YY transformer with floating primary

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Haji said:
Hi mivey!
What is the primary current for the unloaded, neutral floating Y-Y transformer with one shorted secondary of the above type? Thanks.
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With no load it is less interesting but is about:
IA = 0.017<105
IB = 0.017<45
IC = 0.030<-105
 
Phil Corso said:
you left out two calcs necessary for a full analysis
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Well my goodness, I gave the values you asked for. :D

Case 1:
a) Neutral-Shift or Neutral-Displacement: ~none
b) Load over-voltages: ~none

Case 2:
a) Neutral-Shift or Neutral-Displacement: ~none
b) Load over-voltages: Van = 119.9<-0.15, Vbn = 120.1<-120.1, Vcn = 120.1<119.9

Case 3:
a) Neutral-Shift or Neutral-Displacement: ~none
b) Load over-voltages: ~none

Case 4:
a) Neutral-Shift or Neutral-Displacement: Vng = 42.73<-89.42
b) Load over-voltages: Van = 126.9<19.52, Vbn = 86.07<-134.8, Vcn = 158.8<112.3
 
mivey said:
With no load it is less interesting but is about:
IA = 0.017<105
IB = 0.017<45
IC = 0.030<-105
Click to expand...
Thanks, mivey. The interesting point is the presence of loads in other phases are the cause overloading of the transformer and not the shorting out of one secondary phase of the transformer is the cause of the overloading directly, when the primary neutral is floating. This is the point of my post#39. :)
 
Haji said:
There is very small increase in primary current in the phase corresponding to the shorted secondary phase as that primary phase current has to flow through the other two primary phases on that condition during primary neutral floating. So there would be no appreciable change in core flux of the transformer.
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Does this same principle apply to highside delta connected xfmrs?
 
Bugman1400 said:
Does this same principle apply to highside delta connected xfmrs?
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I do not think so because for each pair of delta connected secondary leads, there corresponds a pair of primary leads and so for any secondary phase short, substantial over current would flow.
 
Haji said:
I do not think so because for each pair of delta connected secondary leads, there corresponds a pair of primary leads and so for any secondary phase short, substantial over current would flow.
Click to expand...

Who said anything about delta connected secondary leads? What about the same secondary g-wye leads? I guess my question stemmed from the thinking that if an un-g-wye primary leads lose reference to ground then delta-connected primary leads would follow the same principle and have the same effect.

What say you?

Maybe Phil can chime in here and start talking about centroids again. Has anybody heard from Phil lately......Phil, still here?
 
Bugman1400 said:
Who said anything about delta connected secondary leads? What about the same secondary g-wye leads? I guess my question stemmed from the thinking that if an un-g-wye primary leads lose reference to ground then delta-connected primary leads would follow the same principle and have the same effect.

What say you?
Click to expand...
Sorry, Bugman. I misunderstood you referred to high leg delta. Still, I think a phase short on the star secondary (Phase to Neutral)would cause substantial over current on the delta primary side as the short circuit current could flow freely between any two line leads of the delta primary.
 
Summarizing,

The Part 1 calc, illustrated that the voltage-triangle of an unbalanced Y-connected load supplied from a 3-ph, 4-wire, source, will remain intact!

The Part 2 calc, illustrated that the voltage-triangle of an unbalanced Y-connected load supplied from a 3-ph, 3-wire, source, will became distorted, and its neutral-point displaced or shifted!

And now Part 3!

How does an isolated-neutral affect performance of a Y-Y transformer? For simplification purposes I will ignore harmonic effects!

First consider insulation! Typically, if wye-connected, and neutral-grounded, then insulation (between core and coil) is usually reduced for economic reasons! But, if the neutral is isolated, then full-voltage insulation is required!

Secondly, and more importantly, is the imbalance of excitation-current for each of the three primary-windings connected to the source! When the 3-winding mid-point is connected to source-neutral, therir excitation-currents, Iea, Ieb, Iec, magnitudes are (almost*) equal, and their sum (almost*) zero! But when their mid-point is isolated, their Ie parameters are unequal, so that, for example, Iea = Ieb + Iec, failure is certain!

Note:

The (almost*) term represents the fact that Ie magnitude is dependent on the circuit-model used! If Ie components are located at the Xfmr?s primary-winding terminals, the so-called ?Pi? model, then Ie is affected by only source-voltage! But, if Ie components are located after the primary winding-impedance, the so-called ?T? model, then Ie is affected by both source-voltage and load-current!

Regards, Phil Corso
 
Gentlepeople,

As a result of off-forum requests for additional information following is an an explanation of the "Pi" and "T" circuit-models of a transformer!

The caveat... it's like defining how a ratchet works without using you hands!

Typically, excitation-current components are usually presented as a 'lossy' reactor having two parameters; one resistance-related, the other, inductance-related!

They are configured as a resistor in parallel with an inductor! Their placement in the model varies, depending on the degree of importance necessary for the problem at hand! (Or the can be configured as elements in series!)

In the "Pi" Model, the two pralleled-elements are placed at the primary-winding terminal (left vertical-leg of the 'Pi' symbol!) The series connection of both primary and secondary impedances follow (horizontal-leg of the 'Pi' symbol!) Finally, the secondary-load impedance is shunt connected (right vertical-leg of the 'Pi' symbol!)

For the "T" model, the two paralleled-elements are placed between the primary-winding and secondary-winding impedances!

If additional information is needed, contact me!

Regards, Phil
 
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