I will attempt to answer your question from a technical perspective and disregard the cost aspect as you requested. Cost data for constructing various voltages of overhead and underground transmission lines is readily available online and doesn’t need to be rehashed here.
You raised the first issue to work through, which is the use of shunt reactors to compensate for charging current due to the high shunt capacitance of the cables. Reactors are used to supply charging current to the line capacitance so the power system doesn’t have to supply the charging current - essentially a way to increase the efficiency of the transmission circuit by minimizing the losses that have to be supplied by generation.
The next limitation is that the cable itself has to carry the charging current in addition to the power we are trying to move through the circuit. Each cable design has a continuous rating that is calculated from heat transfer equations based on the thermal conductivity of the surrounding soil and other things that may heat the soil, like other nearby cables. For a particular cable design with some calculated continuous current rating, that current rating will be taken up partially with cable charging current and partially with load current. So the longer the cable is, the more charging current it will have to carry at the ends, then a smaller portion of the ampacity rating is available to carry load current. In the extreme case, if we make the circuit length long enough the charging current will be equal to the circuit ampacity, leaving no copper available to carry load current. From a practical perspective an economic study can be done to determine the relationship between maximum load current and length for each cable design. If we consider a particular cable and find that it can’t carry the amount of load current want for the required distance we can try selecting a larger cable.
At some point we will still run into some limits where we cannot move the desired amount of power the distance it needs to go. The next option is selecting cables with less charging current - DC! Of course this requires converter stations at both ends of the line, which are large, expensive, and require maintenance. With DC links right now there are essentially no power and distance limits - reliability and redundancy concerns tend to crop up and cause us to select multiple smaller lines before DC link technology constraints limit the capacity of the circuit.
As to the viability of an all cable transmission system, some folks are headed in that direction. There are some countries and areas where it is virtually impossible to get approval for an overhead transmission line so underground has essentially become the standard. Offshore wind generation plants use extensive networks of 66 kV collector circuits and transmission level export cables. Some of the export cables have lengths and power requirements in excess of what any AC cable can provide so offshore DC converter stations are being installed and a DC subsea export cable is used. We have made some conceptual studies of more extensive all cable transmission networks and they appear to be technically feasible.
Sent from my iPhone using Tapatalk