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:D

[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?!:p

[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

[BigClive]

It's quite common for the less experienced to get caught out by backfeed from an open line on a three phase load. once you've been tripped up once (or twice) you learn how to recognise the symptoms.

It's also surprising how many get caught out testing feeds with a high impedance tester and measuring inductive or capacitive coupling from adjacent live cores onto a dead core. I recently came across an isolator that someone had written on... "Red phase was only 50V so I connected load to yellow phase." The red fuse had been blown at the source. (Old UK colours.)

Now a duff transformer... Could it really be getting full primary voltage but have a low secondary voltage on one leg? If transformer windings had bridged out internally it would be getting pretty hot, and if there was a bad internal connection delivering load with a significant drop then that fault would surely be showing heat or pressure symptoms.

[tramp67]

If the primary or secondary winding opened on one phase, and the customer had some good size three phase motors running, you could be getting backfeed through the motors. Once a three phase motor is running, you can often lose one leg and not have any noticeable effects with the motor. Current will increase on the two remaining legs and the motor might run a little warm, but it will keep spinning with just two phases. The motor won't start without all three phases on it's own, but you can give a single-phasing three phase motor a spin in either direction and it will continue to run in that direction.:cool:

[playtime]

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?

What did you find as the problem here? Are we close?

[linemanfrog]

Another lineman and I got a call to a church one night. Flickering lights/low voltage. The church was being fed by a 75kva 3 phase URD padmount TX. After doing our initial checks at the TX we determined that c phase was about 10 volts lower than either a or b on the secondary side. I checked the fuses feeding the TX on the pole about 125ft from where the padmount was located. The fuse pole was a mainline pole and all the fuses were in so we got a phase meter out. Upon checking the primary voltage (we were thinking that possibly a regulator had gotten stuck) we were still scratching our heads. Then we checked primary voltage at the TX itself...same readings. Finally remembered another situation that was similar that another crew had happen to them. Turns out the weak link fuse was blown inside the TX tank. Our TX shop came out and field tested the TX and varified the problem and changed the internal fuse.

This is a good thing to remember. Turns out we have had a few of the same size TXs made by the same company were the internal fuse fails for no apparent reason. These TXs were made within a few months of one another and we concluded that the manufacturer of the TXs had gotten a bad batch of fuses.