Cable Sizing for 3000KVA Load

Is it cheaper to run all those 690V cables and buy 2 extra 380/690 V transformers than to run 11 kV cable and only use one 11 kV/380V transformer?

Somewhere around 300 to 500 mm2 (rough estimate) per phase should do it but it isn't worth the cost. And it also depends on the manufacturer data as cables need cooling.

For 11 kV you can get away with only 70-90 mm2.

we are going thru this scheme since there are some loads that are near the entrance as well, we could use the needed 380V here, instead of feeding from 800 meters away. (we might rewire these loads in the future). Then from here, we need to step-up to 690V thinking we could save on cable running 800 meters to where most of our loads are.

So, you say 300-500mm² is enough? but how many cables per phase? is 12 enough?

Also, if we go 11kv straight, how many cables per phase of 70-90mm².

thank you very much.

chadi.

<re-subscribes>

OK. Who is up for free cable sizing, without having seen the job, without knowing the ambient conditions and without knowing the method of installation, with no warranty either expressed or implied <rhetorical question - NNTR>?

Seriously, hire someone locally who knows what to do and is prepared to warrant the design. Ideally, before someone gets hurt/killed by not knowing. It will be one less thing for the hospital and its associated morgue to attend to.

<re-unsubscribes>

Could you tell everyone what your regulations say about cables? There is where you get the answer from, not us.

11 kV run allows you to run fewer and/or thinner cables.

I would go for 11 kV transmission as described by Tornado:

"This overall organization might be more sensible:
11kV genset with "small" transformer for local lower voltage loads,
11kV transmission line for the 800m run,
"Large" transformer for the distant lower voltage loads."

<sigh>

British Standard 7671 gives full protocols for cable sizing.

<unsubscribes>

"One of our concern is get the correct cable size"

Hire an engineer!!!!!!

"Hire an engineer!!!!!!"

I'd like to ad to that. Hire a competent engineer!!!!!!

This overall organization might be more sensible:
11kV genset with "small" transformer for local lower voltage loads,
11kV transmission line for the 800m run,
"Large" transformer for the distant lower voltage loads.

use MV cable

While the suggestion of using 11kV cable over the 800m is good to reduce cost, reduce volt drop and ease installation problems there are issues with this approach which require further exploration by an engineer.

1. MV protection. Fusing to protect the generator, cable and or transformer. The set up of the generator may include this but no information is provided.

2. Isolator at the generator. Is a ganged 3 phase switch required for controlling ferroressonance?

These types of considerations may push you to using a fused oil switch with relay in it at the generator end of the line. You probably want to keep away from the SF6 based gear as one installation is not sufficient to cover the additional costs of protection, handling, procedures, training etc.

The operation of this type of equipment requires specific training that an electrician will not normally have. And you will need to work out how to carry spares and undertake maintenance on it. Consider partnering with other local users and having common equipment.

I strongly agree with above recommendations to hire a specialist in this field.

In my opinion cristle is right. However I'll focus on the cables.

First of all the cable ampacity [current carry capacity or current rating for 100% load] in the underground run it depends on earth thermal resistance [RHO] and earth temperature.

If cables are running directly in the ground then partial drying-out of the soil occurs [as per IEC 60287-1-1 CH.1.4.2.See [for instance]:

http://ieeexplore.ieee.org/xpl/articleDetails.jsp?reload=true&arnumber=5255465

From this article an average value of 1.6-1.7 k.m/W it could be expected in your region. The IEC 60364-5-52 standard takes into consideration RHO 2.7 [K.m/w],but NEC only 0.9.

If the cables will run in concreted cable ducts no drying-out of soil will be expected but the temperature drop from cable to conduit [or duct] will reduce the cable ampacity.

At a depth of 0.7 m at 2.7 k.m./W in one row of 12 columns of 3 cables of copper conductor 500 mm^2 XLPE insulation 1/0.6 kV [3 cables in trefoil] and 350 mm distance center-to-center of two bunches the ampacity will be 290 A. If you need only 209 A you may use 300 mm^2 but the voltage drop will be more.

For 500 mm^2 copper conductors voltage drop will be 3.2%[at 690 V and 0.8 pf] [only on the cables].The voltage drop through two transformers [380/690 V and 690/380 V] has to be taken into consideration.

In my opinion it is not a good solution.

The voltage drop is 15-17% at full load. You need to increase the voltage at secondary from 380/690 V to 380/785 V [at no-load] in order to receive 370 V at generator panel [at full load].However at no-load the voltage at generator panel will be about 430 V.

If RHO it is no more than 200 ,distance between cable bunches 500 mm,30 dgr.C Earth,12 conductors/phase the ampacity could be 380 A[4560 A/phase=3000 kVA/380 V]. You don't need any other transformer except 11/0.380.

In order to receive 370 V at generator panel at full load the transformer ratio has to be 11/0.415 V. However at no-load the generator panel will receive 415 V.

Sorry, wrong calculation of voltage drop.

Let's say the transformer will be located at middle distance from generator to service point. The transformer voltages will be 11/0.415 kV. From the transformer panel the consumer located near the utility service point 3000 kVA will be supplied and to transformer 3000 kVA from generator will be supplied. Using 12 parallel copper cables of 500 mm^2 in both directions the voltage drop will be as above.

But, the problem will be if you intend to supply directly from generator to the consumer near the service point .In this case the generator will have to increase the supply voltage to 430 V.

Since we have not enough data -as PWSlack said-all these are speculations.