This thesis describes transition of a part of a power system of Gorenjska to islanded operation mode and further operation in such mode. Islanded operation mode with advantages and disadvantages and main differences from interconnection operation mode are described. In next chapter frequency, voltage active and reactive power regulation is described. Part of power system of Gorenjska is described as well as its model in Matlab/Simulink simulation environment. In next chapter simulation results with various actions to stabilize islanded operation mode are described. Two scenarios of transition to islanded operation mode are simulated. Conclusions as guidelines and recommendations are presented in last chapter.