertapped...or...) but the MV and LV xfmrs would *have* to be the same voltage as the inverter(s) on their inverter connection sides.

I'm gonna try again- is this correct?
You can NOT connect one OCPD for any PV inverter(s) to two transformers, as in grid and load? You can go PV inverter-->OCPD breaker-->junction box-->>meter-->POCO xfmr, but you have to use a breaker *before* any load transformer(s)?

Going for two correct statements in a row here!
 
Status
Not open for further replies.
Top