This paper considers the problem of assessing and predicting the quality of three-phase electric power systems by introducing an integral criterion. The criterion is based on applied and fundamental research in the field of modern power system management, conducted at the Center of Digital Solutions for Smart Grid, the Trapeznikov Institute of Control Sciences, the Russian Academy of Sciences. The integral assessment criterion is based on several key indicators: frequency deviation, slow voltage changes (in each phase), and the total harmonic distortion (in each phase). Practical results of applying this criterion are presented for the data collected in a real electric power system since 2021. The criterion expands the capabilities of assessing the quality of electric power systems compared to those accepted in the industry.