12.47 kV - 480Y/277 V transformer hookup on a grounded wye utility system

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wirenut1980

wirenut1980

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Plainfield, IN
Hi everyone, Happy Friday!

I am an engineer at a utility, and we have an unusual situation potentially. All of our padmount transformers are grounded wye-grounded wye. And all of our distribution circuits are grounded wye 12.47/7.2 kV. We have a customer where we may replace some wye-wye padmount transformers with delta-wye padmount transformers. The reason we are considering this is to reduce harmonic current distortion generated by the customer loads. They have 6 pulse VFD's, and also 6 pulse induction furnaces. The strategy is to replace half of the wye-wye transformers with delta-wye transformers. The 30 degree phase shift will help cancel much of the 5th and 7th harmonics. So it would be possible to have some wye-wye transformers, and some delta-wye transformers in an underground loop. The primary concentric neutral at the wye-wye transformers would remain bonded to secondary neutral, and grounded at the transformer.

I am getting questions about what we do with the primary (12.47 kV) concentric neutrals at the delta-wye transformers if we go ahead with this. Do we ground them at the transformer and bond them to the secondary neutral, or do we float them? The secondary neutral will be grounded for 480Y/277 V services at all transformers.

I came across the thread below where someone suggests bonding the primary and secondary neutrals at the delta-wye transformers. My personal feeling is that it could be done this way and any unbalanced load current could return on the primary neutral if the primary and secondary neutrals were bonded together, no harm done.

Or the primary neutral could be floated at the delta-wye transformers, also no harm done, as long as the primary neutral is continuous to accommodate any unbalanced load from the wye-wye transformers.

Is there an advantage one way or the other with respect to what to do with the primary neutral at the delta-wye transformers? Or is there only one correct way to do it?

Thanks in advance!


http://forums.mikeholt.com/showthread.php?t=161313&page=2
 
The delta-wye units I have seen fed from an MGN wye do everything identical to a transformer wired Ygr-Ygr.

I see no benefit in an isolated ground for the secondary.
 
Does the NESC have any requirements that would apply? From a NEC perspective you would need an egc run with the primary to bond the transformer case.

I don't immediately see what bonding the MGN to the secondary neutral would do, other than using it as a "good" system ground.
 
Thanks for the replies. The NESC does not get into specifics like this unfortunately. The word "delta" is only mentioned a handful of times in the entire document. I'll do some more research, but I think I will bond and earth the primary and secondary neutrals.
 
It sounds like you have a multi-grounded neutral distribution system; if so, it is necessary to have an electrical connection to earth at least 4 times per mile to keep the voltage on the multi grounded neutral from exceeding approximately 25 volts. The fact that you had a Y-Grd/y-grd, changing to Delta/y-grd, you should probably maintain the neutral on HV side to grd.

Check NESC Rule 096 C for multi-grounded neutral conductor information. The pros and cons of this type of system are well documented.
 
you have a d primary and yg sec
your supply is yg

you have to tie the supply g to the hv enclosure to clear a prim ph fault
but either way you elevate the sec frame potential to 7200 on a prim g fault
and depending on g rtn Z it may not clear quickly (or at all)

I would not bond it to the sec g, although it will be inherently (via util g system, sec gnd system and case bonding)

from the sec run 3 ph, g, n
remove xfmr x0 to g bonding jumper
bond g to lv case
isolate n
at service install gnd bed
bond bed, n and g (n and g in pnl)
 
Besoeker said:
YN. You may tie the N to ground.
Click to expand...
his supply is 4w not 5, and continuously grounded at multiple points

obviously it is tied to equip frame ground at the xfmr as Instated
but not to directly bond it to the sec x0 (it is indirectly though)
if you have a prim l-g fault the entire sec frame voltage is raised to 7200

we never allow wye-wye xfmrs without a sec tertiary winding
in our case it is required by law
 
Besoeker said:
My point is simpler. You can have a Dyn11 and you may the star point to ground. That's why I would call the secondary yn.
Click to expand...

the sec IS intentionally grounded
your point misses the point, not discussing vector group notation, but safety of an
installation: bond or not bond grounded prim n to the sec grounded n
if you directly bond the prim g to the sec x0 upon a prim l-g fault the sec g frame is elevated to 7200

that is why we take steps to isolate the prim and sec g reference as much as possible
2 beds, physically isolated, ngr, etc
the higher the Z between them the higher the v drop the lower the residiual voltage on the sec g frame
 
Ingenieur said:
if you directly bond the prim g to the sec x0 upon a prim l-g fault the sec g frame is elevated to 7200
Click to expand...

Relative to what though? Technically speaking, phase and MGN will have about equal Z, if not lower on the MGN dues to parallel paths and grounding. This would mean that during the fault only 3,600 volt will appear to the frame and remote earth. When this happens the customer's grounding/bonding system should bring everything to near zero potential (relative to one another) inside the building, ie similar to a swimming pool following article 680. Due to the low Z the 7kv fuse will open very fast, in a cycle or two at most.


we never allow wye-wye xfmrs without a sec tertiary winding
in our case it is required by law
Click to expand...


Tertiarys are not used at this voltage level by POCOs, and while for another thread the trend has been to eliminate them even at the 115 and above transformation level- there is just no need for them. As long as you spec a 5 limb core to deal with blown fuses, the transformer will handle everything else Y-Y or D-Y.
 
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