Abstract: The main steam overpressure protection systems of the AP1000 nuclear power unit and the 1000MW ultra supercritical fossil fuel power unit were introduced; the thermal parameters, the protection devices and the control functions of the two protection systems were analyzed comparatively. The main steam overpressure protection of AP1000 nuclear power unit is realized by the safety valve, the steam turbine bypass system and the atmospheric relief valve，it has the dual functions of temperature adjustment and pressure adjustment, which can ensure safe outage of the reactor in emergency. In 1000MW fossil fuel power unit, the high pressure bypass valve is used to replace the superheater safety valve, which not only can eliminate the high intensity noise caused by the safety valve discharge, but also can recover the working medium as far as possible and improve the start performance of the unit.

Key words: AP1000 nuclear power unit, 1000MW fossil fuel power unit, atmospheric relief valve, safety valve, high pressure bypass, turbine bypass system