Islanded power networks comprising a small number of large power plants are subject to perturbations leading to important transients, and, hence need high performance governors. The design of these governors requires a detailed model of the system. This is even more important in the case of hydroelectric power plants featuring surge tank and long penstock. For a complete understanding of the dynamic behavior of the system, the model must also take into account the interaction with the electrical power network. This paper presents such a modeling for a 1 GW hydroelectric power plant and the use of an efficient method for the governor parameter determination. Assessment of the governor performances is carried out for different power ratios between hydroelectric and whole network.