| 摘要 |
For filtering off of low frequency and a periodic transient phenomena in relay protection devices for electric high voltage networks, the signals of which occur in the event of a fault in the network and which are superimposed on the signals supplied to measuring relays arranged in the relay protection device, a normally short-circuited filter is included in a normally open alternative path for the measuring signal which is parallel to a part of the normal signal path. A supervisory circuit includes a time circuit, a fast fault indicator and an undervoltage detector. A signal from the fault indicator breaks the short-circuit of the filter and, upon a signal from the undervoltage detector, connects the filter in the path of the measuring signal. The measuring relay is blocked during a time interval which is equal to the difference between the pulse time of the time circuit and the time of operation of the undervoltage detector. This is accomplished by an AND-gate which has its inputs connected to the fast fault indicator and to the undervoltage detector and its output connected to a make-and-break contact for closing the alternative current path. |
