Arc fault calculations from single phase to 3 phase

[PVfarmer]

Originally Posted by PVfarmer
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?

Hold on!
So grid, PV output
Technically, "grid" there should be "service voltage supplied by grid". (Right? Service voltage = the LV side of the POCO interconnection xfmr?)

 

[jaggedben]

Technically, "grid" there should be "service voltage supplied by grid". (Right? Service voltage = the LV side of the POCO interconnection xfmr?)

Correct.

The service voltage doesn't have to match the inverter, but it must at least be capable of providing the correct reference voltage through a properly configured transformer. A single phase service cannot provide properly phased reference voltages for a 3-phase inverter.

 

[PVfarmer]

Correct.

1.The service voltage doesn't have to match the inverter, but it must at least be capable of providing the correct reference voltage through a properly configured transformer.
2. A single phase service cannot provide properly phased reference voltages for a 3-phase inverter.

Two in a row!
Thanks.
1. Got that, but any gains you got from using a higher AC voltage inverter (and maybe therefore over 600VDC) would probably be lost because every kWh was going through a xfmr- which is on the customer side of the meter.
2. Right, I'm still not sure what the OP is trying to do.
"it will tie into a single phase panel" sort of sounds like it might be just for 240V loads.

How about this idea? If there's no 120V involved, a 480V 3ph to 240Y/139V 3ph xfmr? If say some motors are looking for 240V 1ph, which is 2 legs of 120V and neutral to make 240V, you could also use two legs of 139V to make the 240V 1ph.
Instead of using a 208Y/120V 3ph xfmr and running the 240V motors on 208V.

Or...am I pushing it here?

 

[ggunn]

Two in a row!
Thanks.
1. Got that, but any gains you got from using a higher AC voltage inverter (and maybe therefore over 600VDC) would probably be lost because every kWh was going through a xfmr- which is on the customer side of the meter.
2. Right, I'm still not sure what the OP is trying to do.
"it will tie into a single phase panel" sort of sounds like it might be just for 240V loads.

How about this idea? If there's no 120V involved, a 480V 3ph to 240Y/139V 3ph xfmr? If say some motors are looking for 240V 1ph, which is 2 legs of 120V and neutral to make 240V, you could also use two legs of 139V to make the 240V 1ph.
Instead of using a 208Y/120V 3ph xfmr and running the 240V motors on 208V.

Or...am I pushing it here?

You need to look at the PV and the motors as two independent grid connected systems. The motors need to work with the grid, and the inverter needs to work with the grid. With those conditions met the PV will work with the motors when they are all connected together.

 

[PVfarmer]

You need to look at the PV and the motors as two independent grid connected systems. The motors need to work with the grid, and the inverter needs to work with the grid. With those conditions met the PV will work with the motors when they are all connected together.

Thanks- so that is a viable idea then? A 240V motor doesn't care whether the two legs feeding it are 120V of 139V, as long as the two legs together make 240V?

 

[ggunn]

Thanks- so that is a viable idea then? A 240V motor doesn't care whether the two legs feeding it are 120V of 139V, as long as the two legs together make 240V?

Viable? I have no idea, but if the inverter is happy with the motors disconnected and the motors are happy with the PV disconnected, then they probably will both be happy with everything connected. One concern, though, may be the inrush current when the motors start up; that may cause the voltage at the inverter to sag enough to cause it to drop off line. I have never seen a 240/139V wye service and I don't know a whole lot about motors.

 

[PVfarmer]

One concern, though, may be the inrush current when the motors start up; that may cause the voltage at the inverter to sag enough to cause it to drop off line. I have never seen a 240/139V wye service and I don't know a whole lot about motors.

I've never seen 240Y139V service from a POCO either, but I meant it like this: (480V > 240Y/139V transformers are somewhat available)

If there's no 120V involved, a 480V 3ph to 240Y/139V 3ph xfmr? If say some motors are looking for 240V 1ph, which is 2 legs of 120V and neutral to make 240V, you could also use two legs of 139V to make the 240V 1ph.
Instead of using a 208Y/120V 3ph xfmr and running the 240V motors on 208V.

as in:
If you have 480V inverter(s) (delta or 480/277 wye), and the POCO service is also 480V (I guess they do 480/277V wye now, not 480 delta- around here anyway), BUT you're trying to feed a single phase load from both inverters and grid, it depends on what voltage the "major" load is.

If the load is all 120V, you'd just use a 480v 3ph to 208/120V 3ph xfmr and not even use the 208V.
If it's all 240V 1ph, as in 2 legs of 120V, you could do the thing in italics.
If it's 120V AND 240V, then you have to check the plates on the motors- they'll say 208V if it's ok to use it, or so I'm told.
If not OK to use 208V on a bunch of 1 ph motors that are already installed, then you might want to do the italic thing, if your motors are 15kW of 240V 1h load and the 120V load is only 500w of lighting or something like that, you could use a small 139V > 120V transformer.

Just a typo here, I meant OR about motors:
whether the two legs feeding it are 120V of 139V,

 

[jaggedben]

I have a hard time taking seriously suggestions of nominal voltages I have never heard of, from someone who is not an electrical engineer.

 

[ggunn]

I have a hard time taking seriously suggestions of nominal voltages I have never heard of, from someone who is not an electrical engineer.

I also have trouble taking seriously a Rube Goldberg system of three phase inverters, single phase loads, and who knows what kind of service all tied together by transformers.

 

[electrofelon]

I've never seen 240Y139V service from a POCO either, but I meant it like this: (480V > 240Y/139V transformers are somewhat available)

If there's no 120V involved, a 480V 3ph to 240Y/139V 3ph xfmr? If say some motors are looking for 240V 1ph, which is 2 legs of 120V and neutral to make 240V, you could also use two legs of 139V to make the 240V 1ph.
Instead of using a 208Y/120V 3ph xfmr and running the 240V motors on 208V.

as in:
If you have 480V inverter(s) (delta or 480/277 wye), and the POCO service is also 480V (I guess they do 480/277V wye now, not 480 delta- around here anyway), BUT you're trying to feed a single phase load from both inverters and grid, it depends on what voltage the "major" load is.

If the load is all 120V, you'd just use a 480v 3ph to 208/120V 3ph xfmr and not even use the 208V.
If it's all 240V 1ph, as in 2 legs of 120V, you could do the thing in italics.
If it's 120V AND 240V, then you have to check the plates on the motors- they'll say 208V if it's ok to use it, or so I'm told.
If not OK to use 208V on a bunch of 1 ph motors that are already installed, then you might want to do the italic thing, if your motors are 15kW of 240V 1h load and the 120V load is only 500w of lighting or something like that, you could use a small 139V > 120V transformer.

Just a typo here, I meant OR about motors:
whether the two legs feeding it are 120V of 139V,

I dont think I would ever set up a 240Y/139 system - it would be a rather bastard thing IMO. Siemens doesnt even list that system in their panelboard catalog, and there are a ton of systems on that list I have never heard of. I would either do a corner grounded 240 delta, or a 240 delta with a grounded center tap.

 

[PVfarmer]

I have a hard time taking seriously suggestions of nominal voltages I have never heard of, from someone who is not an electrical engineer.

ggunn- I also have trouble taking seriously a Rube Goldberg system of three phase inverters, single phase loads, and who knows what kind of service all tied together by transformers.

electrofelon- I would either do a corner grounded 240 delta, or a 240 delta with a grounded center tap.

Geez guys, gimmie a break- I have an engineer.
240Y/139V xfmrs are offered standard in the same KVAs as 208Y/120V models, there must be a reason they sell them. (Ok, sure Temco is Canadian, that may be a factor?)
If I'd said a 277Y/160V xfmr, that would have been Rube-ish.
http://www.temcoindustrialpower.com...Co_480V_Three_Phase_Isolation_Transformers#10

First of all, here's a thread from this forum (6 years ago) with 2 guys saying yes, go ahead and run 240V 1ph loads with a 240Y/139V xfmr.
http://forums.mikeholt.com/showthread.php?t=114933

electrofelon- If you do 240DCT/120, the 120V load can only be 5% of the xfmr kVA, correct?

I'm speaking hypothetically, if that helps. That way you can at least say "I've heard about that before" if you haven't actually seen it.
Not trying to construct a Goldberg device at all. Every customer's load is a different amount of kWh, and usually different percentages of two voltages. (208/120V 3ph...240DCT/120...120/240 1ph, etc.)
Therefore, there's a different xfmr setup for every situation.

I mentioned a service voltage- 480V 3ph. I may have been vague there- it could be wye or delta. My POCO doesn't offer 480V delta as standard anymore, but people still have it- you can't "expand" it, which I assume means you can't get more kVA of 480V delta, but you can use what you have for a smaller PV system.

ben, you said- "look at the PV and the motors as two independent grid connected systems. The motors need to work with the grid, and the inverter needs to work with the grid."
So- with 480Y/277V service, you could be using isolation transformers for both PV inverters (to change 480D to 480Y/277V) and motor load, or the inverter voltage could be the same as the nominal grid voltage (as in 3 legs of 277V and neutral for 480Y/277V grid connection).

But I don't see how the voltage on the low/load side of the isolation xfmr for loads (when grid and inverter itself *or* the xfmr for inverter are = voltage) matters to the grid xfmr or inverters with of without xfmrs- if grid and inverters are feeding the 3ph primary of the load xfmr(s) at their nominal voltage, the important things are: that the secondary of load xfmr(s) is 3ph, the same voltage as the load (obviously), and balanced. The load/secondary could be 240CTD/120, or 220Y/127V, or 120 delta (those xfmrs are all offered as standard.)
If the load was 100% 277V lights fed from a 480D inverter, you could use a 480D to 480Y/277 xfmr for both lights and grid.

Here's a question which I find interesting-
if a 3ph delta xfmr is beneficial due to "trapping harmonics" in its windings (which is why the POCO wants one at some point in the connection), you must lose that benefit when using a bank of 1ph xfmrs wired in 3ph/delta?

This seems to be saying "go ahead and run 1 ph from 3ph service, but you have to use 3 1 ph xfmrs if the load is >10kW".
And the 2nd part is saying "we'd prefer if you connected the PV inverters on the LV side of our 12.47kVA delta (or other delta) MV/grid xfmrs at 480Y/277V or 208Y/120V."
Which makes sense, as 480Y and 208Y are the standard services offered.

Consult the Company on existing 3-phase delta services no longer standard where the service
conductors shall be insulated from the service equipment according to the NEC and grounded
only at the Company’s supply transformer.
Where lighting or other reduced-voltage equipment is permitted from existing 3-phase, 3-wire, delta non-standard
services, isolation transformers are required. The secondaries of these isolation transformers shall be properly
grounded. The minimum number of single-phase transformers that may be used to serve the reduced-voltage
load on a 3-phase, 3-wire service is shown in the following table
<5kW=1
5-10kW=2
>10kW=3
---
Effectively Grounded, Four-wire Multi-grounded 3-phase Wye EPS:
The Company requires that all interface transformers be configured to have a
wye connected primary winding with a fully insulated neutral, and the secondary
winding to have a delta connection.
• If infeasible, an alternative that is subject to Company acceptance review is a
primary wye grounded - secondary wye grounded transformer requiring a
grounded source permitted under specified conditions.
• Any DG or aggregate DG below 500kW in a Customer’s facility may be
permitted to utilize a primary delta - secondary wye grounded transformer.
• Where any DG or aggregate DG is 500kW or greater in a Customer’s facility,
ungrounded transformers (i.e. primary delta or Yg-Yg with ungrounded
source) are not permitted on any Company

 

[ggunn]

Geez guys, gimmie a break- I have an engineer.
240Y/139V xfmrs are offered standard in the same KVAs as 208Y/120V models, there must be a reason they sell them. (Ok, sure Temco is Canadian, that may be a factor?)
If I'd said a 277Y/160V xfmr, that would have been Rube-ish.
http://www.temcoindustrialpower.com...Co_480V_Three_Phase_Isolation_Transformers#10

First of all, here's a thread from this forum (6 years ago) with 2 guys saying yes, go ahead and run 240V 1ph loads with a 240Y/139V xfmr.
http://forums.mikeholt.com/showthread.php?t=114933

electrofelon- If you do 240DCT/120, the 120V load can only be 5% of the xfmr kVA, correct?

I'm speaking hypothetically, if that helps. That way you can at least say "I've heard about that before" if you haven't actually seen it.
Not trying to construct a Goldberg device at all. Every customer's load is a different amount of kWh, and usually different percentages of two voltages. (208/120V 3ph...240DCT/120...120/240 1ph, etc.)
Therefore, there's a different xfmr setup for every situation.

I mentioned a service voltage- 480V 3ph. I may have been vague there- it could be wye or delta. My POCO doesn't offer 480V delta as standard anymore, but people still have it- you can't "expand" it, which I assume means you can't get more kVA of 480V delta, but you can use what you have for a smaller PV system.

ben, you said- "look at the PV and the motors as two independent grid connected systems. The motors need to work with the grid, and the inverter needs to work with the grid."
So- with 480Y/277V service, you could be using isolation transformers for both PV inverters (to change 480D to 480Y/277V) and motor load, or the inverter voltage could be the same as the nominal grid voltage (as in 3 legs of 277V and neutral for 480Y/277V grid connection).

But I don't see how the voltage on the low/load side of the isolation xfmr for loads (when grid and inverter itself *or* the xfmr for inverter are = voltage) matters to the grid xfmr or inverters with of without xfmrs- if grid and inverters are feeding the 3ph primary of the load xfmr(s) at their nominal voltage, the important things are: that the secondary of load xfmr(s) is 3ph, the same voltage as the load (obviously), and balanced. The load/secondary could be 240CTD/120, or 220Y/127V, or 120 delta (those xfmrs are all offered as standard.)
If the load was 100% 277V lights fed from a 480D inverter, you could use a 480D to 480Y/277 xfmr for both lights and grid.

Here's a question which I find interesting-
if a 3ph delta xfmr is beneficial due to "trapping harmonics" in its windings (which is why the POCO wants one at some point in the connection), you must lose that benefit when using a bank of 1ph xfmrs wired in 3ph/delta?

This seems to be saying "go ahead and run 1 ph from 3ph service, but you have to use 3 1 ph xfmrs if the load is >10kW".
And the 2nd part is saying "we'd prefer if you connected the PV inverters on the LV side of our 12.47kVA delta (or other delta) MV/grid xfmrs at 480Y/277V or 208Y/120V."
Which makes sense, as 480Y and 208Y are the standard services offered.

Consult the Company on existing 3-phase delta services no longer standard where the service
conductors shall be insulated from the service equipment according to the NEC and grounded
only at the Company’s supply transformer.
Where lighting or other reduced-voltage equipment is permitted from existing 3-phase, 3-wire, delta non-standard
services, isolation transformers are required. The secondaries of these isolation transformers shall be properly
grounded. The minimum number of single-phase transformers that may be used to serve the reduced-voltage
load on a 3-phase, 3-wire service is shown in the following table
<5kW=1
5-10kW=2
>10kW=3
---
Effectively Grounded, Four-wire Multi-grounded 3-phase Wye EPS:
The Company requires that all interface transformers be configured to have a
wye connected primary winding with a fully insulated neutral, and the secondary
winding to have a delta connection.
• If infeasible, an alternative that is subject to Company acceptance review is a
primary wye grounded - secondary wye grounded transformer requiring a
grounded source permitted under specified conditions.
• Any DG or aggregate DG below 500kW in a Customer’s facility may be
permitted to utilize a primary delta - secondary wye grounded transformer.
• Where any DG or aggregate DG is 500kW or greater in a Customer’s facility,
ungrounded transformers (i.e. primary delta or Yg-Yg with ungrounded
source) are not permitted on any Company

Well, all I can say is good luck with all that. It's not a situation I would want to be part of. I haven't dived that deeply into it (and I'm not going to) but my gut tells me that there has got to be a simpler way of satisfying your customer's requirements.

 

[PVfarmer]

One concern, though, may be the inrush current when the motors start up; that may cause the voltage at the inverter to sag enough to cause it to drop off line.

Thanks for that point.
I learned a lot this morning!
This is from 2004, but the basic idea with inrush seems to be- cut it of at the pass!

http://powerelectronics.com/site-fi...archive/powerelectronics.com/mag/404PET20.pdf
Power transformers for isolation
and voltage step-up (or down) are
among the most dated technologies
still in use. Today’s ac power grids
evolved from the early days of the last
century, when Tesla’s generators faced
off against Edison’s dynamos to decide
whether ac or dc power distribution
would rule. Much has changed in
a century; much has not. Materials
have improved, manufacturability has
increased and designs have moved
from engineer’s charts to computer
software. Nevertheless, power transformer
theory and math have remained
relatively unchanged since
Tesla and Edison’s time.
An excellent transformer reference
source is the 1954 Radiotron Designers
Handbook, published by RCA,
which bears a remarkable magnetic
similarity to today’s reference books.
The design approaches, rules-ofthumb
and decision-making criteria
it offers could be considered as contemporary
to the design approaches
offered today. However, a quick comparison
of electrical steels reveals the
improvements in the last century. Although
advanced for their day, the
steels listed in the RCA 1954 Radiotron
Designers Handbook can be characterized
by as lossy, low in permeability,
and as having soft hysteresis loops
with long linear portions.
Today’s grain-oriented steels work
with a fraction of the losses, at higher
permeability, and they can be driven
harder at elevated induction levels.
But there are consequences. The mag-
netizing curve is no longer soft and
round, because it has turned square
and hard. However coincidental, the
combination of high permeability and
square loop comprise a major component
of the formula for inrush.
Improved steels have enabled
smaller, lighter and less costly transformers.
Yet, those same improvements
have created a generation of
transformers that draw immense
amounts of current at start up.
Although it probably was not a problem
in 1954, inrush current is definitely
a problem today—one that concerns
every primary circuit designer.

 

[electrofelon]

I dont think I would ever set up a 240Y/139 system - it would be a rather bastard thing IMO. Siemens doesnt even list that system in their panelboard catalog, and there are a ton of systems on that list I have never heard of. I would either do a corner grounded 240 delta, or a 240 delta with a grounded center tap.

Thought about this some more....I'll amend/clarify that response and say that I could see doing that where the neutral conductor was not brought to the distribution equipment. In other words only grounding the wye point to meet code for system grounding and have the benefits of a grounded system. One still couldn't use slash rated breakers though.

 

[PVfarmer]

Thought about this some more....I'll amend/clarify that response and say that I could see doing that where the neutral conductor was not brought to the distribution equipment. In other words only grounding the wye point to meet code for system grounding and have the benefits of a grounded system. One still couldn't use slash rated breakers though.

Being a bastard thing isn't always bad- Keith Richards for instance!
So you're saying you can't use 240/120V breakers for any of it, but as long as the 240/139V neutral stays on the load side of the xfmr(s), the supply side of the xfmr(s) could be any 3ph voltage.
And if the load is (mostly) 1ph motors that want 240V and not 208V, those motors would prefer 2 legs of 139V L-N making 240V vs. 2 legs of 120V L-N making 208V.
Thanks.

 

[GoldDigger]

I do not see why slash rated breakers could not be used in the solidly grounded wye service scenario even if the neutral is not extended to any load equipment. The NEC requires such an unused neutral to be brought as far as the service disconnect and be bonded there.

 

[electrofelon]

Being a bastard thing isn't always bad- Keith Richards for instance!
So you're saying you can't use 240/120V breakers for any of it, but as long as the 240/139V neutral stays on the load side of the xfmr(s), the supply side of the xfmr(s) could be any 3ph voltage.
And if the load is (mostly) 1ph motors that want 240V and not 208V, those motors would prefer 2 legs of 139V L-N making 240V vs. 2 legs of 120V L-N making 208V.
Thanks.

Yes I think motors would be a little happier on 240 than 208. A little less current and voltage drop, potential smaller wire and pipe for your inductive loads.

I do not see why slash rated breakers could not be used in the solidly grounded wye service scenario even if the neutral is not extended to any load equipment.

I believe a 240/120 slash rated breaker could not be used on a 240Y/139 system. What am i missing?

 

[GoldDigger]

Yes I think motors would be a little happier on 240 than 208. A little less current and voltage drop, potential smaller wire and pipe for your inductive loads.



I believe a 240/120 slash rated breaker could not be used on a 240Y/139 system. What am i missing?

Nothing. I somehow was thinking slash rated was 150V to ground.

 

[electrofelon]

Nothing. I somehow was thinking slash rated was 150V to ground.

Maybe you were thinking of the 150 volts in 250.20 (B)
?