Transformer

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SolaRon

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Hello all, if this question should be in a non- solar forum just let me know...
I want to make certain my math is right on sizing and overcurrent protection so bare with me here. My NEC book is in my work truck 150 miles away making it a bit harder.

I'm installing a 142.1kW solar system on a building. Using (5) 3 phase 277/ 480V inverters totalling 240A (with 1.25 OC protection included)
The grid voltage is 120/ 208V and I'll be using a step up transformer to the 277/ 480V.
This is my math 240*480*1.732 = 199,526/ 1,000 = 199.5 So I'm planning on using a 225kVA transformer, which is the next standard size up... Am I all good so far?
Then for determining the primary amps I'm not sure if I should use the 199.5 or the 225kVA in my math... So coming up with the following
199.5*1,000 = 199,500 208*1.732 = 360 199,500/360 = 554A Rounded up to 600A fuse size
OR
225*1,000 = 225,000 208*1.732 = 360 225,000/360 = 625A Rounded up[ to 650A
OR am I just totally wrong on both??? They both sound kinda high to me compared to a similar one I just installed.

I'm trying to help the designer meet a deadline with plans and I will be installing all major components.
 
I am not at all familiar with solar installations. So I don't quite follow your description. Do I have this part right:
  • The solar panels of the PV system give you DC voltage.
  • Your inverters take the DC and are giving you 240 amps at 480 volt three phase.
  • You wish to change that to a 120/208 volt for distribution at the facility.
  • You would connect the 480 volt to a 225 KVA transformer, and you are trying to figure out the current (and therefore the fuse size) on the 208 volt side of the transformer.

That is what makes sense to me, but it seems to be backwards from your description. So please clarify for us.

It may be that the short answer to your question is that you have just under 200 KVA of power available from your PV system. Thus, you use that value (and not the total 225 KVA rating of the transformer) for all subsequent math.

Welcome to the forum.
 
One more small correction that you may add in is the magnetizing current on the lower voltage side of the transformer. The current in that winding will be the phasor sum of the transformed inverter output current and the magnetizing current.

mobile
 
jaggedben said:
I think the current calculation is really much simpler. 240 * 480/208 = 553. Thus the 600A fuse size is correct.
Click to expand...

The only thing that rings an alarm bell for me is the statement that the "total" amperage out of the three phase inverter is 240A.
In this particular case the power spec on the inverter confirms for us that this is the line current in each of the three phase lines. (As opposed to the phase current on each side of the delta?)
That makes your calculation completely applicable.
 
jaggedben said:
I think the current calculation is really much simpler. 240 * 480/208 = 553. Thus the 600A fuse size is correct.
Click to expand...
A 600A fuse will work, but the maximum fuse rating is based on the transformer rating per 450.3 per 690.9(F), thus...

225kVA ÷ 360 × 125% = 781A, so 800A max.

(F) Power Transformers. Overcurrent protection for a
transformer with a source(s) on each side shall be provided
in accordance with 450.3 by considering first one side of
the transformer, then the other side of the transformer, as
the primary.

Exception: A power transformer with a current rating on
the side connected toward the utility-interactive inverter
output, not less than the rated continuous output current of
the inverter, shall be permitted without overcurrent protection
from the inverter.
Click to expand...
 
jaggedben said:
Gotcha, but does he need a 225kVA xf for a 1.4kw system?
Click to expand...
I just reviewed OP and he said a 142.1kW system...

Would need to know the number of inverters and their continuous output current rating to be certain, but a 150kVA xfmr would handle a 142kW system. Tre trannie only has to be rated for 100% of the total output, not 125%. See the Exception to 690.8(F) that I quoted earlier for confirmation.
 
Smart $ said:
I just reviewed OP and he said a 142.1kW system...

Would need to know the number of inverters and their continuous output current rating to be certain, but a 150kVA xfmr would handle a 142kW system. Tre trannie only has to be rated for 100% of the total output, not 125%.
Click to expand...

Does he not need to multiply by 1.73? Seems to me he did that right if he's starting with current.

He did say total inverter output is 240A with the 1.25 factor included. Comes out to 160kW of nameplate inverter power by my calc. If 142.1 is DC kW, it would be unusual to have AC rating higher than DC, but not impossible. (He did say he was trying to help a designer meet a deadline.)

See the Exception to 690.8(F) that I quoted earlier for confirmation.
Click to expand...

I think you meant 690.9(F), which was renumbered to (D) in the 2017 cycle.
 
Don’t forget you have to size the OCPD for the transformer inrush current. That’s why the NEC allows OCPD that are larger than needed just based on the normal operating current. If you use too small an OCPD it will blow sometimes when you connect the transformer to the utility, depends on the part of the cycle you are in when it connects.
 
jaggedben said:
Does he not need to multiply by 1.73? Seems to me he did that right if he's starting with current.
Click to expand...
No. Total inverter kVA converts direct to minimum transformer kVA

jaggedben said:
He did say total inverter output is 240A with the 1.25 factor included. Comes out to 160kW of nameplate inverter power by my calc. If 142.1 is DC kW, it would be unusual to have AC rating higher than DC, but not impossible. (He did say he was trying to help a designer meet a deadline.)
Click to expand...
He said it is a 142.1kW system first... then 240A incuding 125% factoring... so the total inverter output could be higher than 150kVA. 240A ÷ 125% × 480V × sqrt(3) = 160kVA.

It is impossible to have an actual output higher than the input. But we are not talking actual. We are talking ratings. The inverters could very easily be rated for 160kVA out while only having a max PV source of 142.1KW.


jaggedben said:
I think you meant 690.9(F), which was renumbered to (D) in the 2017 cycle.
Click to expand...
That is correct. I meant 690.9(F), 2014NEC
 
Smart $ said:
No. Total inverter kVA converts direct to minimum transformer kVA
He said it is a 142.1kW system first... then 240A incuding 125% factoring... so the total inverter output could be higher than 150kVA. 240A ÷ 125% × 480V × sqrt(3) = 160kVA.
Click to expand...

Same math as I did. :D Anyway if the inverters are 160kW nameplate he still needs a 225kVA transformer. Either the inverters are oversized or there's a typo.

It is impossible to have an actual output higher than the input. But we are not talking actual. We are talking ratings. The inverters could very easily be rated for 160kVA out while only having a max PV source of 142.1KW.
Click to expand...

Actually it's possible for PV source circuits to output higher than nameplate, but as you said... It makes sense for the code to focus on inverter rating.
 
The inverters and modules have been changed slightly... So now it will be 5 inverters total- 4 of them are 36.2A each and 1 at 24A. So 36.2*4 = 144.8+24 = 168.8A * 1.25 = 211A With 4 having 50A breakers the other one having a 30A breaker. The module wattage has been down graded so it's now 320W panels * 412 of them = 131.84kW system. I 've been trying to jump on this post just been swamped trying to meet some deadlines. I'll still be using a 225kVA transformer and (3) 250A fuses in the 480V/ 277V disconnect after the combiner electrical panel with the (5) breakers. Then (3) 500A fuses in the 208/ 120V disconnect between transformer and grid. Thank you all for your responses. Feel free to let me know of any screw ups.
 
SolaRon said:
The inverters and modules have been changed slightly... So now it will be 5 inverters total- 4 of them are 36.2A each and 1 at 24A. So 36.2*4 = 144.8+24 = 168.8A * 1.25 = 211A With 4 having 50A breakers the other one having a 30A breaker. The module wattage has been down graded so it's now 320W panels * 412 of them = 131.84kW system. I 've been trying to jump on this post just been swamped trying to meet some deadlines. I'll still be using a 225kVA transformer and (3) 250A fuses in the 480V/ 277V disconnect after the combiner electrical panel with the (5) breakers. Then (3) 500A fuses in the 208/ 120V disconnect between transformer and grid. Thank you all for your responses. Feel free to let me know of any screw ups.
Click to expand...
168.8A × 480V × sqrt(3) = 140,338VA

You can easily get by with a 150kVA transformer and 225A fuses... and smaller wire. :happyyes:

500A fusing and wire still okay for 208/120V side... but you can go up to 600A if you want.
 
I am curious why you are specifying 140 kW of inverter for 132 kW of PV array. Seems like the four 30 kW inverters would be more than enough.
 
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