There are three kinds of limitations for loading capability of transmission system:
Thermal capability of an overhead line is a function of the ambient temperature, wind conditions, conditions of the conductor, and ground clearance.
There is a possibility of converting a single-circuit to a double-circuit line to increase the loading capability.
Dieletric Limitations From insulation point of view, many lines are designed very conservatively. For a given nominal voltage rating it is often possible to increase normal operating voltages by 10% (i.e. 400 kV — 440 kV). One should, however, ensure that dynamic and transient overvoltages are within limits.
Stability Issues. There are certain stability issues that limit the transmission capability. These include steady-state stability, transient stability, dynamic stability, frequency collapse, voltage collapse and subsynchronous resonance.
The FACTS technology can certainly be used to overcome any of the stability limits, in which case the final limits would be thermal and dielectric.