The need for a quality supply of electrical energy is becoming increasingly pronounced in today's life. It is very important that the elements of the power system are well protected against the defects occurring in the electrical power system. Large costs may occur in the case of a poor protection. The purpose of this thesis is to show the importance of protection in the electric power system, with an emphasis on its importance in power plants. Therefore, we first describe the topics that show us defects on various elements of the power system and the types of protection that isolate such defects from the other part of the electric power system, thus preventing damage to the elements in the system. After general overview of defects and protection, demonstration of the protection of hydro power plant Sindila in Uganda is shown. Relays and protection parameters of the hydro power plant are described. Each protection also needs different parameters for its proper functioning. These parameters are specified in the relay, which then protects elements. Finally, in the Simulink software environment, the differential protection of the transformer is shown, which we carried out with our own model. This protection is modeled through a simplified protection model of the hydro power plant Sindila and represents the first major protection of the hydro power plant. The simulation is made from ideal voltage and current meters where we ignore the errors. Ideal circuit breakers are also used, with no reaction time and cable connection without impedance. However, basic parameters and protection parameters from the hydro power plant Sindila are taken into account in the simulation. The model shows the operation of the protection without time delay and with time delay. From the simulations it can be seen that in the event of damages, there are rather large currents that cause heating and losses, therefore it is very important that the protection switches off the protected eqipment as soon as possible, or at a predetermined time lag. With the delay, we want to prevent possible early shut downs of hydro power plant in cases when load loads occur or in switching maneuvers.