Solar Transformer Selection

[TheElectrician]

Hey Guys, I am using a 3 phase transformer to convert 480V to 208V. The total current from the 10 inverters I am using is 602A. So the KVA is calculated to be 500.47 KVA

In this case, can I still use a 500KVA transformer or should I upsize it to 750KVA? Any insights would be helpful.

Thank you

 

[gadfly56]

Hey Guys, I am using a 3 phase transformer to convert 480V to 208V. The total current from the 10 inverters I am using is 602A. So the KVA is calculated to be 500.47 KVA

In this case, can I still use a 500KVA transformer or should I upsize it to 750KVA? Any insights would be helpful.

Thank you

Is that peak, average, or minimum?

 

[Carultch]

Is that peak, average, or minimum?

The current rating of an inverter is based on its maximum limit. If the array attempts to exceed that, the inverter will shift the DC voltage away from the IV curve sweet-spot, so the panels don't output their full capacity, and would clip the potential power it could produce. In otherwords, part of the sunlight "stays on the roof" in the form of heat.

You could source it with a tracker, or with a heavily saturated DC-to-AC ratio, and it might output full nominal AC capacity for several hours. By contrast, if you sized it as close as you could go without clipping, it would be the peak of the day's "bell curve" of power production.

 

[Carultch]

Hey Guys, I am using a 3 phase transformer to convert 480V to 208V. The total current from the 10 inverters I am using is 602A. So the KVA is calculated to be 500.47 KVA

In this case, can I still use a 500KVA transformer or should I upsize it to 750KVA? Any insights would be helpful.

Thank you

The transformer KVA comes from the total of the inverter kVA ratings on the datasheet. Or the total kW rating, if there isn't a kVA rating in addition to the kW rating. Some manufacturers provide application notes to recommend a margin above the inverter rating for the transformer, such as transformer kVA should be 105% of the total inverter rating. If there is no such information, there is no NEC rule to require this margin. The ampere rating isn't what needs to be involved in this calculation, because some inverters have a higher amp rating than what corresponds directly to its kVA at nominal voltage, to allow it to still produce full power at lower than nominal grid voltage.

602A sounds like what you get after rounding from 500 kVA at 480V to the nearest ampere. I would expect that the kVA ratings of these particular inverters would add up to exactly 500 kVA.

 

[electrofelon]

That's a tough call. On one hand, in practice I would say it is a 500 is totally fine and perfect. On the other hand (while I don't necessarily agree and wish manufacturers would just stick to making inverters) often a manufacturer will "require"/suggest a transformer be oversized to some degree. Also a larger transformer will likely have better voltage regulation which can help if voltage creeps up too high to the point where the inverters are at risk of tripping out. (Although you will likely have taps that can solve that).