Transformer Delta to Y

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here is something I found via google. I cannot say this is valid, because it does not match what I believed to be true - although this is not my area of expertise.


"WYE-WYE: (No Isolation, least secure design) the primary and secondary coils are normally both grounded and some are supplied with an integral ground common to the primary and secondary. Environmental noise and background noise will enter the users facility unimpeded to every machine, drive and control as it is directly coupled from input to output.
An isolation transformer filters low frequency noise by the inductive coupling between primary and secondary and an electrostatic shield can be inserted between the primary winding and the secondary winding to filter high frequency noise that is coupled by capacitive interaction.
The WYE-WYE transformer connection has no filtering and all the noise on your power system is shared. The WYE secondary is very susceptible to harmonics. WYE-WYE systems generate considerable interference with communications systems and some jurisdictions prohibit their use. With the neutral not used or not connected to ground in a WYE-WYE system, the phase voltages become unstable and the entire system is susceptible to damage from transient voltage events and the harmonics now travel in the phase conductors. It is best to never try and parallel a WYE transformer with a DELTA transformer because they have a 30-degree phase differential"
 
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weressl said:
My orignal comment was to Larry's comment that you need to ground to maintain voltage stability. I answered to Larry that it is assured by the grounding of the supply transformer.
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My bad, I was unclear. I didn't mean the voltages relative to earth, I meant keeping the secondary neutral effectively at zero volts relative to the phases' voltages, instead of acting like any service with a faulty neutral.

In other words, a Y primary needs to have H0 connected to the supply neutral IF the secondary is also a Y. Otherwise, the secondary could behave as if its supply neutral were broken, and the phase voltages could vary.
 
LarryFine said:
In other words, a Y primary needs to have H0 connected to the supply neutral IF the secondary is also a Y.
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Kind of.

If the utility service is a MGN then it is possible that no neutral conductor is actually brought to the transformer; instead the H0 bushing is jumper to the X0 bushing which in turn is bonded to ground. This connection is fairly common in this area.
 
jim dungar said:
Kind of.

If the utility service is a MGN then it is possible that no neutral conductor is actually brought to the transformer; instead the H0 bushing is jumper to the X0 bushing which in turn is bonded to ground. This connection is fairly common in this area.
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Wouldn't that create (what should be) neutral currents on the EGC?

Unless, of course, you're talking about the utuility primary where, like a service, there is no separate EGC.
 
5-legged core

5-legged core

utilizing a 5-legged core helps deal with the circulitating currents in the neutral?
 
LarryFine said:
Wouldn't that create (what should be) neutral currents on the EGC?

Unless, of course, you're talking about the utility primary where, like a service, there is no separate EGC.
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Yes, I mean there is no utility neutral conductor.

Utilities in this area prefer to use triplex cores instead of five legged ones.
 
LarryFine said:
Actually, there is one; it just happens to be bare.
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I really mean it, in many cases the utility does not run any neutral conductor to the transformer. They run (3) shielded phase conductors only. They are not using URD or any other concentric neutral cable. They are relying simply on the H0 and X0 bushings being connected to ground on the transformer secondary. If there will be no H0-X0 jumper (i.e. a high resistance grounded secondary), they do provide a primary neutral conductor.
 
jim dungar said:
I really mean it, in many cases the utility does not run any neutral conductor to the transformer. They run (3) shielded phase conductors only. They are not using URD or any other concentric neutral cable. They are relying simply on the H0 and X0 bushings being connected to ground on the transformer secondary. If there will be no H0-X0 jumper (i.e. a high resistance grounded secondary), they do provide a primary neutral conductor.
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If the primary is Delta, no neutral is necessary.

However, the grounded primary conductor is usually considered a neutral.
 
LarryFine said:
If the primary is Delta, no neutral is necessary.

However, the grounded primary conductor is usually considered a neutral.
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I know the difference between a neutral, a grounded, and an ungrounded conductor. I also know that not every wye-wye connected transformer is fed with a primary side conductor connected to the H0 bushing.
 
weressl said:
The more important question is: why?

All SHOULD be or all SHOULDN'T be connected?:smile:
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Y-D, no. Y-Y, yes.

480/277 to 208/120 Y-Y, for example. Doesn't H-0 have to be connected to the source neutral to maintain each secondary at 120v?
 
weressl said:
LarryFine said:
Y-D, no. Y-Y, yes.

480/277 to 208/120 Y-Y, for example. Doesn't H-0 have to be connected to the source neutral to maintain each secondary at 120v?
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.......No.
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Let's examine that...

To start, take an extreme example...

100A, 120V load on "a" winding, i.e. X1 to X0... no other secondary loads. With H0 connected vs. not connected, what are the [simple theoretical] currents on each of the three primary windings?
 
Smart $ said:
100A, 120V load on "a" winding, i.e. X1 to X0... no other secondary loads. With H0 connected vs. not connected, what are the [simple theoretical] currents on each of the three primary windings?
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I say 43.32a, 0a, and 0a with the neutral connected.

Without, I don't have time to figure it out now. But, I don't believe it's the same.

In other words, there would be less than 277v across H1-H0.
 
weressl said:
.......No.
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Floating neutral....
Not good.
If you don't connect the transformer primary neutral to the supply neutral, you can't be sure that the transformer line to neutral voltages will be the same as the supply line to neutral voltages.
 
Besoeker said:
Floating neutral....
Not good.
If you don't connect the transformer primary neutral to the supply neutral, you can't be sure that the transformer line to neutral voltages will be the same as the supply line to neutral voltages.
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. . . which in turn will allow the secondaries to vary.

3-ph Y-Y transformers behave as three independent units, I've always thought.
 
LarryFine said:
3-ph Y-Y transformers behave as three independent units, I've always thought.
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This is where it confuses me. Clearly if you have three separate single phase transformers then the system behaves like three separate units.

But if you have a three leg transformer core, the flux going through each leg is the result of all three primary coils. Imagine for a moment that you did have the primary neutral (H0) connected, but you disconnected one of the primary phases (say H1). So in this imaginary case you have H0 connected to the supply neutral, H2 connected to supply phase B, and H3 connected to supply phase C. I believe that on a three leg core, you would have flux that goes through leg 1, and voltage developed on all three secondary phases. But I'm stuck figuring out how much voltage would be developed and what its phase angle would be.

-Jon
 
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