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Another Trany Q, Y to open delta?

U

uray

Jan 1, 1970
0
Hello,

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?

Thanx for any help,

uray
 
U

uray

Jan 1, 1970
0
s falke said:
Not likely. Would you detail "...150 amp load on the ground..."?

What are the transformer-nameplate numbers?

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
 
U

uray

Jan 1, 1970
0
| 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.

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

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.
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?
| 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.
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.

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
 
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