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open delta distribution transformer

open delta distribution transformer

open delta distribution transformer

(OP)
Has anyone seen a distribution transformer used as grounding transformer connected star-open delta with resistor on open delta , as a low impedance grounding system?.


 

RE: open delta distribution transformer

Open delta wouldn't accomplish anything.  Broken delta, on the other hand, could certainly be used for that application, though I'd only try it for high impedance grounding.  You'd need too large a transformer and too high a power rating for your resistor for low impedance grounding.

RE: open delta distribution transformer

(OP)
You right. I am talking about broken delta.
BUT:
I know how to use Yn-broken delta Voltage Transformer.
I do not  know how to use Yn-broken delta Distribution Transformer:
How are the symmetrical components of this type of transformer connection?
 

RE: open delta distribution transformer

Only zero-sequence circulates in the delta, therefore only zero-sequence can exist in the wye winding.  So, if you draw it out, you would have an open on the wye side of the transformer in the positive- and negative-sequence networks and the resistor in the zero-sequence network.

RE: open delta distribution transformer

(OP)
I do not agree!
During a fault-to-earth on generator system you always have
If = I1 + I2 + I0 at point of short circuit to ground.
The question is you are reducing I0 by resistor at zero sequence.
I imagine this resistor is seen in the neutral of Yn due to reflection by actual resistor inside delta.
That means,
The resistor in the Yneutral is equivalente to resistor in delta multiplied by Vy/Vd.
I am not sure yet.

RE: open delta distribution transformer

(OP)
You are right davidbeach.
I forgot the symmetrical components should be analised starting at the point of fault. I found this explanation about  Yn-d (closed) grounding transformer bank:

"A primary phase to neutral fault near the grounding bank approach very closely to the conditions which would exist if primary leads of one transformer in the grounding bank were shorted. Under such conditions the impedance of the faulted phase drops to a value approximating the short circuit impedance of the transformer bank in that phase.

The resulting unbalance voltage causes a circulating current in the delta secondary of the grounding bank. This circulating current causes an induced voltage  in the primary of the shorted grounding bank which induced voltage causes current to flow between the faulted phase  and the neutral, the current being inversaly proportional to the sum of the impedances of the grounding bank and the faulted line section.
...........
The unbalanced phase to neutral load divides equally between the transformers of the grounding bank , each transformer carrying one third of the total".
reference: grounding banks - determining the required capacity- L.B. Stahler-Florida Power & Light Company-
sept 1943
  

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