In a power system with very high penetration photovoltaic power (PV) generation, the curtailment of PV power will be necessary to maintain a power supply and demand balancing. PV power output can be effectively utilized by controlling the curtailment at a certain interval based on forecasting and nowcasting of aggregated power output of PV systems. By using an optimum unit commitment scheduling model, this paper assesses the effect of a scheduled curtailment control at 30 min interval on the reduction of curtailed PV power output in comparison with a fixed curtailment control throughout a day. Then, by using an electricity supply - demand simulation model, this paper discusses the effect of ramp rate control of curtailed PV power output for avoiding the steep change in grid frequency due to the stepwise change in curtailment.