Contemporary power systems tend to be unreliable without use of the modern computer technology. In order to ensure the safe and stable operation of the power systems, many practical problems must be solved. Some of the tasks are associated with calculations of the steady state power flow on mathematical models of power systems. Calculations allow predicting the grid responses on the possible changes in the future with sufficient accuracy. The challenges of improving the quality, observability and speed of calculations in load flow studies for the power system planning in short term are the subjects of this doctoral thesis. Load flow calculation program could be extended for the target of creating simulator for operators training. This study was also contributing successfully to practical implementation of dispatcher training simulator in Latvian transmission system operator, based on MUSTANG program. Furthermore in this study, the simplified method for possible fault calculation in case of insufficient input data was developed. The method was shown on well-known IEEE testing model.