The article discusses the low-frequency oscillations of active power and reactive power recorded during real black-start testing in the power system in Slovakia. The occurrence of these oscillations is analyzed by the theory of calculating RMS values of voltage and current, as well as by measuring the mean value of active and reactive power. The mathematical analysis and theoretical simulations described in this paper confirmed the supposed cause of the measured low-frequency oscillations. These oscillations were caused by inappropriate time window for RMS values as well as active and reactive power evaluation because of two different dominant power frequencies existed. Mutual interconnections between power lines installed on one four-system power line were responsible for the occurrence of these two different dominant frequencies because one line was operated in anisland and three other lines were operated in the interconnected grid with frequency 50 Hz. Understanding the cause of low-frequency oscillations occurrence is necessary for the process of finding technical solutions for its elimination.