Wye-Delta transformer bank

[schellersp]

We recently had an incident that I was hoping someone could help shed some light on. We have three single phase transformers connected wye on the high side delta on the load side. 50 kva center pot with 25 kva wing pots serving three phase power to a restaurant and twin single phase residences. We had disconnected the three phase service so they could do some work at the restaurant leaving only the single phase load connected. About 4 hours after the three phase load was disconnected the A phase transformer blew the the lid off. We replaced A and B phase transformer and re-energized the transformer bank. While checking voltage at the residence the voltage started out normal 120/240 and it continued to rise. We opened the bank back up left the wing pots open and tied down the H2 bushing and re-energized the B phase transformer to serve the single phase load. Is it possible that with no three phase load connected that a circulating current existed in the floated high side that induced more voltage in the secondary? We don’t know if it was just coincidence that the transformer blew when it but don’t understand why the voltage continued to rise after the transformer bank was rebuilt. After the restaurant is reconnected, if they open their main breaker for some reason is it going to cause the high voltage again? Thanks

 

[synchro]

Was the center of the wye left floating, or was it connected to a neutral on the HV side? If a neutral was connected then that could cause high circulating currents in the delta windings, and therefore cause the 25kVA pots to fail before a 50kVA one would.

 

[kwired]

Was the center of the wye left floating, or was it connected to a neutral on the HV side? If a neutral was connected then that could cause high circulating currents in the delta windings, and therefore cause the 25kVA pots to fail before a 50kVA one would.

Don't that depend on if you have single core transformer or multiple cores like an overhead bank typically would be?

 

[schellersp]

Was the center of the wye left floating, or was it connected to a neutral on the HV side? If a neutral was connected then that could cause high circulating currents in the delta windings, and therefore cause the 25kVA pots to fail before a 50kVA one would.

The center of the wye was floated

 

[LarryFine]

The center of the wye was floated

We do that (single 3ph unit) , but the utilities normally don't (three 1ph units), from what I understand.

 

[schellersp]

We went back out today moved the single phase service to another pole and connected a small three phase motor to the transformer bank and the voltage steadied out to 120/240/208. Hopefully someone could explain why this bank needs a three phase load to settle the voltage.

 

[petersonra]

We went back out today moved the single phase service to another pole and connected a small three phase motor to the transformer bank and the voltage steadied out to 120/240/208. Hopefully someone could explain why this bank needs a three phase load to settle the voltage.

The 3 phase load is probably masking the problem. My guess is something is wired wrong.

 

[schellersp]

The 3 phase load is probably masking the problem. My guess is something is wired wrong.

This bank has been in place for 15+ years with no issues until the three phase load was disconnected. Each H1 goes to a phase. H2’s floated. Subtractive polarity pots. X3’s tie to x1’s. B phase pot center grounded. Pretty simple bank to wire.

 

[LarryFine]

The 3 phase load is probably masking the problem. My guess is something is wired wrong.

This still sounds plausible. A 3ph motor can act as a rotary inverter, mimicking a 3ph source.

You should still read your voltages even without a 3ph load.

I'm still inclined to believe the primary H2s should be tied to the neutral with three separate transformers.

 

[Hv&Lv]

This bank has been in place for 15+ years with no issues until the three phase load was disconnected. Each H1 goes to a phase. H2’s floated. Subtractive polarity pots. X3’s tie to x1’s. B phase pot center grounded. Pretty simple bank to wire.

Wing pots X2 grounds lifted?

 

[synchro]

Perhaps the A or C phase feeding the bank is open, most likely from a blown upstream fuse. If so, then the missing phase could be being regenerated when there are 3-phase motor loads in the same way that a rotary converter does, as Larry has suggested.
This could then make the L-L voltages across the delta secondary windings more equal, and thereby stabilize the 120/240V single phase voltage as was observed.

Before disconnecting the 3-phase service, were single pole switches opened upstream of the bank? If so, that might've initiated ferroresonance, especially if the switch was a significant distance from the bank. Ferroresonance could then damage a transformer and/or blow a fuse.

 

[LarryFine]

Could be what should be a closed delta behaving like an open delta.

 

[Hv&Lv]

This bank has been in place for 15+ years with no issues until the three phase load was disconnected. Each H1 goes to a phase. H2’s floated. Subtractive polarity pots. X3’s tie to x1’s. B phase pot center grounded. Pretty simple bank to wire.

We recently had an incident that I was hoping someone could help shed some light on. We have three single phase transformers connected wye on the high side delta on the load side. 50 kva center pot with 25 kva wing pots serving three phase power to a restaurant and twin single phase residences. We had disconnected the three phase service so they could do some work at the restaurant leaving only the single phase load connected. About 4 hours after the three phase load was disconnected the A phase transformer blew the the lid off. We replaced A and B phase transformer and re-energized the transformer bank. While checking voltage at the residence the voltage started out normal 120/240 and it continued to rise. We opened the bank back up left the wing pots open and tied down the H2 bushing and re-energized the B phase transformer to serve the single phase load. Is it possible that with no three phase load connected that a circulating current existed in the floated high side that induced more voltage in the secondary? We don’t know if it was just coincidence that the transformer blew when it but don’t understand why the voltage continued to rise after the transformer bank was rebuilt. After the restaurant is reconnected, if they open their main breaker for some reason is it going to cause the high voltage again? Thanks

So I got to ask here…
you disconnected the three phase service…
you cut just the secondary three phase loose, or did you pull the fuses on the wing pots and cut the center leads loose?
If you pulled the fuses on the wing pots there’s your problem.

 

[schellersp]

Wing pots X2 grounds lifted?

X2 is open. Tank ground only

 

[schellersp]

So I got to ask here…
you disconnected the three phase service…
you cut just the secondary three phase loose, or did you pull the fuses on the wing pots and cut the center leads loose?
If you pulled the fuses on the wing pots there’s your problem.

Wing pot fuses were left in, opened main in restaurant, disconnected hot legs only on the three phase service.

 

[Hv&Lv]

Wing pot fuses were left in, opened main in restaurant, disconnected hot legs only on the three phase service.

You guys use a neutral grounding switch for the floating wye, opening it after all three fuses are in?

 

[schellersp]

Perhaps the A or C phase feeding the bank is open, most likely from a blown upstream fuse. If so, then the missing phase could be being regenerated when there are 3-phase motor loads in the same way that a rotary converter does, as Larry has suggested.
This could then make the L-L voltages across the delta secondary windings more equal, and thereby stabilize the 120/240V single phase voltage as was observed.

Before disconnecting the 3-phase service, were single pole switches opened upstream of the bank? If so, that might've initiated ferroresonance, especially if the switch was a significant distance from the bank. Ferroresonance could then damage a transformer and/or blow a fuse.

All three phases are energized. No upline fuses this particular transformer bank is about 1/2 mile from the substation.

 

[schellersp]

You guys use a neutral grounding switch for the floating wye, opening it after all three fuses are in?

No we don’t. I know of places that do, but it is not our practice to do that.

 

[Hv&Lv]

No we don’t. I know of places that do, but it is not our practice to do that.

This is going to sound bad, but please understand, I’m on the internet, not there to check it out..
Don't take it personally…

It sounds like something was inadvertently tied down wrong when disconnecting the three phase.
This bank, like you say is pretty easy to wire and extremely reliable as long as it is done correctly.

You tied the H2 down and basically made a single phase XF now by pulling the wings, just be careful of backfeed on the bottoms of the fuses now.

Check out the caution note below on the left…

 

[schellersp]

This is going to sound bad, but please understand, I’m on the internet, not there to check it out..
Don't take it personally…

It sounds like something was inadvertently tied down wrong when disconnecting the three phase.
This bank, like you say is pretty easy to wire and extremely reliable as long as it is done correctly.

You tied the H2 down and basically made a single phase XF now by pulling the wings, just be careful of backfeed on the bottoms of the fuses now.

Check out the caution note below on the left…

View attachment 2559570

As stated in the caution.. we had only one secondary neutral bushing grounded as I said in a previous the x2 bushings on the wing pots were “open” had nothing connected to them. Also, as I stated earlier the H2’s were not tied to system neutral but were floated. This happened at night 4 hours after the three phase service was disconnected. Nothing else changed except three hot legs disconnected from the transformer bank. Later that night after the transformer blew, we isolated the B phase transformer at which point we had to tie the H2 to ground to continue to serve the single phase load.