Reversing transformers

[sparkey1003]

Is it allowable to backfeed a 480 delta to 120/208 wye three phase transformer? What should be done with the grounding systems?

 

[LarryFine]

Sparks, welcome to the forum!

It is, and here are a few poiunts to ponder:

1. If the now-primary is a wye, meaning it has a neutral (X-0), let it float; connect nothing to it. Treat the primary as if it was a Delta load.

2. Unless you have ground detection and a need, you'll need to ground a secondary conductor.

3. Some transformers have a slightly lower-than-exact primary-to-secondary turns ratio to counter voltage drop, and will end up with a lower-than-expected now-secondary voltage.

4. Due to internal construction, a reversed transformer will have a higher-than-usual inrush current.

 

[hillbilly1]

To add to Larry's reply, you may be able to adjust the output voltage if it is a larger transformer with adjustable primary (now secondary taps) Also keep in mind you will not be able to get 277 volts with this set up.

 

[ceb58]

The transformer will also have to be listed as suitable for reverse connection.

 

[kwired]

Seems like this has been a common topic recently, at least every time it is asked the answers are fairly consistent with other threads on the topic.

 

[LarryFine]

Seems like this has been a common topic recently, at least every time it is asked the answers are fairly consistent with other threads on the topic.

Gee. Maybe we're right, then.

 

[iwire]

The transformer will also have to be listed as suitable for reverse connection.

Reference please.

 

[jrannis]

The transformer will also have to be listed as suitable for reverse connection.

I have done this several times and did not know transformers were listed for that application or not. I just used it as a transformer. High side/ low side. I don't think I ever saw and arrow point out the entrance and exit.

The big debate is usually whether to corner ground the Delta output.

 

[ceb58]

Reference please.

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.

 

[stevebea]

Dont forget to take the ground strap from the XO terminal to the case off.

 

[brian john]

Is it allowable to backfeed a 480 delta to 120/208 wye three phase transformer? What should be done with the grounding systems?

And while your installation can be compliant, a 208 delta - 480/277 wye is (STRICTLY MY OPINION) a better solution.

 

[DavidA]

Dont forget to take the ground strap from the XO terminal to the case off.

Okay, I'm in a similar situation and am curious what would happen if that strap were to be left in place.

 

[kwired]

Do a search on this forum, it has been brought up a few times fairly recent. You will see some people had problems with undesired current flow in the primary circuit, and burned out windings.

By bonding the XO you are creating a path that is not needed for the circuit to operate, this path has little impedance comapared to the normal current path - so if current is to flow through it (and it will) it can increase beyond normal levels easily.

A perfectly balanced secondary load in theory should net zero current on this conductor. Primary impedance would need to be perfectly balanced as well as input voltages would need to be identical. Any slight imbalance of any of these items will magnified greatly by the low imedance and overcurrent will happen very easily.

I do not have any specific reference for facts I presented, that just seems to be very logical to me of what is likely happening.

The same rule would apply to a single phase transformer with multiple taps on both primary and secondary. You do not connect a center tap of a primary winding to ground even if it should normally operate at ground potential.

 

[Open Neutral]

I'm following with interest because I need to do this. A set of 3-sync'ed battery-fed 120V out inverters will feed a step-up transformer to get 3ph 480.

But when the sun is out; separate grid-tie inverters will kick in. They are @ 277/480. And when their output exceeds the load demand; they backcharge the battery plant via the transformer and inverters.

I want wye on the 480 side, which means delta on the LV one. I'm trying to get a declarative statement from the inverter manufacturer that this meets their approval.

 

[jtester]

Okay, I'm in a similar situation and am curious what would happen if that strap were to be left in place.

Grounding the 208 side will allow unbalanced currents on the 208 system side to flow through the transformer in an effort to balance that side of the system. Lifting the strap interrupts the circuit. If you have a perfectly balanced 208 system you'll have little trouble. If your 208 side is unbalanced you might have trouble, depending on the magnitude of imbalance.

I studied this in a graduate class longer ago than I care to admit, but have witnessed it a couple of times also.

Jim T

 

[Open Neutral]

Grounding the 208 side will allow unbalanced currents on the 208 system side to flow through the transformer in an effort to balance that side of the system. Lifting the strap interrupts the circuit. If you have a perfectly balanced 208 system you'll have little trouble. If your 208 side is unbalanced you might have trouble, depending on the magnitude of imbalance.

But here I was told the opposite, I thought.

 

[jim dungar]

But here I was told the opposite, I thought.

That reference is about the L-N point of the PV inverters. It does not say to 'ground' the X0 connection on the transformer itself.

 

[iceworm]

Grounding the 208 side will allow unbalanced currents on the 208 system side to flow through the transformer in an effort to balance that side of the system. Lifting the strap interrupts the circuit. If you have a perfectly balanced 208 system you'll have little trouble. If your 208 side is unbalanced you might have trouble, depending on the magnitude of imbalance.

I studied this in a graduate class longer ago than I care to admit, but have witnessed it a couple of times also.

Jim T

First let me say:
I do not advocate connecting the primary Wye point of a transformer to the system neutral.

The few reverse fed that I have put in I connected as is suggested by Jim (and like minded others)

Posters keep talking about "grounding" the transformer Wye point. I'm hoping they mean "connect the wye point to the system neutral".

Utility Distribution is not in my area of expertise

Jim (and like minded others) -
Why does connecting the primary wye point to the system NEUTRAL (differentiating from ground) causes problems?

The utility regularly uses Wye primary transformers. They don't seem to have any trouble with so-called unbalanced currents.

Now let's look 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)

So tell me about this unbalance, where it comes from, and why is it bad?

Secondary Q: If the this unbalanced current in a Wye primary comes from unbalanced L-N voltages, would the same unbalanced L-L voltages cause similar circulating currents (negatige sequence) in a Delta primary?

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.

ice

 

[jim dungar]

First off, utility MV systems behave differently than do our 480Y and 208Y systems do, so do not try to compare them.

If a reverse fed transformer (wye -> delta) has the X0 terminal connected to neutral, or ground, there is a possibility of overloading/overheating one of the windings of the transformer. A load imbalance on the delta connection will cause different currents to flow in each of the wye windings. These different currents will cause attempt to cause a different L-X0 voltage across each winding. If the X0 point is allowed to float, the different voltages can be tolerated. If the L-X0 voltage is fixed, by connecting X0 to N or G, then additional current must circulate in order to 'fix' voltage difference.

 

[iceworm]

First off, utility MV systems behave differently than do our 480Y and 208Y systems do, so do not try to compare them..

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