This thesis deals with the changes of the control elements of a turbine bypass system for improving the stability of the thermal power plant facility. The turbine bypass system and its major control elements, especially equipped in Ha-dong power plant, are first investigated. The functions and operations of the high and low pressure turbine bypass systems (HPBP and LPBP) and the safety valves are then presented. A linear variable differential transducer (LVDT) are employed to improve the control stability and reliability which replaces the conventional contact type sensors. The changes of the limit switches and control logic are also considered to prevent the abnormal operation of the system In order to verify the stability improvement, the test operation for Ha-dong power plant Unit 3 are performed for the starting and stop conditions, and load variations. The results show the stable operation of the facility under the test conditions.