Ampacity of Cu 15kV Cable in Covered Cable Tray - Triplex Single Conductor

Your best bet is to contact the manufacturer of the particular cable that you are using. There are a number of factors that go into derating the cable from its free-air ampacity, including such things as the ambient conditions, tray orientation/material, covered/vented, distance from/proximity to other conductors and their arrangement, etc., etc. Most manufacturers also offer handbooks and catalogs that cover these and other ampacity considerations.

Usually the raised cover purpose it is to protect the cables from the sun radiation. NEC does not deal with sun radiation.

This system is common for power station outdoor cable tray.

Here the NEC recommendations are not compulsory.

So a special calculation was developed.

The calculation is based on thermodynamics-heat produced in the conductor when current flows through it and heat evacuation by radiation and convection to the environmental air.

Yep, check out the cable manufacturers website for the cable datasheet and/or application notes (or possibly the product line guide catalogue or similar).

Generally (in one of these documents) they will have the particular de-rating factors relevant for the cable, cable configuration and cable tray.