Wye-Delta bank susceptibles to burnouts? 2

[fredpar]

Hi:
I was told that closed Wye-Delta banks are prone to burn-outs on primary faults. Can you explain me why and how to avoid this?.
[I'm closing an open wye - open delta bank, 25 and 50 kva, adding a 25 kva to avoid complaints for unbalanced voltages]
Thanks!

 

[jghrist]

If the primay neutral of a Wye-Delta bank is grounded, the transformer will contribute to ground fault current. This may result in blown tranformer fuses or overheating of the transformer for primary ground faults. It may also play havoc with coordination of primary ground fault protection. If there is unbalanced phase-to-neutral load on the primary system, the bank will be subject to increased loading because it will transform power from the two less loaded phases to the third more heavily loaded phase.

 

[apowerengr]

Use transformers with two primary bushings and float the primary neutral for normal operation. Usually, I'd design a copper primary tap with a hot-line clamp on the transformer side and solid connection to the system neutral/pole ground. Ground the primary neutral when energizing or de-energizing the bank but once all three source cutouts are closed lift the hot-tap thus floating the primary neutral.

If the crews are not used to this they should be instructed in the proper operating sequence for this bank. You also have to make sure any primary arresters are connected to the line-side of the supply cutouts, otherwise you could expose an arrester to full line-to-line voltage.

Both GE's Distribution Transformer Manual and ABB's Distribution Transformer Guide discuss the connection and the issues with grd-wye/delta banks supplied by unbalanced primaries.

I hope this helps.

 

[waross]

The primary wye point must be floated.
Maybe the best way to explain it is to start with the open delta with which you are familiar.
At no load conditions, the voltage across the open delta will equal the phase voltages.
Now connect just the secondary of the third transformer. The primary of the third transformer will supply a voltage equal to the primary voltage.
here is where the problems start.
If you lose a primary phase, the transformer on that phase will try to replace the missing phase.
What happens next depends on the load on that phase.
If the load is less than the transformer bank excess capacity, (That is the capacity of the two energised transformers, considered as an open delta bank.) the circuit will be backfed from the transformer bank (with the aid of any similar banks). This may happen with a circuit feeding a small amout of residential loads and a large industrial plant with an oversized wye/delta bank.
If the load on the phase that lost a fuse is over the excess of the transformer bank, up to about 250% or 300% of the transformer capacity, the transformer will probably burn out. Primary fuses are often selected at about 250% to 300% of rated primary current to withstand inrush currents.
If the load on the circuit phase that lost power is more than about 250% to 300%, the fuses have a chance to save the transformer.
In the event of a ground fault, the transformer on the faulted phase will pull energy from the two good phases and add to the fault current. Apart from scrambleing someones arc-flash calculations, the fuses will have a good chance to save the transformer.
Possibly the worst case, in that it may be chronic and undetected, is unbalanced primary voltages. If the voltage goes low on one phase, the wye delta bank will try to raise the voltage. This will result in circulating currents in the transformer bank. A moderate voltage unbalance may cause circulating currents in the transformer bank that may be in excess of 100% load current. Depending on load on the transformer bank and the magnitude of the unbalance, the life of the transformer bank may be drastically shortened. I have seen several new transformers fail in a few months.

Another interesting effect is two phases missing. Now one transformer backfeeds into two transformers in series. With approximately equal residential loads on the two outed phases, the transformer bank will backfeed approximately 50% voltage to the residential customers on the circuit. Every time this happens, several refrigerators and/or freezers will die.

The explanation of what happens when the primary wye point is floated is a lot easier.
IT WORKS.

Float your primary wye point and you will not have any problems.
If you search the RUS website (Used to be the Rural Electrification Authority.) you will see a diagram for the connection of a wye delta transformer bank, with the terse notation thatb says in effect: Caution, grounding the primary wye point may result in transformer burn-outs"
respectfully

 

[ePOWEReng]

Floating the primary wye is a good point. Also, when floating the wye and performing single phase switching on the transformer the last phase will experience a high over voltage. If you use lightning arresters on your transformers becareful, you should temporarily ground the wye point and then remove the ground when the transformer is completely switched in.

There is a paper out there called, "Self-generated voltages due to open-phasing of un-grounded wye delta bank" I think you can find it on IEEE explore.

 

[fredpar]

Thank you all for your help!
Apowereng:
I went to check it out this morning and I think our crews have been instructed properly and the primary neutral is floated.
Hold6448:
I'd like to see that paper but with my limited IEEE membership I don't have access to it.
Waross:
I did check the RUS Bulletin 50-3 Standard D 804 and it says: "DO NOT DISCONNECT TRANSFORMER NEUTRAL WITHOUT FIRST DISCONECTING PRIMARY". [If you can provide me with the link to the RUS website that you mentioned I'd appreciate it] Also Waross let me ask you: What do you mean by
"The primary of the third transformer will supply a voltage equal to the primary voltage"
Thanks Again

 

[ePOWEReng]

Here is a link to a similar paper:

http://hubbellpowersystems.com/powe...phase switching floating wye open delta bank"

 

[waross]

Hi fredpar;
Consider an open delta. Then connect a distribution transformer across the open delta. It will have rated secondary voltage applied across the low voltage winding. The transformer will now act as a step-up transformer and the high voltage winding will develop the rated high voltage. This should equal the phase angle and voltage of the third high voltage phase. When it doesn't, problems arise and transformers burn out.
Stated another way, when the voltage on any one phase goes low for any reason, the transformer on that phase will start to backfeed current into the line.
respectfully