Procedure and device to operate an HVDC transmission short coupling in the event of power system faults

procedure for operating a HVDC transmission short coupling between two multi-phase a.c. power networks in the event of fault in one of the two a.c. power networks each connected by a static converter to a d.c. transmission line portion of the HVDC transmission short coupling comprising the steps of:

• detecting a fault-included voltage amplitude change, at the at least one phase of the power network which is malfunctioning due to a network fault and converting the fault induced voltage to a first pilot control variable;
• reducing a HVDC voltage under control of a converter control variable, said control variable being a voltage reference value and comprising the sum of said first pilot control variable and a first control voltage of the static converter connected to the faulty power network;
• and
• obtaining a second pilot control variable for a second control voltage controlling the other static converter and compensating the voltage difference at the connection points of the d.c. transmission line to the static converter in dependence upon said voltage difference.