Voltage for undergrounded distribution

mbrooke

Senior Member
Evaluating the under-grounding of miles of distribution circuits in some new builds. Is a higher voltage like 34.5kv better or a lower voltage like 2.4-13.8kv better? My understanding is that reactive currents are greater as voltage increases. Also the odds of resonance and ferroresonance go up.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
You have to define "better" in each case. Cost, efficiency, etc.
 

mbrooke

Senior Member
It depends on the load requirement. Search the internet "economic voltage" and the length of the run.
The thing is I can do it either way. I can either have more 11.5kv circuits or fewer 34.5kv circuits. 25kv looks like a good economic compromise -BUT- reactance has me wondering. I don't want shunt reactors out on the network.
 

Hv&Lv

Senior Member
Your worried about ferroresonance and reactance... just how long are the cable lengths??

low capacitance cable??
 

mbrooke

Senior Member
Your worried about ferroresonance and reactance... just how long are the cable lengths??

low capacitance cable??

5 miles on average with branches. 11.5kv will produce shorter runs 34.5kv longer runs.

3 phase cable would control reactive currents better, but unsure of the cost in both terminating it and capitol. I am used to single conductor UD from overhead riser to pad-mount.

Also whether I should have "backbones"- 600amp main feeders looping through PME/PMH pad mounts spawning 100-200 amp branches- VS- 200amp feeders that just loop through padmounts.
 

Hv&Lv

Senior Member
5 miles on average with branches. 11.5kv will produce shorter runs 34.5kv longer runs.

3 phase cable would control reactive currents better, but unsure of the cost in both terminating it and capitol. I am used to single conductor UD from overhead riser to pad-mount.

Also whether I should have "backbones"- 600amp main feeders looping through PME/PMH pad mounts spawning 100-200 amp branches- VS- 200amp feeders that just loop through padmounts.

how loaded are the circuits going to be? Will there be light or no loads at the ends?


From a reactance standpoint increase the cable insulation size.
Something like 34kv cable with 12 kVA on it.
yes, it will be more expensive...
 

mbrooke

Senior Member
Sorry... 12kV on a 35kV line.
spellcheck threw kVA in there...

I was thinking increase the insulation thickness to decrease the capacitance.
Smart idea- I'll consider it.

Typically, what type (magnitude) of capacitance do you see on 15, 25 and 35kv class UD cables? How does it effect the circuit?
 

Hv&Lv

Senior Member
Smart idea- I'll consider it.

Typically, what type (magnitude) of capacitance do you see on 15, 25 and 35kv class UD cables? How does it effect the circuit?
Haven’t really calculated but one three phase line just to explain why it arced a little when it was put or took off energized. The cable was only 500’ though.


your talking miles here. Not sure of the lengths and the transformers involved with the loading. Long lines with no load on three phase padmounts equals series capacitors and inductors as I’m sure your aware.
 

mbrooke

Senior Member
Haven’t really calculated but one three phase line just to explain why it arced a little when it was put or took off energized. The cable was only 500’ though.


your talking miles here. Not sure of the lengths and the transformers involved with the loading. Long lines with no load on three phase padmounts equals series capacitors and inductors as I’m sure your aware.
Unfortunately, miles and many circuits. Burying 120-200MWs (peak load) worth of distribution. Cost taking precedence. I have no doubt some transfers will see 1% loading at night.



Series capacitors? Don't you mean shunt capacitors and series inductors?
 

Hv&Lv

Senior Member
I mean a lightly loaded long three phase line going to a pad mount will essentially be a capacitor in series with an inductor(transformer).
ferroresonance will be a concern then.
of course, as long as one phase will never open at a time there shouldn’t be a concern. ( I think)
 

mbrooke

Senior Member
I mean a lightly loaded long three phase line going to a pad mount will essentially be a capacitor in series with an inductor(transformer).
ferroresonance will be a concern then.
of course, as long as one phase will never open at a time there shouldn’t be a concern. ( I think)
I've read cases where it has been a concern, (some type of resonant condition) but my understanding is thin.

Its this which is having me consider 11kv maybe even 4.8kv.
 
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