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nightfox1925 (Electrical) (OP)
6 Mar 09 1:40
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)?

Helpful Member!  waross (Electrical)
6 Mar 09 8:46
The primary Neutral must be connected.
If the primary wye point of a wye/wye bank is left floating, the secondary voltages will be unstable. A heavy load or fault on one phase will cause the voltages on the unloaded phases to approach 173%.
You must either connect the primary wye point to the primary system neutral or use a delta/wye transformer.

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

7anoter4 (Electrical)
6 Mar 09 11:18
I am not sure the primary grounding will help. I think the secondary neutral is not properly grounded or the neutral feeder is broken or disconnected. Instead of actual neutral an artificial neutral is spontaneously created and its potential floats with the load unbalance.
racobb (Electrical)
6 Mar 09 12:27
Bill is correct, ungrounded Y (high) to grounded Y (sec) is not capable of providing stable secondary voltage.  Primary should be grounded.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner.  Liberty is a well armed sheep!
Ben Franklin

edison123 (Electrical)
6 Mar 09 12:48
waross is right. An unconnected primary neutral in Y/y will result in "swinging neutral" and your case is the proof.
nightfox1925 (Electrical) (OP)
6 Mar 09 13:01
Thank you for all the prompt feedback.

If it is not much to ask, may I know how an unconnected neutral on the primary of a Y-yn transformer would result into a "swinging neutral" even though the secondary is effectively and solidly neutral grounded?

I was always under the impression that the third harmonic currents component of the magnetizing current are restricted when the primary neutral is not connected to ground and this distorts the L-N voltage at the secondary even at NO LOAD conditions. Is my understanding correct?

 

Helpful Member!  davidbeach (Electrical)
6 Mar 09 13:29
With the primary neutral ungrounded, there is nothing to hold the primary neutral in place with respect to the phase voltages.  The secondary neutral is held at ground, but there is nothing to force a relationship between the phase-neutral and phase-phase voltages on the secondary.  Unbalanced load on the secondary moves the primary neutral around with respect to ground, which in turn moves the secondary neutral around with respect to the phase voltages, but not with respect to ground.

Draw up the three phase circuit diagram, put a phase-phase load on the secondary, and solve for all of the winding voltages and currents.  
pwrtran (Electrical)
6 Mar 09 15:03
Is there other thing to cause the problem since it was measured at no-load conditions?
jghrist (Electrical)
6 Mar 09 15:27

Quote:

Is there other thing to cause the problem since it was measured at no-load conditions?
There is always some load, even if it is only cable capacitance to ground.  If the capacitance is unbalanced, then the neutral can move with respect to the phases.
waross (Electrical)
6 Mar 09 19:08
The word grounded has been used a lot here when the correct term should be connected to the primary neutral. Although the primary neutral is invariably grounded and connecting to the primary neutral in effect grounds the wye point, the connection to the wye point is the important issue.
Connecting the primary neutral to an isolated ground grid will result in neutral currents returning through the ground rather than a conductor. This may result in unexpected and possibly lethal step and touch potentials in unexpected locations in the event of a single phase fault on the transformer secondary.
If a primary neutral conductor is not available, you should consider changing to a delta primary transformer, or possibly cheaper than replacing a power transformer, installing a zig-zag transformer to stabilize the secondary neutral.

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

racobb (Electrical)
6 Mar 09 20:35
Bill is correct.  If it is an actual 3 wire supply, you need a delta/grd Y transformer.  If it is a 4 wire supply, ground the supply side.  Notice no reference to high/low or primary/secondary!

And yes you could do it with a zig/zag but there is another piece of equipment to maintain.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner.  Liberty is a well armed sheep!
Ben Franklin

waross (Electrical)
6 Mar 09 21:07
Three transformers connected wye:delta on the load side of the transformer with the neutrals connected will stabilize the voltages. The transformers must be capable of carrying all unbalance current and be able to withstand fault currents until the protection clears.
I know, this is not economical but it will work.
This connection will also correct the displaced neutral on a distribution circuit with uneven single phase loading. The downside of this on a distribution circuit is that the transformer bank has a distressing tendency to either blow fuses or burn up if the distribution primary loses a phase.

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

racobb (Electrical)
6 Mar 09 21:24
Yes you are right in your solution, but as you say, all kinds of other problems with it....not to mention more to maintain and losses.

Alan

Democracy is two wolves and a sheep deciding what to have for dinner.  Liberty is a well armed sheep!
Ben Franklin

busbar (Electrical)
7 Mar 09 18:53

Offhand, I'd put some load on the secondary [even 5%] and see if the secondary ø-n voltages don't balance better.
  
 
edison123 (Electrical)
7 Mar 09 19:31
welcome back, busbar.
nightfox1925 (Electrical) (OP)
9 Mar 09 15:18
Thank you gentlemen for the insights..I'll take your guidance and move further.

The secondary neutral is solidly grounded and the connection to ground is ok per contact resistance and ground resistance tests.

The primary power supply neutral is connected to ground and the L-N loads at the secondary is 100% balanced.
 

jghrist (Electrical)
9 Mar 09 22:03
A ground fault on the secondary will result in essentially zero current and 400 volts to ground on the unfaulted phases.
 
waross (Electrical)
9 Mar 09 22:15
The primary wye point should be connected to the system neutral. That will probably ground it, but grounding it with a ground rod or ground grid doesn't cut it. IT MUST be connected to the system NEUTRAL.
Just grounding it without a direct connection to the system neutral may result in intolerable step and touch voltages in the event of a phase to ground fault on the secondary.

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

jghrist (Electrical)
9 Mar 09 23:01

Quote:

the L-N loads at the secondary is 100% balanced
All of the time?  Are the loads constant?  Can't someone turn off a load?  Any momentary imbalance will cause the unbalanced L-N voltages in your OP.  You must tie the primary transformer neutral to the primary system neutral, change to a delta-wye transformer, or tie a separate grounding transformer to the neutral.
 
nightfox1925 (Electrical) (OP)
12 Mar 09 8:34
You've got a good point there Jighrist.

Since the L-N loads are getting the strikes from these unbalanced voltages, I believe we can provide a Delta-Wye isolation transformer since the secondary L-L voltages of the Y-Yn transformer are balanced. Unfortunately, the Y side primary does not have any provision bring out the neutral. Creating the connection would mean that the transformer should be disconnected, opened and internally modified...a work that we don't have time and money.

Thanks for all the comments and enlightenment.

waross (Electrical)
12 Mar 09 10:45
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 (Electrical) (OP)
12 Mar 09 11:24
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..

[IMG]http://i532.photobucket.com/albums/ee327/primo_beltran/IsolationTrafoApplication.jpg[/IMG]

waross (Electrical)
12 Mar 09 12:00
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 (Electrical) (OP)
12 Mar 09 12:27
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 (Electrical)
12 Mar 09 13:25
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 (Electrical)
12 Mar 09 15:01
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 (Electrical)
12 Mar 09 15:15
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 (Electrical) (OP)
12 Mar 09 17:08
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?  

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