In recent years, a substantial amount of photovoltaic (PV) generations have been installed in power systems. However, the power output from the PV is random and intermittent in nature. Therefore, the PV generation poses many challenges to the power system operation. To evaluate the impact of the behavior of PV on power supply reliability, we have developed a power supply reliability evaluation model considering a large integration of PV generation into power system. As a result, the power supply reliability is getting worse as the PV penetration level increases. To solve this issue, we have proposed that pumped storage hydro power plant (PSHPP) is used effectively to improve the reliability. In this paper a cooperative scheduling method for the PSHPP and thermal power plants that makes it possible to improve both reliability and economy is presented.