This thesis attempts to present reconstruction of the relay-protection of the generators applied in the nuclear power plant in Paks based on the assessment of generator labelled 42 SP. This reconstruction process is part of the project aiming to lengthen the lifespan of the power plant, therefore, after briefly discussing the structure and functioning of the power plant, the relations between the reconstruction and the aforementioned project are presented. The following chapter deals with the secondary network-topology of the power plant and, in close connection with that, the structure and specialities of the generator TVV-221 are presented. Furthermore, the requirements for the protection system are presented. In connection with the out-of-dated protection system, the main functions of the protection system are described; against what they must protect regarding the generator, and how these functions are realized. Then, the machinery and the software and hardware setup of the new complex system is presented, alongside with their usage. Also, the way it handles the aforementioned protection functions are presented as well as the parameters the system requires for the satisfactory operating. In the closing chapter, the trials and assessments that are required before the manufacturing and commissioning in of the protection system are described, based on which the normal operating may be considered reliable.Finally, the expectations in connection with the outcome of the reconstruction are presented.