Spin Up test question, open-delta Transformer

Below is the question asked, I looked up in Wildi but his answer of 86% wasn't an available option, so i looked through another reference (PPI Camera) and found what it was looking for. Am I reading Wildi's wrong , or why is it different? If this was PE and 86.6% was offered, I would have chosen it and moved on. 



This is Wildi's Screenshot: 



 And PPI Camera:

@SparkyBill What does the previous page of Wildi's book say?

I don't see a derivation in either of those pictures, but the available power in an open delta connection is 1/√3 = 57.7%.

I found this in some googling. Maybe there's a difference between an open delta transformer vs two transformers connected open delta.

The question asks for an open delta transformer, but the Wildi example is two transformers connected open delta.

Chattaneer PE said:

@SparkyBill What does the previous page of Wildi's book say?

I don't see a derivation in either of those pictures, but the available power in an open delta connection is 1/√3 = 57.7%.

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Yeah I found the answer in PPI, but am I understanding Wildi wrong or is it an error? He is calling his 87% "load capacity", don't know if its a play on words or if i lack the understanding of spin up asking for "total power available" 

Really going to suck if this happens on PE. I pray that they are a little more concrete than some of these spin up questions.

Chattaneer PE said:

I found this in some googling. Maybe there's a difference between an open delta transformer vs two transformers connected open delta.

The question asks for an open delta transformer, but the Wildi example is two transformers connected open delta.

View attachment 15907

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Yup, I think that's it too. I couldn't find anything else in Wildi about open delta from index and flipping. 

I really hated PPI for FE, but man that camera book is VERY GOOD if you're lost. That index is amazing. I am using it a lot when I "don't know" questions more than Zach's course binder and my Engineering pro guiides binder even tho it has an index. 

Well I learned something new today. I guess a transformer with only 2 windings, or a three winding transformer that has lost a phase has an available power of 57%. However, when two identical transformers are used for open delta, the available power is 87%.

Chattaneer PE said:

Maybe there's a difference between an open delta transformer vs two transformers connected open delta.

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That's the key here! I opened my references and had made that note so I would know how to distinguish between the 2 on the exam. Zach has a really good article on his website about open delta xfmrs. I don't remember if he discusses the 2 xfmr in open delta configuration but it covers a 3 ph delta and one of the phases have failed.

https://www.electricalpereview.com/open-delta-transformer-connection/

Wildi is right but you are thinking about this wrong. This is an open delta with two transformers each at 50kVA. You added those up to get a total of 100kVA and after seeing the answer of 86.6kVA assumed the total output should be 86.6%. The 58% number which would be the correct answer for your question comes from what would be the percentage of total power if you used three equivalent transformers in a delta. In Wildi's example if you had 3 equivalent 50kVA transformers in Delta, you'd have a total of 150kVA. 58% of 150kVA is 87kVA.

bdpalmer said:

In Wildi's example if you had 3 equivalent 50kVA transformers in Delta, you'd have a total of 150kVA. 58% of 150kVA is 87kVA.

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If there are 3 single phase transformers, they cannot be connected open delta. If there were 3 single phase transformers originally connected, and 1 failed, then 86.6% would be appropriate.

If there was one 3 phase transformer that lost a winding, then 58% would be used.

daydreambeliever said:

That's the key here! I opened my references and had made that note so I would know how to distinguish between the 2 on the exam. Zach has a really good article on his website about open delta xfmrs. I don't remember if he discusses the 2 xfmr in open delta configuration but it covers a 3 ph delta and one of the phases have failed.

https://www.electricalpereview.com/open-delta-transformer-connection/

Click to expand...

This might should be updated to include two transformers connect as open delta, instead of just 1 open delta transformer.

Chattaneer PE said:

This might should be updated to include two transformers connect as open delta, instead of just 1 open delta transformer.

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iirc he goes over that in the class. i don't have my notes handy to check.

LyceeFruit PE said:

iirc he goes over that in the class. i don't have my notes handy to check.

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Very good point and all very good points on this post.  Be prepared for either kind of open-delta question (1 open delta transformer or two transformers connected in delta).