What is the maximum length of underground cable that can be laid without providing reactive compensation?

The extent that compensation is required has to be evaluated on a case by case basis as the need of reactive compensation is dependent on the cable circuit length and on the status of the transmission grid. Large powerful grids may need compensation every 30-40 km of cable circuit. Very weak grids may require compensation every 5-10 km. If necessary, reactive compensation may be applied inside the sub-stations at the extremities of the cable circuit.

In a recent study into the 69 mile mixed OHL/UGC link in Connecticut (USA) between Middleton and Norwalk, an assessment was made of the maximum technological feasible length of 345kV underground cables that would enable the line to meet operability and reliability requirements. The report concluded that XLPE cables were preferable to fluid filled cables and that a single section of 24 miles (40km) in a mixed OHL/UGC link was acceptable in the context of the project. In April 2005, the Connecticut Siting Council approved the plan for a mixed link of 45 miles of OHL and 24 miles of UGC (between Milford and Bridgeport).