Far Away Load

If the steady state copper loss cable loss can be tolerated, the other consideration to take into account is sudden change of load or motor load starting.

Motor starting draws very high current, if your line impedance is high (due to long cable), the voltage dip may be very severe, your other electronic equipment may not like it.

If voltage drop in cables equates to IR, do take into account that under exceptional light loading conditions there will not be the desired voltage loss in the cable hence any electrical equipment in service at that time will get something like 440 v at the terminals and might burn.

Under regular loading conditions when you can expect 380 v at the load end, do take into account that the I² R loss in cable is ending up in heating the cables and depending on the type of insulation there is a limit to heating of the cables.

Do take note of the applicable codes, rules and regulations.

You are right, regarding the change of voltage during a light load. My other idea is to use a voltage stabilizer at the load side, to maintain the 380V. That way I will only use 1 MCCB/ACB at the outgoing, 1 voltage stabilizer, and 1 MCCB/ACB at the incoming. It's much better than using 1 step up transformer, 1 step down transformer, and some LV and MV breakers.

Is the equipment you are powering actually going to be underground in the mine? Will it be movable? (I mean, will it be moved occasionally while the mine is in operation, or will it be at a permanent location?)

I can guess that this is a load outside the actual mine, like a ventilation fan, a crusher, the motor(s) for a conveyor belthead, or a hoist or something like that?

For loads like that, actually outside the mine, I would consider running a high voltage overhead line from your high voltage source and then hanging (well, maybe setting an oil filled transformer) outside the mine to power the equipment. We did that for loads like I mentioned in the US 35 years ago, and I like that solution better than running a big cable and an improvised power boost of any sort. (Well, maybe improvised isn't the best choice of words, but it just seems a little kludgy.)

700 meters is almost a half mile. (I have trouble "thinking" metrically.)

How would you route the big cable that you'd need at 380 volts? If you'd hang it anyway, you'll need poles and hangers. (I guess you could use a suitable outdoor rated power cable (how many would you need) and a galvanized messenger wire and sister hooks (I'm envisioning running around the outside of a hill like we might do in West Virginia--if the land is flat would you dig a trench and bury the cable?

Again I guess and with the comments that the load is on surface in a coal mine.In a coalmine flameproof features are required which is not mentioned. Normally in a mine there is not so much space to draw a HT overhead line. If this is so HT can be transmitted through power cable and transformer can be installed nearby the load. Economically this may be a better option taking into consideration LT related problems afterwards.