Long Distance Supply at Low Voltage 4

[bigamp]

Is there any practical limit to LV circuit length, and what are the limits?

A small offshore platform will have 24V DC loads that look like they will translate to no greater than about 3 amps per phase at 415V/50Hz. The platform is to be about 12 km from the nearest 415V point of supply.

It appears possible to install a mixture of solar panels, wind turbines and batteries but I wonder if it would be practical to cable in a 415V supply (accepting that reasonably large cross sectional area conductors would be required and that installation would be very expensive).

Any thoughts?

Regards

 

[jbartos]

Suggestion: LV limits for a particular voltage are found out by calculations and comparisons with higher voltage power supplies. There are several factors that have to be considered, not just voltage and size of the conductor, e.g. equipment price, equipment availability, weight, physical dimensions, safety, alternate power sources, equipment compatibility, etc.

 

[jghrist]

Voltage drop of #4/0 AWG (107 mm2) cable in steel conduit would less than 4% for 3 amps over 12 km. 125 mm2 armoured submarine cable would have less voltage drop. This is not excessive, so it seems that service at 415 v would be possible. I suspect that the cost would be higher than a solar panel for this size load, however.

 

[dcdtn]

3 Amps at 415V translated into only about 2.2kW. If this load is going to be served for only a short period of time, why not buy a small generator? For long periods of time a wind/solar system would be attractive.

 

[bigamp]

dcdtn,

Yes, its a very small load. It is very attractive to go solar/wind with lots of batteries. It is just that I am a little wary of such things and it would be good to have the option of "proper" power. I am checking out solar/wind vendors, can you suggest any?

The whole of the platform will be an IEC zone 1 hazardous area (North American class 1 div 1) so whatever is installed will need to be Ex rated equipment. A (fossil fuel?) powered generator is not realy being considered because of reliability concerns (the platform is to be unmanned and very infrequently visited).

An earlier version of the platform would have had more load so supply was to be at 11kV with an on-board Ex rated transformer. Now the load has shrunk so much that solar/wind/batteries may well be viable. It's just that if I wanted to provide proper power, it seems silly to put in 11kV and transformer for such a small load. So, I was wondering if supply at 415V over 12km would be practical (if not laughable)? Is there anything other than voltage drop that would come in to play?

Regards

 

[jghrist]

Voltage drop, economics, and equipment availability are the only things that I can think of. Can you get a 12 km reel of 480 volt submarine cable or reliable splices for undersea?

 

[RajT]

I would recommend the selection of a sub sea cable for the application as stated by Jghrist. I would also allow for a dedicated transformer at the sending end.
The volt drop is not an issue at all for a 3 amp feeder noting that the transformer at the sending end can also have its taps set down to say -5% to help the situation.

 

[akhaider]

1) Solar Cells, Wind and Battery Bank arrangement, all appear to be make shift arrangements, considering the fact that the facility you have mentioned is not manned and the atmosphere is hazardous.
2) Your load is very small and voltage drop does not seem to be a problem, however length of cable is.
3) I think dont go for short cuts. Calculate appropriate size of cable for this length. Buy submarine cables, suitable jointing kits and take 415 V to that location.
4) Otherwise you need a step up transformer to step up voltage from 415 V to 11 kV. Carry 11 kV to your Offsite. Step down from 11 kV to 415 V.

 

[jbartos]

Suggestion: The solar cell panels appear to be protected for salt, water, wind, etc.; however, the solar cell panels and combiner boxes are not advertised explosion proof. Also, the wiring from cells to the combiner box is not advertised explosion proof. E.g. visit

http://www.backwoodssolar.com/Catalogpages2/pvmount2.htm#WIRING

for more info.
Maybe, there is a custom-way to make the solar cell panel and associated electrical installations explosion-proof.

 

[jbartos]

Suggestion: Check with

http://www.kellysearch.com/product-7003801.html

solar cell suppliers for explosion proof solar cell panels

 

[bigamp]

Akhaider, thank you for your thoughts. I fully agree with what you have to say.

Jbartos, thanks for the references. As it happened I had already contacted one of them and a response is due this week. It will be interesting to see how they comply with the hazardous area requirements.

Regards

 

[SooryaShrestha]

Drawing 3 kw at 11 kv with 12 km cable will have appreciable charging current. It seems not only not advisable but also, serious problem might arise. I have not done the calculations, but the losses will also be appreciable. There is possibility of supplying by non-standard voltage of say 1000V/600V,which could prove to be cost effective compared to 11 kV. But if it is offshore, solar PV would be ideal in the case of unmanned station.