This work concerns the addition of the few critical elements of a protection system to the Electromagnetic Transients Program (EMTP). The EMTP is one of the most widely used programs for the simulation of transients in power systems. The EMTP contains models for almost every major power system component. A protection system consists of instrument transformers, relays, and circuit breakers. Models for current transformers and capacitor voltage transformers are developed, validated, and incorporated in EPRI/DCG EMTP Version 2.0. The user can define the values of the current transformer and capacitor voltage transformer parameters. Total Fortran capability has been added to EMTP; new subroutines and an inbuilt structure to allow the linking of user defined Fortran subroutines with the main EMTP are explained. This capability is necessary to simulate computer relay algorithms. The outputs of the algorithms can be passed to the EMTP, which enables the study of the dynamic interaction between the power system and the protection system. Models of specific relays for line protection (SLY12C) and transformer differential protection (BDD15B) are also available. The relay models can be used with different settings. These new features in EMTP together constitute the critical elements of a protection system. Thus, it is now possible to simulate the dynamic interactions between a power system and a protection system. / Ph. D.