| 摘要 |
PURPOSE: To prevent erroneous function of an analog monitoring circuit due to fluctuation of frequency by comparing a current sampling data subjected to A/D conversion with a data sampled predetermined cycles before and locking the output of a decision circuit for comparing the negative phase sequence component with a decision level when the difference exceeds a predetermined level. CONSTITUTION: A difference appears between a sampling data at current moment of time and a data sampled one or more integer cycles of basic frequency before as the frequency varies. A monitoring/locking circuit 3 detects the difference and delivers a signal when the difference exceeds a predetermined level. Since a negative phase sequence component detection circuit 1 detects the negative phase sequence component using the current value of sampling data and data sampled 4 (120 deg.) and 8 (240 deg.) samples before, the circuit 1 detects an apparent negative phase when the frequency varies. A decision circuit 2 delivers a signal when the detection value exceeds a predetermined level. Since the monitoring/locking circuit 3 is delivering a signal at that time, an AND circuit 4 locks the output of the decision circuit 2. |
