Another Trany Q, Y to open delta?

Not likely. Would you detail "...150 amp load on the ground..."?

What are the transformer-nameplate numbers?

--s falke

The transformer started out as a 480 to 240 volt 500kva transformer. It was modified by an electrical house into a 480-575(600) volt open delta using two of the windings. The load on the ground is precisely that, the transformer is grounded and the ground wire heading back to the main panel is carrying 150 amps. The transformer is wired with a separate disconnect, the ground wire is bonded there, and continues back to the main feeder. The load only shows up to the bonding point in the disconnect switch, the wire going back to the main shows no load. I did not do the actual wiring and I am not sure if the neutral is bonded to ground at the main panel or not.

I'm not sure what to make of it. I do have 480 in, and 600 out. Why the case ground is carrying so much current escapes me. Although I did not have a chance to check the windings with a megger yet, a standard VOM shows no shorts or grounds.

I ran out of time to devote to it today and will continue checking it out tomorrow. I am suspicious of the ground and may need to pull the covers on the main and see exactly where the electricians made their connections. The transformer may have developed an internal problem as well, a megger will help me determine that.

uray

| I'm still having transformer problems. I've got a 480 to 575 open delta | transformer, it is fed from a 480Y panel. The neutral does not come into | the transformer at all. I'm getting my 600 volts OK, but I am also showing | a 150 amp load on the ground line. The casing of the transformer is also | heating up, although the actual coils are not. I am still checking all the | wiring, it seems OK though. I still want to take a megger to it. | | My question is regarding the 480Y service that is feeding it. Is the fact | that it is 480 Y and not 480Delta significant?

| The transformer started out as a 480 to 240 volt 500kva transformer. It was | modified by an electrical house into a 480-575(600) volt open delta using | two of the windings. The load on the ground is precisely that, the

Is that a 3 bar core with 2 bars used for windings and 1 bar not? If so, you are going to have some induction problems inside the case. That could explain the case heating up. That is bad engineering. Open delta should be done on 2 separate cores.

| transformer is grounded and the ground wire heading back to the main panel | is carrying 150 amps. The transformer is wired with a separate disconnect, | the ground wire is bonded there, and continues back to the main feeder. The | load only shows up to the bonding point in the disconnect switch, the wire | going back to the main shows no load. I did not do the actual wiring and I | am not sure if the neutral is bonded to ground at the main panel or not.

If there is a short, it would not be drawing a mere 150 amps; it would pull all available fault current until your overcurrent protection trips off.

Is that 150 amps only under load, or does the 150 amps flow even if the load is not pulling any current?

What I am concerned about is that it is a 3 bar core (3 common cores), with two cores in use, and a winding remaining on the 3rd bar of the common core connected in some way to be draining flux off the core to ground and som other wire.

| I'm not sure what to make of it. I do have 480 in, and 600 out. Why the | case ground is carrying so much current escapes me. Although I did not have | a chance to check the windings with a megger yet, a standard VOM shows no | shorts or grounds.

I'm assuming you have a clamp on ampmeter. Get a reading on _every_ wire going in or out of the transformer. Is the ground load wire going through the transformer (if so, read current on both ground wires) or around it (read it anyway). Do this with the load ON _and_ with the load OFF for every wire. And for grins, also do it with the breaker feeding it opened.

There are other tests I have in mind, but they involve creating dangerous situations (instantly lethal voltages could be on the case).

| I ran out of time to devote to it today and will continue checking it out | tomorrow. I am suspicious of the ground and may need to pull the covers on | the main and see exactly where the electricians made their connections. The | transformer may have developed an internal problem as well, a megger will | help me determine that.

If a primary line shorts to the case, you'll have more than 150 amps. But if a secondary shorts to the case, funny things can happen.

Getting an "as wired" schematic of this thing would help. But if you post it, be sure to give thorough "as is" detail, separate from any comments as to why you think something might be there, or what you are considering changing. The actual wiring _and_ amp readings of every wire (in on and off cases) will help to give an idea what is going on.

Yes, it is a common core with the center winding unused.

The 150 amps is with no load on the transformer. I am reading that 150 on the ground wire, since the case is also grounded through the conduits the total current may much higher.

I megged the center winding and it seems to be isolated. When energized I am reading about 700 volts on one side and about 180 volts on the other side. I've been wondering if I should do something with that center winding, ground it perhaps?

With the breaker off I see no load on any of the wires. This thing is wired up with multiple conductors, three per phase, which makes it difficult to measure with the equipment I have here.

I will see if I can post a copy of the hookup on the web somewhere. Right now I am still talking to the company that modified it. They seem to be about as confused as I am. It is starting to look like there is a problem in the transformer somewhere. They may be able to come by tomorrow with some better test equipment. They would like to do a thermal image of it to see where exactly it is heating up.

uray

| Yes, it is a common core with the center winding unused.

I'd like to see exactly how this is wired to know what the effect will be.

| The 150 amps is with no load on the transformer. I am reading that 150 on | the ground wire, since the case is also grounded through the conduits the | total current may much higher.

Then that's more than just the ground wire paralleling some other. This is some kind of medium impedance fault.

|> What I am concerned about is that it is a 3 bar core (3 common cores), | with |> two cores in use, and a winding remaining on the 3rd bar of the common | core |> connected in some way to be draining flux off the core to ground and som |> other wire. | | I megged the center winding and it seems to be isolated. When energized I | am reading about 700 volts on one side and about 180 volts on the other | side. I've been wondering if I should do something with that center | winding, ground it perhaps?

What center winding? I guess I still don't have a correct and complete picture of all this.

If there is an unused winding, it should present some voltage end to end, and if put under load, have no voltage to ground from either end. But I can't say if grounding is right for it unless I have the whole picture. Given the huge ground current you have right now, grounding this now might be a serious hazard. Get the other problem resolved first (which this may be a part of, anyway).

|> | I'm not sure what to make of it. I do have 480 in, and 600 out. Why | the |> | case ground is carrying so much current escapes me. Although I did not | have |> | a chance to check the windings with a megger yet, a standard VOM shows | no |> | shorts or grounds. |>

|> I'm assuming you have a clamp on ampmeter. Get a reading on _every_ wire |> going in or out of the transformer. Is the ground load wire going through |> the transformer (if so, read current on both ground wires) or around it |> (read it anyway). Do this with the load ON _and_ with the load OFF for |> every wire. And for grins, also do it with the breaker feeding it opened. | | With the breaker off I see no load on any of the wires. This thing is wired | up with multiple conductors, three per phase, which makes it difficult to | measure with the equipment I have here.

Hmm.

|> Getting an "as wired" schematic of this thing would help. But if you post |> it, be sure to give thorough "as is" detail, separate from any comments as |> to why you think something might be there, or what you are considering |> changing. The actual wiring _and_ amp readings of every wire (in on and |> off cases) will help to give an idea what is going on. | | I will see if I can post a copy of the hookup on the web somewhere. Right | now I am still talking to the company that modified it. They seem to be | about as confused as I am. It is starting to look like there is a problem | in the transformer somewhere. They may be able to come by tomorrow with | some better test equipment. They would like to do a thermal image of it to | see where exactly it is heating up.

That would help, too. Maybe they will run it opened up. Be sure everyone assumes the case has lethal voltage when power is applied. That could very well be the situation if the ground comes loose.

Wow, That sounds serious, expecially if you have found "heating" at no load. Some 240V-delta secondaries come with what the drytype guys call a "lighting tap." That should most likely be abandoned in your case, I guess. It's impossible to tell from here what was done in the 'conversion.' Don't burn anything down.

Megger, indeed. Megger soon. Megger often.

--s falke