Wye or delta primary

[ptonsparky]

Discussion with a POCO area lineman about his primary connections. Breathe easy, I made no suggestions for his side. They were there to add a transformer to the customers Open Delta service with a hi leg, making it a closed Delta. Added motor load.

He was trying to explain why they should use a Wye primary or connect them Delta primary. He was talking it thru more for his benefit than mine and I was on the customers dime.

Why would one matter more than the other, or does it?

 

[electrofelon]

Discussion with a POCO area lineman about his primary connections. Breathe easy, I made no suggestions for his side. They were there to add a transformer to the customers Open Delta service with a hi leg, making it a closed Delta. Added motor load.

He was trying to explain why they should use a Wye primary or connect them Delta primary. He was talking it thru more for his benefit than mine and I was on the customers dime.

Why would one matter more than the other, or does it?

I am interested in this topic, but by no means an expert. I believe one consideration in a delta vs wye primary, particularly in 25KV class and above, is ferroresonance. Then there is this article "the whys of the wyes" which has been referenced on the forum several times:

http://apps.geindustrial.com/publibrary/checkout/GET-3388B?TNR=White Papers|GET-3388B|generic

And most simply, it depends on the distribution system. I dont notice many, if any, delta connected transformers connected to wye primaries.

 

[Smart $]

...
Why would one matter more than the other, or does it?

The connection uses the same transformer. :blink::slaphead:

 

[ptonsparky]

His attempted explanation was more on what happens when one of the primary fuses blow.

The referenced resource Electrofelon gave was written by someone with a sense of humor. He describes a process as "painfull" even for the experienced. The Right side of my brain was working overtime at this first scanning.:dunce:

 

[kwired]

If they are single bushing transformers (which majority of them are) I don't see they have a choice, they need to connect line to neutral.

 

[ptonsparky]

If they are single bushing transformers (which majority of them are) I don't see they have a choice, they need to connect line to neutral.

They have the choice and it was up to him and his partner to make the decision based on the need for the single phase to stay On should the added transformer fail. My understanding.

FWIW, it has only been in the last few years that the third primary phase has been added.

 

[rian0201]

I agree with wired, it depends on the voltage rating of the bushing. If they are using single bushing, no choice, wye, but if double they have the option to connect it in delta.

If in delta, then wye grounded sec, it is good, due to the connection of its zero sequence impedance, the load with be un affected if a fault occurs on the high side.


Sent from Mars

 

[mivey]

The connection uses the same transformer. :blink::slaphead:

They use different transformers. Take 7.2/12.47 kV to 480 V delta. With wye primary the transformer ratio is 7200:480. With delta primary it is 12470:480.

 

[Smart $]

They use different transformers. Take 7.2/12.47 kV to 480 V delta. With wye primary the transformer ratio is 7200:480. With delta primary it is 12470:480.

I know they use different transformers. The question is why one connection vs. the other would matter. For instance, if you have a 7200V PRI pot, are you going to connect it wye or delta? Rhetorical question... just pointing out that it matters.

 

[electrofelon]

They use different transformers. Take 7.2/12.47 kV to 480 V delta. With wye primary the transformer ratio is 7200:480. With delta primary it is 12470:480.

Right, i was going to say the same thing. Assuming the transformers are there on the truck, they have to be connected one way - even if two bushing. Now if they have both transformer winding ratio and proper bushing configuration in the yard or are ordering them, then they can connect primary either way they choose theoretically. I don't recall ever seeing a delta primary bank connected to a wye line, but maybe it's done....

 

[mbrooke]

It matters like this mostly: delta primary or ungrounded wye primary= ferroresonance


Grounded wye primary detla secondary= fuse blowing on faults out on the line.

For cost, ease and reducing ferroresonance (as a risk) grounded wye primary grounded wye secondary is customary for POCOs.


Of note, 3 phase switching and protection eliminates the ferroresonance risk.

 

[rian0201]

Ferroresonance in delta primary? Maybe it depends on the secondary connection. Is there ferroresonance in delta primary, wye grounded sec?


Sent from Mars

 

[ptonsparky]

A bit more info.

When I talked to the lineman, the three pots were on the pole and operational. Whatever he did, or intended to do, was with what he had on hand.

240 secondary.

 

[Smart $]

A bit more info.

When I talked to the lineman, the three pots were on the pole and operational. Whatever he did, or intended to do, was with what he had on hand.

240 secondary.

And if he only had one, or one type of transformer, it will determine which way he is going to hook it up... and why I said what I said earlier.

If how it has to be hooked up is not the right way to hook it up for that situation, back to the yard he goes...

 

[mbrooke]

Ferroresonance in delta primary? Maybe it depends on the secondary connection. Is there ferroresonance in delta primary, wye grounded sec?


Sent from Mars

Unfortunately there is. Any ungrounded primary can ferroress regardless of the secondary.