Optimal voltage 2

[stevenal]

Guess I got on the wrong forum last week.
Utility customer wants to take a service at 240 V 200 A, step it up to run 1 mile underground, then step it back down to feed a residence. Any rules of thumb for determining an optimum step up voltage, or pointers on where to start? Thanks.

 

[itsmoked]

50kVA

One mile... If the cost to do this approaches off-grid costs then I wonder if off-grid wouldn't make more sense.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[stevenal]

Keith,

Question concerns voltage, not apparent power. No thought of going off grid.

 

[davidbeach]

I'd strongly suggest trying to keep it to 600V or less if the customer is responsible for it. On the other hand, if the customer will contract all maintenance to you, put in two of your standard 50kVA padmount distribution transformers and run a single 100% URD conductor between.

 

[jghrist]

I agree with David. MV maintenance will be a big problem if the customer cannot arrange a maintenance agreement with the utility. If the utility does the maintenance, the voltage should be the utility standard MV so that stock cable and transformers can be used.

But then, to step up to something less than MV seems ridiculous. Far better to get service at a higher voltage to start and avoid a second transformer.

For the utility to put in two 50 kVA padmount transformers and a mile of URD cable also seems ridiculous. Surely, some out-of-pocket cost reimbursement can be arranged so that the utility can run MV all the way to the residance without transforming down to 240 volt and then immediately back up to MV.

 

[stevenal]

David, Jim,

All this has been discussed, and may ultimately rule. Still need an answer to the original question, though, regarding an optimal voltage. Lotsa variables starting from scratch. Hoping to find a rule of thumb or other pointers to start from.

 

[davidbeach]

Steve, You've been in this business long enough to know that optimal doesn't exist. If it has to be this way, jghrist is right about just running the MV all the way in, leaving it all at LV is optimal if the customer is responsible for maintenance, Joe's Electric won't have a clue about MV. If you're going to be doing the maintenance, make it as close to the rest of what your crews have to work with. Therefore, at least two "optimal voltages".

At LV, 480V transformers might be enough lower in cost that 600V transformers to make up for the increased wire size, maybe not. Transformers will probably need different taps to maintain decent voltage.

 

[stevenal]

For this exercise, please consider "optimal" to minimize first cost to customer, and losses on the customer side of the meter. It excludes any considerations of maintenance.

It may help to consider the question hypothetical in nature, since the end result will most likely look like an extension of the distribution line. Even so, the question remains.

 

[davidbeach]

Only four cases worth considering; outcome very site specific.

1. Move the meter to the load. Probably less costly than option 4, maybe least cost overall.

2. 480V. Principal advantage is transformers are readily available. Unconsidered maintenance advantages as this is what Joe's Electric is most likely to be familiar with. Possibly lowest initial cost, definitely highest losses.

3. 600V, limiting case of low voltage. Transformers available, but not readily. Can use smaller wire than 480V option, or lower losses for same size wire. Lower current, less voltage drop than 480V option. Depending on transformer cost/wire cost balance could be less expensive than 480V.

4. Distribution L-N voltage. Lowest losses, by far. Probably not the cheapest, though. Needs only one cable if 100% neutral URD is used, 480V and 600V need two conductors.

Don't see how anything between 600V and distribution could be lower cost than both boundary voltages so no point considering anything in between. Cost considered is total cost, not customer cost as separate from utility cost.

No longer having handy-dandy voltage drop calculator available, I've used DC resistance for a unity power factor load and allowed a 5% voltage drop, one transformer tapped up one step, the other down a step; anything more and regulation will be a significant problem. For 480V you need 300kcm or larger conductors. For 600V you could use 3/0. At 7200V (assuming a 12.47kV distribution system) voltage drop ceases to be a consideration (18 AWG has a low enough resistance) and the minimum size 2 AWG is more than sufficient.

My choices (in order) 1, 4, 3, 2. (But, all but 1 are rather stupid.)

Price up (very round numbers) 10.6kft of 300kcm Cu (350kcm may be less as it is more common), 10.6kft of 3/0 Cu, 5.3kft of #2 Al URD, and 2 transformers at each voltage. I wouldn't be surprised (much) if with half the length, smaller size, and Al conductor that the URD turns out to be less cost; certainly lower losses.

 

[alehman]

Probably no rule-of-thumb applies to this odd scenario. If I were doing it, I would start by looking at the cost of the major pieces for various scenarios - cables, transformers, installation labor. To get to this point you will need to do some rudimentary engineering to determine suitable sizes for load and tolerable voltage regulation. That may decide it for you.

Next I would look at cost of energy losses. For that you will have to assume some average load, life span and energy cost per kWhr.

Run these calculations for a number of designs, keeping in mind that the installation cost is fixed but the energy cost will most likely continue to escalate.

 

[jghrist]

For this exercise, please consider "optimal" to minimize first cost to customer, and losses on the customer side of the meter. It excludes any considerations of maintenance.

alehman's response is the best if these are the ground rules. A star for him.

 

[waross]

One thing to consider:
Why go from distribution voltage to 240V, and then back up to 480V or 600V. Why not just go from distribution voltage to 480V or 600V? There are digital kWHr meters available that will accept any voltage between about 69V and 600V.
By the way, 600 volts is a standard Canadian voltage. Any supplier with a Canadian branch should be able to locate a 600V transformer.

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

 

[davidbeach]

Aw, geez, Bill, now you go and make me add options 2a and 3a to my list, take service at 480V or 600V and dispense with the first additional transformer. Downside is that the utility probably isn't set up to deal with single phase services above 240V.