ggunn
PE (Electrical), NABCEP certified
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- Consulting Electrical Engineer - Photovoltaic Systems
Transformer xo bonding
- Thread starter nickelec
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MTW
Senior Member
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- SE Michigan
Nope, the OP showed us the tag earlier, it's a standard stepdown unit, 480V Delta primary x 208V Y secondary. Hooked up in reverse.
I see it as a case where the pulling crew, installed a 5W 208Y feeder, they were not sure on what the equipment needed, so they pulled all 5 conductors, to cover themselves.
The transformer purchaser, also not sure what was exactly needed, ordered a standard Delta/Wye stepdown unit, cause their standard and readily available for less money. A Delta/Wye, 208/480 step up unit is what was needed, but those have lead time, and cost more generally. So you get the standard unit.
Then along comes the terminator. Faced with one too many wires on the input and output circuits, and no where to properly terminate them on the given transformer terminals, takes a guess, and splices the neutrals through. But what we haven't seen yet, is how those are treated at the load end, how are they terminated, or are they? It makes a difference.
Now we have the common of the 208V wye service transformer, substituting at the load end, for the common of a 480V delta setup transformer. In reality that delta output coil is floating, it has no reference to the ground or the 208V wye service common.
Any current placed on the load end neutral or ground, be it load operating current, or fault current, has no good low impedance path, to get back to that delta 480 coil. It's not trying to seek the 208 service common, it's seeking that isolated 480 delta coil. The only path that current has, to get back to that isolated delta coil, is through the magnetic coupling of the step up transformer. Its a high impedance connection for sure, and has a long way to go to get there. From the fault, back to the service, then back to the step up windings.
It's a setup that won't conduct any real fault current, or operating current. It will conduct some, but nothing useful. What it can do though is setup a ringing high voltage on the system though, when there is arcing fault current at the load end. You have lots of capacitance and inductance with the added wiring length and transformer coils, add in an arcing fault current as a spark generator, and you can result in a ringing voltage on the system.
As it sits the 480V system is running isolated, no Wye point to ground, corner not grounded, and no ground fault detection. There is no good way to get fault current back to that delta coil. It does have a way to start resonating though with arcing current. That's why the code says to corner ground it, to give a low impedance path for fault currents to flow.Then it can trip the overcurrent device properly on a the first ungrounded line fault. Trying to use the option of ground fault detection is really not an option in most cases. You need trained personnel on staff to monitor and react to the first fault indication. Most installations do meet that requirement.
So you end up with corner grounding to meet the rule. Electro mechanical controls and motors have no problems with corner grounding. But electronic loads like VFD's, can take exception to it. Most of them are designed for a wye input, the MOV surge suppression system in the drives use that wye common for a reference. With a corner ground these MOV's can see more voltage than they are rated for. Their rated for line to common voltages, not line to line. Combine that with the possibility of high voltage ringing on a ungrounded coil, and you can easily see drive damage, sooner than expected.
The better case would be to use a proper step up unit with a wye on the output side. 208V delta primary/ 277/480 wye secondary. Then you have a neutral, a symmetrical voltage reference to ground point. Connect that derived neutral to the building ground electrode system and you are good to go, everybody's happy.
With corner grounding, the loads compatibly need to be investigated, and it requires different wiring techniques and more knowledge on how to deal with it.
MTW
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- Licensed Electrician
mbrooke
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That and the NEC now requires it if the transformer is not listed for back feed. Dumb rule IMO, but it might be to deter this very issue. Still should not have been mandated however.
JFletcher
Senior Member
- Location
- Williamsburg, VA
I haven't seen an xfmr above 15kva that isnt listed for backfeed. I'm sure they exist, just havent seen one.
and thank you MTW, your post explained to me why the primary 208V neutral wont work for the 480V delta secondary side as its neutral.
MTW
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- SE Michigan
Yes, that would be ideal, but that's not what the OP has in use. That's what all that other stuff is about, what he has existing. Trying to get him to understand why not to replace the gear with a like setup, it's not good as is, and doesn't meet code.
Reverse feeding has been in use for a long time, but there can be issues involved with it. And that's why the code now requires it to be listed as such. Newer energy efficient designs have way more inrush current than the older models, and are even higher when reverse connected. The windings may not be strong enough to withstand the mechanical forces if not constructed for it.
If you noticed the OP setup shown was a bit under protected on the 208V input. 150KVA unit, rated 416A. But his feeder breaker is rated at 800A, a bit overkill for the standard 25% adder. But if you take a look at these links you can begin to understand why, the standard 125% rating may not hold when energized.
https://www.hammondpowersolutions.com/faq/can-a-transformer-be-back-fed-or-used-in-reverse/
http://apps.geindustrial.com/publib...nsformer2|PDF&filename=XformerWhitepaper2.pdf
https://www.ecmag.com/section/codes-standards/putting-it-reverse
https://www.schneider-electric.us/e...cale=en_US&searchid=1517217382219#__highlight
MTW
mbrooke
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mbrooke
Batteries Included
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Ok- my stupid is showing here- but where do you normally find the listing? :dunce: I've never payed attention to that.
mbrooke
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MTW
Senior Member
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- SE Michigan
Magnetic trip curves are one issue. If you use fuses for protection, then you have a different curve to be concerned with.
Using the proper transformer makes things a whole lot less complicated.
MTW
Using the proper transformer makes things a whole lot less complicated.
MTW
mbrooke
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MTW
Senior Member
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Agreed, just trying to get the OP and others to realize their is something to consider, breaker trip curves and types, as well as fuse curves.
I believe in one of the links given, one manufacturer talked about inrush being up to 10-12 X the full load current. For their reverse connected units. The combination of inrush from a newer energy efficient type, and the reverse connection adder (energized winding closer to the core).
250% or FLA X 2.5, is the max allowed overcurrent device, IF there is primary AND secondary protection. A 1000% inrush spike can be a hard thing to get through any overcurrent device, if its max rating is 250% FLC
Its just another reason to avoid using a reverse connected unit, if you can avoid it.
MTW
I believe in one of the links given, one manufacturer talked about inrush being up to 10-12 X the full load current. For their reverse connected units. The combination of inrush from a newer energy efficient type, and the reverse connection adder (energized winding closer to the core).
250% or FLA X 2.5, is the max allowed overcurrent device, IF there is primary AND secondary protection. A 1000% inrush spike can be a hard thing to get through any overcurrent device, if its max rating is 250% FLC
Its just another reason to avoid using a reverse connected unit, if you can avoid it.
MTW
mbrooke
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JFletcher
Senior Member
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- Williamsburg, VA
Spec sheet like this one:
http://www.larsonelectronics.com/images/product/specsheet/64254.pdf
bottom of page 2:
Reverse Connectable: Yes
for this xfmr:
http://www.larsonelectronics.com/p-...e-primary-120240v-single-phase-secondary.aspx
which also says it's reverse connectable.
mbrooke
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Thanks- for a second I thought it said that on the label I was like
Anyway- few would pay attention to that before the code mandate. No issue.
jumper
Senior Member
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Thanks- for a second I thought it said that on the label I was like
Anyway- few would pay attention to that before the code mandate. No issue.
Not sure if you know this but the code does not require the label be marked, just that the instructions say reverse feed is okay.
mbrooke
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Useless IMO. If it can feed forward, it can feed backward.
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- Placerville, CA, USA
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- Retired PV System Designer
If it can do one it can do the other, a consequence of the basic physics of transformers.
However, it definitely does not follow that if it does one efficiently it will do the other efficiently.
Among other things, the position of a winding on the core (inside or outside the other winding) can make an enormous difference in the inrush current on energization (after correcting for the turn ratio.) This effect is even more pronounced with modern high efficiency designs.
mbrooke
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That is correct, but its up to the designer to take into account the extra inrush much like someone working with motors. The code should not be delving into this issue unless its direct, strong hazard to life and property.
kwired
Electron manager
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- NE Nebraska
:thumbsup:
Sounds like design issues to me.
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