In order to assess sufficiency of Latvian power system, research simulation model has been created. It embraces Latvian, Estonian and Lithuanian systems and parts of Russian and Belorussian power systems. The software LDM-AD has been applied for analysis performance on reliability of transmission network substations and switchgear units, as well as technical and economic analysis of several dynamic development process options observing reliability economic indicators. The researches performed on planning methodology development indicated that system sufficiency estimation for 5 and 10 horizon is not capable to ensure sustainable development strategy. In order to ensure long-term development, entirely different planning methodology and other tools for analysis are required. The planning process shall be sustainable taking into consideration rather long prospective period from 30 to 50 years.