This paper presents the data preprocessing algorithm for real time digital simulation of large power systems based on energy management system data. Recently, the power systems have become complicated due to increase of distributed power, including renewable energy and High Voltage Direct Current & Flexible AC Transmission System. In order to overcome the technological limitations, there are more and more cases of analyzing real time operation characteristics through Real-Time Digital Simulator. However, because the Real-Time Digital Simulator has a load unit limit per rack, it is difficult to simulate large scale power systems. When analyzing large scale power systems, we often use data obtained from actual power system operation data through Energy Management System. In this paper, we propose a solution to solve various errors and reduce the load unit limit for real time digital simulation by processing Energy Management System data.