Earth fault protection for direct current transmission

Claim 1

A HIGH VOLTAGE D.C. POWER TRANSMISSION SYSTEM PROVIDED WITH A PROTECTION MEANS FOR DETECTING LINE FAULTS WHICH IS CHARACTERISED BY FIRST AND SECOND VOLTAGE SENSITIVE MEANS THE INPUTS OF WHICH ARE CONNECTED TO THE VOLTAGE OF THE TRANSMISSION LINE, THE FIRST VOLTAGE SENSITIVE MEANS BEING RESPONSIVE TO A CERTAIN DECREASE OF THE LINE VOLTAGE AND THE SECOND VOLTAGE SENSITIVE MEANS BEING RESPONSIVE TO A LINE VOLTAGE BELOW A PREDETERMINED LOW LINE VOLTAGE LEVEL, A TWO STAGE DELAY MEANS CONNECTED TO SAID FIRST VOLTAGE SENSITIVE MEANS, TO RECEIVE A SIGNAL THEREFROM, THE FIRST STAGE OF SAID DELAY MEANS INCLUDING MEANS TO PRODUCE A DELAY CORRESPONDING TO THE DAMPING TIME FOR A VOLTAGE OSCILLATION OCCURRING IN THE TRANSMISSION LINE, THE SECOND STAGE INCLUDING MEANS RESPONSIVE TO THE OUTPUT FROM THE FIRST STAGE TO PRODUCE A TIME DELAY CORRESPONDING TO THE PERIOD OF AN A.C. NETWORK CONNECTED TO A CONVERTER STATION OF THE SYSTEM, AN OUTPUT STAGE OF SAID PROTECTION MEANS INCLUDING MEANS TO INITIATE A CORRECTIVE ACTION ON THE OCCASION OF A LINE FAULT, A SWITCHING MEANS CONNECTING SAID OUTPUT STAGE TO SAID SECOND DELAY STAGE, SAID SWITCHING MEANS INCLUDING MEANS RESPONSIVE TO THE OUTPUT OF SAID FIRST DELAY MEANS TO RENDER SAID SWITCHING MEANS OPERATIVE AT LEAST AT THE END OF THE DELAY PRODUCED BY THE FIRST DELAY MEANS TO CONNECT THE OUTPUT STAGE TO THE SECOND DEALY MEANS, SAID SWITCHING MEANS INCLUDING HOLDING MEANS NORMALLY OPERATIVE TO MAINTAIN SUCH CON-