| 摘要 |
PROBLEM TO BE SOLVED: To make a decision whether the frequency of a power system falls within a predetermined range or not with high accuracy by making the decision based on a value corresponding to the frequency of the power system. SOLUTION: A deciding section 1200 takes in a value corresponding to the frequency derived from a second frequency converter 16 and makes a decision whether it is larger than a predetermined value K2 within a band derivable from a first frequency converter 11. If it is larger than the value K2, a value corresponding to the frequency derived from the first frequency converter 11 is inputted and if it is larger than a predetermined value K1, an output is produced. If it is smaller than the value K2, a value corresponding to the frequency derived from the second frequency converter 16 is inputted and an output is produced. When the deciding section 1200 produces an output and a UV detecting section 13 does not produce an output, an AND circuit 15 produces an output. When the UV detecting section 13 produces an output and the deciding section 1200 does not produce an output, the AND circuit does not produce an output thus making a decision with high accuracy.
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