This study is dedicated to the development of a transmission power system model to find optimal technical, economic, and ecological solutions for its operation under rapid integration of renewable energy sources (RES). The developed in the MATLAB environment modeling algorithm let determinate optimal power flow distribution across transmission lines to improve power system operational efficiency and security by considering the variable nature of generation and consumption, dynamic character of restriction changes of transmission line capacities, and electricity market prices, based on the day ahead regime planning schedules. Experimental results indicate that this approach, by avoiding immediate extensive grid modernization, enables to maximize green RES generation share in transmission power system and reduction in operational costs without additional risks to system operational security. The developed algorithm can further be applied in large-scale modern transmission power system models for operational planning and management.