The quality of the electric power delivered to customers by utilities may not be acceptable for some types of sensitive loads, which are typically power electronics- and computer-based loads controlling industrial processes. There are cases where the increasing use of power electronics to enhance process efficiency and controllability creates other problems. To evaluate the promising solutions offered with the growing use of power electronic devices in transmission and distribution systems, as well as to analyze their interaction and impact on either the load or the network side, computer programs based on the EMTP (Electromagnetic Transients Program) are becoming more useful. The development of new EMTP-based models for representation of controls and power electronic devices has been the main subject of this Ph.D. thesis project. Its main contributions were the development of a "simultaneous solution for linear and nonlinear control and electric power system equations" (SSCPS), through the compensation method and the Newton-Raphson iterative algorithm. Hopefully, the models developed in this work may help on the electrical compatibility assessment among loads and power system.