three phase padmount proublem

[topgroove]

A call comes in to customer service that the local Kmart has partial power/ one leg dead.regional control dispatches a truck to the problem.the crew arrives on the scene and drives behind the store to the three-phase pad mounted transformer. the crew inspects the conducters and connections and takes voltage readings. the crew leader looks at the fuses on the main line riser pole that feeds the underground primary to Kmarts three-phase 7.6/13.2 KV pad mounted transformer. The three cutout doors are in the closed position.An electrician comes out and tells the crew leader his mains are open to protect his three phase equipment. the crew leader opens the padmount and takes secondary readings. the normal voltage for this customer is 120/208. using a fluke he reads the following voltages... x1 to nuetral=121.8 x2 to neutral=120.9 x3 to nuetral = 80 volts x1 to x2=208 x1 to x3=175 x2 to x3=175. The crew leader tests the primary at the transformer bushings and tests h1 to nuetral and gets 8kv, h2 to neutral 8kv h3 to neutral is 6kv. the crew feels the primary voltage is withen limits and therefor they deside there must be a problem internally with the transformer and feel it must be replaced. Do you aggree with there decision?

[PA BEN]

The fuse in cutout #3 might be blown but not open. He could be reading back feed

[PA BEN]

You said the mains are open. Might not be back feed at the trans. Down the line on the overhead pirmary, a phase might be open but, between the open and the padmount is a Y delta bank with the H2's tied down. This will backfeed the open phase with a lower voltage.

[dbrown20]

That's too low. Check your source voltage and the dip. If it's okay and the fuse is okay, then yes you have a bad pot. If source voltage is bad, then trace it down and fix it. dbrown20

[Livewire]

Had this same problem once. We we're installing a new 500kva 277/480 pad mount. After making everything up and checking voltage we had 277-neutral, 277-neutral, and 195-neutral. I can't remember phase to phase reading, but i think it was in the low 400 volts range.

Ours turned out to be bad connection on a jumper a few poles away. Reattached it and transformer checked good.

So like Dbrown said, Check your source voltage and the dip. If it's okay and the fuse is okay, then yes you have a bad pot. If source voltage is bad, then trace it down and fix it. dbrown20

[playtime]

Had a situation similar to this once. Talked to a transformer manufacturer about back feed on a Y secondary. We were told that it is possible to get a backfeed through the three phase transformer because a three phase tub has three individual windings inside that are connected together. Even with the consumers disconnect open on the secondary side a backfeed is possible. Check the fuses in the cutout before you replace the tub.

[dbrown20]

Had a situation similar to this once. Talked to a transformer manufacturer about back feed on a Y secondary. We were told that it is possible to get a backfeed through the three phase transformer because a three phase tub has three individual windings inside that are connected together. Even with the consumers disconnect open on the secondary side a backfeed is possible. Check the fuses in the cutout before you replace the tub.

You know I was intrigued by backfeed on Wye/Wye setups. I think I read something once that suggested that in a Wye/Wye padmount that it actually wasn't backfeed when there was no load attached. I believe the term was "mutual inductance". I suppose it's the same as backfeed though. It seems when everything is in the same body together this "mutual inductance" causes a backfeed effect. Something to be leery of.

Now on an overhead Wye/Wye bank there isn't usually backfeed. Beware though as it can happen with the right setup. If some of the customers have a dry transformer that is hooked in a delta configuration, then there will be the possibility of backfeed. I always considered anything that was hooked phase to phase and was turned on in the customers' premises, might under the right circumstances backfeed someway. Can't be to careful about backfeed. dbrown20

[PA BEN]

Now on an overhead Wye/Wye bank there isn't usually backfeed. Beware though as it can happen with the right setup. If some of the customers have a dry transformer that is hooked in a delta configuration, then there will be the possibility of backfeed. I always considered anything that was hooked phase to phase and was turned on in the customers' premises, might under the right circumstances backfeed someway. Can't be to careful about backfeed. dbrown20

The last big storm I worked the water Dept. called after we heated up the primary and said a pump was single phaseing. No cutouts open, checked primary voltage had voltage. Upped the scale on the meter and didn't get a reading, I don't remember what the voltage was { we were up to our ears in work at the time} Three cutouts up the line was a blown fuse no open door. I do remember getting about 80 volts or so on the open pot. 277/480 bank

Another time I was driving down an alley and saw an open cut out, this was on a main feed. We had a lighting storm the night before. But no one called with no power. This feeder had a laterel three phase tap that ended with a 3 phase pirmary feeding a Y delta URD bank. The diff was this bank was made with three 75 KVA Padmount Transformers can't float the high side, with the open phase the bank was open dalta and back feeding the overhead pirmary. I wish we would have checked voltage and amps but the boss didn't want to take the time.

[tramp67]

You know I was intrigued by backfeed on Wye/Wye setups. I think I read something once that suggested that in a Wye/Wye padmount that it actually wasn't backfeed when there was no load attached. I believe the term was "mutual inductance". I suppose it's the same as backfeed though. It seems when everything is in the same body together this "mutual inductance" causes a backfeed effect. Something to be leery of.

Now on an overhead Wye/Wye bank there isn't usually backfeed. Beware though as it can happen with the right setup. If some of the customers have a dry transformer that is hooked in a delta configuration, then there will be the possibility of backfeed. I always considered anything that was hooked phase to phase and was turned on in the customers' premises, might under the right circumstances backfeed someway. Can't be to careful about backfeed. dbrown20

Many of the three phase padmounts share a common core. With the pole mount transformers, you have individual cores for each phase, and the tanks also shield each other from magnetic fields. Isn't electricity fun?!

[dbrown20]

similar thing happen once. Had an open phase, and downstream from it there was a 3 phase that ran to a building. Some UG primary ran to a building with a 277/480 padmount. On the effected phase past the aforementioned tap, a customer still had dim lights. The only connectiion he had was thru the padmount which was single phasing. Didn't check the voltage, but I know it was low, but still enough to give him lights. Beware backfeed. dbrown20