In order to protect the primary equipment of electricity networks, we install protection relay systems. These aim to minimize the impact of failures on network operation, prevent damage to the equipment and consequently reduce the costs. Differential protection is one of the most important functions of protection relays. It compares the currents on the primary and secondary sides of the protected element, whose sum must be close to zero according to the 1st Kirchhoff's law. In case of big differential current the protection is activated and causes the disconnection of the protected element from the network.
We developed a model of differential protection for a power transformer at a medium voltage level. Power transformers convert primary voltage in a given ratio to secondary one, and are among the most important and expensive elements of a power system. Therefore, their protection must be properly planned and implemented.
Our algorithm was developed using Simulink, the simulation package in Matlab. We took into account the relevant interfering factors that occur in power transformers (inrush currents, overexcitation conditions and current transformer saturation). These were identified with a method based on harmonic analysis of differential current. Their influence on reliability of the protection was adequately eliminated.
We simulated the response of our algorithm on failures within and outside of the protected zone. We observed the influence of current transformer saturation on protection activation, and demonstrated compliance with time requirements. Finally, we provided recommendations for further improvements of the developed algorithm.