Open Delta

[72.5kv]

A small store had a single phase 50KVA 120/240 pole mount tranformer feeding its service. The city installed an irrigation pump that is 240 3 phase. To supply the pump, the utility changed the single phase service to open delta. It is now a 75KVA with a 25KVA on the open side. How is the total output of the bank computed?

 

[bob]

A small store had a single phase 50KVA 120/240 pole mount tranformer feeding its service. The city installed an irrigation pump that is 240 3 phase. To supply the pump, the utility changed the single phase service to open delta. It is now a 75KVA with a 25KVA on the open side. How is the total output of the bank computed?

The 75 kva will carry 100% of the single phase load and each transformer will carry 57% of the 3 phase load.

 

[LarryFine]

. . . each transformer will carry 57% of the 3 phase load.

Isn't that 114%?

 

[eric stromberg]

Isn't that 114%?

Larry,
You ever do a project with someone where you do half the work, the other person does half the work, but you spend 14% of the time fighting with each other and getting no work done? :grin: :grin:

 

[LarryFine]

Larry,
You ever do a project with someone where you do half the work, the other person does half the work, but you spend 14% of the time fighting with each other and getting no work done? :grin: :grin:

Well, yes and no. It's usually: the other guy does 14% of the work, I do the rest, and we argue half of the time.

 

[JohnJ0906]

Larry,
You ever do a project with someone where you do half the work, the other person does half the work, but you spend 14% of the time fighting with each other and getting no work done? :grin: :grin:

Only 14% ? :roll:

Usually much more!

 

[bob]

Isn't that 114%?

Larry, let me reword my remarks. It was late and I had been celebrating Auburn's route of mighty Arkansas 9 to 7. I'll use the OP's information.
If you have a 3 - 25 kva transformers, the capacity is 75 kva. If one of the transformers is removed, as with an open delta, the capacity is 57.7% of
the 3 transformer capacity. So, 0.577 x 75 kva = 43.4 kva. Each of the open
delta transformers can only be loaded 21.7 kva. In this case you have a 75 kva and a 25 kva. The 25 kva limits the 3 phase load to 21.7 kva. The single phase load will be carried by the 75 kva.
If you assume that the 3 phase primary current is 1 amp on the 3 transformer bank, when you remove the single transformer from the bank, the sqrt of 3 no longer applies. So you must derate the capacity by 1/1.73 = 0.577.

 

[72.5kv]

thanks Bob for the insight. The open delta configuration is something I was in the fog about.