Transformer sizing for step up and step down transformers

[zman990]

I need to have a better understanding of sizing the kva rating
Also which type is best for solar dry or liquid filled transformers.
I have an example for step down
100amps of 480/277 from solar and need to tie in to customers service at 240/120.

480v X 100 amp X 1.73 3phase power factor = 83 KVA X 1.25 percent rule 103.8 KVA transformer

Do I really need to add another 1.25% multiplier?




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[kwired]

Transformers are generally rated in KVA not KW and AFAIK rating is essentially output rating though efficiency is generally high enough it can almost be disregarded. So a 100 kVA unit can be loaded to 100kVA. Imbalancing or harmonic currents may present some issues though.

 

[don_resqcapt19]

...
100amps of 480/277 from solar and need to tie in to customers service at 240/120. ...

Is the customers service really a 240/120 3 phase 4 wire delta?

 

[ggunn]

I need to have a better understanding of sizing the kva rating
Also which type is best for solar dry or liquid filled transformers.
I have an example for step down
100amps of 480/277 from solar and need to tie in to customers service at 240/120.

Whoa, back the truck up. Is the customer's service 204/120 single phase? You can't interface a 480/277 three phase inverter with a 240/120 single phase service. A transformer cannot help you. You can connect single phase inverters to three phase services but not the other way round.

 

[jaggedben]

It could be a high-leg delta. He didn't say single or three phase. We definitely need some clarification.

 

[zman990]

http://www.civicsolar.com/product/s...-vac-transformerless-inverter-w-afci-se333kus

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[zman990]

I find is very funny that u say we can't do it when I already have a ge transformer sized at 75kva that will take it down to single phase so what is difference.
Between me taking 480 to 208 then 208 to single phase I am wondering why if it is not possible ge makes a transformer that does it.
Why make it

 

[zman990]

Hope u can read it

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[jaggedben]

The inverter needs a three-phase voltage reference and even if that transformer transforms 3-phase to single phase in one direction, it won't work the other way. We just had another thread on this.

If the service is single phase you should be using single phase inverters, even if you need two or three.

A line diagram or other description would clear things up.

 

[ggunn]

View attachment 13925
I find is very funny that u say we can't do it when I already have a ge transformer sized at 75kva that will take it down to single phase so what is difference.
Between me taking 480 to 208 then 208 to single phase I am wondering why if it is not possible ge makes a transformer that does it.
Why make it

Transforming 480V 3P down to 208V 3P and splitting 120V 1P out of it is not the same thing. Not even close.

 

[ggunn]

It could be a high-leg delta. He didn't say single or three phase. We definitely need some clarification.

It could be, but from context I do not think that is the case.

 

[zman990]

Every one is saying it won't work well I need more than a feeling I need math and proof because this is how people do this on large farms in Midwest

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[kwired]

Every one is saying it won't work well I need more than a feeling I need math and proof because this is how people do this on large farms in Midwest

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How everyone in midwest does what? Derive three phase from single phase? That is something that happens more often. There isn't as much need for converting three phase to single phase until you bring renewable energy sources into the picture. There is no simple transformer that will convert single phase to three phase, you need additional components, whether mechanical or electronic, not saying you can't find an assembly that does this - but it is not a simple core and coil only type of thing.

 

[ggunn]

Every one is saying it won't work well I need more than a feeling I need math and proof because this is how people do this on large farms in Midwest

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I can assure you that they are not connecting three phase inverters to single phase services. Large farms typically have three phase services, hence three phase inverters work for them. Residences typically have single phase services; three phase inverters do not work for them. I cannot give you "math and proof"; one cannot prove a negative. I do, however have several years experience designing PV systems for both single phase and three phase systems.


It's very simple: you can connect single phase inverters to either single phase or three phase services, but you can only connect three phase inverters to three phase services. Don't believe me? Call the tech support of any manufacturer of three phase inverters; they will tell you the same thing.

But let me ask you this: Even if it were possible, why would you want to do such a thing? The cost of this magic transformer would surely make the system so much more expensive that it would negate any benefit you think you would get by using a three phase inverter over a single phase inverter.

 

[PVfarmer]

100amps of 480/277 from solar and need to tie in to customers service at 240/120.

I already have a ge transformer sized at 75kva that will take it down to single phase

Hold on here- still not sure what's going on.
You have 120A of PV, not 100, if you're using one of the 33kW models from the pdf you posted. Max. Continuous Output Current (per Phase) 40A

So that's 99.6kVA of PV.

If the service is 1ph 120/240V, as in two 120V Ls and a N, it has to be what, 800 amps of 120V? That seems really large.

25kVA of 120V is 208.3 amps, 100kVA is 833.3 amps of 120V- but either way it seems like a 100kVA service transformer would be too small for 99.6kVA of PV.

 

[ggunn]

Hold on here- still not sure what's going on.
You have 120A of PV, not 100, if you're using one of the 33kW models from the pdf you posted. Max. Continuous Output Current (per Phase) 40A

No. 40A per phase is 40A, period. I don't know why they put that "per phase" in there; it just confuses people. 33kW at 480V 3P is about 40A total current. Power splits between phases, current doesn't. 33kW 3P is 11kW per phase. 11kW/277V = about 40A, again.

 

[kwired]

Quote Originally Posted by zman990 View Post
100amps of 480/277 from solar and need to tie in to customers service at 240/120.


I already have a ge transformer sized at 75kva that will take it down to single phase

Hold on here- still not sure what's going on.
You have 120A of PV, not 100, if you're using one of the 33kW models from the pdf you posted. Max. Continuous Output Current (per Phase) 40A

So that's 99.6kVA of PV.

If the service is 1ph 120/240V, as in two 120V Ls and a N, it has to be what, 800 amps of 120V? That seems really large.

25kVA of 120V is 208.3 amps, 100kVA is 833.3 amps of 120V- but either way it seems like a 100kVA service transformer would be too small for 99.6kVA of PV.

You might be on to something before even considering conversion between three phase and single phase, based on what OP said with :"100amps of 480/277 from solar", that alone is about 83 kVA, yet he says he already has a 75 kVA transformer. Something don't seem right already and I am not even experienced with PV applications.

 

[PVfarmer]

120/240V Delta with a center tap between A and C phases for 120V (4 wire "stinger leg"); still provided for small commercial users with >10% 120V 1 phase loads
But really, step 1 of this process is to contact the engineering department at the inverter manufacturer and confirm the compatibility of their inverter with the High Leg Delta service. Most inverters over 25 kW can be ordered with custom output transformers or preconfigured to match the most cost-effective isolation transformer that will connect them to the high leg delta service.

http://www.civicsolar.com/forum/309...uired-step-down-240v-delta-service-connection

If this is a center tapped service, I don't see how you can get around the 120V / 10% thing without "customizing".
If it is CT service, why not drop the center tap and connect the 480V inverters to grid at 240V delta, and use a 1ph xfmr for the small-ish 120V loads?

 

[PVfarmer]

No. 40A per phase is 40A, period. I don't know why they put that "per phase" in there; it just confuses people. 33kW at 480V 3P is about 40A total current. Power splits between phases, current doesn't. 33kW 3P is 11kW per phase. 11kW/277V = about 40A, again.

Yes! Good point. I knew that.
But he doesn't have 2.5 of them, so...it'd have to be, um...no idea. There's no way to make 100A out of those options.
Four of the 20kW models would be 96 A of PV.

 

[ggunn]

120/240V Delta with a center tap between A and C phases for 120V (4 wire "stinger leg"); still provided for small commercial users with >10% 120V 1 phase loads
But really, step 1 of this process is to contact the engineering department at the inverter manufacturer and confirm the compatibility of their inverter with the High Leg Delta service. Most inverters over 25 kW can be ordered with custom output transformers or preconfigured to match the most cost-effective isolation transformer that will connect them to the high leg delta service.

http://www.civicsolar.com/forum/309...uired-step-down-240v-delta-service-connection

If this is a center tapped service, I don't see how you can get around the 120V / 10% thing without "customizing".
If it is CT service, why not drop the center tap and connect the 480V inverters to grid at 240V delta, and use a 1ph xfmr for the small-ish 120V loads?

This is getting way off the beam. Unless we are shown otherwise, the OP is talking about a 240/120V split (single) phase service. Talking about 240V high leg services just muddies the water; connection to such a service with a three phase inverter may or may not work, depending on whether or not the three phase inverter needs a neutral (if it does it cannot use the center tap as such and therefore would require an isolation transformer) and whether the transformer driving the high leg of the service can take the output of the inverter (it's typically a lot smaller than the transformer(s) driving the other two phases).

If he has a 240/120V split phase service, no transformer will allow him to interconnect a three phase inverter to it.