Transformer Primary Protection Requirement

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imisspla07

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We have a design firm designing our incoming power with parallel 115kV / 13.8kV transformers. The transformers don't have primary protection but come together at a bus with primary protection upstream. I thought each transformer needed its OWN protection?
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Is the downstream load more than one transformer (redundant transformers) worth?

The upstream protection needs to protect the withstand of the conductors and the primary protection (where ever it is) needs to be compliant with table 450.3(A)
 
If your service point is ahead of the transformers, you'll have to comply with 450.3 requires primary protection for each transformer or bank of single-phase transformers operating as a unit.
 
Smart $ said:
If your service point is ahead of the transformers, you'll have to comply with 450.3 requires primary protection for each transformer or bank of single-phase transformers operating as a unit.
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I don't see it that way. I see where each transformer must have primary protection, but I see nothing that prevents one OCPD protecting two transformers.
Looking at Table 450.3(A) I would think the install is Code complaint if its a supervised location.
 
Smart $ said:
If your service point is ahead of the transformers, you'll have to comply with 450.3 requires primary protection for each transformer or bank of single-phase transformers operating as a unit.
Click to expand...
That said, if there's a "bypass" switch ahead of the transformers so that only one transformer can be energized at a time, then you can get by with one OCPD.

However, unless there is also a "bypass" switch on the secondary side, neither transformer will be truly de-energized, such as locking out for maintenance and such.
 
augie47 said:
I don't see it that way. I see where each transformer must have primary protection, but I see nothing that prevents one OCPD protecting two transformers.
Looking at Table 450.3(A) I would think the install is Code complaint if its a supervised location.
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Ahh yes... but Table 450.3(A) uses terms Transformer, Primary, and Secondary... all singular form. It is left a little open in the general statement because of instances where multiple 1? xfmr's comprise a bank acting as one.
 
Smart $ said:
That said, if there's a "bypass" switch ahead of the transformers so that only one transformer can be energized at a time, then you can get by with one OCPD.

However, unless there is also a "bypass" switch on the secondary side, neither transformer will be truly de-energized, such as locking out for maintenance and such.
Click to expand...

Assuming you are designing per the NEC - Where do these requirements come from?

If you are designing per the IEEE - what are your references?

ice
 
imisspla07 said:
We have a design firm designing our incoming power with parallel 115kV / 13.8kV transformers. The transformers don't have primary protection but come together at a bus with primary protection upstream. I thought each transformer needed its OWN protection?
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assumption: The meter is shown on the xfm primary. Service is 115KV. MET is primary metering.

I have only worked a few MV/HV services. However, this one is typical. Ron cited the applicable code section.

ron said:
...The upstream protection needs to protect the withstand of the conductors and the primary protection (where ever it is) needs to be compliant with table 450.3(A)
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Everything past that is design preference.

This design has:
CB on xfm primaries, should be set (as ron said) per 450.3(A). xfm FLA at 14MW is ~70A. Primary protection will likely be between 200A and 300A. It has to be sufficient to meet the xfm inrush.

Each secondary has OCP. likely settings are 1400A.

Looks good to me.Only thing I have usually seen different is the secondary tie. It is common in industry, where there are redundant transformers, each transformer will feed a separate switchgear.

Still, nothing wrong with talking to the design group and discussing the design phillosophy.

ice
 
Smart $ said:
Ahh yes... but Table 450.3(A) uses terms Transformer, Primary, and Secondary... all singular form. It is left a little open in the general statement because of instances where multiple 1? xfmr's comprise a bank acting as one.
Click to expand...
I agree.. but, to me, it says (#1) transformer will have OCPD. Transformer #2 will have OCPD. It never states the OCPD devices will be individual for each transformer.
It is commonplace around here for one feeder to supply more than one transformer.
Industrial folks do it all the time with the 600 V or less. It has been my understanding that the 250% alloablr primnary (450.3(B)) is there for that reason.
 
augie47 said:
I agree.. but, to me, it says (#1) transformer will have OCPD. Transformer #2 will have OCPD. It never states the OCPD devices will be individual for each transformer.
It is commonplace around here for one feeder to supply more than one transformer.
Industrial folks do it all the time with the 600 V or less. It has been my understanding that the 250% alloablr primnary (450.3(B)) is there for that reason.
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Not sure what you mean by #1 and #2 in your first sentence.

I agree one feeder can supply more than one transformer. But then you either have feeder taps somewhere, or you have to run full size conductors to the transformers. On the latter, as long as the primary OCPD is sized to protect the smallest primary, they can have at it. :p
 
Smart $ said:
If your service point is ahead of the transformers, you'll have to comply with 450.3 requires primary protection for each transformer or bank of single-phase transformers operating as a unit.
Click to expand...

Smart $ said:
I agree one feeder can supply more than one transformer. But then you either have feeder taps somewhere, or you have to run full size conductors to the transformers. On the latter, as long as the primary OCPD is sized to protect the smallest primary, they can have at it. :p
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Sounds like you changed horses... :)
I agree with then 2nd quote.
 
augie47 said:
Sounds like you changed horses... :)
I agree with then 2nd quote.
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Nope... still same horse. Read last sentence of second quote.... that's primary protection for each transformer. Can't size OCPD to total primary, just smallest.
 
Smart $ said:
...I agree one feeder can supply more than one transformer. But then you either have feeder taps somewhere, or you have to run full size conductors to the transformers.
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just curious. For the OP's case. Have you ever seen it any other way (than full size conductors from the OCP to the xfm primaries)?

ice
 
iceworm said:
just curious. For the OP's case. Have you ever seen it any other way (than full size conductors from the OCP to the xfm primaries)?

ice
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To be honest, I don't believe I've ever seen a 115kV primary up close and personal. Usually when I see any lines at that voltage it's off the secondary.
 
Is it "over current protection" or "short circuit protection?"
The primary device I think would be intended to take the transformers of line should either of the transformers fail. As such the only negative would be is the lack of coordination, that is if one pof the transformers fails it affects the other.
Overload protection is provided by the secondary OCPD located on the secondary of both transformers.
 
Hopefully the one-line in the OP is not "the" one-line because it is missing a crap load of information, and a 115kV breaker is not going to be a draw out breaker.

It would also be very odd that a design firm would be doing the 115kV and transformers that would not be well versed in the utility interconnection requirements. I would assume there have been many meetings between the two to determine those requirements since you don't just plunk in a 115kV feeder.

Due to the small size of the 115kV conductor, your probably running the minimum size, which would be the same for both and plenty of current carrying capability to handle the total.
 
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