PURPOSE: To provide a high speed and high sensitive operation, by applying a transformer circuit, that comprises a filtering and settling circuit likely for a removal of higher harmonic, with a voltage input converted into a suitable level.
CONSTITUTION: A transformer circuit 8, for making a filtering operation like a removal of higher harmonic with a secondary voltage converted into a suitable level, shifting circuits 9 to 11, for making a shifting to an angle of θ, θ-π/3, θ+π/3 with an output of said circuit 8 made as an input, squared circuits 12 to 14, for producing an output proportional to the square of the above output, an adder circuit 15, for obtaining the sum with the total outputs of those of the above squared circuits inputted, and detecting circuits 16 to 18, for detecting a production of change in the output of the adder circuit, are provided. In this way, an intermittent output of "1", "0" is not produced from the level detector 18 at the normal time as usually made likely before, and an output of the level detecting circuit is always "0", causing no requirement to remove the intermittent output by using like a time limit operating circuit, consequently, the detection is correctly made simultaneously at the time of fault occurrence, resulting in an operation capable at high speed.
SUZUKI SHIGERU
NAKASHIMA MASUO
