Why Why Wye Wye

bwat

EE
I must need to revisit some of my old text books.

If there's a Yg-Yg step up transformer from a 3 phase generator where the generator neutral is NOT grounded, if there's a single line to ground fault on the distribution side of the transformer, how is it possible to have over-voltage on the unfaulted distribution phases?

When looking at the zero sequence circuit, I can see how there's the lack of a return to the generator, but I'm having a hard time conceptually grasping how over-voltage can happn since the neutral on the distribution is grounded.

Hypothetical setup is something like this:

(gen)---(Yg)-(Yg)---(dist)

Here's a white paper that is related to this. See Figure 8 and discussion around it.

electrofelon

Senior Member
All I can say is the wye wye has been tormenting people for hundreds, if not thousands of years......

synchro

Senior Member
As you've indicated, if the generator neutral is ungrounded then it presents an open circuit to zero sequence voltages and currents. The generator output will drive the Yg-Yg with line-to-line voltages and can have a relatively low source impedance for positive or negative sequences, but otherwise it's "floating" relative to ground.

Now looking into the Yg-Yg from the generator side, one of the phases will have a low impedance to ground because of the ground fault on the corresponding phase on the distribution side of the transformer. Therefore if we drive the Yg-Yg with the generator, one phase of the generator will be essentially grounded with this low impedance and so the voltage to ground on the other two generator phases will be close to the line-to-line value. Then the Yg-Yg will transfer this higher voltage directly over to the corresponding two "unfaulted" phases on the distribution side.

bwat

EE
As you've indicated, if the generator neutral is ungrounded then it presents an open circuit to zero sequence voltages and currents. The generator output will drive the Yg-Yg with line-to-line voltages and can have a relatively low source impedance for positive or negative sequences, but otherwise it's "floating" relative to ground.

Now looking into the Yg-Yg from the generator side, one of the phases will have a low impedance to ground because of the ground fault on the corresponding phase on the distribution side of the transformer. Therefore if we drive the Yg-Yg with the generator, one phase of the generator will be essentially grounded with this low impedance and so the voltage to ground on the other two generator phases will be close to the line-to-line value. Then the Yg-Yg will transfer this higher voltage directly over to the corresponding two "unfaulted" phases on the distribution side.
Thank you for this response. I’ve been trying to chew on this. It still is seeming a little like 2+2=5 though to me. I can’t get it to click.

Which of these things that I’m holding onto is incorrect in this fault situation? To put numbers to it to make it easier to describe, let’s say it’s 480LL on the gen side and 13.2KV LL on the dist side.

1. The gen side windings still are 277V at 120 degrees apart with respect to the center point
2. The coupling of each winding from one side of the transformer to the other still maintains itself so the dist windings still are 7260V at 120 degrees apart with respect to center point

If one of those are wrong, and I could grasp why (ha!), this may go a long way.

I feel like I almost get there sometimes if like think about what would happen if I put a dead short on the dist side terminals from H0 to H1. I wouldn’t have much of voltage drop across X1 to X0, so it would all (LL) go from X0 to X2 and X3 so the transformer would want to put LL across H0 to H2 and H3. Anywhere close?

synchro

Senior Member
Which of these things that I’m holding onto is incorrect in this fault situation? To put numbers to it to make it easier to describe, let’s say it’s 480LL on the gen side and 13.2KV LL on the dist side.

1. The gen side windings still are 277V at 120 degrees apart with respect to the center point
That is correct from the scenario you described. The 120 degree relationship is established by the rotating magnetic fields applied to the stator windings in the generator.
2. The coupling of each winding from one side of the transformer to the other still maintains itself so the dist windings still are 7260V at 120 degrees apart with respect to center point
That will depend on how well the loads on the distribution side are balanced, or if there's a a grounding transformer or wye-delta to stabilize and help center the neutral point. A wye-wye without a tertiary winding will have a relatively high impedance to zero sequence load currents drawn on the distribution side.

I feel like I almost get there sometimes if like think about what would happen if I put a dead short on the dist side terminals from H0 to H1. I wouldn’t have much of voltage drop across X1 to X0, so it would all (LL) go from X0 to X2 and X3 so the transformer would want to put LL across H0 to H2 and H3. Anywhere close?
I agree. That puts in more specific words what I was trying to say. H2 to H0 and H3 to H0 would be 60 degrees apart because the fault is basically forcing the three LL voltages to appear across H2, H3, and H0. This assumes there's not a grounding transformer or equivalent that will provide zero sequence currrents to oppose this and therefore reduce the amount of overvoltage. Also the transformer core legs for H2 and H3 may saturate and could limit the amount of overvoltage and/or cause other effects. In reality the 3-phase transformer legs do not behave totally independently if they are on a common core, but that's getting beyond this level of discussion and also my amount of direct experience on this particular issue.

synchro

Senior Member
I just noticed that for your questions #1 and #2 you were asking for the case when there's a L-G fault. My answer to #1 will still apply. But obviously the fault will be a severely unbalanced "load". So no it will not maintain the 120 degree relationship in this situation on the distribution side. As I mentioned in the last paragraph of my response it will depend to some extent on whether a grounding transformer is present. If it is not and there's only the wye-wye then there would be a 60 degree relationship as I described above. If there is a grounding transformer or equivalent then I'm guessing it would be somewhere between 120 and 60 degrees.

bwat

EE
Thank you, synchro. You are very close to making it stick for me.

So if I were then to add a (Yg-Delta) grounding transformer on the distribution side, how does this help? I would expect a bolted fault from H1 to H0 would still be refelcted back to the generator side of the transformer as a near short from X1 to X0, so I'd think you'd have the same issue of developing LL voltage from X0 to X2 and X3. And then that would show up as LL voltage from H0 to H2 and H3 which is precisely the problem we're trying to mitigate.

This likely has something to do with the circulating zero sequence current in the delta winding of the grounding transformer, but the reasoning for what this does/how it helps to change what I think is happening at the step up transformer eludes me.

One of my earlier posts was misleading. I stated this and you agreed with it, but I meant the generator side windings [of the transformer] and [in the faulted situation]. I think you would say this is false now. You would not have 277 from X0 to X1. It would be next to 0.
1. The gen side windings still are 277V at 120 degrees apart with respect to the center point

GoldDigger

Moderator
Staff member
If the incoming power is wye and not known to be very perfectly balanced in voltage, adding a wye input transformer with a delta winding (even internal only with no external leads) can cause severe circulating currents, even greater than normal full load currents, if the wye point of that transformer is grounded.

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mbrooke

Batteries Not Included
What if we simply added a delta tertiary winding?

GoldDigger

Moderator
Staff member
What if we simply added a delta tertiary winding?
If you add a delta tertiary winding to a wye primary with a solidly grounded wye point, you are asking for problems. If the primary wye point of the transformer is left floating the winding voltages will adjust so that the delta voltages form a closed loop without driving high secondary currents.

mbrooke

Batteries Not Included
If you add a delta tertiary winding to a wye primary with a solidly grounded wye point, you are asking for problems. If the primary wye point of the transformer is left floating the winding voltages will adjust so that the delta voltages form a closed loop without driving high secondary currents.
Why would there be problems? Thats actually a good thing.

GoldDigger

Moderator
Staff member
Yes, leaving the primary wye point floating is a good thing.

mbrooke

Batteries Not Included
Yes, leaving the primary wye point floating is a good thing.
There you go

bwat

EE
So to take it back to my question in post #8. How does adding a Yg-D grounding transformer mitigate the over voltage described because the reasoning for why the over voltage could happen seems to still apply?

(selfish bump)