We solve the robust control problem for a network of electric power generators whose mathematical model is represented by a system of third order differential-algebraic equations with parameters that are not known a priori. In our solution, we assume that only relative angular velocities of generator rotors are available for observation. We obtain a control algorithm that ensures network synchronization with the necessary accuracy in both standard mode of operation and in emergencies related to abrupt changes in the transmission line resistance. Operation of the proposed scheme is demonstrated with a numerical example dealing with a network of three generators.