Wye-Wye Transformer and grounding 3

[bspace123]

Hi All

I have been reading for hours and still confused about the grounding concepts for wye-wye transformers.

1. Say we have a wye-wye transfrormer and the primary side neutral is not connected to ground but the secondary side neutral is connected to ground, if we have a phase-ground fault on the secondary, will fault current flow from phase -> ground -> neutral of the secondary transformer? And will this fault current be seen as phase current on the primary side?

2. Say we now connect the primary side neutral to ground, will this change anything?

3. Is it common practice to bond the STAR points together if they are connected to ground for this type of transformer? If so, why?

4. If we have both the primary and secondary side neutrals connected to ground, but the primary side is via a neutral earthed resistor, if we have a ground fault on the primary side, will fault current still be limited by the resistor? or will the bonding mentioned in point 3 compromise this?

 

[waross]

5. On delta(pri)-wye grounded(sec) systems if we have a phase-ground fault on the secondary, fault current will flow from phase -> ground -> neutral of the secondary transformer?

Yes.

6. Could you please provide some links or name articles further explaining your answer # 1?

This is not an accepted connection and I have not seen it addressed in texts.
Possibly some others may have a reference.

7. Is there a way to intuitively understand why if a fault occurs on the secondary ,in a delta(pri)-wye grounded(sec) transformer the fault current will flow, but in a wye ungrounded(pri)-wye grounded(sec) transformer the fault current will not flow?

In the ungrounded wye primary, fault current will flow but the voltage across the primary of the faulted phase will be determined by the voltage at which the healthy transformers saturate.
Actually, look at the saturation curve of a typical transformer or iron core induction coil.
Determine the current when 1.73 times rated voltage is applied to the transformer.
The fault current will be less than this amount, but this will give an indication of the order of magnitude of the fault current.
The fault current will be less than if the primary wye point was grounded.
The three phase case is more complicated than the single phase case.

8. If transformer is wye grounded-wye grounded. A fault on primary side will not be visible on secondary side?

Any reduction in voltage on the primary that is caused by the fault will be seen on the corresponding secondary phase (with consideration of the transformer ratio.)

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Ferdi86]

Hi,

Having read through all the answers in this thread it is still not clear to me what the answer to the original question (1) is. I can't understand how a floating neutral on the primary can prevent the earth fault current on the grounded secondary. As the neutral on the secondary is grounded, the earth fault current will return to the secondary neutral there, thus completing the circuit. This phenonema is contained within the secondary circuit. I have also simulated this in simulink, confirming that a ground fault currend ideed will flow:

Trying the same circuit with a grounden primary does not change the current. THe same circuit with a grounded generator, will however increase the fault current substansially.

Anyway, a ground fault is pretty much the same as a single phase load on the secondary, and of course it is possible to supply a single phase load on the secondary?

What is do not understand is how come the zero sequence equivalent circuit for the transformer configurantion show and open circuit for the zero sequence current? Both a ground fault and a normal unbalanced neutral current would consist of zero sequence current. I suspect the equivalent circuit is showing the zero sequence flow from primary to secondary only?

 

[stevenal]

The secondary is magnetically coupled to the primary, so it cannot be considered in isolation. Ampere-turns must sum to zero in a transformer.
No, you cannot supply a line to ground single phase load on such a transformer. Connect your single phase loads line to line instead.

 

[waross]

Consider an A phase to neutral fault on the secondary.
That will cause the impedance of the primary winding to drop.
The primary current path is from A phase through the A phase winding to the wye point. From the wye point through the B phase and C phase windings to B phase and C phase.
However, the B phase and the C phase windings have a much higher impedance.
The current through the A phase winding will be limited by the higher impedance of the B phase and the C phase windings.
For simplicity of explanation, regard the B phase and C phase windings resolved into one winding in phase with the A phase winding and of equal impedance to either B phase or C phase.
We now have a high impedance in series with a low impedance with the same current flowing in both impedances.
The effective voltage will be 87% of line to line voltage causing a little drop in current.
As the impedance of the fault increases, the the current will increase proportionally, until the the rising voltage saturates the B phase and C phase cores. From that point, the voltage and the current in A phase and in the fault will start to rise but will still be less than it would be if the primary neutral path was complete.
Grounding the generator neutral will not cause a change.
Grounding the transformer primary neutral will not cause a change.
Grounding both the generator neutral and the transformer primary neutral will complete the primary neutral path and the fault current will rise to a value determined by the line to neutral voltage.
Note: Beware the term Grounded when discussing theory. Plant electrical persons typically make a distinction between a neutral and a ground.
A neutral serves a different purpose than a ground.
Utilization systems at 600 Volts or less, typically use separate conductors for ground and for neutrals and the code makes important distinctions between a neutral and a ground.
If the generator wye point and the transformer wye point are connected by a conductor, it is immaterial whether the circuit is grounded or not. It is the connection of the wye points that we should be considering.
Why not grounded?
Distribution systems often use a multiple grounded neutral conductor.
The neutral is connected to a ground rod at frequent intervals.
But, the grounding serves to dissipate lightning strike energy, it serves to avoid a higher voltage on unfaulted phases and it serves to avoid the damage that may be caused by a discontinuous ground fault. (An arcing ground fault generating high voltage and high frequency transients.
The connection between the generator neutral and the transformer primary neutral serves to stabilize the transformer wye point voltage.
This connection between wye points may be a metallic conductor, it may be a return through the ground rdos or most typically a combination of a metallic conductor and multiple ground returns.
But it is the action of connecting the neutrals, not grounding the neutrals that stabilizes the transformer wye point.


Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Ferdi86]

Thank you for your answer.
I agree that the current should return through the B and C windings. My understanding is that this current would also "see" the magnetizing inductance of windings B and C? The increased current i windings B and C does not create an equal and opposing current on the secondary (assume no load on these winding on the secondary, the load won't change because of the increased current in primary winding B and C) in windings B and C, and therefore the magnetizing inductance would oppose this current. Therefore the fault current would be severely limited by this rather large inductance (not considering saturation).

What I still do not understand is how this is possible considering the transformer should block all zero sequence currents. The single phase fault current/neutral current on the secondary is to my understanding zero sequence current only.

According to multiple articles online, a single phase load on the secondary would require a delta tertiary to flow. E.g. refer to figure 8 in this article:

https://electrical-engineering-portal.com/transformer-connections

How does these two concepts fit together? The first explanation (yours) does not seem to require a tertiary winding for a single phase current to flow on the secondary (still assuming no neutral/ground return path on the primary). Although limited, a current would flow. But the symmetrical components approach suggests a delta winding would be required (or at least a phantom delta in the transformer chassis) for any current to flow?

 

[waross]

What will happen?
The voltage across the unfaulted secondary phases will rise to about 120% to saturation voltage.
The voltage across the faulted phase will drop accordingly.
The primary currents will be limited by the transformer impedance of the faulted transformer in series with the combined saturation currents of the unfaulted phases.
The primary voltages will stabilize at around 90%, 120% and 120% due to saturation. The fault currnt will be around 90% of the Available Short Circuit Current.
Due to the non-linear effect of transformer saturation current and voltages, the values given are reasonable estimates, not exact values.
Transformer saturation must be considered.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[waross]

In regards to the article:
I don't agree with all of the comments.
First, to reconcile my comments with the comments in the article:
The addition of a delta winding stabilizes the voltages.
But you asked about a Y/Y transformer, not a Y/Y/D transformer.
The wye/delta connections may have advantages for transmission circuits but it is a disaster on distribution circuits.
I spent over 15 years in a land that was stuck in a time warp and made extensive use of the grounded wye/delta distribution connection.
The wye/delta connection was responsible for untold numbers of refrigerator burnouts.
I spent a lot of time devising methods to protect customer's equipment from the damage that the connection could and did cause to consumer's equipment.
There may be some situations where a wye/delta connection shows some benefit, but in the majority of cases, the problems with a star/delta are far worse than the problems without a star/delta connection.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[VMMF]

Could you provide some technical details for this? What could be causing the customer equipment damage e.g burnt refrigerators? Was the delta facing the grid and the wye facing the customers? Was there any kind og earth/ground fault resistor (NGR) on the wye? Were this pole mount or pad mount transformers?

I have a mine customer with 138 kV to 4160 V delta-wye transformer which gas a 75 A NGR and he seems to be working fine but it is an industrial not residential customer and it has a lot of protection relays making sure everything runs smooth

 

[waross]

Grounded wye primary, delta secondary. Single phasing, or more common, single pole switching.
In my experience, most line maintenance was done on dead lines.
Sunday power outages were common and frequent.
Disconnection was done in the field with fused cutouts.
The scenario:
The power has been off for several hours.
All of the refrigerators have closed their thermostat switches and are waiting to start.
Phase one cutout is closed.
Now 1/3 of the customers have power, no problem.
2/3 of the customers have about 50% voltage.
The refrigerators try to start, but at 50% voltage they start to turn but as the discharge pressure builds, they stall.
Phase two cutout is closed.
Now, 2/3 of the customers have stable power, but many of the compressors remain stalled.
1/3 of the customers now have 80% or 90 Percent voltage, depending on the size of the wye/delta transformers back feeding the circuit.
Many of the compressors remain stalled.
When a refrigerator compressor is stalled, the Klixon over temperature device will disconnect it before it burns out, until it doesn't.
Note: One star/delta transformer bank on a residential circuit will affect all of the single phase residential services on the circuit.
Another effect is that a grounded primary phase may cause primary fuse blowing in downstream wye/delta banks.
Friends don't let friends use wye/delta distribution transformer banks.
Oh, and did I mention that after you calculate the single phase Available Short Circuit Current in the normal way, you should then double it. Rated full load current / %Impedance voltage becomes Rated full load current / %Impedance voltage, times two
Friends don't let friends use wye/delta distribution transformer banks.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[golan]

The zero sequence scheme, related to the ground fault, looks different in 2 important references. In the book "Symmetrical components for power systems engineering" by J.L. Blackburn page 54, figure 4.9 f) the scheme is as follows:

However, in the IEC 60909-4 standard, page 12, table 1, 1a) the scheme is as follows:

The first scheme, which I think is the correct one, if the neutral of a star is not grounded, it would not allow the current to ground in case of a fault in the star that has the neutral grounded. In the second scheme it would be possible.

I think that the first scheme is the correct one since in the star that is not grounded, to allow the flow of current to ground in the grounded star (the other winding), currents should also flow in the other 2 phases, so that the sum of the 3 currents would be zero in the ungrounded star.
However there are no ampere turns in the phases without fault of the opposing ungrounded star.

Therefore, in answer to the first question from bspace123, the answer is no, there will be no fault current in the secondary winding (the grounded star winding)