Dual primary transformer impedance

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skeshesh

Senior Member
Location
Los Angeles, Ca
Good day everyone.

I'm working on a remodel of an existing building (several floors getting torn out and rebuilt). We're going to have a bunch of new equip in there and the question related to the KAIC rating of this equip. Distribution class will switch over from 4.8kV to 12.5kV in the future and I'm unsure of the effect on the system.

Existing dual primary transformer is rated at 12.47/4.8kV. In my understanding from the nameplate the %Z (6.1@rated kVA) does not change with the switchover. I'm trying to understand this concept better. I thought the %Z is determined by the mfg. by shorting the secondary and applying test voltage until rated current is circulating and then taking the ration of that voltage with the rated. If I'm correct about the procedure, then how do we have one %Z for the dual primary transformer? Am I correct in believing that the available short circuit downstream of this transformer will not change if the primary feed is changed to 12.5kV?
 

bob

Senior Member
Location
Alabama
Good day everyone.

I'm working on a remodel of an existing building (several floors getting torn out and rebuilt). We're going to have a bunch of new equip in there and the question related to the KAIC rating of this equip. Distribution class will switch over from 4.8kV to 12.5kV in the future and I'm unsure of the effect on the system.

Existing dual primary transformer is rated at 12.47/4.8kV. In my understanding from the nameplate the %Z (6.1@rated kVA) does not change with the switchover. I'm trying to understand this concept better. I thought the %Z is determined by the mfg. by shorting the secondary and applying test voltage until rated current is circulating and then taking the ration of that voltage with the rated. If I'm correct about the procedure, then how do we have one %Z for the dual primary transformer? Am I correct in believing that the available short circuit downstream of this transformer will not change if the primary feed is changed to 12.5kV?

Your understand of %Z is correct.
AS I understand your question, you have one transformer that is presently connected to the utility 4.8 kv system. For some reason you are going to change the service to 12.5 kv using the same transformer.
1. What will the secondary voltage be? I assume that it will be the same.
2. What is the rating of the transformer at 4.8 kv?
3. Does it have a different rating at 12.5 kv on the name plate?
4. If the KVA rating is the same, You may have a different KAIC
if the available fault KVA at 12.5 kv is higher that the available fault KVA
at 4.8 kv.
If you use the infinite fault method, the fault current would depend on questtion 2 and 3.
 

BJ Conner

Senior Member
Location
97006
Darn
Just when you think you know it all, along comes a distribution transformer with dual windings.
Who made it?
The primary is voltage system is being changed from 4.8 KV to 12.5 ?
whats the secondary?
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Your understand of %Z is correct.
AS I understand your question, you have one transformer that is presently connected to the utility 4.8 kv system. For some reason you are going to change the service to 12.5 kv using the same transformer.
1. What will the secondary voltage be? I assume that it will be the same.
2. What is the rating of the transformer at 4.8 kv?
3. Does it have a different rating at 12.5 kv on the name plate?
4. If the KVA rating is the same, You may have a different KAIC
if the available fault KVA at 12.5 kv is higher that the available fault KVA
at 4.8 kv.
If you use the infinite fault method, the fault current would depend on questtion 2 and 3.

1. The secondary voltage (in this case) will be 120V/208Y and will remain unchanged. There's another transformer of similar size with 277V/480Y secondary but let's focus on one for ease of discussion.
2 & 3. The rating of the transformer is at 750kVA for both primary ratings.
4. This is where I disagree because if the %Z is the same, then the available short circuit value at the secondary will be the same regardless of primary, Hence the confusion.

There's apparently something called a "dual voltage switch" installed on it. The nameplate also mentioned a tap changer but I think that's just to adjust a few taps and doesnt have to do with the dual rating. Imagine of nameplate is attached for Ref.

primarytransformer.jpg
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Darn
Just when you think you know it all, along comes a distribution transformer with dual windings.
Who made it?
The primary is voltage system is being changed from 4.8 KV to 12.5 ?
whats the secondary?

Haha. That's what keeps it interesting though right? The primary is being changed over from 4.8kV to 12.5(12.47)kV and the secondary in this case is 120V/208Y although there are other transformers around.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Update: Just spoke with the Mfg of transformer. The person said the nameplate rating indicated the %Z calculated based on the higher primary voltage (12.5kV in this case) so basically if I calculate the available fault on the secondary based on that I should be good for the worst case possible when they change the feed. I'm still slighty unsure though, cause this pretty much means if you have a dual primary transformer your equipment will be sized for the maximum possible fault even if the tranformer is feed the lower primary voltage - now i'm not saying this is not the wiser approach but using the lower rating (if indeed fed from lower voltage source) could save a bunch of money and it doesnt make the system less safe, just less flexible.
 

BJ Conner

Senior Member
Location
97006
Unless this transformer has 6 primary bushings I wouldn't call it a dual primary winding.
Dual secondarys I know. Transformers with 18.8 and 4.16 KV secondaries I've worked with. Both secondaries were energized at the same time.
I have worked around MGM transformers and they are good. They can build anything for mere money.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Unless this transformer has 6 primary bushings I wouldn't call it a dual primary winding.
Dual secondarys I know. Transformers with 18.8 and 4.16 KV secondaries I've worked with. Both secondaries were energized at the same time.
I have worked around MGM transformers and they are good. They can build anything for mere money.

Don't think I ever claimed it's called "dual primary winding". You may call it what you want, call it a chair or a mountain if you like, fact is it takes two significantly different voltage class inputs. I need to research further though - I'm not really sure how it exactly works and what operating the dual voltage switch accomplishes.
 

nollij

Member
Location
Washington
The XFMR secondary fault current would increase as the per unit impedance on the primary side of the XFMR would be lower with the increased distribution votlage.
 

nollij

Member
Location
Washington
How do you figure that the %Z will be lower if the primary side uses the higher distribution voltage?

If the distribution was at the 5kV voltage before and is now at 15kV, the per unit impedance of the distribution system up to the transformer would be lower (not the transformer impedance). Thus, the short circuit current on the transformer secondary system would be higher.
 

BJ Conner

Senior Member
Location
97006
Don't think I ever claimed it's called "dual primary winding". You may call it what you want, call it a chair or a mountain if you like, fact is it takes two significantly different voltage class inputs. I need to research further though - I'm not really sure how it exactly works and what operating the dual voltage switch accomplishes.

Still looks like from the name plate is only got one set of HV bushings. If your going to ask for information on a form with lots of helpful people you might as well call it a chair, a mole hill or big gray thing that hums.
It will help you get a better answer faster.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
If the distribution was at the 5kV voltage before and is now at 15kV, the per unit impedance of the distribution system up to the transformer would be lower (not the transformer impedance). Thus, the short circuit current on the transformer secondary system would be higher.

My mistake I misunderstood your statement that makes sense.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Still looks like from the name plate is only got one set of HV bushings. If your going to ask for information on a form with lots of helpful people you might as well call it a chair, a mole hill or big gray thing that hums.
It will help you get a better answer faster.

Didn't mean any disrespect, your reponse is appreciated. What I meant is that although proper verbage is pretty important in communicating technical matters, I'm not too hung up on nomenclature for this case, just more interested on how it physically works.
 
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