Reversing transformers

[kwired]

Okay. Let's start with this one. Just why would a Utility grounded wye behave differently than a customer owned grounded wye?

I have never been a believer in the electrons behaving differently depending on which side of the service disconnect they were on.

ice

I don't claim to be an expert on all aspects of transformers, but the utility transformers that are wye connected primary with the system neutral connected to the center of the wye are three individual core transformers and the dry type we commonly see indoors are common core with all windings on the same core. I don't know why but I think that may have something to do with why POCO doesn't have any trouble with their wye primary transformers. I think all of their padmounts are delta primary and a single core.

 

[jim dungar]

Okay. Let's start with this one. Just why would a Utility grounded wye behave differently than a customer owned grounded wye?

I never said there was a difference between a utility owned transformer and one owned by the customer.

I said that a Medium Voltage wye system behaves differently than 480Y and 208Y ones. You need to read about things like multi-grounded neutrals and wye-wye transformer connections.

Off the top of my head I cannot remember ever analysing a grounded wye -> delta utility connection.

 

[hurk27]

OK, let me rephrase that. Suitable for reverse connection. That is what is stated by the mfg. in transformers that I have installed for this type application. Acme transformer co.

isn't that where the coyote gets his from to shock the roadrunner

 

[kwired]

I never said there was a difference between a utility owned transformer and one owned by the customer.

I said that a Medium Voltage wye system behaves differently than 480Y and 208Y ones. You need to read about things like multi-grounded neutrals and wye-wye transformer connections.

Off the top of my head I cannot remember ever analysing a grounded wye -> delta utility connection.

What from an electrical theory point of view is different. The only difference is insulation methods for the voltage involved and number of turns and conductor sizes which also vary on low voltage transformers depening on kVA rating. Otherwise you put X volts in and you get what ever the ratio of turns = Y volts out. A wye circuit is a wye circuit and a delta circuit is a delta circuit regardless of voltage.

 

[LarryFine]

I don't claim to be an expert on all aspects of transformers, but the utility transformers that are wye connected primary with the system neutral connected to the center of the wye are three individual core transformers and the dry type we commonly see indoors are common core with all windings on the same core. I don't know why but I think that may have something to do with why POCO doesn't have any trouble with their wye primary transformers.

I concur.

 

[jim dungar]

A wye circuit is a wye circuit and a delta circuit is a delta circuit regardless of voltage.

But grounded wye to grounded wye, grounded wye to delta, wye to delta, delta to wye, delta to grounded wye, and delta to delta are all different transformer connections, each with subtle quirks surrounding unbalanced loading .

 

[jim dungar]

I don't know why but I think that may have something to do with why POCO doesn't have any trouble with their wye primary transformers.

This is definitely one reason.

 

[kwired]

But grounded wye to grounded wye, grounded wye to delta, wye to delta, delta to wye, delta to grounded wye, and delta to delta are all different transformer connections, each with subtle quirks surrounding unbalanced loading .

But those quirks do not change just because the supply is 12.5Kv vs 480v

The configuration of wye to delta or wye to wye, etc would have to change before the quirks will change.

 

[iceworm]

but the utility transformers that are wye connected primary with the system neutral connected to the center of the wye are three individual core transformers and the dry type we commonly see indoors are common core with all windings on the same core. .

This is definitely one reason.

I don't think so. Not because I know nor because I am a transformer whiz (cause I don't and I'm not).

Plenty of POCO Y-Y tansformers ar single shell padmount. I don't know the internal construction but since they are single shell, I highly suspect they are a common core.

I never said there was a difference between a utility owned transformer and one owned by the customer.

Glad to hear that. Otherwise I would have a laugh, cease torturing bits, and file this conversation as less than usless.

Off the top of my head I cannot remember ever analysing a grounded wye -> delta utility connection.

I never have either. I can't think of a reason why I would. POCO uses y-y. Industry uses D-Y, D-D (ocassionally)

This bears exactly on my point:

I read a lot of the previous posts on this subject and there a lot of posts that agree the primary wye point should not be connected to the system neutral (or ground) - nobody says why not.

Paraphrasing Larry, most of the posters are saying the same thing. However, if any understand the physics or science they are keeping it a secret - not the sicence or physics, just that they understand

So, paraphrasing: The utility does not have difficulty with Wye primaries because the transformers they use are not Y-D, but are Y-Y. That was my guess, but I have not done an analysis on a Y-Y.

So, on to the next question.

ice

 

[iceworm]

What from an electrical theory point of view is different.

you put X volts in and you get what ever the ratio of turns = Y volts out. A wye circuit is a wye circuit and a delta circuit is a delta circuit regardless of voltage.

Good way to put it

But grounded wye to grounded wye, grounded wye to delta, wye to delta, delta to wye, delta to grounded wye, and delta to delta are all different transformer connections, each with subtle quirks surrounding unbalanced loading .

Okay, let's examine some of the subtlities. Once we understand them, they won't be quirks.

look(ing) at the currents in a 480 Delta primary/208 Wye secondary connection. If the loads on the 208 side are a combination of 3ph, 1ph 208, 1ph 120, there will be current on the transformer neutral, equal to the vector sum of the phase currents. And, this unbalanced current will be reflected in the Delta primary current. I guessing all know this and this unbalance is considered okay.

Now let's turn the transformer around and supply power to the Wye side and put the loads on the Delta side. We will set the loads such that the currents are the same as the previous example. Are the Wye side primary currents going to be the same as in the previous example? I think they are not (but I don't know that - so I won't get too adamant about why that is)

Edit to add:
One issue I didn't mention is all of the D-Y transformers I have looked in (dry types with the covers off) were three leg cores with one primary winding over one secondary winding on each leg. This likely colors my concepts of transformer theory

ice

 

[ceb58]

isn't that where the coyote gets his from to shock the roadrunner

Along with a million other things that dont work except for the transformers

 

[jtester]

But grounded wye to grounded wye, grounded wye to delta, wye to delta, delta to wye, delta to grounded wye, and delta to delta are all different transformer connections, each with subtle quirks surrounding unbalanced loading .

Jim has hit the nail on the head. The high side of the transformer acts differently with the low side depending on the individual configurations, delta and wye, and grounded or not, on each side. A portion of symmetrical component theory utilizes different network connections to analyze the different types of transformer connections, delta wye, delta delta, etc., to determine current flows, impedances, etc. A study of these network connections will reveal why certain transformer connections work, and certain ones have undesirable effects.

Most utility transformer connections are grounded wye on the high side, to reduce the cost of insulating the high side windings, L-N is cheaper than L-L. They won?t provide a delta low side with a wye high side, in those instances, and some others, they will provide a delta high side. This is particularly true in padmounted installations.

Jim T

 

[davidaengelhart]

Could someone please tell me how they expect fault current (phase to case;277V) to flow on the 480V secondary if there is no return (grounded bonding conductor) to the 480V windings??
Situation being: 208V grounded branch circuit feeding the primary of a reversed connected step down to step up connection for a single 480V load (no 277V load). If there is no HO connection on the secondary side (480V) for the phase to case 277V fault, just how is current going to flow on the equipment grounding conductor that is connected to the 120/208Y source.
Is it not true that the 120/208V primary is not the source for the 480V system return path??

 

[ptonsparky]

Could someone please tell me how they expect fault current (phase to case;277V) to flow on the 480V secondary if there is no return (grounded bonding conductor) to the 480V windings??

How do you get phase to case voltage of 277 if the secondary is not bonded?
If it is not bonded the voltage will float until the first ground. After that it would be anywhere from 0 volt to 480v.

[/QUOTE]Situation being: 208V grounded branch circuit feeding the primary of a reversed connected step down to step up connection for a single 480V load (no 277V load). If there is no HO connection on the secondary side (480V) for the phase to case 277V fault, just how is current going to flow on the equipment grounding conductor that is connected to the 120/208Y source.[/QUOTE]

Current won't flow on the primary sided EG.

[/QUOTE]Is it not true that the 120/208V primary is not the source for the 480V system return path??[/QUOTE]

Yes. I think, The double negative terms always got me even back in grade school.

Some day I'll figure out how to split these replies up properly. Maybe.

 

[kwired]

Could someone please tell me how they expect fault current (phase to case;277V) to flow on the 480V secondary if there is no return (grounded bonding conductor) to the 480V windings??
Situation being: 208V grounded branch circuit feeding the primary of a reversed connected step down to step up connection for a single 480V load (no 277V load). If there is no HO connection on the secondary side (480V) for the phase to case 277V fault, just how is current going to flow on the equipment grounding conductor that is connected to the 120/208Y source.
Is it not true that the 120/208V primary is not the source for the 480V system return path??

Like ptonsparky said the secondary does not have a 277 volt point. Until something is grounded it floats in relation to ground. Now to have a NEC compliant installation you either will need ground fault monitoring equipment or ground one of the phases creating a grounded phase system. You still have to run equipment grounding conductors and bond all non current carrying parts just like with any other system.

 

[tom baker]

Reference please.

Sq D makes a 208 Delta primary to 480/277 wye secondary, listed for the application.
While it can be done to take a step down and use it as a step up, this becomes a wye - delta, the secondary being a delta, at so a standard slash rated 480/277 breaker could not be used, it would have to be full voltage rated.

 

[LarryFine]

Could someone please tell me how they expect fault current (phase to case;277V) to flow on the 480V secondary if there is no return (grounded bonding conductor) to the 480V windings??

It wouldn't. An ungrounded secondary system will flow no (appreciable) current through ground until another system conductor faults to ground.

With an ungrounded system, the OCPD would not open due to a ground fault unless a second conductor faults to ground.

 

[fdt1973]

reverse feeding a 120/208v wye 277/480v delta

reverse feeding a 120/208v wye 277/480v delta

If I am trying to run (2) 480 volt garage heaters each pulling approximately 10 amps and my service is 120/208v

I have a 30 kva transformer and I am backfeeding it with 50 amps on the secondary or 277/480v side.
? How do I calculate the availble current on the primary or 120/208v side.


What I am experiencing is that the fans run but the heaters are not really heating they just warm up a little.
Now keep in mind these are used heaters and they are about 15 years old.
? could it be that the heaters are bad or am not feeding my transformer right.

 

[Open Neutral]

If I am trying to run (2) 480 volt garage heaters each pulling approximately 10 amps and my service is 120/208v

(I'm rewording this to help dissect the problem.)

So you are drawing 2*480*10 watts.

You have 208-120 service, single phase I assume.

I have a 30 kva transformer and I am backfeeding it with 50 amps on the secondary or 277/480v side.

You have a transformer capable of up to 30 KVA. You are feeding it with voltage; but I assume you must mean you are feeding at the lower voltage. You ain't feeding it current; it shall draw current as needed by the load.

How do I calculate the availble current on the primary or 120/208v side.

Remember this guy?? To paraphrase, watts is watts. Your heaters draw X watts; you need X watts [plus a smidgen for transformer losses, but set that aside. for now...] in; irrespective of the voltages.

What you seem to be asking is not available current, but needed supply current to run the heaters. And that's easy.

At 480 V;

X watts
--------   = Y amps.
480V

At 208 V:

X watts
--------   = Z amps.
208V

Now, is X within the transformer's rating? Is Z within what you can supply?

What I am experiencing is that the fans run but the heaters are not really heating they just warm up a little.
Now keep in mind these are used heaters and they are about 15 years old.

could it be that the heaters are bad or am not feeding my transformer right.

Well, measure the voltage at the heaters when running. [Obviously, in a manner that won't kill/maim you; maybe power down, clip meter onto terminals, then power on at a safe distance & while looking away...and only THEN read the meter.]

 

[kwired]

If I am trying to run (2) 480 volt garage heaters each pulling approximately 10 amps and my service is 120/208v

I have a 30 kva transformer and I am backfeeding it with 50 amps on the secondary or 277/480v side.
? How do I calculate the availble current on the primary or 120/208v side.


What I am experiencing is that the fans run but the heaters are not really heating they just warm up a little.
Now keep in mind these are used heaters and they are about 15 years old.
? could it be that the heaters are bad or am not feeding my transformer right.

First let me say that you are sort of starting a new topic on an old thread although they are kind of related you are still basically asking a new question. You should probably have started a new thread.


In answer to your question:

30000VA / 208V / 1.732 = 83.27 amps

30000VA / 480V / 1.732 = 36.09 amps

This would be rated currents for both sides of a 30kVA transformer.