autotransformer bank for 3 mini splits

[electrofelon]

I have a 277/480 system that I want to run 3 single phase 240V mini splits from. I was thinking I would put in a 3 pole 480 breaker and run a MWBC to feed 3 autotransformers and each would buck the 277 down to 240. I havent designed the specifics yet, but was just first wanted to confirm that is a good way to go vs using an isolation transformer and also if there is anything I should watch out for. Never have used buck boosts in this manner. Thank You.

 

[electrofelon]

Ok played around with this a little more. The voltage was confusing me. I think I have it but wouldnt mind some confirmation:

1. So I have to convert the desired L-N voltage to L-L when selecting the transformer from the charts. For example, under the three phase Y category, they show units that will take 480 down to 424, which would be 245 L-N.

2. For KVA, I select the combined KVA of all three units.

3. If the MCA of each unit is 25, the charts show a bank that does 26 amps so that should be good.

4. Now for my 450.4 protection I use 125% of the INPUT amp rating plus next size up, which I calculate if they only show load amps.

5. What about the neutral conductor? Does that just pass through to the load or do I hit the Y point of the bank with it also?

 

[topgone]

Ok played around with this a little more. The voltage was confusing me. I think I have it but wouldnt mind some confirmation:

1. So I have to convert the desired L-N voltage to L-L when selecting the transformer from the charts. For example, under the three phase Y category, they show units that will take 480 down to 424, which would be 245 L-N.

2. For KVA, I select the combined KVA of all three units.

3. If the MCA of each unit is 25, the charts show a bank that does 26 amps so that should be good.

4. Now for my 450.4 protection I use 125% of the INPUT amp rating plus next size up, which I calculate if they only show load amps.

5. What about the neutral conductor? Does that just pass through to the load or do I hit the Y point of the bank with it also?

Suggestion: please post a sample sketch. that would be easier to digest compared to a long-winded narrative of what you intended to design.

 

[electrofelon]

Ok here is the diagram. I think this clarifies things. The initial diagram (from a different manufacturer) I had been referencing just said
"may be grounded" with an arrow pointing to the Y point of the H1's. This one clearly states HV neutral, LV neutral, and Y point are all common. Thats what I want right? So HV will be 480 from 3 pole breaker, each LV line will go to an AC unit's L1 terminal, and neutrals, all common to the Y point, will go to each unit's L2 terminal. Transformer specs for this configuration from the square D calculator are : configuration 3 phase WYE, HV 480, LV 424, load current 25.46. Loads are 3 mini splits MCA 25A.

If that is good, the final question is: in regards to my max ocpd size on the nameplate, can I meet that with that 480 breaker feeding the buck bank or do I need an OCPD between the bank and each unit?

 

[topgone]

Ok here is the diagram. I think this clarifies things. The initial diagram (from a different manufacturer) I had been referencing just said
"may be grounded" with an arrow pointing to the Y point of the H1's. This one clearly states HV neutral, LV neutral, and Y point are all common. Thats what I want right? So HV will be 480 from 3 pole breaker, each LV line will go to an AC unit's L1 terminal, and neutrals, all common to the Y point, will go to each unit's L2 terminal. Transformer specs for this configuration from the square D calculator are : configuration 3 phase WYE, HV 480, LV 424, load current 25.46. Loads are 3 mini splits MCA 25A.

If that is good, the final question is: in regards to my max ocpd size on the nameplate, can I meet that with that 480 breaker feeding the buck bank or do I need an OCPD between the bank and each unit?

That clears up things here.

You have a bucking autotransformer, connected wye-wye, rated at 18.7 kVA (25.46 x 424/1.732)*3 = 18.7 kVA). As mentioned, your secondary current will be 25.46 amperes. Your primary current will then be 18,700/(1.732 x 480) = 22.5 amperes.

IMO, the problem with wye-wye configuration is the possibility of allowing positive sequence currents to get from the secondary to the primary side during faults. Grounding solidly the neutrals helps but unless your transformer has additional tertiary winding that you could connect in delta, expect positive sequence currents to be a problem as well as third harmonics. Other posters could have a better perspective than mine, of course.

 

[electrofelon]

That clears up things here.

You have a bucking autotransformer, connected wye-wye, rated at 18.7 kVA (25.46 x 424/1.732)*3 = 18.7 kVA). As mentioned, your secondary current will be 25.46 amperes. Your primary current will then be 18,700/(1.732 x 480) = 22.5 amperes.

IMO, the problem with wye-wye configuration is the possibility of allowing positive sequence currents to get from the secondary to the primary side during faults. Grounding solidly the neutrals helps but unless your transformer has additional tertiary winding that you could connect in delta, expect positive sequence currents to be a problem as well as third harmonics. Other posters could have a better perspective than mine, of course.

Thanks. Another option is to just use three single phase units, one for each AC .....but wait I think that ends up being the same thing as a y bank....