Numerical simulations of the transient operation of an islanded power network subject to load rejections are performed. The islanded power network case study consists of a 1-GW hydroelectric power plant featuring four generating units, a long penstock, and a surge tank connected to four 1.3-GW thermal power plants and passive consumers. The modeling of every network component is described and special care is paid to the modeling of the hydroelectric power plant. In particular, the high-order modeling used for these investigations enables the detailed simulation of waterhammer, mass oscillations, and nonlinear hydraulic characteristics of the turbines interacting with the electrical components of the network. Moreover, the stability analysis of the systems is performed for different load conditions and the damping performances of a power system stabilizer IEEE PSS2B are investigated.