Transformer sizing - Load is a mix of AC and DC consuming equipments

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antonio00

Member
Location
Saudi Arabia
Hi,

My load is a mix of AC and DC voltage consuming equipment as given below:

  • AC voltage Load = 2KVA (230V AC , 1 phase)
  • DC voltage Load = 2KVA (48V DC)
  • AC voltage Load = 5KVA (380V AC , 3 phase)

If I want to calculate the size of my transformer, then to do the load calculation, I am doing the following (but I am confused when calculating the DC load, need your help)


  • For AC load, I can calculate as following:

5K+2K+2K+2K = 11KVA (Even though I am not gonna use 2K+2K on other 2 phases, but as this is a three phase system, i have included 2K+2K at the end, please correct me If i am wrong here)

Hence, here I have got 11KVA AC voltage 3 phase load


  • For DC load, Is this the correct way to calculate?

2KVA/48V = 41.6A
41.6A x 380V x 1.732 = 27.37 KVA 380V AC 3 phase (Is this correct)??


  • Now finally adding all together = 11+27.37 = 38.37KVA (380V, 3 phase AC load)

Hence Transformer sizing = 38.37 x 1.25 = 47.9 = 50 KVA Transformer of 380V 3 Phase

Can anyone please correct me for the mistakes..Thanks :)
 

antonio00

Member
Location
Saudi Arabia
Further to my above post, I did a little research, and the update to my post is as given below (please check and correct)


  • For DC load, Is this the correct way to calculate?


2KVA @ 48V DC has to pass through a rectifier and a step up transformer, hence assuming the efficiency of (Rectifier+Step up transformer) to be 80%, then

2KVA / 0.8 = 2.5KVA @ 380V 3 phase AC

****Power is Power, either its DC or AC, only the efficiency of the converter shall be taken care******


  • Now finally adding all together = 11+2.5 = 13.5KVA (380V, 3 phase AC load)


Hence Transformer sizing = 13.5 x 1.25 = 16.87 = 20 KVA Transformer of 380V 3 Phase

Can anyone please correct me for the mistakes..Thanks :)
 

Besoeker

Senior Member
Location
UK
[*]For DC load, Is this the correct way to calculate?
[/LIST]

2KVA/48V = 41.6A
41.6A x 380V x 1.732 = 27.37 KVA 380V AC 3 phase (Is this correct)??
At 48V out, the DC will not take 41.6A. That's the output, not the input current

It will use a step down transformer, rectifier, and probably a voltage regulator or possibly be a switch mode power supply. Depends on how stable you need it to be.
 

Ingenieur

Senior Member
Location
Earth
your load is 9 kva

1 ph = 2000/230 = 8.7 A AC
DC =2000/48 = 41.7 A DC
3 ph = 5000/(1.732 x 380) = 7.6 A AC

what ac phase and voltage feed the DC device?

assume 1 ph will load A - N 2 kva
assume DC will load B - N 2 kva (assume 230 1 ph)
assume 3 ph load A, B and C - 1667 kva each

A total ~ 3.667
B total = 3.667
C total = 1.667
total = 9 kva

min size 3 x 3.667 = 11 kva
use next standard size
 

antonio00

Member
Location
Saudi Arabia
your load is 9 kva

1 ph = 2000/230 = 8.7 A AC
DC =2000/48 = 41.7 A DC
3 ph = 5000/(1.732 x 380) = 7.6 A AC

what ac phase and voltage feed the DC device?

assume 1 ph will load A - N 2 kva
assume DC will load B - N 2 kva (assume 230 1 ph)
assume 3 ph load A, B and C - 1667 kva each

A total ~ 3.667
B total = 3.667
C total = 1.667
total = 9 kva

min size 3 x 3.667 = 11 kva
use next standard size

Thanks Ingenieur, :) The way you have provided detailed calculation has really helped me to understand the complete calculation steps. Thanks again!!
I believe if I just add all KVA (either it's 3 phase and single phase or DC) I will reach only 9KVA. But this will lead me to a small transformer than required.

As you have shown in the last step to take highest load on any phase and then multiply by 3 to reach for transformer size, helped me understand!! :)
 

jumper

Senior Member
Thanks Ingenieur, :) The way you have provided detailed calculation has really helped me to understand the complete calculation steps. Thanks again!!
I believe if I just add all KVA (either it's 3 phase and single phase or DC) I will reach only 9KVA. But this will lead me to a small transformer than required.

As you have shown in the last step to take highest load on any phase and then multiply by 3 to reach for transformer size, helped me understand!! :)

Yes, Largest load on any phase x 3.
 

antonio00

Member
Location
Saudi Arabia
A small NEC trivia note, when we do not want to actually calculate the load for an existing building, for whatever reason, we can monitor it for 30 days at 15 minute intervals and use the highest peak load of any phase as a basis to start. Section 220.87. A few other factors come into play for final numbers.

Noted. Thanks :) , I will keep it in mind when doing the calculation for an existing building.
 

topgone

Senior Member
Yes, Largest load on any phase x 3.
My calcs tell me he needs a 15 kVA!
Assuming the rectifier for the DC load is a three-phase bridge, you have to spread out the loading to the three-phase supply. Here's my calculations:
ABC
LOAD 1(SINGLE-PHASE)2,000 VA--------
LOAD 2(3-PHASE RECTIFIER)667 kVA667 kVA667 kVA
LOAD 31,667 kVA1,667 kVA1,667 kVA

MAX PHASE LOAD = 4,333.33 kVA; 3-PHASE LOAD = 3 X 4,333.33 = 13,000
TOTAL SIZE BASED ON MAX PHASE LOAD = 13000.00 SAY 15 kVA
 

jumper

Senior Member
My calcs tell me he needs a 15 kVA!
Assuming the rectifier for the DC load is a three-phase bridge, you have to spread out the loading to the three-phase supply. Here's my calculations:
ABC
LOAD 1(SINGLE-PHASE)2,000 VA--------
LOAD 2(3-PHASE RECTIFIER)667 kVA667 kVA667 kVA
LOAD 31,667 kVA1,667 kVA1,667 kVA

MAX PHASE LOAD = 4,333.33 kVA; 3-PHASE LOAD = 3 X 4,333.33 = 13,000
TOTAL SIZE BASED ON MAX PHASE LOAD = 13000.00 SAY 15 kVA

I didn’t calc anything, I simply agreed about largest phase load x 3.

So, whatever number is correct x 3 is used.

Up to OP to size correctly, the methods are the answer.
 
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