In this paper, we would like to review and introduce technologies which mitigate problems in an electric power grid caused by installation of renewable energy sources (RES). We classified these technologies into three types of technologies in an analogy of 'medical therapy'categorization, which are 'causal therapy', "symptomatic therapy', and 'infection prevention'. As for the causal therapies, active-power-based control for reducing short-term output power fluctuations and a hybrid system of battery storage and wind turbines for long-term output power fluctuation are described. For the symptomatic therapy, a reactive power control is mentioned. In terms of the infection prevention, harmonic reduction technology is introduced.