The main objective of this paper is to show the ways of improving electricity generation and transmission long-term planning methods. Paper reveals differences in approach to long-term planning in different market organization models, showing similarities and differences in tasks and procedures. Performing of some procedures is a complicated task, which requires creation of special computer models of interconnected systems. There is given a small test case, providing model for justification of building real interstate link – “LitPol”. Since the calculations are simplified and the results might be inaccurate, there are given only a review of the method. The model is developed using newest development of Laboratory of Power System Mathematical Modelling - PSPlanner software that also is going to be briefly presented.