Reverse fed step down transformer

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kwired said:
Pretty much all the banks here are Y primary built out of one bushing transformers the MGN must connect to the neutral of the primary if that is the case

Secondary side can be wye or delta - depends on what service is being supplied, and of course either setting taps or using right transformer to get desired secondary voltage.

Open delta systems are fairly common around here as well, the primary side of those banks are two phase conductors again connected to single bushing transformers and MGN connects as well - so you have an "open wye" so to speak for the primary side.

You don't see two bushing transformers around here, if you do it is probably some older setup, they don't use anything but single bushing transformers for pole mounted transformers.
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So, you have actually seen a Y Primary (all four leads connected - not "open Y") and Delta Secondary (not "open D)? Could be, but I don't know of any reason why one would want to do this.

I'm definitely out of my area of expertise and I rarely ever work with separate single phase transformers - all I've seen were pretty small, a few hundred KVA or less. Been staring at this stuff a lot of years, still, lots of stuff out there I haven't seen.

Now, back to the OP case:
I've never heard of a common core deliberately connected as a 4WY primary-closed D secondary.

ice
 
iceworm said:
So, you have actually seen a Y Primary (all four leads connected - not "open Y") and Delta Secondary (not "open D)? Could be, but I don't know of any reason why one would want to do this.

I'm definitely out of my area of expertise and I rarely ever work with separate single phase transformers - all I've seen were pretty small, a few hundred KVA or less. Been staring at this stuff a lot of years, still, lots of stuff out there I haven't seen.

Now, back to the OP case:
I've never heard of a common core deliberately connected as a 4WY primary-closed D secondary.

ice
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I'm a little out of my area of expertise as well, I don't install these, but they are right there in the open hanging on a pole - all of them are utilizing the MGN when made into a three phase bank. Have no clue what is in most padmounts, but have seen on occasion single phase padmounts networked together to make a three phase bank as well. All of these were POCO owned/maintained transformers.
 
Besoeker said:
On the wrong direction of the wind or a bad case of flatulence or whatever.
I still don't see why connecting the neutral to the star point is a bad thing.
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I am not sure what I can do to convince you. Did you do the analysis I suggested with an extremely unbalanced wye input? Although that situation is absurd it should convince you that a more credible wye imbalance will also create a problem.
A closed delta with two legs at 208V line to line with 120 degree phase difference and the third leg at zero volts will have some really high circulating currents!
 
Besoeker said:
Not at all.
Where would you get an extremely unbalanced wye input?
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Why, how would not connecting the neutral fix that?
What if the WYE input was perfectly balanced and the delta output load was extremely unbalanced - where would that put the unconnected input star point?
 
Besoeker said:
Why, how would not connecting the neutral fix that?
What if the WYE input was perfectly balanced and the delta output load was extremely unbalanced - where would that put the unconnected input star point?
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Simple. Not connecting the neutral would allow it to float to create a balanced wye where each winding has an equal voltage line to neutral. That balances the three delta voltages, just at a lower value than with a proper balanced solid wye.
A severely unbalanced load on the delta side may result in a couple of volts shift in the star point voltage with a floating center. But as I mentioned earlier the sharing of single phase load current across all three windings (2:1:1) will tend to leave the star point voltage fairly close to where it would be with no load at all.
The key fact is that any star point shift resulting from voltage drop in the windings will not cause any problems.
 
Besoeker said:
Exactly why/how would it do that?
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It would do that because it is the only solution for the "variable" star point voltage that satisfies the constraint equation imposed on the delta side.
We have to assume that for practical purposes the voltage ratios on each transformer are fixed except for load induced VD.
 
GoldDigger said:
It would do that because it is the only solution for the "variable" star point voltage that satisfies the constraint equation imposed on the delta side.
We have to assume that for practical purposes the voltage ratios on each transformer are fixed except for load induced VD.
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Each transformer?
 
Bes:
The neutral not to be grounded in a Y-D transformer because for an unbalanced supply voltages, the vector sum of fluxes due to those voltages still equals zero on the delta secondary side when neutral is not grounded i.e not connected to the source. But when neutral is grounded, that vector sum differs from zero by the amount of neutral to ground voltage drop outside the transformer core and causes circulatory current in the delta winding.
However many utilities operate their Y-D transformers,neutral grounded to afford lightning protection to the transformers as only when supply voltages are unbalanced, there would be current in the neutral and neutral to ground voltage drop.
 
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