The major difficulty in modeling aerial transmission lines and underground cables is derived from the fact that they are highly nonlinear in nature; due mainly to the frequency dependency of conductors (skin effect), as well as the ground or earth return path. The ability to represent these systems accurately and efficiently plays an essential part in the electromagnetic transient simulation of power systems as a whole.
Transmission systems are frequency-dependent and so it makes sense to solve their parameters in the frequency domain. This is only suitable for those instances when the frequency domain characteristics are of primary interest. In order to accurately represent a frequency-dependent line when simulating with EMTDC (which of course operates in the time domain), these parameters must be convolved into their equivalent time-domain characteristics. The techniques required for this convolution are quite complex, and are one of the primary attributes that differentiate the various transmission system modeling methods available today.
PSCAD includes a few different modeling techniques, each with their own pros and cons. The most accurate of these, the Frequency Dependent (Phase) model, is one of the most precise in the world.