Increasing integration in the Baltic electricity market and Europe-wide interconnection establishment makes it necessary to consider markets principals for power system development planning. In this paper is given a method for modelling of zonal prices with application and long-term development planning strategies. Proposed method of zonal prices is based on Interior Point Method (IPM) for finding maxima of Social Welfare subject to power system constraints. The main parts of this task are improving existing method for criterion calculations in development tasks with market conditions as well as analyze advantages and disadvantages of Method. The results of this research could contribute to development of methods appropriate in sustainable development management of power system.