We need to get internet access on top of a mountain, and all we have is a high voltage electrical line used to power a ski resort lightning system. We'd be able to get 220v outlets in each end, but we're unsure whether the homeplugs will be able to send signal as far as 1,5-2km, and if the power line will be suitable to transfer the signal.
What do you mean by high voltage?
If there are any transformers in the way, I don't think it will work at all.
If the 400V is just three phase with a neutral, then one of the phases should be able to carry the signal (although the range will vary from device to device).
1.5km is a long way, and I doubt that you'll get a signal. In fact, I would expect there to be 11kV (or whatever the local distrobution voltage is) to the mountain top and a pole-top transformer there.
Also, you would need both plugs to be on the same phase.
You may be better off with a directional WiFi link. Shouldn't be much interference up there, and mountaintops tend to be low on the obstacles.
The standard you need to dig though that this stuff is based on is called IEEE 1901. The limit on distance is 1.5km but I suspect someplace in the fine print there are other limitations. The largest issue I suspect is getting though the transformers. Most these do a very good job of blocking this signaling. The other issue you have is with that long a distance you are very likely to get other interference from external devices or maybe even the lighting being powered.
I am going to suspect it won't work mostly because of the distance but there are not a lot of other option that do not involve running fiber or putting up small wireless radio towers.