# Poco transformers



## Big John (May 23, 2010)

Might be a totally different setup.

Any single line tap is going to be a single-phase secondary.

But they can put a style of 3 phase transformer on two primaries and run a 3 phase business.

But it could just be a single phase transformers connected across the line. That would reduce voltage drop for the load.

-John


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## niteshift (Nov 21, 2007)

Big John said:


> Might be a totally different setup.
> 
> Any single line tap is going to be a single-phase secondary.
> 
> ...


I'd agree with this .


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## OKelectric (Mar 6, 2012)

Julius793 said:


> Around my area all the poco transformers tap one of the primaries to power them. However I was just I new jersey and I noticed the transformers tap two of the primaries, can anyone explain why this is and what are the advantages? Thanx


Single phase "Y" connected just connect to 1 hot phase. 

Single phase "Delta" connected must be connected to 2 phases.

Delta systems have NO "System Neutral", so xfrmers MUST be connected to 2 or more "phases".


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## don_resqcapt19 (Jul 18, 2010)

If utilities were required to connect all of their transformers line to line, on source of "stray voltage" would go away. That source would be the voltage drop on the primary grounded conductor. With all of the transformers connected line to line there would be no voltage drop on the primary grounded conductor to create the stray voltage.


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## don_resqcapt19 (Jul 18, 2010)

OKelectric said:


> Single phase "Y" connected just connect to 1 hot phase.
> 
> Single phase "Delta" connected must be connected to 2 phases.
> 
> Delta systems have NO "System Neutral", so xfrmers MUST be connected to 2 or more "phases".


You can get single phase from a wye system with the primary of the transformer connected line to line or line to neutral. It is a design choice of the utility and often line to neutral is used because a single bushing transformer is cheaper.


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## guest (Feb 21, 2009)

don_resqcapt19 said:


> You can get single phase from a wye system with the primary of the transformer connected line to line or line to neutral. It is a design choice of the utility and often line to neutral is used because a single bushing transformer is cheaper.


Correct. And one reason single-bushing transformers are cheaper: "Graded insulation."

Graded insulation is where the dielectric and impulse voltage withstand values are adjusted (or graded) as the winding gets farther away from the hot side and closer to the grounded side. This can, especially with very large transformers, result in considerable cost and weight savings. Not a huge factor for pole-mounts though. 

Another fun fact: many POCOs operate their pole-mount transformers at 300% or MORE of rated capacity. They rely on the oil cooling, loss-of-life adjustment factors and load diversity to minimize burnouts or major failures. In residential areas load diversity usually allows the transformer to operate for periods of high overload then cool down before the next heavy load. A heat wave with everyone's A/C cranking (or in cold climates with electric heat, a cold snap with heaters running continuously) will result in the weaker transformers failing, often violently.


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## randas (Dec 14, 2008)

mxslick said:


> Another fun fact: many POCOs operate their pole-mount transformers at 300% or MORE of rated capacity.


So true, I share a 15KVA with roughly 8 of my neighbors.. And I don't live in the ghetto....


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## OKelectric (Mar 6, 2012)

Yeah! I always loved going back into an easement @ 6M Middle of July 100+ degree day,to change-out an old cast iron pot puking hot oil all over the place. (this was before easement dollies)

Of course it would be an old crooked leaning pole with bare open wire secondary, 8-9 services all kinds of telco and catv drops on it.

Always send in the grunt or apprentice first to fight off the yard jumping dogs...:laughing:


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## guest (Feb 21, 2009)

OKelectric said:


> Yeah! I always loved going back into an easement @ 6M Middle of July 100+ degree day,to change-out an old cast iron pot puking hot oil all over the place. (this was before easement dollies)
> 
> Of course it would be an old crooked leaning pole with bare open wire secondary, 8-9 services all kinds of telco and catv drops on it.
> 
> Always send in the grunt or apprentice first to fight off the yard jumping dogs...:laughing:


We still have a few of the old square Westinghouse pots humming along in my area...on the 1930's era- 4kv primary circuit. 

The "modern" 16kv circuit in my 'hood, which has failed over 12 times in the last seven years just had another breakdown...a crossarm caught fire when one of the LAPP insulators tracked badly. It took the local POCO over 14 HOURS to replace it. 

I have pics of the arm before the repair, am going back to get the after pics, and the crew gave me the insulator that started it all. I will get a new thread up soon with the carnage.


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## noarcflash (Sep 14, 2011)

mxslick said:


> Another fun fact: many POCOs operate their pole-mount transformers at 300% or MORE of rated capacity. .


 
I think because of efficiency. There are more internal loses when they run under their rated capacity.


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## guest (Feb 21, 2009)

noarcflash said:


> I think because of efficiency. There are more internal loses when they run under their rated capacity.


Ummmmm.......maybe but I doubt it. POCO transformers are designed for very low no-load losses. 

I think it is mainly due to the cost of a transformer in all facets, purchase, installation, no-load losses and maintenance. They try to get the maximum bang for their buck, which often results in a literal "bang."


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## user4818 (Jan 15, 2009)

It's also cheaper for a poco to run one HV wire and one MGN rather than two HW wires (less insulators, cutouts, cross arms, etc.)


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## BBQ (Nov 16, 2010)

mxslick said:


> They try to get the maximum bang for their buck, which often results in a literal "bang."


Often?

Realy, it seems like somebody is getting dramatic. :laughing:

Considering that the power companies are providing power to most every structure in the US I think there are few bangs.


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## guest (Feb 21, 2009)

BBQ said:


> Often?
> 
> Realy, it seems like somebody is getting dramatic. :laughing:
> 
> Considering that the power companies are providing power to most every structure in the US I think there are few bangs.


Depends on the area you're in....around here we have a lot of breakdowns and blown pots especially on the "new" 16kv circuits.

But yes, considering the overall scope and number of customers served/transformers in use, the percentage is most likely very small.


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