120/240 backfeed to create 7200 causes 240 volt breaker to trip.

[11bgrunt]

Recently I built a demo that fed 240 volts into the secondary of a conventional wet transformer, 120/240 - 12470Y/7240. I wanted primary voltage to feed back into a MV regulator to show operation and test methods for training. I had visited with folks at another utility that had built their high pot booth and discussed the problems they had run across putting it all together. The problem was that if the neutral was connected to the center tap of the secondary, the breaker protecting the 240 would always trip on magnetic. They pulled the neutral from the secondary coil, served the transformer with 240 only and the backfeed produced the MV 7240 as desired. I built mine using the 240 only and had no problems. Today a group called because they are building a new arcing demo and could not keep their 240 breaker from tripping. I advised them to take the neutral out of the low side and then all worked fine. I have had my hands in many transformers over the years and know this 120/240 volt transformer uses two 120 volt coils in series with a center tap for the neutral. We preach daily that backfeed will kill you because the low voltage goes through the transformer and energizes the primary. Been saying this for years.
Why does the neutral connected to the center tap of the 120/240 coil cause this problem when we backfeed?
I have drawn this out but still can't explain it.

 

[GoldDigger]

Any chance that the supply voltages on L1 and L2 are even a little unbalanced?

 

[11bgrunt]

Any chance that the supply voltages on L1 and L2 are even a little unbalanced?

Can't see where your headed.
I would say balanced 120-120-240

 

[GoldDigger]

Imbalanced applied voltage on a center tapped winding can cause very high circulating currents. Same reason you do not connect star point on a wye primary for three phase.

 

[11bgrunt]

Imbalanced applied voltage on a center tapped winding can cause very high circulating currents. Same reason you do not connect star point on a wye primary for three phase.

I think I can see that. I was thinking along the lines that the source was from a 208Y three phase system in their case. 120 versus 180 degrees.

 

[templdl]

Recently I built a demo that fed 240 volts into the secondary of a conventional wet transformer, 120/240 - 12470Y/7240. I wanted primary voltage to feed back into a MV regulator to show operation and test methods for training. I had visited with folks at another utility that had built their high pot booth and discussed the problems they had run across putting it all together. The problem was that if the neutral was connected to the center tap of the secondary, the breaker protecting the 240 would always trip on magnetic. They pulled the neutral from the secondary coil, served the transformer with 240 only and the backfeed produced the MV 7240 as desired. I built mine using the 240 only and had no problems. Today a group called because they are building a new arcing demo and could not keep their 240 breaker from tripping. I advised them to take the neutral out of the low side and then all worked fine. I have had my hands in many transformers over the years and know this 120/240 volt transformer uses two 120 volt coils in series with a center tap for the neutral. We preach daily that backfeed will kill you because the low voltage goes through the transformer and energizes the primary. Been saying this for years.
Why does the neutral connected to the center tap of the 120/240 coil cause this problem when we backfeed?
I have drawn this out but still can't explain it.

From what you said the KVA of the transformer is and the rating of the circuit breakers the breaker will most likely trip bec as use of the transformer inrush.

 

[11bgrunt]

From what you said the KVA of the transformer is and the rating of the circuit breakers the breaker will most likely trip bec as use of the transformer inrush.

I don't understand.
Transformer does get energized and will produce MV 7200 with no neutral connected to the 240 volt coil center tap. 100 amp breaker trips when the arc is maintained long enough to take breaker out on thermal. Add neutral to 240 side and that breaker trips instantaneous. I can offer many examples where they did use three wire and all worked fine.

 

[kwired]

Different impedance in half winding then in full winding - your problem is likely more the surge current upon energizing and you are getting more current in each 120 volt segment then you are if only applying 240 across the entire series. If you get it to hold during starting it may otherwise be fine.

I can understand the desire to connect the neutral so you still get full voltage if you bring it to a place where 208 is available. You might be better off using a buck boost transformer to convert from 208 to 240 when at those sites, if 208 is not going to work for you but something tells me it might work for most of what you are doing, and in either case not use the neutral tap of your 120/240 x 7200 transformer.

 

[GoldDigger]

Different impedance in half winding then in full winding - your problem is likely more the surge current upon energizing and you are getting more current in each 120 volt segment then you are if only applying 240 across the entire series. If you get it to hold during starting it may otherwise be fine.

I can understand the desire to connect the neutral so you still get full voltage if you bring it to a place where 208 is available. You might be better off using a buck boost transformer to convert from 208 to 240 when at those sites, if 208 is not going to work for you but something tells me it might work for most of what you are doing, and in either case not use the neutral tap of your 120/240 x 7200 transformer.

When you have a single core with a center tapped 240/120 winding you cannot connect two different 120V phase lines and the neutral!

 

[kwired]

When you have a single core with a center tapped 240/120 winding you cannot connect two different 120V phase lines and the neutral!

Do you mean in the 208/120 instance, I think it should work with 120/240 you just may have different starting surge then with 240 only, then as you said any voltage imbalance between the 120 lines would also introduce problems.

 

[GoldDigger]

Do you mean in the 208/120 instance, I think it should work with 120/240 you just may have different starting surge then with 240 only, then as you said any voltage imbalance between the 120 lines would also introduce problems.

1. No way can you connect the neutral with 208Y/120. That is a non starter.
2. With no current in the neutral the surge from balanced 120/240 three wire will be no different with or without the neutral.
3. If L1 is 118 and L2 is 120 you will be connecting a stiff 118 supply to a stiff 120 supply via a 1:1 transformer with very low impedance. Enough current will flow that the IR drop balances the two applied voltages. Very simple.

 

[templdl]

I don't understand.
Transformer does get energized and will produce MV 7200 with no neutral connected to the 240 volt coil center tap. 100 amp breaker trips when the arc is maintained long enough to take breaker out on thermal. Add neutral to 240 side and that breaker trips instantaneous. I can offer many examples where they did use three wire and all worked fine.

To my point. , A 100a breaker feeding the (?) KVA transformer. Now, What is the KVA of the transformer and the fla at 240v? Maybe, just maybe it can be be determined what inrush can be expected that would conflict with your 100a breaker's instantaneous trip.

 

[GoldDigger]

To my point. , A 100a breaker feeding the (?) KVA transformer. Now, What is the KVA of the transformer and the fla at 240v? Maybe, just maybe it can be be determined what inrush can be expected that would conflict with your 100a breaker's instantaneous trip.

Which should not vary at all depending on whether or not the neutral is connected.
Now if the two half windings are very asymmetrical in position relative to the core there a chance that the inrush will be different with the neutral connected. Running the transformer in reverse already causes higher percentage inrush on energizing than with normal drive.

 

[templdl]

Which should not vary at all depending on whether or not the neutral is connected.
Now if the two half windings are very asymmetrical in position relative to the core there a chance that the inrush will be different with the neutral connected. Running the transformer in reverse already causes higher percentage inrush on energizing than with normal drive.

If there is a neutral on the LV side of a transformer being used for a step up it should not be connected to the neutral from the source and s hi would also b insulated and isolated from the ground. But, this is a rabit trail Dr oes not address the nuisance trip issue. There seems to be and effort to avoid stating what the KVA of the transformer is. I finally know that a 100at breaker is being used to supply the transformer. If the OP would only advise what the KVA of the transformer is but for some reason the OP doesn't feel that the size of the transformer is something that is significant to know. May the OP just does't know what it is.

 

[kwired]

A local POCO has a demonstration trailer they take around to different places - mostly to do demonstrations to the general public for safety concerns and will show a few faulting conditions as well as cook a hot dog across the high voltage lines. (I really need to go see their demonstration sometime, until then I can only repeat what I have heard or read about).

I have seen the trailer just never been there when they are doing a demonstration, I believe they only used 3 maybe 5 kVA transformers - basically the smallest ones they typically stock for their system, anything else is overkill and even the 3 or 5 sort of is overkill for what they are doing.

 

[iwire]

They could probably use a neon sign transformer but then it would not look familiar to the public.

 

[kwired]

They could probably use a neon sign transformer but then it would not look familiar to the public.

That is part of the idea - they have actual pole pigs on wood poles and a short section of line all attached using usual hardware found on utility poles so general public sees what they would looking at a utility pole. They use hot sticks to introduce different items into the danger areas to demonstrate what happens. I think several area rural power companies share this demonstration trailer, they take it to all kinds of public events to do demonstrations, county fairs, home shows, schools, etc.

 

[11bgrunt]

A local POCO has a demonstration trailer they take around to different places - mostly to do demonstrations to the general public for safety concerns and will show a few faulting conditions as well as cook a hot dog across the high voltage lines. (I really need to go see their demonstration sometime, until then I can only repeat what I have heard or read about).

I have seen the trailer just never been there when they are doing a demonstration, I believe they only used 3 maybe 5 kVA transformers - basically the smallest ones they typically stock for their system, anything else is overkill and even the 3 or 5 sort of is overkill for what they are doing.

The transformer is/was a 15kVA. This was modeled off another demo at another utility and they were using a 15. They managed the high secondary currents by limiting the display time. A couple good arcs and they called it done.
Our 240 breaker never ever tripped on inrush. That was never a problem. GoldDigger had it right. The wye 208 system that is in the building is the problem, if the secondary neutral is connected. If they had connected to a single phase source, probably never another thought.
The transformer kVA has been reduced to a five to reduce the available fault current and consequently the current back to the low side.
If transformer impedance was adjustable, the 15 could have been left on the demo trailer. We are looking at putting 3 transformers in a series configuration to get the impedance higher than available on a normal single transformer. Today the arcs on the 7.2kV side are suitable for any demonstration involving phase to ground contact. If I knew how to add a video to this posting, I would. I have had these big arcs over my head but not because I wanted to build an arc.
Thanks to all that contributed.

 

[kwired]

The transformer is/was a 15kVA. This was modeled off another demo at another utility and they were using a 15. They managed the high secondary currents by limiting the display time. A couple good arcs and they called it done.
Our 240 breaker never ever tripped on inrush. That was never a problem. GoldDigger had it right. The wye 208 system that is in the building is the problem, if the secondary neutral is connected. If they had connected to a single phase source, probably never another thought.
The transformer kVA has been reduced to a five to reduce the available fault current and consequently the current back to the low side.
If transformer impedance was adjustable, the 15 could have been left on the demo trailer. We are looking at putting 3 transformers in a series configuration to get the impedance higher than available on a normal single transformer. Today the arcs on the 7.2kV side are suitable for any demonstration involving phase to ground contact. If I knew how to add a video to this posting, I would. I have had these big arcs over my head but not because I wanted to build an arc.
Thanks to all that contributed.

How are you planning to put three in series? If you are talking putting the low voltage side in series you will only have 1/3 of rated volts across each coil, if you boost input voltage by a factor of 3 you sort of end up with same thing you had to begin with. Maybe a line reactor would do what you want here?

 

[11bgrunt]

How are you planning to put three in series? If you are talking putting the low voltage side in series you will only have 1/3 of rated volts across each coil, if you boost input voltage by a factor of 3 you sort of end up with same thing you had to begin with. Maybe a line reactor would do what you want here?

Attached is a design that is in service at a training facility. Our folks have something that does the job for the time being. The increased impedance with the three pot design should allow for use in more venues.