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]

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]

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]

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]

waross is right. An unconnected primary neutral in Y/y will result in "swinging neutral" and your case is the proof.

 

[nightfox1925]

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?

 

[davidbeach]

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]

Is there other thing to cause the problem since it was measured at no-load conditions?

 

[jghrist]

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]

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]

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]

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]

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]

Offhand, I’d put some load on the secondary [even 5%] and see if the secondary ø-n voltages don’t balance better.

 

[edison123]

welcome back, busbar.

 

[nightfox1925]

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]

A ground fault on the secondary will result in essentially zero current and 400 volts to ground on the unfaulted phases.

 

[waross]

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]

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]

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.