WYE-DELTA Step up x-frmr connections

[photon]

Am installing a delta/wye transformer (back fed, but approved by the manufacturer). Primary = 120/208V WYE from the utility service disconnect; Secondary = 480V Delta; 112.5kVA unit;
Question: Manufacturer says "do not connect X0 to ground / grounded conductor when back feeding"(Using as step up instead of step down); Which phase should be grounded on the secondary(high side - 480v)? I think "C" phase. Comments?

 

[curt swartz]

Re: WYE-DELTA Step up x-frmr connections

What are you feeding with this transformer? You don't necessarily need to ground any of the phases.

 

[jim dungar]

Re: WYE-DELTA Step up x-frmr connections

The standard is "grounded B" phase.

On average, I specify 5->6 of these systems every year. For single machines/motors I suggest an ungrounded system. For panelboard or multiple machines I recommend a grounded system.

Note:
Grounded phase circuits require circuit breakers with special short circuit current ratings.

Intentionally ungrounded systems should include a "ground detector" to indicate or alarm when a ground fault occurs.

 

[templdl]

Re: WYE-DELTA Step up x-frmr connections

The 208y/120v connection should be may by 1) unbond the X0 and ground, the manufacturer usually bond the XO and enclosure with a bonding strap, make your 208 connections to X1, X2, and X3. DON'T run a neutral to the X0. Ground the enclosure of the transformer.
The 480v delta side can be run ungrounded depending upon what you are feeding. Such a configuration can be extremely dangerous should 2 phases go to ground. Jim has pointed out a ground detector system which can warn if one phase goes to ground but it is a method that must be monitored by qualified people.
I personally would suggest that it be corner grounded.

 

[bennie]

Re: WYE-DELTA Step up x-frmr connections

I suggest getting the manufacturer to sign off on this set up, or buy the correct transformer.

Actually, I would buy the correct transformer.

 

[curt swartz]

Re: WYE-DELTA Step up x-frmr connections

Bennie

I know you don't feel that a transformer should be used this way but I have done it before and the instructions that came with the transformer specifically said the transformer could be used in this manner. The few times I have had to step 208-480 for individual pieces of equipment I have tried to obtain 208 delta - 480Y/277 transformers but I have always been told it would take 4+ weeks to get one. My customers didn't want to wait that long to get the equipment running and since the manufactures approve of using a standard 480 delta - 280Y/120 transformer in reverse I don't see a problem.

 

[tom baker]

Re: WYE-DELTA Step up x-frmr connections

If you will look in a Sq D catalog you'll note they have a 480 to 208 xfmr and a 208 to 480. if you use a 480 to 208, which is delta wye, and wire it 208 to 480, it comes out wye delta. If you don't need a neutral then you are OK.
However I agree with Bennie, if the transformer is not specifically identified as being able to be used backwards it would be a violation.

And the 208 to 480 I orderd from Sq D I had in a week, it is stocked in their distribution system.

 

[jim dungar]

Re: WYE-DELTA Step up x-frmr connections

The last information I got from Square D never addressed any listing violations, they claim it is an application issue for their customers. Their transformer nameplates now only have high voltage and low voltage, they no longer list primary and secondary.

Considerations include:

1. Backfeeding is not recommended for general purpose transformers smaller than 3kVA, and is not allowed for any Industrial Control Transformers of all sizes, because windings are compensated. Backfeeding will result in lower than expected output voltage.

2. Backfeeding causes very high excitation inrush, making coordination to breakers or fuses difficult without exceeding the limits set by the National Electrical Code. Avoid backfeeding wherever possible for this reason.

3. If a Delta-Wye transformer is to be backfed so that the Wye side is the input, do not connect the neutral terminal to the primary system neutral, nor should the neutral terminal be connected to ground.

4. If the transformer has taps, when backfed the taps do not help compensate for poor (other than nominal) source voltage to provide the appropriate magnetic flux levels in the core as they are designed to do.

 

[templdl]

Re: WYE-DELTA Step up x-frmr connections

Jim,

2. Backfeeding causes very high excitation inrush, making coordination to breakers or fuses difficult without exceeding the limits set by the National Electrical Code. Avoid backfeeding wherever possible for this reason.

The inrush is the proportional, that is if the inrush in the HV side is 10x the HV FLA then it will be 10x the LV FLA on the LV side. A study that I have done with inrush values as provided by a transformer design engineer to compare them with the instantantaneous trip calibration of molded case circuit breakers shows that a 75kva 480v delta-208y/120 dry type distribution transformer will have in HV inrush of about 900a. Since the LV has a FLA of 208a then if the LV inrush would be 2080a if the transformer were to be reverse fed. Yes, the inrush is higher on the LV side but so is the FLA.
Please note that the magnetic trip calibration of a common indutrial MCCB is 10x the rating of the breaker +-20%. MCCBs with adjustable magnetic trips will have an adjustable range of 5-10x the rating.

4. If the transformer has taps, when backfed the taps do not help compensate for poor (other than nominal) source voltage to provide the appropriate magnetic flux levels in the core as they are designed to do.

A transformer commonly may have (2)2-1/2%FCAN and (4)2-1/2% FCBN taps as viewed from the HV side. From the LV side then the taps would be concidered just the opposite. Then the disadvantage is that there would be only (2)2-1/2%FCBN taps ot compensate for a voltage lower than 208v instead of 4.

 

[photon]

Re: WYE-DELTA Step up x-frmr connections

I really appreciate all the input from everybody. As I stated in my first post, the manufacturer (FPE)provided specific instructions sent with the unit specifying how to back feed this Xformer - which I assume would be an approved listing. It is intended to supply an infrared furnace (custom designed / built by a qualified engineering company) at +/- 68kW / 480v 3ph. I have corner grounded the high side (secondary), and have 200k rated fuses at the secondary disconnect. The manufacturer also states that the high in-rush current will most likely trip the primary breaker (it does), but to reset it quickly and it will hold after the field stabilizes(!), and this is normal(!!).
Question: Why do I not supply Xo with a neutral connection?

 

[bennie]

Re: WYE-DELTA Step up x-frmr connections

A floating wye is to mitigate the heating effects of third order harmonics.