delta secondary supplying single phase system

[UtilityGy]

Gentleman,

My second question on this forum. My first question response was of immense help.

I have a 27.6 kV primary and 13.8 kV secondary. It is a Star/Delata transformer. Secondary has a grounding transformer.

My question is how realistic is to provide a single phase supply to customers with a distribution line 13.8 kV configuration fed from delata secondary. Let mee rephrase, If it can be done.

I am sure you guys have seen and done it all. I am just
experimenting some ideas and possibilities.
I would appreciate your input.

Thanks

 

[waross]

If your supply is weak and if you have the primary neutral connected, the single phase load may cause unequal voltage drops on the primary conductors and/or a voltage drop on the primary neutral. That may result in circulating currents in the delta. The circulating currents may limit themselves by correcting the unbalance.
A 10 KVA single phase load will load all transformers to 5 KVA.
Another way of saying this is that you may only use 2/3 of a delta bank rating when loading all on one phase.
Yes you may be able to do it. There are a lot of effects that depend on the firmness of the system, the relative size of the transformers, the frequency of phase loss, the loading, other loads on the system. It depends, but will probably work.


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

 

[UtilityGy]

Thanks Waross. I am glad that your responded. I have read u r post responses and they are brilliant.

What I understood that the 27.6 kV primary star neutral which is grounded could have an impact on secondary voltage balance.

In regards to a 13.8 kV Delta secondary supplying a single phase load, how would this work ? If I have supplying this I will use one phase an then ground the secondary point of the pole mounted transformer. What if the ground breaks what would this mean to the secondary of pole mounted transfromer. Its configuration will be 120/240 V.

Will 13.8 kV secondary have to have a grounding transformer to use single phase loading option.

I would appreciate, if you could help.

Thanks

 

[waross]

The issue with a primary neutral connected Wye/Delta bank is primary voltage unbalances.
To visualize this, break the transformer bank into an open delta (two transformers "A" Phase and "B" Phase) and a single phase (one transformer,"C" Phase.
The open delta will form a virtual transformer in place of "C" Phase. This virtual transformer will have the same characteristics as the single phase transformer on "C" Phase.
The voltages will be equal and in phase so that it may be safely connected to close the delta.
Consider however what will happen if the primary voltage is low on "C" Phase. The real transformer voltage will no longer be equal to the virtual transformer voltage. This will cause a circulating current in the delta. This voltage may be small but the current will be limited by the transformer impedances and a voltage pu error of much more than the impedance voltage of the transformer may cause current greater than the full load current of the transformer.
BUT if you already have the wye/delta bank in service that will indicate a good voltage balance and few missing phase issues. BTW in the event of a missing phase the affected transformer will back feed and attempt to replace the missing phase. What happens next "It depends". A warning to service men is in order. When a single phase feed cutout is opened the fuse holder will still be hot on both sides until all three phases are cleared.
For your proposed installation, it will probably not add any complications.
By aware that the in phase transformer ("C" will supply 50% of the current and the virtual transformer ("A" and "B" in the example) will supply 50% of the current. The current in "A" and "B" will be leading in one and lagging in the other. The power factors will be 50% leading and 50% lagging.
Grounding or floating the 13.8kV system will have no effect on secondary of the 13.8kV/120:240V system. Ground the center tap of the 120:240 Volt transformer in accordance with good practice.
Note, loss of one primary phase, any primary phase (27.6kV system) will still leave full voltage o the 120:240 volt system possibly indefinitely, possibly not, it depends.
Loss of the primary phase feeding the single phase transformer and any other primary phase (27.6 kV system) will leave the unloaded voltage on the single phase transformer approaching 50%, 60:120V. Any load will tend to drop this voltage. How much? It depends. Possible saturation of the other unfed transformer in the 27.6/13.8kV bank may hold the voltage up quite a it higher than a simple load/transformer impedance ratio calculation may indicate.
*It depends: Source impedance, other loads on the 27.6 kV system, other loads on the 13. kV system, fusing, human factor (In areas where this the four wire wye/delta connection is used and single phase outages are common, the crews respond to numerous trouble calls to the same transformer bank by either over fusing or removing one primary fuse completely. Overfusing often leads to transformer burnout.)
But, as noted, these are issues with your existing system and apart from the 50% voltage with two phases missing should not affect your addition of a single phase transformer.

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

 

[pwrengrds]

You can connect a single phase transformer 13.8kV phase to phase with whatever secondary phase voltage is needed, this would not have the neutral current issues.