Transformer kVA rating calculation for small rooftop PV system

Status
Not open for further replies.
P

PV314

New member
Location
san antonio TX
Hello all,

If I have (2) 20 KW, 3 phase, 480V, solar inverters, how do I calculate the transformer kVA rating to step the voltage down to 208/120?

Am I doing this right? Or am i missing something?
Grid = 208/120, 3 ph, 4 w
AC system size = 40 kW
Voltage = 480 V
Current = 48 A
Min kVA raitng = 48x480x1.732 = 39.9 kVA
90% xfr rating = 39.9 / 0.9 = 44.3 kVA

So, it would be best to use a 45 kVA, dry type, wye to wye, 480/208 step down transformer for this PV application? Thanks for your help !
 
Smart $

Smart $

Esteemed Member
Location
Ohio
FWIW, you can shortcut some calculating there by just replacing "kW" with "kVA"... but you should really base your transformer size on the rated inverter output. Using the kW value of the system will, however, provide a good estimate.

Also, it is my understanding that the utility side of the transformer should be configured delta.
 
G

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I believe that in a pinch you can also use a wye winding as long as you leave the wye point floating. (I.E. only three connections made whether the winding is wye or delta.)
The POCO supply will usually be a grounded wye, but that does not change anything.

Sent from my XT1585 using Tapatalk
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
PV314 said:
Hello all,

If I have (2) 20 KW, 3 phase, 480V, solar inverters, how do I calculate the transformer kVA rating to step the voltage down to 208/120?

Am I doing this right? Or am i missing something?
Grid = 208/120, 3 ph, 4 w
AC system size = 40 kW
Voltage = 480 V
Current = 48 A
Min kVA raitng = 48x480x1.732 = 39.9 kVA
90% xfr rating = 39.9 / 0.9 = 44.3 kVA

So, it would be best to use a 45 kVA, dry type, wye to wye, 480/208 step down transformer for this PV application? Thanks for your help !
Click to expand...


Is your "90% transformer rating" from the inverter manufacturer? I have heard that stated before by some manufacturers, not sure why that is. It is not an NEC issue.
 
G

gray.one

Member
Location
Reston, VA
Transformer kVA rating calculation for small rooftop PV system

electrofelon said:
Is your "90% transformer rating" from the inverter manufacturer? I have heard that stated before by some manufacturers, not sure why that is. It is not an NEC issue.
Click to expand...

Can't vouch for 90%, but you can undersize PV system transformers because power only flows for part of the day. There is an IEEE standard somewhere that explains that the lifespan of a transformer is based on the loading. Root mean squared of the demand current over every hour (or other period) if I remember correctly.


Sent from my iPhone using Tapatalk
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
gray.one said:
Can't vouch for 90%, but you can undersize PV system transformers because power only flows for part of the day. There is an IEEE standard somewhere that explains that the lifespan of a transformer is based on the loading. Root mean squared of the demand current over every hour (or other period) if I remember correctly.


Sent from my iPhone using Tapatalk
Click to expand...

yeah that topic has come up here now and then. In theory there is no "hard limit" on what a transformer can do (unless we are talking a bolted short on the secondary), it just comes down to life span. In practice, IMO, an inspector is within his rights to call a 110.3(B) if the transformer nameplate KVA was less than the inverter outputs. Note also that considering the 125% rule for OCPD's, the transformer protection requirements in 450.3(B), and that PV is a continuous load, you would be hard pressed to find a situation where you could undersize the transformer even if 110.3(B) was not called (its late, but I think the only way to do it is to have the perfect combination of equipment and use note 1 of 450.3(B)).
 
Sahib

Sahib

Senior Member
Location
India
A transformer life may be estimated as below. (Rated transformer life/Actual transformer life)=(Actual transformer current squared/Rated transformer current squared).
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
electrofelon said:
Is your "90% transformer rating" from the inverter manufacturer? I have heard that stated before by some manufacturers, not sure why that is. It is not an NEC issue.
Click to expand...
I believe that the 90% rating refers to how big the PV system can be compared to the utility owned transformer powering the site. We ran afoul of this a couple of years ago when we designed commercial systems for a customer's two businesses on adjacent properties without realizing that the properties were supplied with power by the same single transformer. The 90% number is what the POCO quoted to us when they were telling us that we had to pay to replace the transformer. Since the systems were already installed, we had no bargaining power. One more lesson learned.

For 480 to 208 step down transformers, I just make sure that the kVA rating of the transformer is equal to or greater than the kW rating of the inverter(s).
 
Status
Not open for further replies.
Top