connect three (one-phase 10KVA transformers) in three-phase Y connection; explain what the KVA rating is

[cmoreno58]

It is a lengthy question. You have a 10KVA single-phase transformer, 1:1 turns ratio, with 240Vac applied at primary terminal; I = 10000/240 = 41.66 amps. You now arrange three of these 10KVA transformers in 3-phase Y connection. You now apply 480Vac (across phase to phase) of the 3-phase Y connections. what is the KVA value for this 3 phase transformer ? ( if that answer is just 30 KVA...... then I really have a different question. what is the KVA of the 3-phase transformer if I apply 480 V (phase to phase) and the current drawn is 41.66 amps ???

 

[electrofelon]

It is a lengthy question. You have a 10KVA single-phase transformer, 1:1 turns ratio, with 240Vac applied at primary terminal; I = 10000/240 = 41.66 amps. You now arrange three of these 10KVA transformers in 3-phase Y connection. You now apply 480Vac (across phase to phase) of the 3-phase Y connections. what is the KVA value for this 3 phase transformer ? ( if that answer is just 30 KVA...... then I really have a different question. what is the KVA of the 3-phase transformer if I apply 480 V (phase to phase) and the current drawn is 41.66 amps ???

In a wye configuration, your phase to phase voltage would be 415 from 240v single units

 

[LarryFine]

10 + 10 + 10 = 30, regardless of voltages or configuration.

 

[cmoreno58]

Thank you. That 30KVA value is based on three transformers of 10KVA rating each. Then. If these 3 transformers are connected in 3-phase Y, and we apply 480V (phase to phase) on the primary side, what is the current that can be drawn on the primary side (to satisfy the 30KVA rating) ? - - ----- is the solution: 30,000 divided by ( 480 x 1.732) ?? 36 Amps ?

 

[winnie]

The question is flawed.

Transformers are mostly limited by their rated current below a maximum allowed voltage. Halve the operating voltage (vs transformer design voltage, and you halve the kVA.)

But once you exceed design operating voltage you start to get saturation effects. There is some headroom above rating, but not much.

If you bank the transformers Y:Y, then when you apply 480V three phase, you are putting 277V across your 240V coils.

This will likely saturate the transformers, leading to excessive magnetizing current and heating.

If the transformers are not damaged by the excessive voltage (in that design headroom) then you will might get more than 10VA out of each unit (>30kVA total) because of the higher voltage at maximum current.

Finally, as a Y:Y transformer you will see problems with neutral voltage stability. You can pretty much only supply L-L loads.

Jon

 

[electrofelon]

If you bank the transformers Y:Y, then when you apply 480V three phase, you are putting 277V across your 240V coils.

This will likely saturate the transformers, leading to excessive magnetizing current and heating.



Jon

Perhaps the OP can clarify if is actually inquiring about applying 480 to this bank which would be intended for 415?

 

[jim dungar]

Thank you. That 30KVA value is based on three transformers of 10KVA rating each. Then. If these 3 transformers are connected in 3-phase Y, and we apply 480V (phase to phase) on the primary side, what is the current that can be drawn on the primary side (to satisfy the 30KVA rating) ? - - ----- is the solution: 30,000 divided by ( 480 x 1.732) ?? 36 Amps ?

Sounds like a homework question.
Your math is correct for three phase, I=kVA/(Vx√3)

 

[winnie]

Revisiting the original question about the current and kVA rating of a wye bank of 10kVA transformers.

The given is that the transformers are single phase, rated 10kVA at 240V. This means that the rated current of the transformer is 41.7 A.

If you took this same transformer and supplied it with 120V on the primary, the rated current would still be limited to 41.7A. This means that when used at 120V, this same transformer only has a capacity of 5kVA.

If three of these transformers were arranged in a wye bank and connected to a 208V three phase supply, then 120V would be placed across each transformer. The rated current would remain 41.7A, and the capacity of this three phase bank would be 15kVA. In general you can use a transformer below its rated voltage, but you get reduced kVA capacity.

If instead you used a 480V three phase supply, then 277V would be placed across each transformer (remember rated at 240V). This means that the transformer would be subject to excessive voltage, would saturate, and would have increased magnetizing current and heating, even at no-load. If, because of transformer design (headroom for saturation) the magnetizing current was not excessive, then you would still expect a current rating of 41.7A. Thus the kVA of this three phase bank would be anything from 0 (excessive heating because of excessive voltage, cannot use) up to 34.5kVA (full rated current at the applied voltage.) The transformers are being used outside of their design specification and their resulting capacity is indeterminate.

-Jon

 

[Joethemechanic]

1:1 turns ratio, with 240Vac applied at primary terminal;

rated 10kVA at 240V

I'm not seeing that in the original post. It only mentions "240 applied", nothing about a rated voltage

 

[winnie]

I'm not seeing that in the original post. It only mentions "240 applied", nothing about a rated voltage

I agree. I assumed that the rating was 10kVA at 240V based on the "240Vac applied" statement.

If we don't know the rated voltage, than the 10kVA number is meaningless.

-Jon

 

[tennsims17]

I know this is an old thread, but it is closely related to my question.
Im installing EV chargers for the state, the utility pole that i am i stalling the panle and meterbox on currently has a 60A 3 phase disconnect for a pump/lift station.
It is being fed by 3-10kva pole mounted transformers 208/120.
Ive called the utility company but cant seem to get any help.
Will those 3-10kva transformers handle a 200a panel that would have 2-50a double poles for the EV chargers and 1-60A 3 pole breaker for the pump.
When i did my calculations i was told that the utiity company plays by different rules and doesnt use NEC like the rest if us. Haha

 

[topgone]

I am assuming the 2 x 50A EV charger loads are supplied at 120V and the chargers draw 40A plus a three-phase load amps of 48A, hence a 60A breaker. My worksheet here tells me your load is ~ 34 kVA. Max line amps of your loading configuration is 123A. 30 kVA transformer bank (FLC= 83 A) is not enough if all your loads are ON at the same time.
However, transformers can be loaded 115% of rated kVA continuously without loss of life when the ambient daily temperature is 59 degrees F (15 degrees C). Or, transformers can be loaded 117% of rated capacity for 4 hours when the initial load was 70% of rated.

 

[augie47]

"When i did my calculations i was told that the utiity company plays by different rules and doesnt use NEC like the rest if us. Haha"

When dealing with the power company that is pretty much the rule. They don't play by the same rule book as we do.
It's not unusual to find multiple 200 amp panels on a 25 kva transformer. They allow there transformers to handle "overloads" where you would not do so on your customer owned transformer as they take into consideration many factors such a duration of the load, diversity, and ambient conditions,.
Best you can do is advise them of the anticipated load and let them determine if they need to upgrade,

 

[tennsims17]

I am assuming the 2 x 50A EV charger loads are supplied at 120V and the chargers draw 40A plus a three-phase load amps of 48A, hence a 60A breaker. My worksheet here tells me your load is ~ 34 kVA. Max line amps of your loading configuration is 123A. 30 kVA transformer bank (FLC= 83 A) is not enough if all your loads are ON at the same time.
However, transformers can be loaded 115% of rated kVA continuously without loss of life when the ambient daily temperature is 59 degrees F (15 degrees C). Or, transformers can be loaded 117% of rated capacity for 4 hours when the initial load was 70% of rated.

Thanks for the reply,
So the chargers "state" 47amps, they are also 208v. And although, im sure they will not be on all at the same time, im not sure how busy this park is during holidays. And the 60a pump has not run in about a year. But it does seem like it will be very close, and definitely wont be able to add anything in the future, should they want to or decide to, without uograding the poco trans.