25kV vs 13.8kV Distribution Systems

[MarkKW]

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.

 

[jghrist]

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/

 

[alehman]

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.

 

[busbar]

 
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.