A new control method based on PID for load frequency control in power systems is proposed in order to suppress frequency deviations due to load and generating power fluctuations occurring by increased penetration of renewable resources such as photovoltaics or wind farms. The variation of renewable resources can be considered to be a kind of load fluctuations. For the appropriate absorption of load fluctuations, these are separated into three components - short-term less than 10 minutes, medium-term ranging 10 to 30 minutes, and long-term sustaining over 30 minutes. In this paper, to absorb these random load disturbances effectively fast generators and slow ones are separately controlled by processing the filtered random frequency deviations. The separation of slower and fast load fluctuations is carried out by low pass filter. The effectiveness of the proposed method is illustrated by a three area power systems.