mivey
Senior Member
To expand: With Z1=Z2 and Z0 ~ 0 (close to a delta-wye transformer) we get the approximation:
If(3) = En / Z1
If(1) = 3 x En / (Z1+Z2+Z0) ~ 3 x En / (Z1+Z1+0) = 3 x En / 2*Z1 = 3/2 * En / Z1 = 1.5 * En / Z1
so If(1) ~ 1.5 * If(3)
Available Fault Current Calculation Question
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If(3) = En / Z1
If(1) = 3 x En / (Z1+Z2+Z0) ~ 3 x En / (Z1+Z1+0) = 3 x En / 2*Z1 = 3/2 * En / Z1 = 1.5 * En / Z1
so If(1) ~ 1.5 * If(3)
Bugman1400
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- Charlotte, NC
2.) I don't know much about low voltage generators. It makes sense that they would be solidly grounded.
3.) I was referring to delta vs wye. I thought you were going in the direction of explaining why buried delta tertiaries provide a significant amount of the zero sequence fault current.
Very good discussion so far.:thumbsup:
Bugman1400
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Can you explain how Z0 would go to zero?
Phil Corso
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- Boca Raton, Fl, USA
Bugman... no!
Now you have me puzzled! I know about an infinite zero-impedance, but never zero zero-sequence! Now how is that accomplished?
Phil
Now you have me puzzled! I know about an infinite zero-impedance, but never zero zero-sequence! Now how is that accomplished?
Phil
Bugman1400
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- Charlotte, NC
Phil, that's my question to mivey.
mivey
Senior Member
I said approximately zero; you still have the transformer zero sequence impedance (Z0T), but when compared to the relatively large impedance on the source side, we can approximate it as zero for determining the maximum L-G fault possible (Z0T -> 0).
In real calcs, you would include the transformer impedance (the transformer leakage impedance). X0 (zero) is about the same as X1 (positive) on shell type transformers. For core type transformers, X0 is about 85-90% of X1 because zero sequence current circulates in the tank as well as the core.
As for the zero sequence impedance on the source side of the transformer, it is not in the sequence network current path. I0 circulates in the delta (thus we include the transformer impedance) but does not circulate in the primary lines so we do not see the source zero sequence impedance (Z0S); in other words, it is blocked from our network.
Thus there is no source impedance in the network current path so Z0 in our fault formula is driven to zero as we drive Z0T to zero for the maximum L-G fault case.
mivey
Senior Member
An infinite impedance in our sequence network path would result in no fault current! The source zero sequence impedance is not in our current path thus is not in our fault formula! The transformer impedance is left in the zero network and as we drive it to zero for the maximum L-G fault analysis (Z0 << Z1) we drive Z0 to zero in our fault formula!
mivey
Senior Member
Because for the wye-delta-wye transformer, the currents circulate in the buried delta like they do with the delta-wye; both transformer configurations act as zero sequence sources for the grounded wye side.
The Y-D-Y will act as a zero sequence source for whichever wye side is grounded (or both sides if they both are grounded). The buried delta can circulate zero sequence currents from either side.
Zero sequence currents will not pass through if either or both sides are ungrounded.
Phil Corso
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- Boca Raton, Fl, USA
Mivey...
It is true that zero-sequence currents don't exist in Y-connected windings unless its neutral is grounded! Furthermore, zero-sequence currents circulated within D-connected circuits don't produce zero-sequence components in connecting lines!
Thus, the lines are open-circuited to zero-sequence currents, meaning zero-sequence impedance is infinite, not zero!
Phil
It is true that zero-sequence currents don't exist in Y-connected windings unless its neutral is grounded! Furthermore, zero-sequence currents circulated within D-connected circuits don't produce zero-sequence components in connecting lines!
Thus, the lines are open-circuited to zero-sequence currents, meaning zero-sequence impedance is infinite, not zero!
Phil
mivey
Senior Member
Check yourself!
The Z0 in the fault formula and how it goes to zero is the topic! That is the impedance in the path of the network current! An infinite impedance makes no sense in that path!
The impedance you want to discuss is not included! Not in the formula! A different discussion! Something else! In a different circuit loop! In a different fault formula!
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Mivey...
So, you don't believe my first sentence, "It is true that zero-sequence currents don't exist in Y-connected windings unless its neutral is grounded!"
So, you don't believe my first sentence, "It is true that zero-sequence currents don't exist in Y-connected windings unless its neutral is grounded!"
mivey
Senior Member
Read my post #28. I already said these currents can be on whatever wye is grounded.
What you need to see is that whatever happens with the zero sequence impedance on the primary side of a delta-wye transformer is irrelevant to Z0 in the L-G fault equation at the secondary terminals. Z0 looking back to the source at that point is composed only of Z0T.
Z0S_primary is not in the fault equation. It can be infinite, non-existent, or two monkies summersaulting on the back of a circus elephant; it has nothing to do with the loop equation and driving Z0T (thus Z0) to zero.
Phil Corso
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- Boca Raton, Fl, USA
Mivey,
Can you illustrate what you mean with an example in which the zero-sequence impedance, Zo, is 0?
Phil
Can you illustrate what you mean with an example in which the zero-sequence impedance, Zo, is 0?
Phil
Last edited:
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Mivey...
My example, using your D-Y example:
1) There are Zero-seq currents in he Y-windings.
2) There is a Zero-Seq current circulating in the D-winding.
3) There are no Zero-Seq currents in the primary supply windings.
My question to you: Does 3) mean that Zero-sequence impedance is '0'?
Phil
My example, using your D-Y example:
1) There are Zero-seq currents in he Y-windings.
2) There is a Zero-Seq current circulating in the D-winding.
3) There are no Zero-Seq currents in the primary supply windings.
My question to you: Does 3) mean that Zero-sequence impedance is '0'?
Phil
mivey
Senior Member
Consider this illustration:
Take a two-terminal voltage supply, V, and several resistors, say Ra, Rb, Rc, Rd, Re. Now connect the resistors a-e in series and put this resistor string across the voltage terminals (having voltage V) and study the current I. Now make the following current equation from the circuit:
I = V / (Ra + Rb + Rc + Rd + Re)
In discussing the fault formula and the sequence network circuit, we have something similar to the current equation above. I am discussing the resistances in the equation, their relationship to each other, and their impact on the circuit.
You want to discuss a resistor Rx that is sitting in the supply bin and not even connected to the string. Stop doing that.
Whether Rx is infinite, zero, or whatever has no impact on the fault equation as it is not even in the circuit.
mivey
Senior Member
No. It means that the source zero sequence impedance is not even in the sequence network circuit.
Ultimately, if you want to get in the weeds, it is external to the balanced circuit, but not because it is an infinite source impedance like you keep thinking. It is external because of Kirchoff's Law since we have a balanced set of currents. In fact, you will find that for the unbalanced network circuit, the source impedance is not infinite but plays an active role.
Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Mivey...
So then, the term zero Zero-Sequence Impedance simply means it doesn't exist!
Phil
So then, the term zero Zero-Sequence Impedance simply means it doesn't exist!
Phil
Bugman1400
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- Charlotte, NC
Well put. This is exactly what I thought you were referring to before.
Calculation SLG fault at secondary side of the delta wye transformer on the wye side:
Considering zero source impedance. Will equal to 3P fault. Unless there is source impedance values, the value would be different. Whether or not SLG is higher, depends on the values of source impedances. But, the zero sequence impedance of the transformer will still be included for a SLG fault at the wye side.
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Considering zero source impedance. Will equal to 3P fault. Unless there is source impedance values, the value would be different. Whether or not SLG is higher, depends on the values of source impedances. But, the zero sequence impedance of the transformer will still be included for a SLG fault at the wye side.
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Phil Corso
Senior Member
- Location
- Boca Raton, Fl, USA
Bugman, Mivey...
"The Y-D-Y will act as a zero sequence source for whichever wye-side is grounded (or both sides if they both are grounded). The buried delta can circulate zero sequence currents from either side."
"Zero sequence currents will not pass through if either or both sides are ungrounded."
The first is correct, but the second... !
Phil
"The Y-D-Y will act as a zero sequence source for whichever wye-side is grounded (or both sides if they both are grounded). The buried delta can circulate zero sequence currents from either side."
"Zero sequence currents will not pass through if either or both sides are ungrounded."
The first is correct, but the second... !
Phil
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