Return Path on Delta Primary and Transformer Windings

[HighVoltage4800]

Hello, I am a distribution lineman with a few questions. I work on the Delta and the Wye system. I have a question on the return path for a Delta system.

In a Wye system the return path is through the system neutral which is tapped at the center of the Wye back at the generator.
In a 3 phase Wye primary, there is 4 wires, 3 primary phases and a neutral.

Now in a Delta Primary, there is 3 wires, only 3 primary phases. So where is the return path back to the source?
Does it use the other phase as they reverse polarity due to the AC? When one is positive on the sine wave, electrons are flowing out through that conductor,
then when the other phase dips down to negative the electrons return back on that phase?

If this is the case, the system neutral on the Wye 3 phase is just for protection? IE phase to ground fault?

Second question is I understand how a transformer works but on our 4800v Delta when we wire a transformer, you put 2 phases of 2760 (phase to ground) into
the primary side of the transformer. H1 gets 2760v and H2 gets 2760v. If the Primary Coil is physically connected between the H1 and H2 how is this not a phase to phase fault?
Or is it not physically connected but induced like the secondary side? Same thing when we wire the same transformer to Wye. We change the tap changer on the transformer,
1 phase of 7620v goes into the H1 and then we put a ground into the H2. The ground is tapped to the pole ground, which is tapped to the system neutral and a driven ground rod.
How could that not cause a phase to ground fault?

Thanks guys.

 

[mbrooke]

Hello, I am a distribution lineman with a few questions. I work on the Delta and the Wye system. I have a question on the return path for a Delta system.

In a Wye system the return path is through the system neutral which is tapped at the center of the Wye back at the generator.


In a 3 phase Wye primary, there is 4 wires, 3 primary phases and a neutral.

Or transformer if fed via typical utility substation. Neutral current on a 12.47kv circuit usually does not result on neutral current on the terminals of a generator due to the substation transformer being delta-wye in most cases.

Now in a Delta Primary, there is 3 wires, only 3 primary phases. So where is the return path back to the source?
Does it use the other phase as they reverse polarity due to the AC? When one is positive on the sine wave, electrons are flowing out through that conductor,
then when the other phase dips down to negative the electrons return back on that phase?

It uses the other phases. Remember 3 phase power is 120* out of phase, so current from one winding is passed to the other two phases in simplified terms.

Look into 3 phase vectors, 3 phase sign wave, and if really up to the task you can try sequence components of 3 phase power.

If this is the case, the system neutral on the Wye 3 phase is just for protection? IE phase to ground fault?

Fault current is one reason, neutral current that results in connecting something in wye is the another. In delta primary imbalance from the transformer secondary gets transferred to the other phases, in wye primary, imbalance on the secondary goes to the primary neutral. Basically you are putting the same package in a different place.

Second question is I understand how a transformer works but on our 4800v Delta when we wire a transformer, you put 2 phases of 2760 (phase to ground) into
the primary side of the transformer. H1 gets 2760v and H2 gets 2760v. If the Primary Coil is physically connected between the H1 and H2 how is this not a phase to phase fault?
Or is it not physically connected but induced like the secondary side? Same thing when we wire the same transformer to Wye. We change the tap changer on the transformer,
1 phase of 7620v goes into the H1 and then we put a ground into the H2. The ground is tapped to the pole ground, which is tapped to the system neutral and a driven ground rod.
How could that not cause a phase to ground fault?

Thanks guys.

The primary winding is isolated from the grounded core. Also, when the primary coil "magnetizes" (induces a magnetic field in) the iron core, this creates a counter flux which limits the current on a winding that would otherwise pull a lot of current due to low resistance.

 

[winnie]

A very brief response:

Yes, the other phases are the return to source, in both a wye and a delta system.

In a wye system the neutral is an _additional_ path back to the source, but when the loads are perfectly balanced there is very little flow on the neutral. With a delta system all current must flow back on the other phases.

When you make your connections on the transformer, there is a direct metallic connection through the transformer coils between the phases or phase-neutral.

In the same way that the primary induces voltage in the secondary, the primary induces voltage in itself. This is called 'self induction'. This self induced voltage acts to block the primary current flow.

What ends up happening is that secondary current flow reduces the primary induced voltage, so that the current flow on the primary ends up being just what is needed to support the secondary current flow, plus a little bit for maintaining the magnetic field.

There are more details, so the above is only an approximation, but it should give you the general idea; the changing magnetic field in the transformer core acts both to limit primary current and to supply the secondary current.

-Jon

 

[HighVoltage4800]

Thanks guys that clears up some things.

Basically a transformer is a load, just like a light bulb. So that makes sense.

Now this brings up a few other questions.

On Delta we have Single Phase (2 conductors) and 3 phase (3 conductors).
A transformer connected Delta has a phase going into the H1 and H2 bushings.
So it makes sense that the return patch is on the other phases.

How ever in a Wye we have Single Phase (1 primary conductor, 1 system neutral) Open Wye (2 primary conductors 1 system neutral) and 3 phase (3 primary conductors 1 system neutral).
When a transformer is wired Wye it has 1 primary phase on the H1 and a ground on the H2.
So how can the return path be through the other phases as there is no connection through the other phases?
Even on a Wye-Wye or Wye Delta bank, closed or open.
So it must use the neutral as the return path on a Wye system correct?
If we have a system neutral come down we have to wear our primary gloves to handle it because it can have up to primary (7620v) voltage on it.

Once again I appreciate it guys.

 

[GoldDigger]

Thanks guys that clears up some things.

Basically a transformer is a load, just like a light bulb. So that makes sense.

Now this brings up a few other questions.

On Delta we have Single Phase (2 conductors) and 3 phase (3 conductors).
A transformer connected Delta has a phase going into the H1 and H2 bushings.
So it makes sense that the return patch is on the other phases.

How ever in a Wye we have Single Phase (1 primary conductor, 1 system neutral) Open Wye (2 primary conductors 1 system neutral) and 3 phase (3 primary conductors 1 system neutral).
When a transformer is wired Wye it has 1 primary phase on the H1 and a ground on the H2.
So how can the return path be through the other phases as there is no connection through the other phases?
Even on a Wye-Wye or Wye Delta bank, closed or open.
So it must use the neutral as the return path on a Wye system correct? [See #2 below]
If we have a system neutral come down we have to wear our primary gloves to handle it because it can have up to primary (7620v) voltage on it.

Once again I appreciate it guys.

The current flowing on one wire will return to the source on one or more other wires.
Imagine a delta source connecting to the primary of a single phase transformer:
I will label the source wires X, Y, and Z.
1. I connect X to H1 and Y to H2. Current leaves the X-Y winding of the source on H1 and returns to the other end of that winding on H2.
Change the source to be wye instead.
2. I still connect H1-X and H2-Y. Current leaves the X-N winding on H1, returns to the source on H2 to the top of the Y-N winding, through that to N and the bottom of the X-N winding.
With wye source, I choose to connect H1-X and H2-N.
3. Now current leaves the X-N winding on X, returns to the other end of that winding on N.
Finally what happens when I connect H1-X and H2-N with a delta source?
4. I can't do that since there is no N in a delta source.

That covers all the possibilities.

 

[Besoeker]

So it must use the neutral as the return path on a Wye system correct?

Except when you have a balanced three phase load.
If you have a three-phase cage induction motor, you don't run a neutral to it, just the three line voltages.

 

[mbrooke]

Thanks guys that clears up some things.

Basically a transformer is a load, just like a light bulb. So that makes sense.

Yup- another load.

On Delta we have Single Phase (2 conductors) and 3 phase (3 conductors).
A transformer connected Delta has a phase going into the H1 and H2 bushings.
So it makes sense that the return patch is on the other phases.

How ever in a Wye we have Single Phase (1 primary conductor, 1 system neutral) Open Wye (2 primary conductors 1 system neutral) and 3 phase (3 primary conductors 1 system neutral).
When a transformer is wired Wye it has 1 primary phase on the H1 and a ground on the H2.
So how can the return path be through the other phases as there is no connection through the other phases?

Well, not to confuse, you have transformers out in the system connected to the other phases and then to the MGN (multi grounded neutral). I will explain further. But in any case the return is indeed the neutral. Current flows from the connected phase, into H1 and out through to H2 and then onto the neutral.

Now to get technical: As current flows through the neutral back to the substation, the actual current will increase and decrease as transformers connected to various phases and varying loads "add" or "deduct" current. What do I mean by this? Lets say you have a single transformer with 10amps of primary current draw at the end of a 3 mile line connected to phase C, the current flow on the MGN will measure at 10 amps. Now at 2 miles you have a transformer with 15 amps of load connected to phase B. 15 amps will leave and travel down to the MGN, but due to the sine wave being 120* out of phase, the current before the 2 mile mark will be 13 amps. Not 10, 15 or 25, but 13. Now at 1 mile you have a 10 amp transformer connected to phase A. Again due to the sign wave being 120* out of phase, the current will drop to 5 amps. I guess you could say current that can, or is allowed to, is getting back up onto phase A from B and C. As the line heads toward the substation, the neutral current will be 5 amps. So at the substation feeder you will measure:

Phase A: 10 amps
Phase B: 15 amps
Phase C: 10amps
Neutral: 5 amps

In a nut shell a 3 phase system is always trying to balance itself. However, that still does not stop the neutral from being the return. In a single phase lateral it is a return in every sense of the word, and in our example, (or in any wye system for that matter) if you opened any two phases, the loads connected to the remaining phase will function without issue. So if we opened A and B phase in our example but only left C phase closed, C phase would have 10amps of current and the neutral would carry 10 amps of current at all points. If we opened A and C but left B, B phase would measure 15 amps and the neutral would measure 15 as well. Same for a real world scenario. If a 3 phase lateral blew 2 fuses with all L-N connected loads, what ever the phase carries the neutral will carry, ie 89 amps on C and 89amps on N.



{ And- if we really wanted to get real/technical Because I know you will see this in the field... A clamp on amp meter might read 100amps on a single phase lateral or one-phase-in scenario, but read 80, 50, or even 40amps on the neutral. Why so? Nothing to worry or having to do with our discussion. Because the neutral is grounded at so many points, connected to the telco shields and connected to city water mains at the customer's main disconnect current will often divide between the neutral and everything else thats conductive. But if you were to account (measure) for the earth, pipe and shield current; in addition to the neutral current at the point being measured- it will equal the phase current. So 100 on the phase; 65 on the neutral wire itself, 20 on the water pipes, 5 on the telco/coxial shields and 10 through the soil- would add up to 100amps. }

Even on a Wye-Wye or Wye Delta bank, closed or open.
So it must use the neutral as the return path on a Wye system correct?
If we have a system neutral come down we have to wear our primary gloves to handle it because it can have up to primary (7620v) voltage on it.

If the neutral breaks its possible to only have a few volts if you have good conductors parallel to the neutral itself like phone shields and metal water pipes. But if you do not- yes- that neutral could easily reach 7000+ volts between it and everything else. A neutral conductor can be just as dangerous as a primary phase conductor under the right conditions.

Once again I appreciate it guys.

No problem

And heres the thing, I will try and upload xcel sheets with 3 phase wye and delta current calculators. They do all the complex vector and yada yada yada math for you. Play around it- anyone who does learns a lot about how 3 phase power behaves.

 

[mbrooke]

Except when you have a balanced three phase load.
If you have a three-phase cage induction motor, you don't run a neutral to it, just the three line voltages.

Or a closed wye-delta transformer A closed wye-delta can skip the primary neutral to MGN connection. But thats another post all onto itself.

 

[mbrooke]

Ok- I have Golddiggers (if I am remembering right) Xcel 3 phase calculators, but I have no idea how to upload them (says file is to big). Does anyone know?

 

[Smart $]

...
Basically a transformer is a load, just like a light bulb....

Yup- another load.
...

Only IF you totally ignore the secondary... which sort of defeats the purpose of calling it a transformer.

A transformer is a passive device. With exception of relatively small % of loss, the electrical power that goes in, comes out the other side, only in an altered configuration (or same in some cases).

 

[mbrooke]

Only IF you totally ignore the secondary... which sort of defeats the purpose of calling it a transformer.

A transformer is a passive device. With exception of relatively small % of loss, the electrical power that goes in, comes out the other side, only in an altered configuration (or same in some cases).

True- but for the sake of the argument it does not hurt to call it as such. (IMO) :angel:

 

[Smart $]

Basic to understanding how three-phase transformers work is first understanding how single phase transformers work... because, with a three-phase transformer, all you basically have is three (or two) single-phase transformers connected in different primary and secondary configurations to achieve the desired 'transformation'. This is where winding interconnections, voltage, and phase relationships enter the picture when transforming three-phase and most any other polyphase configuration.

Anyway, I'm just trying to point out basics because everyone else seems to want to jump to the more technical aspects right out of the gate.

 

[mbrooke]

Basic to understanding how three-phase transformers work is first understanding how single phase transformers work... because, with a three-phase transformer, all you basically have is three (or two) single-phase transformers connected in different primary and secondary configurations to achieve the desired 'transformation'. This is where winding interconnections, voltage, and phase relationships enter the picture when transforming three-phase and most any other polyphase configuration.

Anyway, I'm just trying to point out basics because everyone else seems to want to jump to the more technical aspects right out of the gate.

I know, but for the sake of the discussion describing a transformer more than an object drawing X amps provides more room for confusion in an already heavy topic.

 

[Smart $]

I know, but for the sake of the discussion describing a transformer more than an object drawing X amps provides more room for confusion in an already heavy topic.

I think the biggest challenge to his understanding is phase relationship and the fact that one wire cannot provide a phase relationship. In fact, it takes a minimum of three wires for there to be any phase relationship voltage wise. It takes at least two wires just to provide single phase. One wire doesn't even have voltage unless we discuss voltage drop from end-to-end while current passes through it.

 

[Smart $]

http://iisolutionstechnology.com/wp-content/uploads/2013/05/Conexiones_tx_monofasicos1.pdf

 

[mivey]

But in any case the return is indeed the neutral.

In the single-phase case where the primary is connected Line-Neutral, that is. For the the single-phase case where the primary is connected L-L the other phase is the return.

For the other cases having multiple transformers, the neutral carries only the phasor imbalance. For a balanced wye-wye, the return for one phase is the other phases, not the neutral.

 

[mivey]

How ever in a Wye we have Single Phase (1 primary conductor, 1 system neutral) Open Wye (2 primary conductors 1 system neutral) and 3 phase (3 primary conductors 1 system neutral).
When a transformer is wired Wye it has 1 primary phase on the H1 and a ground on the H2.
So how can the return path be through the other phases as there is no connection through the other phases?
Even on a Wye-Wye or Wye Delta bank, closed or open.
So it must use the neutral as the return path on a Wye system correct?

On the single-phase L-N primary, the neutral is the return. On single-phase L-L primary the neutral is not the return.

On a three-phase wye-wye, there is a connection between phases. Remember that all three pots join at the neutral point and that is where phase A can connect to and return current "backwards" through the B and C windings to get to the primary B & C phase conductors.

On a wye-delta we open the neutral after switching in so the primary neutral point floats and the current from one phase returns on the other two phases.

On open wye-delta, we must have the primary neutral and the primary neutral returns the phasor imbalance current.

 

[mivey]

Only IF you totally ignore the secondary... which sort of defeats the purpose of calling it a transformer.

A transformer is a passive device. With exception of relatively small % of loss, the electrical power that goes in, comes out the other side, only in an altered configuration (or same in some cases).

While not what we would consider a load, it was a fair assessment by the OP. It is a "load" like an inductor or a capacitor and not a direct short like a wire. Better to use your term "device" to indicate it is not shorting the source.

 

[mivey]

In a Wye system the return path is through the system neutral which is tapped at the center of the Wye back at the generator.
In a 3 phase Wye primary, there is 4 wires, 3 primary phases and a neutral.

Now in a Delta Primary, there is 3 wires, only 3 primary phases. So where is the return path back to the source?
Does it use the other phase as they reverse polarity due to the AC? When one is positive on the sine wave, electrons are flowing out through that conductor,
then when the other phase dips down to negative the electrons return back on that phase?

If this is the case, the system neutral on the Wye 3 phase is just for protection? IE phase to ground fault?

In the delta, the return for load current is the other phases.

In the multi-grounded wye system the return is the neutral wire and earth (and stuff in the earth). This will split as high as about 70/30 but 50/50 or 60/40 is more typical. We usually just say about 1/2 in the primary neutral and 1/2 in the earth.

If you were connecting all of your banks on the wye system L-L, the neutral would be just for protection.



The key to the return current of delta vs. wye is the following:

With a neutral conductor, the neutral point in unbalanced conditions is forced to stay at the midpoint and that causes unbalance current to appear on the neutal. From a physics perspective, it is a higher net energy state because the system wants to unbalance the neutral point but we don't let it - we add system stress.

With a delta unbalance, we float the neutral point away from the midpoint and the unbalance reaches natural equilibrium among the phases and all return current is in the phases. This is a more relaxed or lower net energy state from a physics standpoint.

 

[mbrooke]

In the single-phase case where the primary is connected Line-Neutral, that is. For the the single-phase case where the primary is connected L-L the other phase is the return.

Correct- I explained that. :thumbsup:

For the other cases having multiple transformers, the neutral carries only the phasor imbalance. For a balanced wye-wye, the return for one phase is the other phases, not the neutral.

Correct