×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Question about using step down transformer in reverse

Question about using step down transformer in reverse

Question about using step down transformer in reverse

(OP)
Hello,
I have a 240 V 3p 4 wire delta service to a building.  In one unit I have a 480V 3 phase load, so I need to step up the voltage.  If I use a step down transformer, wired in reverse, I end up with a delta secondary.  Is there any reason I cannot use this connection, assuming I ground a corner of the delta?  What are the safety concerns?

thanks
EE

RE: Question about using step down transformer in reverse

If you leave the wye point floating you may have issues with voltage transients on energization. If you connect the primary wye point to the system wye point any primary voltage unbalance or phase angle unbalance will cause high circulating current in the delta. If your transformer has 2% impedance voltage then a 6% voltage unbalance or equivalent phase angle error will cause full load current to flow in the delta.
Other than that, it works fine.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Question about using step down transformer in reverse

(OP)
Thanks for pointing out the ground current problems.  I hadn't planned to tie the neutral, but it's good to keep that in mind.

RE: Question about using step down transformer in reverse

This is not ground current but a heavy current circulating in the delta.
A little more explanation:
The secondary vectors are directly related to the primary vectors. The angle is substantially the same and the magnitudes are proportional.
Now look at the vector diagram and simulate a voltage unbalance by shortening one primary vector about 10%. One side of the delta will be shortened by 10%. The delta will no longer close on itself. That 10% will cause a current to flow in the delta which is limited by 3 x Z or three times the PU impedance. This secondary current will be reflected by primary currents on both the phases and on the neutral. These currents will cause voltage drops in the feeder conductors and the source transformer which act to correct the voltage or phase angle error.
On long rural distribution circuits where the loads are mostly single phase and voltage regulators are used, Both conditions may occur. The voltage regulators may correct the line to neutral voltage but at the same increase the phase angle errors.
If your load can accept voltage transients of approximately 200% on energization then go ahead and float the primary wye point. As you suggested ground one corner.
Alternately you may use three small dry type transformers and a resistor to build an impedance grounding circuit. The dry type transformers must be rated for line to line voltage and be large enough to support the current through the grounding impedance. Use a wye:delta connection and connect the wye point to ground through the grounding impedance. The delta secondary is not connected to anything and the secondary voltage is unimportant. This is an instance of using the idiosyncrasy of a wye:delta connection to advantage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Question about using step down transformer in reverse

(OP)
Thank you for the explanation.  I will work that out on paper to convince myself. This is a good exercise.  I have never had to deal with a delta secondary except on a static exciter.

I look at it as zero sequence current in the primary means zero sequence in the secondary.  The delta cannot get rid of zero sequence.  If the WYE is not grounded, there can be negative sequence currents but not zero.  I think your explanation is better.  Thanks.

EE

RE: Question about using step down transformer in reverse

Don't confuse grounds and neutrals.
It doesn't matter if the wye is grounded, but it does matter if the neutral is connected. In distribution circuits the terms are often used interchangeably as (almost) all neutrals are grounded and grounding grounding inherently connects the neutral, but this is often unique to distribution circuits. There is a difference in utilization circuits and improper grounding of neutrals to avoid using a dedicated neutral conductor often leads to issues with protection circuits and other issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Question about using step down transformer in reverse

...I have a 240 V 3p 4 wire delta service to the building...
 Excuse my ignorance but how do we have a system in delta (ungrounded) with 4 wires?

RE: Question about using step down transformer in reverse

(OP)
The source from the utility is a 4 wire delta, with 240V 3 phase.  The center tap of one leg is grounded.  So two phases are 120VAC to gnd, and the odd phase is 208 to ground.  This is the supply that I am going to feed into a 240/480 transformer which is supposed to be 480(D) to 240(Y)

RE: Question about using step down transformer in reverse

Life will be better in the long run if you buy a 240 delta-480/277 wye transformer.

RE: Question about using step down transformer in reverse

(OP)
I agree, and it would be easier for the installation, the labeling, the overcurrent protection....  But it costs 50% more.   

RE: Question about using step down transformer in reverse

Not being cursed by everybody who has to deal with it in the future isn't worth the extra 50%?   

RE: Question about using step down transformer in reverse

(OP)
I think you are correct.  But I didn't make the decision to buy the transformer.  I just have to make sure it's wired and labeled properly.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources