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Stormtrooper
03-06-2008, 02:43 AM
Am I correct in saying with a 3 pot wye-delta bank you connect the primary neutrals together but float them or isolate them from system neutral?
With a wye-open delta 2 or 3 pot bank the primary neutrals are also isolated?

wtdoor67
03-06-2008, 08:52 AM
Stormtrooper. We have beat Wye/Delta banks to death on here. If you want to check the archives you can find all of your answers.

My advice to you is to get a Kurtz lineman's book and a GE Distribution handbook and have a look. Those will answer all your questions. No apprentice should be without the Kurtz Lineman and Cablemans handbook.

PA BEN
03-06-2008, 01:50 PM
Jumper or not to jumper forum:D

lewy
03-06-2008, 05:38 PM
With a 3 pot delta bank you can float them (they are connected to each other but not to neutral) or connect them to the neutral
If you do not float them & you loose 1 tx you would not know & the other 2 would try and carry all the load. when you do float them & you loose 1 your secondary voltage will be lower e.g 520 compared to 600.
If you are just using 2 txs you would have to ground both neutrals & you would have an open delta which would give you 57% of the the original bank or 86% of the total capacity of the 2 txs

johnbellamy
03-06-2008, 10:10 PM
With a 3 pot delta bank you can float them (they are connected to each other but not to neutral) or connect them to the neutral
If you do not float them & you loose 1 tx you would not know & the other 2 would try and carry all the load. when you do float them & you loose 1 your secondary voltage will be lower e.g 520 compared to 600.
If you are just using 2 txs you would have to ground both neutrals & you would have an open delta which would give you 57% of the the original bank or 86% of the total capacity of the 2 txs

You can tie the floated neutral when energizing and removed when energized. Small banks usually you should'nt have to, but the higher the voltage the worse the problem. The larger the bank loaded or unloaded the worser the problem. If you bank anything 50 or over I recommend tieing it to the system neut. A 4th cut out? Ok if the outfit can afford it, sometimes it clutters the pole up depending on the costruction. A gang switch? I dont know who puts that kinda money in to a system. A cheap easy way is to put a hot tap on a piece of 4 or six copper what ever size your pole ground is that is bonded to the system neut, than tie down your floated neut, energize the bank, then remove your hot tap with a shotgun so your highside neut is again floated.

It is common that you float the high side, other benifits include that you can run a regular 120/240 house service and also 3 phase 120/240/208 power that can run someones well or shop. If you blow a fuse on one pot as long as it isn't the 120/240 pot that you tied X2 down. You will loose your 3 phase but the house will stay in service until crews arrive to make repairs.

Special ED
03-15-2008, 04:05 AM
Bellamy:

We would install a solid blade switch on large 3 pot delta banks. You would have to close it before you opened any of the switches and after you closed all the switches you had to bust the ground switch with a load buster. Otherwise it made a pretty nice ball of fire.

1st Class Husker
03-18-2008, 10:57 PM
Am I correct in saying with a 3 pot wye-delta bank you connect the primary neutrals together but float them or isolate them from system neutral?
With a wye-open delta 2 or 3 pot bank the primary neutrals are also isolated?

on a open delta - reference the neutrals back to system neutral!
on a closed delta- float neutral if you want the motor to shut down when a phase is lost, otherwise the wild leg will switch pots !

Fiberglass Cowboy
03-19-2008, 08:14 AM
TO USE A GROUND JUMPER AND GROUND THE FLOATER DOWN BEFORE RE-ENERGIZING. WE ALSO USE PHASE TO PHASE RATED ARRESTORS. SO OUR SYSTEM IS 7200V/12,470V GRD WYE AND ON OUR WYE-DELTA BANKS WE USE 12KV LIGHTNING ARRESTORS. I CLOSE THE 1 TRANSFORMER WITH THE GROUNDED NEUTRAL FIRST,THEN THE OTHER TWO. THEN AT THE END OF MY 8' SHOTGUN (A.K.A. GRAB-ALL) REMOVE THE GROUND JUMPER FROM THE FLOATER ON THE HIGH SIDE THEN REMOVE IT FROM THE NEUTRAL. THESE GUYS ARE ALL CORRECT. FLOATED NOT GROUNDED SO IF YOU LOSE ONE IT (THE REMAINING TX'S) WON'T SEEK TO OPERATE IN AN OPEN DELTA POSITION AND THE WILDLEG COULD BECOME A DIFFERENT LEG STILL WORKING. :cool: BAD FOR THE CUSTOMER....

STAY SAFE OUT THERE

PA BEN
03-19-2008, 09:26 AM
I've been at this utility for 3 years now, and my first morning here for my interview I stepped out of my Motel and looked at the Wye-Delta bank across the street and saw a cutout open on one of the power pots. This was a apartment building so I thought they were single phasing. But no utility people were around? I took a closer look and the three transformers were single bushing transformers. Will I've seen it all now. I guess years ago engineering approved the set-up. We use two bushing transformers on all new Wye-Delta banks. :cool: Every time we have a primary feeder go down that feeds down town, we lose a s#!t load of cutouts and have a s#!t load of 3 pot, open-delta banks that we have to refuse.

lewy
03-19-2008, 05:55 PM
I've been at this utility for 3 years now, and my first morning here for my interview I stepped out of my Motel and looked at the Wye-Delta bank across the street and saw a cutout open on one of the power pots. This was a apartment building so I thought they were single phasing. But no utility people were around? I took a closer look and the three transformers were single bushing transformers. Will I've seen it all now. I guess years ago engineering approved the set-up. We use two bushing transformers on all new Wye-Delta banks. :cool: Every time we have a primary feeder go down that feeds down town, we lose a s#!t load of cutouts and have a s#!t load of 3 pot, open-delta banks that we have to refuse.A couple of questions if the transformers were single bushing are you sure it was a delta secondary & if it was how do you have a floating neutral as it could be alive at line voltage
I am asking this because of the switch you said was open which I understand you have to use on a delta secondary on the higher voltages ( this is something I am not familiar with because we do not have any delta secondary on our 27.6 kv distribution)
I also thought that open delta banks were 2 pot banks, again maybe just something different
All of our delta transformers are also 2 bushing

PA BEN
03-19-2008, 07:48 PM
Yes, they are close delta secondaries. On a single bushing transformer the primary coil is tied down to the case. You have to tie the case to the natural so these no way to float the high side neutral in a bank. So when you lose a power pot, (open fuse), you have a open-delta bank that is 57.7% of the closed bank.:rolleyes:

lewy
03-19-2008, 08:31 PM
Yes, they are close delta secondaries. On a single bushing transformer the primary coil is tied down to the case. You have to tie the case to the natural so these no way to float the high side neutral in a bank. So when you lose a power pot, (open fuse), you have a open-delta bank that is 57.7% of the closed bank.:rolleyes: I should have read it a little closer
Did they also burn out some transformers or was it just fuses blowing

Fiberglass Cowboy
03-19-2008, 09:02 PM
Sounds Like Your Shop Ben Should Spend Some Time Rebuilding Some Of These Old Banks The Way They Should Be Built. Seems Like You Would Benefit From Rebuilding Them 1 At A Time During The Day Rather Than Lose Multiple Banks All At Once,possibily In The Middle Of A Cold Rainy Night During A Thunderstorm. That's Alot Of Lost Revenue For The Company Too. Lost Revenue Is Money The Company Will Never Get Back. Some Of These Dufus Supervisors Just Don't Get It. Surprised They Haven't Had You All Doing That For Awhile Now. But That Makes Too Much Sense... And If Your Utility Is Anything Like Mine, They Are Not In The Habit Of Doing Things That Actually Make Sense...:d

PA BEN
03-20-2008, 09:57 AM
Sounds Like Your Shop Ben Should Spend Some Time Rebuilding Some Of These Old Banks The Way They Should Be Built. Seems Like You Would Benefit From Rebuilding Them 1 At A Time During The Day Rather Than Lose Multiple Banks All At Once,possibily In The Middle Of A Cold Rainy Night During A Thunderstorm. That's Alot Of Lost Revenue For The Company Too. Lost Revenue Is Money The Company Will Never Get Back. Some Of These Dufus Supervisors Just Don't Get It. Surprised They Haven't Had You All Doing That For Awhile Now. But That Makes Too Much Sense... And If Your Utility Is Anything Like Mine, They Are Not In The Habit Of Doing Things That Actually Make Sense...:d
Driving rain in the face wile refusing transformers at night is fun! That's why we get the big bucks:D

joeholl
03-28-2008, 10:37 PM
You're half correct. You float the neutral on the wye-delta, but you have to connect it for an open wye-open delta. Most companies have began using a "fourth cutout" with a solid door on the wye-delta banks. This allows you to close in the fourth cutout when performing switching, which will remove the possibility of ferroresonnance. For example, you would close the 4th cutout first, then close the other three cutouts, then open the 4th cutout. The bank will actually work if the neutral is not floated, but if a fuse ever blows the bank will 2 pot itself, because the neutral is closed in. The customer will not know there is a problem and the other two transformers will eventually go bad due to overload.