This paper describes how to smoothly restore the power system from blackout using a hydro power plant. Various kinds of abnormal phenomena could occur during the period of blackouts and restoration. Taking various possible abnormal phenomena into account, the authors first performed an analytical study of power system restoration processes using the proposed method with a simplified power system model consisting of a generator, some transmission lines and transformers. Based on this study, an actual power system test was then carried out. A transformer voltage goes to increase after its voltage application. This voltage increase causes an over voltage problem to a transformer. The authors are proposing practical countermeasures on the over voltage problem at restoration with a hydro power plant.