Feeding a wye-wye transformer from a corner grouded delta source.

[ESOXmaniac]

I have a customer that wants to feed several 330kVA 480V wye-wye isolation transformers from a 480V corner grounded delta source. No neutral available and no ground connected to the primary neutral point. The primary and secondary neutrals are not connected together and the wye secondary Xo is grounded. Will this work, if so are there any special concerns with this application? To me it would appear to be OK, i.e., the secondary voltges (480Y/277V)would be stabilized with respect to ground. The transformer load has a ~0.98 PF so I am not really concerned about harmonics.

Thanks,
Al

 

[ESOXmaniac]

After a little more thought, I don't think it would work. If the source delta is B-phase corner grounded. Then the phase angle of A & C phases would be referenced to B and to ground. Now grounding the secondary Xo would create a phase shift primary to secondary. Wouldn't this create some pretty serious circulating flux in the core?

Thanks
Al

 

[rbulsara]

If primary and secondary are completely isolated as you are suggesting, I do not see a problem. Personally, I stay away from Y-Y xfmrs, unless there is a technical need for them.

You can also do one of them and see how it goes before investing in others.

 

[davidbeach]

You will never be happy with the results unless:

1) H0 is left ungrounded and has no internal connection to X0.

2) The secondary load is always, Always, ALWAYS, perfectly balanced.

It is quite possible to achieve 1) and essentially impossible to achieve 2). Grounding X0 won't accomplish much as long as H0 is ungrounded. Grounding H0 will thoroughly screw things up.

What you need are delta-wye isolation transformers. rbulsara is correct about the general inapplicability of Y-Y transformers.

 

[ESOXmaniac]

Thanks - The source is a B-phase corner grounded delta, which is feeding these wye-wye transformers. The wye primary H0 is not grounded. The wye secondary X0 is grounded. This means I will now have a direct connection via the equipment grounding conductors and GEC conductors between the B-phase primary and the secondary wye XO. At first glance this doesn't seem like a very good idea, i.e., install a wye-wye then hard jumper input B-phase to output X0.

I've attached a drawing from the customer. Keep in mind that there is a hard copper grounding conductors tieing the two earth/ground points together.

Thanks
Al

 

[davidbeach]

Like I said, unless you can maintain absolute current balance on the secondary you will have voltage imbalance issues. Other than the possibility of a phantom delta in the wye-wye transformer, you don't really have a grounded system on the secondary as it can't produce any appreciable amount of ground current. Bad installation.

 

[rbulsara]

ESO:

The secondary winding is a separate source, its grounding has nothing to do with primary side or many other grounded sources in the world to the same "earth". Nonetheless, if you are having so much heart burn over this, do not do it. Use D-Y units and call it a day.

 

[waross]

Search this site. A few weeks ago a poster was having issues with a similar installation. Any unbalanced load or a ground fault on the secondary will cause the voltage on the loaded or faulted phase to drop and the voltages on the unfaulted phases to rise. In this case the phase to ground voltages may approach phase to line voltage but may be limited by saturation and possibly phantom delta effects.
This is not a good idea.

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

 

[ESOXmaniac]

I'm not really having heart burn. The transformers are owned by the customer - they bought 480V delta-delta 330kVA, when they should have gotten 480V delta-wye. Now they want to convert the delta-delta to wye-wye and derate them to 150kVA.

I know wye-wye is not common in facilities but also is common in the utility arena. Just needed some solid advice. My gut tells me a wye-wye configuration shouldn't be used as shown in the drawing, i.e., with B-phase primary grounded and X0 secondary grounded. Was hoping for a bit more detail if someone had experience with this configuration.

Thanks Bill - I did a search and the other thread poster had an ungrounded delta source.

Al

 

[waross]

They will be much farther ahead using delta:delta and buying an artificial neutral. (Zig-zag transformer).
For wye:wye you need a primary neutral (Not to be confused with a ground although it probably will also be grounded.)
With a delta supply there is no neutral. The system may work but only by accident. You will no longer be able to hope that saturation will help balance the voltages. Some transformer core configurations develop a "Phantom neutral" but that may not be dependable to stabilize the neutral location.
You may want to be positioning yourself towards "no responsibility" while avoiding "I told you so".
As I said, it may work but don't count on it.

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

 

[jghrist]

A wye-wye transformer is not a ground source by itself, whether or not the neutral is connected to ground. It depends on the primary system having a ground source. (Note: Physical ground is not a requirement; I'm talking about a stabilizing neutral, but ground source is more common terminology.)

Look at it this way. The primary currents in a wye-wye transformer are equal to the secondary currents times the turns ratio. If the primary winding neutral is not connected to anything, then the vector sum of the primary currents have to equal zero. There isn't anywhere for the summation of the currents to go. If the primary currents sum to zero, then the secondary currents also sum to zero.

If the secondary neutral is connected to ground, and there is a short of one secondary phase to ground (an no load current in the other phases), then there will be no fault current. If there were, then the phase currents wouldn't sum to zero. With no fault current, the voltage across the faulted phase winding is zero. Essentially, the faulted phase voltage shifts to ground potential and the unfaulted phase voltages to ground become equal to phase-to-phase voltage.

 

[skiier]

If the secondary neutral is connected to ground, and there is a short of one secondary phase to ground (an no load current in the other phases), then there will be no fault current.
Huh? You present a short circuit path to the t/f secondary phase voltage and you expect no current? This current is only limited by the winding impedance, which is usually listed on the nameplate as a percentage value. I would like to see you short cct that t/f and create no problems.
Where did you learn cct theory?

 

[davidbeach]

Calm down skiier. Draw up the circuit and remember current only flows in the secondary winding if it can flow in the corresponding primary winding. How do you have current flowing out of the out of the secondary wye point if it can't flow into the primary wye point? Look at transformer sequence diagrams, there is no zero-sequence path, and without a zero-sequence path there is no ground fault current.

Using ideal transformers what jghrist said, and you objected to, is absolutely correct. When implemented using real 3-phase transformers, there is a certain amount of phantom delta effect and there will be some current, but it will be far less than if the primary neutral were connected to ground.

Best to know you are on solid ground before launching into a disagreement with well established transformer theory.

 

[waross]

In my previous post I inadvertently said "Phantom neutral" when I meant to say "Phantom delta".
The problems you will have will be caused by the lack of a primary neutral, not by the corner grounded delta. That is why I referenced the other thread.
David and skiier;
There may be two factors that may provide a field condition somewhere between your two positions.
Case 1;
In the case of a Y:Y transformer with a floating primary neutral, a line to neutral short on the secondary will cause the corresponding primary winding to have a low impedance. This will cause the voltage across the other two windings to approach 1.73% of rated voltage. If the cores saturate before 1.73% of rated voltage is reached, then the excess magnetizing current which returns through the primary of the shorted winding will drive current through the shorted winding.
Case 2;
If the transformer has a three legged core, the resulting phantom delta will stabilize the secondary neutral some what.
Not all transformers will saturate and not all transformers will form a "phantom delta". Even if the transformer in question does either or both, I would not depend on either effect to create a stable neutral.

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