×
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

25kV vs 13.8kV Distribution Systems

25kV vs 13.8kV Distribution Systems

25kV vs 13.8kV Distribution Systems

(OP)
I work for a utility that has a 13.8kV distribution system.  The utility will be installing two new substations near the edge of our service territory over the next 2 to five years.  The main purspose of the substations to provided capacity relief from existing feeders and substations and also to provide capacity for the undeveloped areas.

I was asked to review the possibility of a 25kV distribution voltage for these two new substations.  I know there are pros and cons of each distribution voltage.

Some of the benifits of a 25kV distribution system include:
[1] Higher voltage circuit has less voltage drop.
[2]Higher voltage system can carry more power for a given ampacity.
[3]For a given level of power flow a higher voltage system has fewer line losses.
[4]Higher voltage circuits can cover a much wider area (more reach).
[5]Due to the longer reach, higher voltage systems require fewer substations.  (The distance can be increased between subs.)

Some disadvantages of a higher voltage system include:
[1]Longer circuits will have more exposure and will mean more customer interruptions.  This can be somewhat mitigated by installing distribution automation.
[2]Generally, the equipment such as transformers, insulators, underground cable, cost more for a higher voltage system.
[3]Higher voltage system are more prone to ferroresonance, for delta-grounded wye trasformer connections.

Does anyone know of a published study evaluating both systems from an operational point of view and from the economical analysis?

If the substations did not have to provide relief to the existing 13.8kV distribution system the analysis would be straight forward.  There is about 12 to 15 MW of load that will either have to be converted or 25/13.8kV stepdows transformers will have to be installed.  (My guess that the economical analysis require a combination of both since there is a lot of 15kV rated direct buried cable would have to be replaced if the area was converted to 25kV.)

Any thoughts and comments are welcome.

RE: 25kV vs 13.8kV Distribution Systems

Also consider voltage sag.  See Consider Voltage Sag When Upgrading System Voltage, Transmission & Distribution World, Vol. 56, No. 12, December 2004, by M. Stephen Daniel, Alabama Power at http://tdworld.com/mag/power_consider_voltage_sag/

RE: 25kV vs 13.8kV Distribution Systems

Consider availability of spares. Does your company stock 25kV equipment. If not, is it worthwhile adding this inventory requirement? Training and personal protective equipment requirements for line crews will be different. The physical size of 25kV equipment will be larger.

RE: 25kV vs 13.8kV Distribution Systems

 
For a US site, you might mull over a few things that could give you a ‘hedge’ at minor cost.

Although 1965 copyright, the Westinghouse “Volume 3 — Distribution” utility engineering book contains some little-changed general design considerations.

Even if the lower voltage is chosen for the immediate system, (a) plan for a 23.9Y/13.8kV multigrounded system with reconnectable overhead or internally switchable-primary distribution transformers, and (b) specify 320-mil dielectric URD cable for any new work with corresponding IEEE-386 fittings.
  

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