Best service type for EV charging?

[LarryFine]

My thought was to recommend 24 or 26, usually the chargers come in pairs. Maybe another reason though.

Divisible by three, as in even distribution among phases.

 

[pipe_bender]

Thanks everyone for the info on these oddball systems, sorry i was busy with other work
A few questions on the two-phase or double split phase concept,
So three phase primary goes in and you get two 120/240 systems out?
The transformer supplier did not even flinch about two-phase he said standard single phase transformers can get wired together on the primary side and output two 120/240 split phase feeds on the secondary and that is not a Scott-T so what is this connection called?
Or you can order a customized transformer called a Scott-T with a 'common core'?
They make them to do something called a back to back T which is way over my head.
Presumably the two feeders have to be balanced at all times?
What if one feeder is fully loaded with chargers and the other is empty?
Would I need a 4-pole disconnect at the transformer or could it just go directly into two single phase 3R panelboards?
They said you need a 200% neutral, but the chargers do not have a neutral so would you run a 200% neutral to each panelboard? The chargers do not need a neutral, but they would see 120 to ground from each leg?
Would there be any other code issues?
Thanks in advance.

 

[winnie]

Ok, the first issue is 'why do you even care'. You could just select a large single phase transformer to feed these chargers and be done.

The problem is that there is absolutely no way to take a single phase load and distribute that load evenly on all three phases using only transformers. (You _can_ balance a single phase load evenly on 3 phases if you have energy storage, but that is even more off in the world of physically permitted fantasy than using 2 phase power.) If your power supply system is large enough, then you might not care; how often does the power company just pick one primary line to supply single phase to a large residence. All of the discussion of 2 or 3 or 6 phases is in service of balancing 3 phase loading. But always ask question 0: is this even necessary.

Thanks everyone for the info on these oddball systems, sorry i was busy with other work
A few questions on the two-phase or double split phase concept,
So three phase primary goes in and you get two 120/240 systems out?

Before thinking about 2 phase, start with a 'simpler' idea: you have 3 separate single phase transformers, each creating a separate 120/240V supply for a subset of the chargers. (Simpler only because 3 phase is common, 2 phase is an oddball.) You could connect the 3 separate primary circuits to different phases, and balance the three phase loading that way. You can see that if you have 3 transformers each feeding 8 chargers you could balance the 3 phase loading if the chargers were used evenly.

Now take those three separate single phase transformers and wind them on a single common 3 leg core. Still 3 separate 120/240V systems, on 3 separate phases, but you are sharing transformer iron, just like a 208/120 transformer feeding a bunch of 120V loads.

Think standard delta:wye transformer, except that you have 3 separate center tapped secondary coils, rather than the secondary coils connected in wye.


The above are the three phase analog to what the transformer distributor was describing.

It turns out that with a proper combination of transformers you can convert _any_ 'polyphase' (say 3 phase) to any other polyphase system, say 2 or 5 or 9 phases. The 'Scott-T' arrangement is a clever coil arrangement that converts between 3 phase and 2 phase. If you have a balanced 2 phase load, it will convert it into a balanced 3 phase load.

If you have a transformer with a Scott-T primary arrangement, you get a 2 phase magnetic field running on the transformer core. Put 2 separate 120/240 coils on each leg, and you get two separate 120/240V systems produced. (Actually, it would be called 2 phase 5 wire, described here: https://forums.mikeholt.com/threads/two-phase-systems.92486/post-1478246 but you are ignoring any voltage relationship between the phases and thinking of the secondary as 2 separate single phase systems.) This is simply the 2 phase version of having separate 120/240V coils on each phase, blended with the Scott-T arrangement to convert to 3 phase.

The transformer supplier did not even flinch about two-phase he said standard single phase transformers can get wired together on the primary side and output two 120/240 split phase feeds on the secondary and that is not a Scott-T so what is this connection called?

Something doesn't sound right on the above. As I described, you could have 3 separate standard single phase transformers to connect 3 single phase systems to a 3 phase system in a balanced fashion, but I don't think you can do this with 2 standard transformers to get the balanced loading. Maybe the supplier is just describing putting two separate single phase transformers on two phases.

Or you can order a customized transformer called a Scott-T with a 'common core'?

A Scott-T with a 'common core' or single phase transformers custom tapping arranged in a Scott-T fashion would do the 3 to 2 phase conversion.

They make them to do something called a back to back T which is way over my head.

This is just a type of 3 phase to 3 phase transformer, that saves money for small transformers by using 2 coils in the Scott-T arrangement. They are going from a 3 phase supply, to a 3 phase load, but via 2 phase magnetic flux. The goal is to save money and be confusing.

Presumably the two feeders have to be balanced at all times?
What if one feeder is fully loaded with chargers and the other is empty?

As mentioned before, if you have a single phase load, no combination of transformers will convert it into balanced 3 phase loading. So if one single phase feeder is fully loaded and the other empty, then the three phase supply will be unevenly loaded. Remember the question: do you care? In this example, when both feeders are fully loaded, the 3 phase supply will be balanced. If one feeder is fully loaded and the other empty, then the 3 phase supply will not be balanced, but at 50% load; since you are not fully loaded maybe you don't care.

Would I need a 4-pole disconnect at the transformer or could it just go directly into two single phase 3R panelboards?
They said you need a 200% neutral, but the chargers do not have a neutral so would you run a 200% neutral to each panelboard? The chargers do not need a neutral, but they would see 120 to ground from each leg?

Not sure on the 4-pole disconnect.

The arrangement describe would give you proper 120/240V single phase, 120V to each leg.

Would there be any other code issues?
Thanks in advance.

Other than maybe needing a 4 pole OCPD for secondary protection, I think it would otherwise be a standard transformer installation.

Personally I'd go with standard transformers; 3 separate circuits to 3 separate single phase transformers used in the standard fashion. That is if 1 standard transformer and unbalanced 3 phase loading was not acceptable.

 

[pipe_bender]

Before thinking about 2 phase, start with a 'simpler' idea: you have 3 separate single phase transformers, each creating a separate 120/240V supply for a subset of the chargers.

Thanks for taking the time to write all this up,
The reasons for considering the double split phase or 'two phase' on a common core are;

• They prefer the 120/240 over the other systems its a standard system.
• Each transformer has to sit on a concrete vault thats about 4 foot square, customer would prefer one vault as there is only one good location in the landscaping where we can put a vault. Possibly a second location but it would take more excavator work and concrete cutting across the a entrance. There are other per transformer costs that add up so a single transformer is best.
• The customer gets hit with some kind of peak demand surcharge. A large peak demand on one phase can bump them into a higher bracket. (You'd think the POCO would charge less per kWH if you use more but not so I guess), so the EV peak load has to be balanced across the 3 phases. The supplier said 'two balanced single phase feeders off a Scott-T would present as a balanced three phase load' used a bunch of fancy terms. I am sure I have no understating how that math works I suck at math stuff.

My boss did also say a 4-pole disconnect is not needed, and 200% neutral not needed other than at the xfromer as they are separate feeders, not sure is he looked it up though.
This is all just sptiballing but it helps me learn thanks again for your input.

 

[winnie]

• Each transformer has to sit on a concrete vault thats about 4 foot square, customer would prefer one vault as there is only one good location in the landscaping where we can put a vault. Possibly a second location but it would take more excavator work and concrete cutting across the a entrance. There are other per transformer costs that add up so a single transformer is best.

If a single transformer is the goal, and you have a good custom transformer supplier, then you could get a single three phase transformer wired to produce 3 _separate_ 120/240V outputs. This would be otherwise the same as my description of having 3 separate single phase transformers.

• The customer gets hit with some kind of peak demand surcharge. A large peak demand on one phase can bump them into a higher bracket. (You'd think the POCO would charge less per kWH if you use more but not so I guess), so the EV peak load has to be balanced across the 3 phases. The supplier said 'two balanced single phase feeders off a Scott-T would present as a balanced three phase load' used a bunch of fancy terms. I am sure I have no understating how that math works I suck at math stuff.

Please notice the critical caveat: 'two balanced single phase feeders'. If you don't have balance between the two single phase loads, then the resulting three phase load is also not balanced.

I think that having the charger loads divided between 2 separate branches instead of 3 separate branches might increase the odds of being close to balanced, but if the load is not balanced then no transformer will fix that. I think this is a point in favor of going to the Scott-T.

My boss did also say a 4-pole disconnect is not needed, and 200% neutral not needed other than at the xfromer as they are separate feeders, not sure is he looked it up though.
This is all just sptiballing but it helps me learn thanks again for your input.

The chargers don't need a neutral; no reason to bring it past the point where it gets bonded to ground and becomes an EGC (with EGC sizing, not neutral sizing).

Let us know what you guys finally decide to do.

 

[hillbilly1]

Make sure the chargers don’t need a neutral, they may have got a deal on them since they dead set on using them. Eaton commercial charging stations required a neutral for the control contactor on the early models, don’t know if they still due.