The use of small hydroelectric power plants has increasingly in the last decades worldwide. In this context, this paper presents a study of an industry of batteries that has a small hydroelectric power plant that feeds their electrical system, where the capacity of the plant coincides approximately with the average industry demand. In periods of high load the own generation is replaced by the power distribution company of electrical energy. Aiming the suitable usage of his generation resources and considering the industry expansion projection, this paper presents real case simulations of the industrial electrical system operation, evidencing the behavior of the frequency of generated voltages and the rotor angle, in this last case by determining the critical times for reconnecting the generator or transmission line to keep the synchronism. The computer simulations are made considering alternatives of isolated operation and the system connected to the network.