| 摘要 |
PURPOSE:To provide an earth trouble detecting device that is not responsive to harmonic currents caused by unbalance in the AC current system and performs stable performance by detecting only direct current component in the current flowing at an earth point. CONSTITUTION:A direct current transformer DCCT6A detects currents of both poles and corresponding signals can be obtained therefrom. This signal has its DC component cut off by a differential circuit 8 and inputs only the AC component to an addition unit 9. The other inputs to the addition unit 9 are the output of a rectifying bridge 62 and the AC components are cancelled each other out. The DC component output of the addition unit 9 is input to a comparator 73 after rectifying by a total wave rectifier 10, and there discrimination of an AC component whether it is caused by unbalance in the AC components or it is a detected component by earth trouble is automatically conducted. In other words, during unbalance in AC components, DC component is small, and furthermore because harmonic components due to the unbalance and detected alternately are cancelled each other out on the input side of the addition unit 9, the detected DC component is smaller than the level for comparison, but the DC component during earthing is larger than the level for comparison and is shown in the output of the comparator 73. |
