Abstract
This paper presents an experimental comparative analysis of electromechanical, static, and microprocessor-based protective relays using the secondary injection testing method. The study evaluates key performance indicators, including pickup current, dropout current, and operating time, under controlled laboratory conditions. The results show that electromechanical relays exhibit pickup current deviations below 5\%, accompanied by relatively high variability in operating time (up to 0.2 s), mainly due to inherent mechanical limitations. Static relays demonstrate improved performance, with enhanced stability, reduced deviations, and limited time variation within ±0.1 s. Microprocessor-based relays achieve the highest level of performance, characterized by minimal deviations, negligible hysteresis, and fast response times of approximately 30 ms. The findings confirm that the technological evolution of protective relays leads to significant improvements in accuracy, response speed, and operational stability. Nevertheless, electromechanical and static relays can still ensure reliable operation when properly maintained and correctly applied. The presented results provide a realistic assessment of relay performance across different technological generations and offer a useful basis for decision-making in the modernization and upgrade of protection systems.