Portable Step Up Transformer??

T

Therealcrt

Senior Member
Location
Kansas City
Occupation
Electrician
I’m using talk to text so I hope this all will make sense

My boss is having me install a three phase step up transformer from 208 to 480 V on a rollaround wagon that’s bolted to it that will be used at an aircraft hangar. So the primary source of power is a 70 amp, thick SO cord with a strain relief connector coming into the transformer wire to the primary. The end of that Esso cord will have a pin and plug style end on it that will plug into a pen and plug receptacle, and that will be the main source of power for the transformer the secondary side I am directly mounting another pendant, plug style receptacle directly to the transformer, coming off the secondary side to feed that there will be no neutral present for that receptacle. I know I’ll still need to bond XO to the ground bar inside the transformer and then also I am mounting a ground lug to the backside of the transformer going to run a bear number six from it inside to my ground lug inside the transformer, and then that ground lug will also be the mechanics way to ground to the building steel or a grounding pod whenever they use this system, that will be the way it is grounded to the building. I’ve never done anything like this. I don’t know if it’s legal. I don’t know if it’s code compliant since there are jet fuel, and stuff involved. I just do what my boss tells me to do. can anyone chime in?
 
My transformer is a wye wye configuration my 480 V load does not need a neutral. The receptacle only has line one line 2 line 3 and ground. The primary is being supplied by a three phase power source with no neutral. My transformer has Xo and Ho I know I bond XO to the ground bar. Do I need anything landed on the HO for this to work correctly?
 
Therealcrt said:
My transformer has Xo and Ho I know I bond XO to the ground bar.
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If I understand what you are doing - you do NOT bond XO to anything. Let it float.

You will normally bond the midpoint of your secondary Wye connected coils to ground. This is typically the X0 connection in a normal step DOWN SDS setup.

You are NOT creating a normal step down setup. You are creating a step UP system, so the midpoint of you SDS secondary is the HO terminal. That is what you bond.

Note: Your Wye- Wye transformer is unusual (though common for a POCO).


Some terminology to remember.

XO is the midpoint of the Low Voltage windings (when wound in Wye).

HO is the midpoint of the High Voltage windings (when wound in Wye).



You want to bond the midpoint of the secondary side of the transformer, this is the SDS side, and in your step UP case, it's the HO terminal.

You almost never want to bond - or connect anything (including a neutral) to the primary side of a transformer that is wound in Wye. Again, it's unusual to have a Wye primary, the most common reason it happens is people looking to reverse feed a Delta to Wye Step Down xfmr. And they don't - or can't - get a proper step Up unit.

If you really have a Wye-Wye xfmr - you want to bond the midpoint of the secondary and let the primary midpoint float. If you are stepping up from 208 to 480, HO is the connection on the secondary that you want to bond.
 
Therealcrt said:
So my 208 primary side goes to X and my secondaries go to H Since I’m stepping up?
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Yes, in your case, the 208 (Low Voltage) connections are the primary at X1, X2 and X3. Again, no connection to X0 at all.

The secondary 480 (High Voltage) connections are at H1, H2, H3 and H0.

Easy to remember if you think "H" for High.
 
MD Automation said:
Yes, in your case, the 208 (Low Voltage) connections are the primary at X1, X2 and X3. Again, no connection to X0 at all.

The secondary 480 (High Voltage) connections are at H1, H2, H3 and H0.

Easy to remember if you think "H" for High.
Click to expand...
Is this standard for any transformer stepping up or stepping down the higher voltage always on the H doesn’t matter if it’s a delta Y or a YY or a Y Delta, etc. whatever the higher voltage is input or output I always land on the H?
 
I believe the H for higher voltage and X for lower voltage is the ANSI standard for low voltage transformers, regardless of configuration.

Something that concerns me: are you sure you have a wye-wye transformer, and not a wye autotransformer?

You might have a problem if you don't bring the neutral to the primary of a wye-wye transformer. If you don't supply the primary neutral, then the secondary will have a high neutral impedance. Even if you properly ground H0 the high neutral impedance will limit fault current severely. This might be acceptable or even desirable (people use impedance grounded systems for a reason) but might not be what you really expect.

If you have a wye autotransformer, then I'm pretty sure you have to bring the supply neutral.

I'm pretty sure that a wye-wye transformer can supply a balanced 3 phase load without the primary neutral.

Any chance you can share a photo of the transformer nameplate, or a link to the transformer datasheet?

-Jonathan
 
Don't forget the transformer's disconnect. 450.14

And GEC. IF you could do a 5 wire SO cord or something, 3 phases, one egc and one to building steel for the primary.

And the classified location stuff.
 
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