The authors are involved in a project devoted to development of new methods for setting up of relay protection (RP) of modern electric power systems (EPS) using detailed mathematical models, which take into account the specific features of RP and processes in transducers. Results presented in the paper show that numerical methods of integration are badly applicable for solving even simple mathematical model of RP. Such conclusion was made due to obvious distortion of the simulated signals. An alternative to numerical modeling is the so-called hybrid approach, on the basis of which a simulator of EPS - Hybrid Real Time Simulator (HRTSim) was created. This approach was applied for development of software and hardware element compatible with the HRTSim. Test of the element was performed and there is no distortion appears in the output simulated signal.