×
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

Connecting LV Mobile Generator to a HV Transformer

Connecting LV Mobile Generator to a HV Transformer

Connecting LV Mobile Generator to a HV Transformer

(OP)
Hi all. I have a 2MW, 480VAC mobile generator that we commonly use in conjunction with our 2500kVA Multi-Tap VanTran Transformer (see attached picture).  The voltage rating for the transformer is 2400(delta) x 720(delta)-480(Y)/277;  4160(GRDY)/2400 x 12470(GRDY)/7200-480(Y)/277.  We use this combination with utility companies as a stand alone system or to parallel to the utility to peak shave/baseload.  My question is, when is it necessay to run in a 480(Y) configuration?  Usually, the utility connects the concentrics of the HV cable from H1, H2, & H3 to the HO bushing (which is typically bonded to the grounding system).  On the 480V side, we usually lift the bond from XO bushing to the system ground and leave the neutral-ground bond in the generator connected.

This question ALWAYS comes up when we use this combination and ANY advice would be GREATLY appreciated!

http://files.engineering.com/getfile.aspx?folder=eb149c20-f949-428d-a813-34483b1d1204&file=2500kVA_Transformer_(2).JPG

RE: Connecting LV Mobile Generator to a HV Transformer

When the power flow is from the wye to the delta in a wye-delta transformer the supply on the wye side must have the voltages and the phase angles equal. Any voltage difference between phases or phase angle errors will cause heavy circulating currents in the delta which are limited by the transformer impedance. This situation may be avoided by floating the neutral on the wye side.
With power flow from a delta to a wye in a wye-delta transformer has no such issues.

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

RE: Connecting LV Mobile Generator to a HV Transformer

(OP)
Thanks for the reply.  Here's a real-life scenario that just recently occured:  We had our portable 2MW generator (480V) feeding our portable 2500kVA (480V/4160V) transformer going into the utilities 4160V switching station.  From the switching station, 4160V was being distributed to a network of 20+ 50kVA 120/240V transformers.  We had the generator connected to the transformer in a Delta conficuration and the HV side connected in a wye configuaration.  At each 50kVA transformer we were getting varying voltages anywhere from 98V to 130V.  We decided to add neutrals to the LV side between the generator and the 2500kVA transformer and checked again, and low-and-behold a nice 120/240V was measured.  Why did this occur?  Is it because of the multitude of 50kVA transformers, did they cause the unbalance?  My question is, when is it necessary to run neutrals on the LV side and when is it not?  Thanks in advance for any advice

RE: Connecting LV Mobile Generator to a HV Transformer

I sounds as if you had the generator connected in wye with a floating neutral.
Don't confuse grounds and neutrals.
With a wye:wye transformer connection, the voltages and currents are reflected from one side to the other.
If the supply side neutral is floating you have the three phase variation of the open neutral condition.
The most familiar open delta scenario is a residential installation in North America (120/240 Volt utilization with shared grounded neutrals for our IEC friends.)
If a shared neutral goes open the voltages are in inverse proportion to the load impedances on both the affected circuits.
On a three phase transformer bank, transformer saturation and phantom delta effects may act to moderate the voltage shifts.

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

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