Wye/Delta step up transformer connection

[Talley1013]

Hey guys,

We are looking at installing temporary power to a pump station with 2-2000KW 480V generators, each one feeding a back fed delta wye 4160/480V transformer. Each transformer will then feed one half of the plant's switchgear (tie breaker open between the 2 halves).

My questions has to do with the transformers. Since they are being backfed, we will have a wye delta step up transformer. I have read on this forum and several other places to not connect the X0 to the the primary neutral or to ground. I brought this up today and unfortunately didn't know enough about it to effectively make the point. Can you guys explain, very simply, why this transformer should be hooked up this way? I read a few threads but for some reason the theory behind it just didn't click. I guess I need it dumbed down a little. Just so everyone knows, I have already received confirmation from the supplier that the transformers are rated for step up operation.

Thanks in advance for the help.

 

[kwired]

Connect your incoming equipment grounding conductor to the frame of the transformer, but do not connect any conductor to the XO terminal, and remove any bonding from X0 to the frame if it exists. This is especially needs followed if it is a single core unit. They can get away with having an X0 connection on pole top transformer banks because there is not a single core like there usually is in a padmount.

 

[Talley1013]

Connect your incoming equipment grounding conductor to the frame of the transformer, but do not connect any conductor to the XO terminal, and remove any bonding from X0 to the frame if it exists. This is especially needs followed if it is a single core unit. They can get away with having an X0 connection on pole top transformer banks because there is not a single core like there usually is in a padmount.

Thanks. Can you explain the theory behind that? I'm sure it's right, just want to know for my own knowledge.

 

[kwired]

Thanks. Can you explain the theory behind that? I'm sure it's right, just want to know for my own knowledge.

I honestly don't know why, but it is a topic that has come up many times on this site, people that have connected neutral to X0 (usually on a 208 - 480 application) often reported high current on the neutral and overheating of transformer windings. But I do know it has something to do with the difference between the common core and separate cores.

 

[GoldDigger]

I honestly don't know why, but it is a topic that has come up many times on this site, people that have connected neutral to X0 (usually on a 208 - 480 application) often reported high current on the neutral and overheating of transformer windings. But I do know it has something to do with the difference between the common core and separate cores.

A very rough answer that may be enough to satisfy some people is that:
A. the delta connected load may try to create unbalanced currents in the three secondary windings, and the common core will translate that directly to unbalanced currents in the primary (low voltage) wye windings. But with unbalanced current, the impedance of those windings will cause a voltage difference between the wye winding phase to neutral voltages and the POCO-side phase to neutral voltages. This will be resolved by additional current flowing in the neutral tap and some of the wye windinigs to equalize the voltages. Also
B. If the wye phase to line voltages from POCO are unequal (even by a few volts) and the delta load is balanced, the same offset between the voltage at the unconnected wye point and the POCO neutral will again cause current to flow through the X0 tap to make the two sets of voltages equal. This particular issue will come up even if there is no load at all on the delta side of the transformer, as long as it is a closed rather than open delta.

As for why it is a bigger problem for a common core than for independent cores, my SWAG is that it has to do with the lower winding to common point impedance in a unit transformer than with separate cans and interconnecting wires, requiring more current to make the voltages equal.
Which leads to the question of whether the difference is really in the common core or just the fact that the three windings are so close together....
Second SWAG is that in some cases the difference in the results with separate cans may be related to whether the step up is done with an open or closed delta configuration.
A common core xformer is going to have all three phase windings on both sides.

 

[SparkyHC]

a lttle theory

a lttle theory

let's consider just the wye connected winding(s) on the primary of a common transformer...
the impedance of an inductor is given as Z(L) = 2 pi f L (f = frequency, L = inductance in henrys)
Look at the equation at time "0" .....Z(L) = 2 pi "0" L = "0" so when you turn on the power with the
frequency at "0" the resistance or impedance of the windings is "0" and you essentially have a short
circuit for a cycle or two. All three windings are now short to ground if of course you have "X0" connected
to ground....
We did this 20 years ago and were surprised when the ground wire literally disappeared in a small cloud
of smoke...

 

[Talley1013]

let's consider just the wye connected winding(s) on the primary of a common transformer...
the impedance of an inductor is given as Z(L) = 2 pi f L (f = frequency, L = inductance in henrys)
Look at the equation at time "0" .....Z(L) = 2 pi "0" L = "0" so when you turn on the power with the
frequency at "0" the resistance or impedance of the windings is "0" and you essentially have a short
circuit for a cycle or two. All three windings are now short to ground if of course you have "X0" connected
to ground....
We did this 20 years ago and were surprised when the ground wire literally disappeared in a small cloud
of smoke...

Thanks Sparky. I think I can make my point with that info. This is outside the scope of my normal work so I appreciate the help. It's always great to learn something new.

 

[rcwilson]

Your generators will have their wye points (neutral) grounded. If you ground the Xo neutral connection on the 480/277V transformer there is a path for current to flow between the neutrals. The generators will produce some third harmonic voltages (180Hz) which can cause 3rd harmonic currents to flow from the generator windings to the transformer and back through the interconnected neutrals.

Since 3rd harmonic currents from phase A, B & C are all in phase and the HV winding is in delta, the currents can freely circulate in the HV winding with minimal impedance. That allows the 480V currents to flow in the wye windings and back through the neutral to the generator. With a 2 MVA generator, I have seen currents over 600A at no load when improperly connected like this.

Also, any imbalance in the 60 Hz phase-neutral voltages will cause circulating currents in the delta winding. Think of it as the three 277 volt vectors being connected end-end to form the delta. If one of the vectors is too short, the triangle doesn't close, leaving a voltage difference that creates circulating currents that try to balance out the 277 voltages. Since your generator voltages are probably relatively well balanced, this may not be a problem.

 

[mike_kilroy]

Z=Jw+R

Z=Jw+R

Talley, pleased don't sink your self with the Z=Jw ONLY argument... that is only PART of it..... resistance is NOT 0 at turn on..... it is indeed low but not zero.... if you go into your meeting and state effective resistance is 0 at turn on you probably will be challenged if any engineers are in the room....

plse see the attached vector picture and understand it before you go say something....

there IS resistance in them there copper wires, so there is no "0" impedance on turn on.

 

[Sahib]

The generators will produce some third harmonic voltages (180Hz) which can cause 3rd harmonic currents to flow from the generator windings to the transformer and back through the interconnected neutrals.

Even when the generator supply voltage is perfectly sinusoidal, 3rd harmonic current appears in the grounded/ interconnected neutrals due to transformer core saturation.

 

[SparkyHC]

Agreed

Agreed

Talley, pleased don't sink your self with the Z=Jw ONLY argument... that is only PART of it..... resistance is NOT 0 at turn on..... it is indeed low but not zero.... if you go into your meeting and state effective resistance is 0 at turn on you probably will be challenged if any engineers are in the room....

plse see the attached vector picture and understand it before you go say something....

there IS resistance in them there copper wires, so there is no "0" impedance on turn on.

That is correct, there is resistance in the copper wires, the copper or aluminum windings in the transformer, the terminations....etc.
But let's agree that all of these other resistances are very small....

What the math defines for us is the inherent physics and behavior of an inductor or a transformer....The resistance of capacitors and inductors is
frequency dependent. Z(L) = 2 pi f L (inductive reactance) Z(C) = 1 / (2 pi f C) (capacitive reactance) where frequency is "f"...check the web, these equations
are real.
So if "f" is "0" we have a DC circuit and an inductor acts like a short circuit....and a capacitor looks like an open circuit...

So maybe you should leave out the details and just strongly recommend removing all bonds from X0 to ground...

 

[Sahib]

So maybe you should leave out the details and just strongly recommend removing all bonds from X0 to ground...

It is the interconnection of neutrals that causes the problem and not individual ground connection.

 

[Sahib]

often reported high current on the neutral and overheating of transformer windings.

Overheating of the generator and not possibly transformer.

 

[SparkyHC]

Your recommendation?

Your recommendation?

It is the interconnection of neutrals that causes the problem and not individual ground connection.

are you recommending that the X0 bond to ground stays connected?

What is your recommendation? Can a Wye/Delta step up transformer be successfully
installed?

There is no neutral connection on the Wye side when connected as the primary...so no problem?

 

[Sahib]

are you recommending that the X0 bond to ground stays connected?

No. Because the generator neutral upstream is grounded.

What is your recommendation? Can a Wye/Delta step up transformer be successfully
installed?

Check the generator capability for harmonic currents especially 3rd harmonics.
Yes. A Wye/Delta step up transformer be successfully installed without a primary neutral connection

There is no neutral connection on the Wye side when connected as the primary...so no problem?

No problem because of delta secondary.

 

[kwired]

Overheating of the generator and not possibly transformer.

The instances I was referring to did not involve a local generator, and I really doubt they had much effect on the utility generator, but they still had high current on X0 and some maybe even damaged the transformer in question, so where you come off saying not possible?

 

[Sahib]

The instances I was referring to did not involve a local generator, and I really doubt they had much effect on the utility generator, but they still had high current on X0 and some maybe even damaged the transformer in question, so where you come off saying not possible?

Sorry, I should have said ''not probably the transformer'' and not ''not possibly the transformer''.
I mistook the case you were referring to applied exactly here.