Lineman Forum: 4 secondary bushings - Lineman Forum

This post follows the one talking about alley cat bad dog titled for "for journeyman 81" http://www.line-man....-81-t18663.html

I was driving through my neighborhood and saw a transformer with 4 secondary bushings, thought I'd share it, I noticed the two center ones were tied together (CB?). Anyone know what the ABB at the bottom of it means or why the other two are just regular 3 bushing transformers? Any thoughts or comments are welcome.

This post has been edited by amnava2: 05 February 2010 - 09:40 PM

I'll start with the easy part,,ABB thats the maker.
From just looking at the pix it looks like it's a wye /delta set up for power and lights, the tying if the middle pot just makes it a 3 bushing like the other two!They've tied and grounded the center bushings deriving a neutral to get 120 volts.
If you look carefully the primary side is hooked up H2's to abc(phases) and H1's to a pole ground,thus making it a wye hook up.
I COULD be wrong!! I don't do a whole lot of transformer banking,,,,,I would like to know if the can on the left is a smaller kva rating the the other two,,,,W.S.

I was thinking that the secondary was wired delta. The left looked smaller (size, so I'm guessing kva would be also), the middle one looked bigger than the other two.

This post has been edited by amnava2: 05 February 2010 - 09:35 PM

dont you float the h1's if its a delta secondary or not in all situations???

edit: duh! on what i wrote above. i thought about it after i posted.

i cant see but on the secondary side, 2 outside cans where is the x3 to x1 tied? unless thats what the others legs coming down are doing out of the picture.

This post has been edited by scratchpad: 05 February 2010 - 03:15 PM

I'll go check it out

This post has been edited by amnava2: 05 February 2010 - 09:41 PM

See Scratch,,,I fucked up!! Good eye,your right ,,,wye /wye you tie,wye/delta you float. See that's why you need to study this stuff,NOBODY knows it all.I hooked that bank up WRONG!!And you caught it,Nice! W.S.

What does 'float' mean?

Nope WS, you was right the first time-- wye/primary and delta/secondary, you can ground or float the neutral depending on bank setup and customer needs.

You have a larger pot in the middle with the single phase lighting load coming off it and you have three phase load off all the pots. Also the bank is set up with the leads configured as below:

Left pot x3 to x1 on center pot, x4 on center to x1 on right pot ---so out of picture it must be x3 on right to x1 on left---classic delta secondary.

the terms
wye/wye you tie
wye/delta you float

refer to the tying to or floating the neutral on the high side of the transformer. so wye primary/wye secondary you tie the h1's or h2's, in your pics its the h1's, to the neutral. wye/delta you float, just connect them together. if that made sense???


Lrod, i dont see a primary neutral. so in what circumstance do you tie and when would you float on wye/delta? Is tying to the pole ground the same as tying to neutral if one was there?

This post has been edited by scratchpad: 05 February 2010 - 10:17 PM

Looks like there is a system neutral to me... looks like 6 or 4 copper.

It's a wye-delta bank, 3 phase 4 wire 120/240.

I'd go with a 99% probability that the wye is floated on top, you'd want to float the wye so that if one of the power pots fail the customer would see no voltage, otherwise if you grounded the wye the other power pot would carry all of the 3 phase load, potentially burning up the other power pot.

In any new installations here we use a 4th cutout to prevent ferroresence... even on older ones when we work on them we'll install the 4th cutout.

On a open wye-open delta bank you'd ground the wye, practice at my utility is to use single bushing transformers in that case instead of two bushing.

The only thing that is different is they used H2 on the primary instead of H1, all that did is change the polarity of each transformer... really no big deal, just make sure you do that on all three if all three are the same polarity.

The lighting pot with the 4 bushings, I've only seen one of them, but to my understanding it's just an external way of changing your coil to utilize if you wanted to cut it two wire for a 120/208 (or 277/480) application (A C B D).

Floating the wye just means lets say on this picture

Each pot has one phase of primary going into H2 ( A B C ) the other top bushing H1 just has a common lead going to each transformer... it just floats, only ties into each H2.

Here's a diagram of floating wye

that cleared it up for me, thank you.

thanks for the explanantion dallas

Been gone awhile but. . .

Way to "GET SATURDAY NIGHT WITH IT" rcdallas Jerry would be proud! Did you just get back from Lancaster or something LOL

This post has been edited by splitbug: 08 February 2010 - 10:31 PM

Naw, I had David!! That was last years class... I just lack leadership... don't wanna go to Dallas too damn far away!!!

This is a wye delta bank, the primary nuetral is floated, the secondary side is a straight 240 bank. The center transformer is center tapped to also give the customer one leg of 120 volts for lighting.

i've wondered about "floating the neutral? does that mean you're NOT connecting the neutral?

thx

Correct, the neutral is connected on all three pots but isolated from ground (neutral). Also I missed it on the first go around but I now see where the neutral's have a connection between them on the high side.

A floating nuetral is very common in distribution for Wye Delta banks. The most common reason for floating the nuetral on the primary is because if you were to ground the float and one of the transformers go bad you will still have perfect three phase power on the secondary but only 2/3 of the available KVA. It makes trouble shooting a nightmare and can overload the customers mains. The most important thing to remember about floating nuetrals is you must ground the float whenever energizing, de-energizing and testing the bank. After the bank is energized,tested and on-line the ground is removed. If you can visualize a wye vector in your head the point where the three legs meet is the nuetral. In most cases like a wye wye , the primary float is grounded... in cases where the primary is a wye and the secondary is a delta, customer load keeps the nuetral in the middle of the wye vector

We have a few of them around here. They are on large pots 167 and 333. We tie the middle bushings together and build a delta bank.