Trouble shooting banked secondary

[Ironmike]

What is the best way to tell if a transformer is working properly if it is banked with another pot?

I always checked the amps on the xfmr leads but noticed that even if the pot is offline, some current still flows.

[Pootnaigle]

What is the best way to tell if a transformer is working properly if it is banked with another pot?

I always checked the amps on the xfmr leads but noticed that even if the pot is offline, some current still flows.

Umm itd possiblr to check voltage pn each indivual pot at the secpndary bushings and you should get the rated voltage for the transformer m,akes no difference if they are banked

[Orgnizdlbr]

When you say "banked" do you mean two single phase pots banked or do you a pot in a bank?

[Ironmike]

This scenario involves at least 2 OH single phase xfmrs paralleled or banked together.

[urban1095]

What is the best way to tell if a transformer is working properly if it is banked with another pot?

I always checked the amps on the xfmr leads but noticed that even if the pot is offline, some current still flows.

The current flow is caused because you are backfeeding the pot with the secondaries and heating up the primary coil and whatever it is connected to! Very dangerous sitiuation!

As far as working correctly nameplate says it all right? Just have to do your calculations and checks.

[Pootnaigle]

Umm
m assuming we are talkng bout a 3 -phase bank consisting of 2 or more single phase pots then the voltage tween x1 n x2 shud be the nameplate voltage on each pot

[Rob]

The current flow is caused because you are backfeeding the pot with the secondaries and heating up the primary coil and whatever it is connected to! Very dangerous sitiuation!

As far as working correctly nameplate says it all right? Just have to do your calculations and checks.

If both pots are on-line there is no backfeed. They must be fed off the same phase and the secondary is tied somewhere between them. So the only way to determine whether one is working or not is to trip the CSP breaker (or open the cut-out), disconnect the harness at the secondary, then trip the pot back on and test for voltage at the bushings.

Tying 2 single phase pots together is certainly a very dangerous condition. I think if you had to do it in an emergency, because of load issues, then caution tags should be hung on both pots. At least you'd be letting the next guy know what he had.

[urban1095]

If both pots are on-line there is no backfeed. They must be fed off the same phase and the secondary is tied somewhere between them. So the only way to determine whether one is working or not is to trip the CSP breaker (or open the cut-out), disconnect the harness at the secondary, then trip the pot back on and test for voltage at the bushings.

Tying 2 single phase pots together is certainly a very dangerous condition. I think if you had to do it in an emergency, because of load issues, then caution tags should be hung on both pots. At least you'd be letting the next guy know what he had.

The reason I addressed the question in this manner is because in a post farther down he said it
involved two single phase pots tied together by secondaries , and that with one offline he still had
current.
I was just trying to bring attention to something that can become a very dangerous
situation if left unattended. We are told to do alot of things in trouble situations by people that do not have to worry about the dangers in field! I will now cut up my soap box and recycle it.:)

[loodvig]

So your saying 2 pots were tied together and left that way? Yes very dangerous! Not a good practice!

[Ironmike]

Where I work there are a lot of single phase 120/240 pots that are paralleled on the sec. I've seen as many as 4 pots banked together and I totally agree that it is very dangerous for numerous reasons.
The situation I was in that made ask the question was this:
Tag said "bad 2400volt 120/240 pot". The pot was banked with another pot that was 3 poles away. The pole with this "bad" pot was a DE pole with 2-3 OH svcs and 1 sec riser. I measured the load on the sec line and it had 80 amps on both legs. Then I checked the xfmr leads and got around 18 amps.
Turns out the pot was fine and the cutout was bad.
I just didn't understand why 18 amps was showing on the leads when the fuse was blown

[Trouble1]

Urban had the answer on that. The secondary coil picking up the primary coil is the reason for amperage on the transformer. If you back fed a transformer with the cutout open with the secondary connectors you will get a nice little pop just from coil load.

I've heard banking all secondaries together is popular in third world countries. I know in Boston we have a pretty big network of banked secondaries, it's actually called The Network.. It works, but it's very old out of date system. It probably feeds maybe 1/6th of Boston. I don't know how many transformers are banked together, but there are a lot.

[T-Man]

A network is a little different in that they have protectors and disconnects. The load determines how many transformers join the network to help the load. The protectors watch current flow so current only flows out of the transformer and when it tries to flow into the tub the protector opens.

[splinter]

Worked several places where they bank their transformers together. You just have got to be careful and realize just because the cutout is open doesn't mean the secondary is dead and that the bottom of the cutout is back fed. Trouble1 is exactly right in that when you back a transformer in (or heat up the secondaries with cutout open) you wil be exciting the coils and on the last leg you will get a big pop. We used to back in alot of pots because they didn't want to buy dual voltage pots and we would hang the 7.2 pots back them in and when we had a cutover just open the 4kv and close the 7.2kv and pick up the load.

[Pootnaigle]

Umm once LONG TIME AGO I wuz kinda new at trubble shootin I hadda call wayyyy out in the boonies n The complaint wuz dim lites so after checkin voltage I got 86/ 86 n I68 Furst thing I thunk t wuz the pot n later on I found a jumper off at the sub... There wuz a ton of banks feedin oil wells n such n most of em wuz 480 3 phase so the backfeed frum them had nuff pzazz to run that pot I thunk wuz bad. sho nuff when put that jumper back it wuz fine she wuz the onlyest residential cust on the hole line far I as i cud determine

[De-energized]

A network is a little different in that they have protectors and disconnects. The load determines how many transformers join the network to help the load. The protectors watch current flow so current only flows out of the transformer and when it tries to flow into the tub the protector opens.

Very well put. Don't think I've ever heard it explained so simple. Spot on!

[Orgnizdlbr]

A network is a little different in that they have protectors and disconnects. The load determines how many transformers join the network to help the load. The protectors watch current flow so current only flows out of the transformer and when it tries to flow into the tub the protector opens.

Protectors I've worked on had demand relays that would work axactly how you described, reverse current relays dumped the protector so the transformer wouldn't be backfed by the system. The system I worked on was very old and expensive to maintain, they don't build them like that where I'm at anymore.

[Brooks]

Protectors I've worked on had demand relays that would work axactly how you described, reverse current relays dumped the protector so the transformer wouldn't be backfed by the system. The system I worked on was very old and expensive to maintain, they don't build them like that where I'm at anymore.

You are describing what I would refer to as a "spot" network. Typically 2-4 transfromers feeding a single building. We would have two subs feed 4 pots with each with each of the two feeds per sub originating from a separate buss.(Current practice in many places)

A "grid" network would be banks tied together with no protection other than the primary fuses. I've only seen it on 120/208. The thought being that 120/208 would not reliably create a "self sustaining" arc and would burn itself clear. (sort of old school)

The two above that I've worked on were UCD.

On overhead, I've seen mulitple pots operated in parallel with seconday fuses and without secondary fuses. ( kinda old school)

[Orgnizdlbr]

You are describing what I would refer to as a "spot" network. Typically 2-4 transfromers feeding a single building. We would have two subs feed 4 pots with each with each of the two feeds per sub originating from a separate buss.(Current practice in many places)

A "grid" network would be banks tied together with no protection other than the primary fuses. I've only seen it on 120/208. The thought being that 120/208 would not reliably create a "self sustaining" arc and would burn itself clear. (sort of old school)

The two above that I've worked on were UCD.

On overhead, I've seen mulitple pots operated in parallel with seconday fuses and without secondary fuses. ( kinda old school)


Actually the system I was talking about fed the entire town of Asbury Park NJ, 3 primary circuits, secondaries all tied together in phase and rotation....... It's all gone now replaced by a URD system.

[Rob]

The reason I addressed the question in this manner is because in a post farther down he said it
involved two single phase pots tied together by secondaries , and that with one offline he still had
current.
I was just trying to bring attention to something that can become a very dangerous
situation if left unattended. We are told to do alot of things in trouble situations by people that do not have to worry about the dangers in field! I will now cut up my soap box and recycle it.:)

I agree with you Urban... I was just trying to answer the original question.

[reppy007]

When banking you tie two banks together you wouldnt want to cut the secondaries on the bank thats going to be taken out of service if the transformer fuses are pulled/blown/opened,Ive always been taught that you would be cutting the primary coils and could see a fire.You would leave the fuses in until the secondaries are cut,then open them...good question....but Ive never seen anything banked together 3 or 4 poles apart....where is this located anyway?

[Pootnaigle]

Umm they do that kinda thang in Dallas I ssed it once... I bettcha thats a trubble shooters nightmare

[Trouble1]

When banking you tie two banks together you wouldnt want to cut the secondaries on the bank thats going to be taken out of service if the transformer fuses are pulled/blown/opened,Ive always been taught that you would be cutting the primary coils and could see a fire.You would leave the fuses in until the secondaries are cut,then open them...good question....but Ive never seen anything banked together 3 or 4 poles apart....where is this located anyway?

You wouldn't cause an fire. It will just feel like you're dropping 20 amps depending on transformer size. Just like if you open a cut-out with a stick and no load connected you draw a small arc. You do get a good pop when you backfeed a transformer with the cut-out open or disconnect the live secondaries with the cut-out open.

You could cause a fire the other way around though if say you had one transformer running a few sections and you wanted to split it in half. Now say you hang another transformer. Leave the cut-out open and connect the secondaries, you get a good spark because you are picking up coil load, but no problems yet. The problem now is you just guessed on the polarity and if you close the cut-out you have a 50/50 chance(and linemans luck is never50/50) of some bad stuff happening, so you have to do a push-pull when you could have banked them together with no outage.

There's almost no reason for doing this obviously because it's dangerous, but you will not cause problems until you close that cut out. If it were open wire secondaries you could probably do this because you could see how the secondaries are connected on the transformer you are banking. Again, no reason for doing this, but theoretically you can if it's on the same phase and you can visually see what wire goes where and the transformers are the same polarity.

Also, just wanted to add that although you won't cause problems disconnecting a banked transformer with the cut-out open, you will probably regret it if you try to do it on a 167.5 kva because those coils are a lot of load to drop.

[1245hand]

Here it's not uncommon to have single phase transformers paralleled thru secondaries in residential neighborhoods. Usually rear easements. Seen up to 4 together before. Unfortunately, this was done a lot in the 60's & 70's when CSP's were common here for single phase transformers. Seems the theory is it increases the secondary load capacity. Cheaper to hang a pot than reconductor backlot secondary.

Problem is no one maps which transformers are paralleled together. Pretty common to have T-man go on an outage to find 2 pots off after testing. Sometimes they reset, sometimes they don't. Sometimes T-man isolates the pot, sometimes they don't. Sometimes they call both of them bad. When they do reset, it's usually only a matter of time til they go out again.

Fortunately, company doesn't allow CSP's paralleled with fused transformers.

Oh yeah, as far as paralleling transformers, I've found after verifying phasing, parallel them while energized. Test voltage between new and old, then connect secondaries. Usually less than 10-12 volts difference each phase on residential. Done it both ways, seems like more load picking up new transformer thru secondaries. (Unless small can).

By the way, I'm on the west coast.

A little off the original post, but that's my 2 cents worth.

Be Safe

[bobbo]

Get your full load current, kva times voltage/ voltage gives you full amperage. So if the pots are banked single phase and should be the same size. So if the one pot can carry the current of the two pots. Isolate the secondary leads off the "net" and check for good voltage. If they are banked together and one physically looks bad, sometimes the fault current of one can will go to another. In CA, there were 4 men passed away by a four wire bank. The one can looked bad, oil leaking, flash marks. But the bad can was the one that didnt absorb the fault, it transferred to the other can. And when they closed in on it. The whole bank went catastrophic. Now they test every can with a transformer tester. But you have to isolate the can to do that. I would break the secondary load and check for good voltage. Here is a trick, if the cans good and you need to pick load up again with a big mechanicaal tape the very end and slam it in your connector. It will act as a loadbreak. And when you take off load from a mechanical just break the bolt enough and cut the tape enough to rip out (with tape at the tail)that lead as fast as you can maybe another way is just to read the load off the primary tap. If the coil is burned open you shoud read no amps on the primary side and the other can is hugging load. Cans that have an open in the coils dont have that buzz sound or chattter.In detroit its all banked together and close looped, they have little fuses that shoot out a flag. Hopefully your not an out of towner, remember they dont ground the cans and you can have full primary votage on those cans if its shorted to the tank. And if you break the load of the good can your going to have an outage. And reppy thats standard in CA, you got to break the secondary first then open the.doors too much circulating current there. To me that clamp amp is my best friend.

[rob8210]

I have never worked on a networked system so I can't make any kind of comment about it. What I do know is banking single phase transformers here is a no no. In some cases we will bank a new transformer in with an old one to change it out or for a pole replacement but we will not leave them baked together. The rule of thumb for banking transformers is, close in high side, check voltages with a meter on both cans to verify that they are the same or within 3 volts, check phasing with a meter to be correct, make secondary taps ( you will pick up some load) , then cut secondary taps of old transformer. I was always told that it was a supreme no no to connect the secondaries of a transformer to a live buss with the high side open.

Also we do have some old installations of 2 single phase cans banked together, where each can only feeds one leg of the buss. These are also slowly being removed

[bobbo]

I never read off the secondary with the amp clamp, in that situation. if the can has no volts, it cant have amps on the secondary either. So I am thinking you can clamp the secondary leads and should read minimal amps. The good can has all the load and the bad can has none of it. And I know if you do all the tests on the can it has to be isolated. If I am wrong just say so. If the can isnt doing its job, it shouldnt have the load, you can clamp either the primary or secondary should have minimal amps in comparison to the other can. Then you ID the bad can. parrallel a new can in and I would split the load somewhere so the next guy doesnt have to deal with it. And your inspector should agree. Out there, they were tied together to keep people hot, install a new can or what ever they need to do and forgot to split the load. Sometimes its like that. Troubleman work fourty hours straight sometimes and they forgot to split the load. I wouldnt say its dangerous only if the impedance was different. I have seen guys work three days straight there doing twenty step switching orders, then trouble calls in between.