In our modern society it is almost impossible to get permits for new overhead lines. The power companies struggle against residents that do not want an overhead line in their neighborhood. This struggle takes years and costs a lot of money. A way of dealing with these problems could be to build an underground AC-cable. The buried cable would probably not cause such struggle to get permits.
When a cable is installed in an existing transmission grid it carries more load than an overhead line should have carried. The reason behind this is that the cable has lower impedance than overhead lines. This lower impedance has to be compensated in some way. The simplest way is to install a series reactor on the cable which increases the impedance.
The amount of reactive power produced by a cable is in the vicinity of 20 times larger than the reactive power produced by an overhead line. This reactive power needs to be taken care of by shunt reactors. The degree of compensation needs to be about 100 %, the degree of compensation is the ratio between created reactive power and shunt compensated reactive power. Most of the reactive power needs to be compensated in order to energize the cable; the voltage would otherwise become higher than the highest voltage for equipment.