FILTER

PURPOSE:To enhance the high-speed response and stability, by combining an analog filter and a digital filter. CONSTITUTION:For a filter to be provided to an AC input of a protection/control device of an electric power system, a digital filter has a high-speed response to the elimination of a low frequency component with an intrinsic speed of response secured for each type of filter and has the upper limit to use 1/2 sampling frequency. While for an analog filter, the elimination of a high frequency component is facilitated and the speed of response varies by the attenuation factor. For the filter of the AC input of a digital protection relay, an analog filter that cuts the frequency of >=1/2 sampling frequency of a digital filter is set previously to a digital filter. The digital filter cuts a low frequency component such as a drift component. Thus the merits of a digital filter and an analog filter can be obtained by combining these two filters.