Long Distance Cabling, 3 Phase Supply but 1 Phase Loads

You're going to run in to problems with volt drop and MPFC.

I've just finish a design for 25KVA @ 240V over 1.2Km. I've specified 3.3KV transformers at each end.

1Ft burial is far too shallow.

At a 7 KVA load there is no need to spread it over all three phases.

As for the underground line around here it is standard practice to bury HV feeder lines down at least three feet.

For what cabling to use I would recommend using standard HV underground direct bury cable that is used by most every utility company and put it in with a cable plow machine 2 - 3 feet deep.

6 km would usually be overhead open wire on poles, due to cost, unless you have to bury it for amenity or security reasons.

Can you dig a 1m deep trench 6 km without encountering rock or a clay which shrugs off dynamite?

Even at 1000V & 35 mmsq. Al, 6km is 10% volt drop @ 7 kVA, this looks like 3.3 kV minimum - there will be drop in transfos & LV distribution too. A normal supply design gives 0 - 100% of capacity with 230V +/- 6% volts at the consumer's meter.

If you have several sites to provide, you need to consider them together - it may be better to consider 3 phase to carry power for several sites to a branch point. The fundamental fact is that 3 wires 35mm², 3 phase balanced carry 3 times the power of 2 wires 35mm² single phase, with half the volt drop. Even if you begin with 1 phase transfos, you may be glad you have an easy path to 3 phase when the loads go up.

Having dug a trench & back-filled or put up poles, the cost of a 3 wire system is less han 3/2 of a 2 wire, so you need to consider all options.

Regarding backfill material, it is far better for you to find what is available & used locally than for anyone remote to suggest a material. Ideally, it has high thermal conductivity, contains nothing sharp/hard which can push into the cable, will not react with cable coverings etc, is light to transport, easy to pour into the hole, available locally at zero cost & lasts for ever.

Are there any rules for step up voltage against cable sizing? I've plugged in the figures to several voltage drop calculators. It appears that its possible to limit voltage drop to 10% if using 185mm2 cable and only 1000volts. However using 35mm2 cable and 3300volts also limits to 10%. Or is it generally just pure economics and considering future expansion for which option to go for. We have to bury our cable for safety and security. Unfortunately people do steal the cable and sell it as scrap metal. We have our own excavators and hence 1m is not an issue. Is sand a suitable backfill? Perhaps digging 1m down and covering in 1/2 foot of sand and remainder backfill with existing soil? One more, we have another site with almost the same figures but this time, the supply side has supply from a local utility company. Is there any issues if we take the supply after the smart meter and then step up to 3300v and run down the 6KM and step down. Do utilities company care about what occurs after the meter as long as its not short circuiting anything before the meter and breaker box?

3.3KV needs a skilled cable jointer. Cable laying is also a skilled job as the outer sheath must not be damaged otherwise the CPC will be compromised.

6Km of 1700/3300V 25mm 3c SWA will require 12 drums of cable each weighing 1750Kg. Can you handle that?

I'm not even going to bother working out the weight of 600/1000V 185mm SWA

The difference in cost between 1000V and 3300V cable is significant due to the reduction in CSA

Depth should be 1m, the trench having sand in the bottom. Once the cable is in, another layer of sand followed by the infill. Raw infill will damage the outer sheath.

There are various tests to be done at each joint. I'll leave them until the OP gets back.

Have you considered using renewable energy sources at the actual domestic area and advanced design battery storage to supply load and minimise cost of diesel backup and defray the intended expenditure to supply from main factory?

Solar and batteries starts to look like a very attractive option for the remote sites.

I have considered solar and if only lithium batteries such as those from Tesla were available now to the rest of the world. The issue we have with solar is on our load profile. Consumption occurs during early morning and evening when the sun is out. No consumption is actually required during the day time save for lunchtime 12 -2pm. This means investment in deep cycle batteries becomes significant and becomes recurring as every 5 years or so, they must all be replaced. If Lithium batteries were now available at the same price, it would suddenly become cost economical for deep cycling requirements such as ours and maybe push out replacements to every 10 years. Hence I'm trying to weigh up whether its more cost economical to lay cable as a pure once off cost where power supply is from our own steam turbine at USD4cents/KWH, or solar with recurring charges every 5 - 10 years. The reason for posting is that local contractors always tend to push us into the most expensive solution and there's no easy way to get a balanced view. We would of course get qualified electricians to connect up and supervise a 3.3KV system. I am trying to piece this project together by direct purchase of cable from China to spec and transformers, and then local contractors to connect it up. We have a network of excavators and equipment/staff and hence working with heavy rolls would be challenging, but manageable. Some back of the envelope calculations still show that laying cable is 30% cheaper than solar. I am being quoted 3*70mm2 cores + 1*35mm2 core @ USD3.00/meter from China.

As suggested in few replies you can consider to have hybrid power- A combination of solar + good back up + Diesel for emergency.

Hi

Distances of 6km/10km are nothing for a properly designed HV distribution system. The cost difference between overhead and underground is going to be about the cost of doing the trenching. Maintenance cost of underground is lower than overhead, until you come to the point of having to repair something.

I have found that there is minor cost difference between 3C and 2C cable types.

Voltage drop over the HV distribution is handled by way of tapping up the HV voltage at the point injection and or adjusting the taps in the step down transformer.

Burying depth and cover has already been covered.

the OP has been silent on his proposed protection scheme. And please consider if ferroresonance will be an issue. If so you will need to have ganged isolating switches.

Probably sound like a job for a good local electrical engineer, one skilled in distribution systems.

The technicalities of this are all routine.

It does seem, to me at least, to be way too much trouble for just 7kVA of load.

Low load remote sites are/were normally served by SWER systems in Oz. Single conductor above ground. pole mounted trafos.

I have an idea for you to explore, use a gas genset, a typical 10kVa by Kohler single phase generator ( easily available in USA). The reasons is, your main factory I presume to be a palm oil mill produce a lot of pomme bio gas ( presently most are being flare off and some mill are using them to generate upto 1MW electricity for feed in tariff). Thus your investment will be some sort of portable gas tanker, compressors and transportation to site, which is far less expensive capital outlay. Anyway the bio gas is free except for the transportation ( can even use the bio gas to run the tractor for delivering the storage gas tanks)

Hi and spot on... We are a palm oil mill with biogas. We have over 12 remote housing estates. Some are within a 7KM radius of the factory; some are much further away and cross land we do not own. Hence no cable can be laid to these. The plan is to connect any remote housing estates nearby the factory to the factory, and other remote estates to a centralised estate where a small 10 or 20KVA gas genset can eventually power this. We can of course have 10 - 12 gas gensets but I'm just concerned about the maintenance cost of all of these. It sounds far easier to have 3 - 4 gas gensets and any estates within a 5KM radius connected to these. The plan subsequently is to upgrade the biogas to natural gas and then fill up gas tanks to power these remote gas gensets and hopefully later on our fleet of vehicles (quite a few government red tape to get through before a vehicle can be converted to natural gas). We don't have a grid nearby and nor is the local utility company bothered to engage with us to install cable up to our factory. Hence, everything is pure off-grid. My solution so far is to step up to 3.3KV, run 3C (3*50mm + 1*25mm) AL cable and then step down to 240v. I need to investigate ferroresonance that Cristle is talking about. Many thanks for this. Could someone help me with one more electrical terminology. The cable I'm being quoted is 0.6/1kv AL/XPLE/STA/PVC power cable. I queried the manufacturer that 1kv is too low because I'm stepping up each phase to 3.3KV. Manufacturer replied that the cable is fine because my working voltage is still 230volts. Is this correct? I'm being quoted USD0.86/meter. Total cost of purchasing 7KM of this is USD6070.00. I'm not sure if I'm missing something but economics of underground cabling (we have issues with theft and hence it must be underground), save for challenging conditions of installing the cable, appears to be much more economical than solar... Could anyone shed some light on whether in one of our remote estates where there is utility power, is there any issues to stepping up to 3.3KV after the meter? Is approvals required in the USA and Europe if we wanted to do something like this internally?

With SWER, I assume that the single cable must be above ground? If I purchased completely insulated single core cable and laid it underground, would this be fine? Need to investigate this now as a single core would of course make the capital outlay even less...

SWER is single wire, earth return. The return current is flowing through the ground. This means the potential gradient is highest near the earth rods for the return i.e. the places where people and animals are likely to be. This can be lethal and result in loss of livestock, which are more vulnerable (leg pairs further apart, more potential, current flow through heart, rather than leg to leg).

Also, there may be a difference in risk between Australia where ground is dry and high resistance, population is low & people wear boots and well populated, wet locations with people in bare feet.

Taking some numbers, 30 amps at 230 V, that is 2 amps at 3.3 kV. Supposing 3 ohm earth rods each end - 6 ohm x 2 amps = 12 volts which is below the Safety Extra Low Voltage Level of 24 volts (which is OK for normal home conditions but not for ponds & swimming pools).

However, what happens if the rod resistance becomes higher or there is a short at the load end - what chance of a dozen people in a paddy field when 500 amps is loose? Not long ago a valuable race horse was killed in England because there was a leak in an 11 kV cable which ran under the paddock - people were affected, but saved from harm by smaller "step potential" & footware.

Can the poster with the SWER system help with the reference of a Code of Practice or Standard for such a system?

A single core cable with armour return or concentric cable with armour seems safer.

I mentioned SWER as a solution I've seen. I have never worked on one.

These were not rare in remote South Australia (Ceduna) and there were no dangerous step voltages associated with them that became part of the local folklore, like there were no tall tales of horror being shared at the pub/club/campsite/tearoom.