Transformer Secondary Voltage Unbalance at NO-LOAD 2

[nightfox1925]

A Pad Mount Transformer Y-yn Configuration, 34.5KV Ungrounded Wye Primary, 400/230V Solidly Grounded Wye Secondary.

When voltage at secondary side at no load was checked, the following were read: L1-L2=396V, L2-L3=398V, L3-L1=396V, L1-N=245V, L2-N=212V, L3-N=239V.

Since the wye primary neutral is not grounded, the 3rd harmonic current component of the transformer magnetizing current is restrained and it may have caused voltage distortion on the secondary L-N due to 3rd harmonic voltages at L-N. This is how I see it, but it would not harm me to consult further.

Another reason can be that the secondary neutral ground is not properly or effectively connected to ground.

Is there any other cause why Line to Neutral Voltage in the secondary side is unbalance?

I always been a fan of having both primary source and transformer primary neutrals grounded. Is it possible to have the Y-y transformer primary and secondary neutral grounded and leave the power source neutral ungrounded ( or a delta primary source)?

 

[waross]

Unfortunately, the Y side primary does not have any provision bring out the neutral.
That's hard to believe. It has been established for many decades that the primary neutral of a wye;wye transformer bank must be connected.
If someone ran a feed into a 120:240 residential panel with only 240 volts and left the neutral floating it would not work properly. Then the choice would be to do the job properly and install and connect the neutral conductor regardless of the cost, or to install a second transformer to develop a local neutral.
As I said before, grounding is not enough. Grounding is to limit voltages, not to carry working currents.
You can change the Pad-Mount to a delta:wye, but if you are going to add a transformer to stabilize the neutral it must be a wye:delta. It must have sufficient capacity to carry all unbalanced currents and withstand faults.
To make the point concerning grounding versus connecting the neutral, Adding a wye:delta transformer to the secondary and connecting the neutrals will stabilize the voltages even if it is not grounded. The neutrals are then grounded for safety, not circuit functionality.
A simple analysis will show that the unbalanced voltages may approach 173%. In real life, transformer saturation, and (in the case of a three phase transformer, flux leakage), will limit the voltage rise to less than the theoretical maximum. However, depending on saturation and flux leakage to partially balance voltages is not an accepted industry practice.

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

 

[nightfox1925]

Hi warross, I have attached a sketch which shows what we are trying to get at. I simplified it for our discussion. Is there any "quirks" on applying a Delta-Wye isolation transformer instead of using a Wye-Delta?

The secondary L-L voltages of the Y-Yn trafo is ok and if connected to a Delta-Yn isolation trafo, I'll have a stable 230V L-N voltages right? or I'm missing something here..

 

[waross]

I'm sorry friend but that may just give you more of the same.
Consider a load from "A" phase to neutral.
That will show up as a phase to phase load ("A" to "C") on both the secondary and the primary of the pad mount. Your primary voltage vector diagram will approach a "T" shape instead of a "Y" shape. The impedances of both "A" and "C" phase will be low. The impedance of "B" phase with no load will be so high that it may be temporarily ignored. Temporarily remove "B" phase and the vector sketch becomes a straight line from "A" phase to "C" phase on the primary.
Each phase ("A" and "C") now receives 50% of line to line voltage rather than the correct 70.7% of line to line voltage.
The open circuit voltage of "B" phase will approach 1.73% of line to line voltage.
The neutral point may be anywhere inside a triangle formed by "A"-"B"-"C", depending on the loading of the individual phases.
If you draw your load from the secondary of the pad mount and float the other transformer on the secondary of the pad mount, connected wye delta it will stabilize the voltages.
The neutral of the second transformer must be connected to the secondary neutral of the pad mount and the there is no load on the delta, it just floats. The wye:delta transformer does not have to be full size.
My solutions in order of preference are:
1> Connect the primary neutral to the system neutral. If the nearest point on the system neutral is so far away to be economically unrealistic, grounding will NOT suffice and may be DANGEROUS.
2> Change the pad mount transformer to a delta wye.
3> Add a stabilizing tramsformer. (I would pay more to use one of the other options, but I realize that there is a limit to.)
Please consider options #1 and #2 and if the cost is exorbitant, come back and we can help with the sizing of a stabilizing transformer.



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

 

[nightfox1925]

Thank you for your patience Waross.

The Yn-Delta transformer you are suggesting will be connected to the secondary output of the Y-Yn transformer. The delta secondary of this yn-D stabilizing transformer will be floating (no load).

I am interested to know the theory on how does this "floating" secondary Delta of the Yn-D stabilizing transformer would operate to stabilize the secondary L-N voltage. is it the same principle like a Y-D-Yn trafo with a deltae tertiary? Is this stabilizing transformer delat secondary act as a path for the 3rd harmonic current to flow and circulate continuously?

 

[waross]

Consider the delta as three separate transformers. The voltage of each will be proportional to the corresponding line to neutral voltage of the primary side. As the voltages are equal, they may be connected in delta. Now when an unbalanced load causes a voltage unbalance in the secondary voltages of the pad mount transformer, this unbalance will be reflected in an unbalance in the secondary delta voltages. This voltage difference causes a circulating current to flow in the delta. This current tends to reduce the higher phase voltages and increase the lower phase voltages. In a three phase distribution bank it will back feed a missing phase on a distribution circuit if one phase is lost at the source.


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

 

[jghrist]

I think nightfox and waross are discussing two different things.

Nightfox is talking about an isolating transformer which would serve the loads from the grounded wye side of a delta-wye transformer. The system between the wye side of the 34.5 kV - 400/230 V transformer and the isolating transformer would be effectively ungrounded (regardless of the neutral ground connection because there is no ground source).

Waross is talking about adding a grd wye-delta grounding transformer as a ground source to make the 400/230V system a truly grounded system. Loads would remain on the existing system.

 

[waross]

Thanks jghrist;
I think you may be correct. One thing though, I am referring to a system with a solid or stabilized neutral rather than a floating neutral. Ground has nothing to do with it. Grounded or not you will get unbalanced voltages with unbalanced loads unless the neutral is stabilzed when the primary neutral is floating on a wye:wye connection.
The effect will carry through a delta:wye transformer.


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

 

[nightfox1925]

Thanks for reminding us Jighrist...its like both of us going sideways.

I good point waross pointed out the wye-delta transformer. I believe he was referring to a creation of a stable neutral using it. This is similar to the so-called NDT (neutral deriving transformers) option right?