Blown Primary Fuse on a Delta-Wye transformer for a Secondary Network

[holychicken]

Hello all, first post.

My education is in computer engineering and have focused mainly on embedded programming, both for processors and FPGA/ASICs. I kind of got thrown into the world of transmission and distribution engineering. So I pre-apologize if some of my terminology is wrong/confusing and if I state things that are extremely obvious. I don't apologize for being wrong, that's why I'm posting here.

Assume a 13kV primary and 120/208V secondary.

I kind of understand what would happen if we have a single delta-wye transformer feeding a single load and a fuse upstream of the primary is blown. However, I work on secondary network distribution systems where we have many delta-wye transformers all simultaneously feeding the same load. I am confused as to what would happen in the case of a blown fuse upstream of the primary winding on a single leg.

A little more information, the protector opens a connection between the secondary side of the Delta-wye transformer and the actual network.

If the protector is open with the blown fuse, I imagine it would look a lot like a single delta-wye transformer powering a single load, just a small load that equals the magnetization current of the transformer.

Where I get confused is when the protector is closed. I would assume the secondary side of the delta-wye transformer would remain in its normal balanced state (ideally, a positive sequence of 120 V @ 0 degrees). But what would happen with the currents? And what would happen with the voltages on the delta (primary) side of the transformer? I am thinking that on one leg you would see normal forward current, but on the other two legs you would see a reverse current equal to the magnetization current of the transformer. Is this thinking even remotely correct?

Thanks for your consideration.

 

[Smart $]

Welcome to the forum :thumbsup:

I read your post earlier, but had other things to do at the time, and then got sidetracked. I am surprized no one else has replied.

I think in order to answer you question with any substantial degree of certainty (and perhaps eliminate a lot of supposition), we need to better understand the scenario.

1) Are the multiple transformers supplied by a common source, or isolated sources, or a combination thereof?

2) If common source, is the blown primary fuse, on the primary itself (affecting multiple transformers), or on the primary tap going to a single transformer?

3) The "protector" mentioned seems to be on the secondary side. Are we talking common switch/trip (affects all three lines), or individual-line acting?

I should note T&D is not my area of expertise.

 

[holychicken]

First, thanks for the response and the help (it is always good to think about things outside of this as well) and I will try to answer your questions as best I can.

1) I'm not entirely sure if there are any isolated sources. However, you will have one feeder that will feed the network through multiple protectors. So in that sense you definitely have at least a common source. But you also have other feeders feeding the same network through their own protectors, I'm just not sure if they come from a common source or isolated sources. The idea is if you lose one feeder (fault or it is taken out for some kind of service) the other feeders will keep the network alive. So, of course, all of the feeders are in phase.

2) This all depends on the set up. But I am speaking of the case where the fuse only goes to one protector.

3) Yes, the protector is on the secondary side of the system (between the wye side of the transformer and the actual network) and it is a common switch.

Thanks again.

 

[wirenut1980]

I am having some trouble visualizing what you are talking about, but here is what I think you are saying.

You have several 13 KV-208Y/120 V 3 phase transformers networked on the secondary side (is it an underground network?). Let's assume all are connected to the same primary 13 KV source. A fuse blows on the 13 KV feeding one of the transformers. As a result of this, a protector on the secondary side of the transformer opens up all 3 phases. So for this transformer, you have two phases energized on the primary, and no load on the secondary. As far as currents, you will likely have just transformer magnetization currents, plus some real current from the copper losses in the transformer on the two primary phases. However, with these sets of circumstances, you may have a recipe for ferro-resonance, which can blow MOV's on the primary of the transformer, or possibly fail the transformer.

If the secondary potector is closed after replacing the blown fuse, everything should operate normally I think. If the secondary protector is closed without replacing the blown primary fuse, then I would think you would measure low/unbalanced voltage on the secondary, but it is hard to say what it would be, because the secondary is networked and all those other transformers would be helping hold up the secondary voltage.

Is this hypothetical situation, or did something happen?

 

[Smart $]

I am having some trouble visualizing what you are talking about, but here is what I think you are saying.

You have several 13 KV-208Y/120 V 3 phase transformers networked on the secondary side (is it an underground network?). Let's assume all are connected to the same primary 13 KV source. A fuse blows on the 13 KV feeding one of the transformers. As a result of this, a protector on the secondary side of the transformer opens up all 3 phases. So for this transformer, you have two phases energized on the primary, and no load on the secondary. As far as currents, you will likely have just transformer magnetization currents, plus some real current from the copper losses in the transformer on the two primary phases. However, with these sets of circumstances, you may have a recipe for ferro-resonance, which can blow MOV's on the primary of the transformer, or possibly fail the transformer.

If the secondary potector is closed after replacing the blown fuse, everything should operate normally I think. If the secondary protector is closed without replacing the blown primary fuse, then I would think you would measure low/unbalanced voltage on the secondary, but it is hard to say what it would be, because the secondary is networked and all those other transformers would be helping hold up the secondary voltage.

Is this hypothetical situation, or did something happen?

Similar, if not the same, as what I was thinking.

Waiting to see what OP'er says...

 

[holychicken]

wirenut, thanks for your response.

You have several 13 KV-208Y/120 V 3 phase transformers networked on the secondary side (is it an underground network?). Let's assume all are connected to the same primary 13 KV source.

Fair enough, I think this is effectively what it looks like anyway.

A fuse blows on the 13 KV feeding one of the transformers. As a result of this, a protector on the secondary side of the transformer opens up all 3 phases. So for this transformer, you have two phases energized on the primary, and no load on the secondary. As far as currents, you will likely have just transformer magnetization currents, plus some real current from the copper losses in the transformer on the two primary phases. However, with these sets of circumstances, you may have a recipe for ferro-resonance, which can blow MOV's on the primary of the transformer, or possibly fail the transformer.

I think I have an idea of what would happen if the protector is already open or opens up. However, I do not believe it will open up based on the current algorithms used to "trip" the protector.

If the secondary potector is closed after replacing the blown fuse, everything should operate normally I think. If the secondary protector is closed without replacing the blown primary fuse, then I would think you would measure low/unbalanced voltage on the secondary, but it is hard to say what it would be, because the secondary is networked and all those other transformers would be helping hold up the secondary voltage.

This is exactly what has me confused. I have a feeling that the voltage on the networked side would stay up and keep the transformer energized (in the sense that the voltages will all look normal) but we would have some weird current patterns. Maybe just small energizing currents (in two legs, normal current in the third). But I'm really not sure which is why I am asking here. (edit here: I did not realize you said "replaced fuse." The protector will not necessarily trip when the fuse blows. My question is what the currents and voltages would look like in the transformer. This may be a very simple question).

Is this hypothetical situation, or did something happen?

This does happen regularly and this is where it gets complicated. There are competing algorithms used for protection now, I am fairly certain one will just ignore this altogether, but a customer wants us to explore the possibility of catching these so they can get some kind of warning, and I just want to make sure I know what I am looking for. Protector opened: no problem. Protector closed: mathematics confuse me a bit (or I am over thinking it).

Thanks again for the thought and consideration. I really do appreciate it.

 

[Smart $]

wirenut, thanks for your response.


Fair enough, I think this is effectively what it looks like anyway.


I think I have an idea of what would happen if the protector is already open or opens up. However, I do not believe it will open up based on the current algorithms used to "trip" the protector.


This is exactly what has me confused. I have a feeling that the voltage on the networked side would stay up and keep the transformer energized (in the sense that the voltages will all look normal) but we would have some weird current patterns. Maybe just small energizing currents (in two legs, normal current in the third). But I'm really not sure which is why I am asking here. (edit here: I did not realize you said "replaced fuse." The protector will not necessarily trip when the fuse blows. My question is what the currents and voltages would look like in the transformer. This may be a very simple question).


This does happen regularly and this is where it gets complicated. There are competing algorithms used for protection now, I am fairly certain one will just ignore this altogether, but a customer wants us to explore the possibility of catching these so they can get some kind of warning, and I just want to make sure I know what I am looking for. Protector opened: no problem. Protector closed: mathematics confuse me a bit (or I am over thinking it).

Thanks again for the thought and consideration. I really do appreciate it.

I don't think there is any one answer without specific values. What happens will be dependent on the "stiffness" of other transformers simultaneously supplying the network. Consider the result with only this one transformer (i.e. no network), then compare to two identical supplying network, then three, four, etc. Then there's the possibility not all transformers are identical. ...

 

[holychicken]

There is a condition where the protector (either due to bad settings or a faulty device) will not trip open when the feeder is taken out of service.

The relay is supposed to trip the protector when it sees the reverse magnetizing current of the transformer and, if this doesn't happen, the network voltages still normally look fine. The network is supposed to function normally even if an entire feeder is lost.

So I think in the case where the protector is closed, we can assume that the network does not suffer from the loss of that source and the network even acts like a source for the remain 2 phases of that transformer.

Does that make any sense?

 

[Smart $]

...

The relay is supposed to trip the protector when it sees the reverse magnetizing current of the transformer and, if this doesn't happen, the network voltages still normally look fine. The network is supposed to function normally even if an entire feeder is lost.

That's just it. The transformer primary still has voltage across each primary winding. The one winding connected across the two energized lines is still normal, while the other two windings act in series with the energized lines voltage, 13kV?2 each but but out-of-phase with each secondary winding... but that is only if there is no influence from the secondary. The secondary voltage will influence these primary windings. If the secondary network voltage is stiff enough, there won't be enough of a difference for fault sensing to pick up on.

So I think in the case where the protector is closed, we can assume that the network does not suffer from the loss of that source and the network even acts like a source for the remain 2 phases of that transformer.

Does that make any sense?

Yes. Just speculating in general, I think if you use a more sensitive adjustment or sensing algorithm on the protection equipment, you risk nuisance tripping. Each actual situation would need a comprehensive analysis to make a risk vs. reward determination.

 

[holychicken]

That's just it. The transformer primary still has voltage across each primary winding. The one winding connected across the two energized lines is still normal, while the other two windings act in series with the energized lines voltage, 13kV?2 each but but out-of-phase with each secondary winding... but that is only if there is no influence from the secondary. The secondary voltage will influence these primary windings. If the secondary network voltage is stiff enough, there won't be enough of a difference for fault sensing to pick up on.

On the voltages, my guess agrees with you and that there would not be enough of a difference to differentiate normal variance from a blown fuse condition if the secondary is stiff enough, which I believe to be true. However, I think we may be able to look at the currents and make a determination from there.

Yes. Just speculating in general, I think if you use a more sensitive adjustment or sensing algorithm on the protection equipment, you risk nuisance tripping. Each actual situation would need a comprehensive analysis to make a risk vs. reward determination.

Possibly. This is kind of what I am trying to hammer out.

But thanks again for the help, this has given me a lot to think about.

 

[gar]

130205-1422 EST

Some comments and associated assumptions.

First, assume the secondary contactor (protector or switch) does not exist. Then all secondaries remain connected to the secondary bus at all times.

Second, assume a large number of transformers feed the secondary bus, and only one three phase transformer (could be three separate single phase transformers) has one primary line lost (opened). Thus, the secondary bus voltage is a very stiff source from all the powered transformers. The secondary source becomes a source to back feed the three phase transformer with one lost primary leg.

Third, magnetizing current under normal conditions is a small fraction of full rated load current on an RMS basis. This is true whether the transformer is forward or back fed.

Fourth, it is reasonable to assume that the instantaneous primary and secondary voltages are approximately related by the turns ratio.

What happens is that the two delta primary windings with the common lost primary leg are back fed with energy from the secondary bus to supply energy (power) back to the primary bus to perform a redistribution of energy (power) so that the primary voltages of the partially disabled transformer are approximately what they would be if the one primary leg was not open. Impedances, loads, and the number of transformers feeding the secondary bus will determine the magnitude of the redistribution.

If you measured the load power separately on each leg of the secondary side of the transformer, then there would be a substantial and measurable change in these power levels from normal conditions to when the one primary line opened. If you had phase sensitive current sensors in each of the secondary lines on the faulted transformer, then you could get the same information. Just RMS current magnitude might not be good enough. However, if you monitored the delta primary lines feeding the transformer, then the current on the open fuse line would go to zero.

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