Unbalanced wye transformer

[bspace123]

Hello

In a wye secondary transformer with the star point connected to ground, if phases are unbalanced the star point remains at ground potential. Does a current flow into to ground if phases are unbalanced? If so, how can it if it has no return path to the transformer itself?

 

[7anoter4]

If the sum of phase currents it is not 0 a zero sequence current will flow through neutral or grounding or both. If the star neutral is connected directly to ground then if an insulated conductor or equipment is accidental connected locally with the ground then the current will return to neutral through the grounding system [electrode, EGC or else].
If the close to neutral ground connection potential is considered zero then at the fault location ground potential will be phase-to-ground [or to neutral] supply system voltage.

 

[waross]

Grounding the wye point is not the same as connecting the neutral of the wye point.
When distribution people talk about "grounding" the wye point, they are generally using the ground as a neutral connection.
If the wye/wye transformer is fed from a source with either no neutral or an ungrounded neutral, the effect is different than if the ground is supplying a connection back to the source neutral.
Three cases:
1/ Most common; the ground provides a good neutral connection back to the source neutral.
The simple solution is to consider this as three independent single phase transformers.
In the real world, you must also consider the type of transformer involved.
If the transformer has a tertiary delta, or if the transformer has a three legged core forming a phantom delta, calculating the load currents is not as simple as it may seem.
The apparently unloaded phases may contribute to the load via the delta.
The ground connection acts as a conductor back to and a connection to the source neutral. The grounding is not important as far as a return path. The connection to the source neutral is important as a return path.
We see these connections in industrial settings where the code makes a distinction between the ground connections and conductors and the neutral conductors and neutrals.
For a load on "A" phase only, the current path is through the "A" phase winding of the source to the transformer under discussion, through the "A" phase winding of the transformer under discussion, and and from the wye point back to the wye point of the source transformer. The path from wye point to wye point may be a wire conductor or it may be via ground connections.
But that may not be the only path.
As I said, a delta, real or phantom makes a difference.
An unbalanced load will cause a circulating current in the delta. Power will flow out of the delta to support the load. Power will flow into the delta from the unloaded phases. This will tend to reduce the current on the neutral/ground.
The return path will split, part through the neutral ground and part through the lighter loaded phases via the delta.
How much current will split off?
That depends.
The Current in the delta will depend on the source impedance and the transformer impedance from the primary to the delta.
The amount of current reduction will depend on the impedance of the delta to the secondary winding.
Any impedance in the return path, be it via ground or a metallic conductor will also be a factor.
With an unbalanced load, the current in the neutral will be reduced.
By how much?
It depends.
There may also be increased currents in the lighter loaded phases.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamburgerHelper]

I don't like the term unbalanced because you can have off voltages that sum to zero, ie have negative sequence present. Saying that you have residual voltage or current too me is more representative of the situation. Imbalances can create zero sequence voltages or currents. If your system is effectively grounded, you'll get zero sequence current but very little zero sequence voltage. If your grounding is restricted because it is ungrounded or grounded through an impedance, you'll get zero sequence voltages that develop as the result the system not being strong grounded. The systems neutral strays from ground. Zero sequence voltage is the voltage difference between the neutral and ground, 3*V0 = Vresidual.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[jghrist]

If the load is ungrounded (no neutral), then unbalanced load currents will result in currents that are unequal in magnitude, but with phase shifts such that they will add to zero. No ground current will flow. For instance, you could have all of the load connected between two phases. There would be equal currents in two phases, with angles 180 deqrees apart, but no current in the third phase or ground.

 

[waross]

I don't like the term Grounded to mean neutral connected to the source neutral.
While grounding is a common way of connecting the wye point back to the system neutral when the system neutral is grounded, it is not always so.
By code, in an industrial plant, if a wye;wye transformer is used to transform 480 Volts down to 2908/120 Volts, the primary wye point must be connected back to the source neutral independently of any grounding.
When an unbalanced load is connected across two phases of the secondary, no current will flow to ground.
However, those two phases will have a current on the neutral conductor, back to the source neutral.
A common distribution scheme is a neutral conductor run below the phase conductors on the poles.
This neutral conductor is typically grounded at intervals, possibly every 1/4 mile.
It serves as both a neutral conductor and as a grounding conductor.
In the case of a neutral current flowing, it is acting as a neutral and will do so even if it were not grounded.
Do distribution workers call a neutral conductor a ground? Yes.
Do inside industrial workers call a neutral a ground? NO it is a separate conductor from the grounding conductor.

There would be equal currents in two phases, with angles 180 deqrees apart, but no current in the third phase or ground.

I don't agree with that. Saturation of the third phase may limit the phase shift so that the angle is much less than 180 degrees.
I understood that the OP was considering a ground on the secondary. The effect mentioned does not happen if the primary has a proper neutral connection to the source.
There may be special circumstances, a crane operating over combustible fibers may be one case, where there is a neutral connection but no ground connection.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

When an unbalanced load is connected across two phases of the secondary, no current will flow to ground.
However, those two phases will have a current on the neutral conductor, back to the source neutral.

I was considering an industrial three-wire circuit with no neutral, serving a load with Ø-Ø connections only and no neutral. The circuit would include an equipment grounding conductor, but this would carry no current unless there was a ground fault. There will be no current in the neutral because there is no neutral.

It is true that a ground is different from a neutral, but in a four-wire circuit, the neutral is always grounded. In the NEC, it is not even called a neutral, it is called a grounded conductor.

 

[cranky108]

It is possible to have the currents on a delta winding to be unbalanced, and no neutral current.

So, it is also possible to have the currents unbalanced on a wye winding, and have no neutral current.

This is called negative sequence load current, which results from having a number of phase to phase connections on the distribution system.
Or a number of unbalanced three phase loads.

But, if you have some number of unbalances on the same feeder, they can tend to balance each other, which reduces the amount of unbalance current seen at the substation.

Even if you do not have a neutral wire on overhead lines, an earth path can still exist. It is a higher impedance, granted, but it still exists.

 

[waross]

Ground.
Neutral.
Two different functions.
May be two different conductors.
One conductor may serve both functions.
But, for a ground to serve as a neutral, the source must have the neutral grounded.
And, for what it's worth, in our area, there are no neutral conductors on most of the distribution circuits. Distribution transformers are grounded on one side and the ground completes the neutral circuit.
Under the Canadian Code, for systems below 150 volts to ground, a four wire circuit must have five wires;
Three phase wires, a neutral wire and an equipment grounding wire or other conductor.
I understand that under the NEC at one time, residential electric ranges shared a single conductor for both neutral and ground.
I was under the impression that that had changed. What is the current regulation?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wroggent]

 

[prc]

Bill, you mentioned the secondary star neutral current will return to source neutral. Why it cannot go to the secondary load neutral and close the path? Suppose it is a delta/star transformer, then there will be no neutral current with single phase load but current will flow through other two star phases to balance the delta current in other phases.

 

[waross]

Thanks wroggent;
Does this mean that new installations must have a dedicated equipment grounding conductor in addition to the grounded circuit conductor, but legacy installations without a separate equipment grounding conductor are grandfathered?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Suppose it is a delta/star transformer
The OP specified a star:star transformer.
A ground on the secondary neutral will only pass current in the event of a ground fault.
However with a line to line load on the secondary, there will be a current in the primary neutral.
From there; It depends.
For a distribution transformer, the neutral path back to the source wye point may be completed in one of the following ways:
1. The neutral path is a conductor running with the phase conductors. This conductor may be grounded at one point for a transformer close to the source but the neutral current will not pass over the ground connection..
2. Most common. The neutral path is a metallic conductor running with the phase conductors that includes multiple safety grounds.
The neutral current may split between the neutral conductor (grounded circuit conductor) and the ground connections.
3. There is no metallic grounded circuit conductor. The neutral current returns to the source wye point over the ground connection.
In an industrial plant, the secondary wye point must be grounded if by so doing the maximum voltage to ground of the phase conductors does not exceed 150 Volts.
In accordance with the rule that a system must be grounded at one point only, (does not apply to utility owned distribution circuits)
an in plant transformer will have one neutral (grounded circuit conductor) and one equipment grounding conductor and no connection between them at the transformer.
In the example cited by jgrist, a wye:wye transformer with a floating primary wye point, the loaded phases will at first look apear to be in series across line to line voltage.
Yes, but the third, unloaded phase winding will go over voltage. The over voltage will be limited to about 115% by saturation and the phase angles will be distorted, but may not reach 180 degrees.
Now if this is a three legged core, the phantom delta will further limit the voltage swings and phase angle distortions.




Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[wroggent]

waross, yes, however jurisdictions adopting the code have the ability to exclude the exception or modify it. I don't know if that's common or not.

 

[jghrist]

In the example cited by jgrist, a wye:wye transformer with a floating primary wye point, the loaded phases will at first look apear to be in series across line to line voltage.

The OP did not specify a wye-wye transformer, he specified a grounded wye secondary. In most industrial cases, this would be a delta-wye transformer. The neutral would be grounded, this being a separately derived source. If the secondary circuit was three-wire (no grounded circuit conductor) plus an equipment grounding conductor, then it could not (by code) serve any load to ground. Any unbalanced load would result in phase currents with different magnitudes, but with phase angles shifted so the vector currents add to zero. There cannot be any neutral current because there is no grounded circuit conductor (neutral) connecting the transformer neutral to the load.

 

[waross]

but with phase angles shifted so the vector currents add to zero.

I understand that this is only in the case of a wye primary with a floating wye point.
Other than that I stand corrected and agree completely with the rest of your post of 21 Feb.

Bill
--------------------
"Why not the best?"
Jimmy Carter