Device for supervising a high voltage converter station

A device in a line commutated valve of a high voltage converter station located on high voltage potential level and having a plurality of valve units each having at least one semiconductor component of turn-on-type in the form of a thyristor and for each thyristor a first control unit located on high voltage potential level and controlling the thyristor, each of said first control units being connected to a valve control unit located on low potential level for communication between the valve control unit and the first control units, and said device for supervising a voltage divider circuit connected across each thyristor including resistances and capacitors connected in series, wherein the capacitors of the voltage divider circuit comprises a self-healing type, in which the capacitance decreases gradually as of the point of time for occurring of a possible defect thereof, the first control unit associated with the respective thyristor is arranged to register the voltage across the thyristor through a member arranged on high potential level through a first resistance of the voltage divider at an instant immediately before the turning-on of the thyristor and by that short-circuiting of the capacitor of the voltage divider and through a member register the voltage across a second resistance in the voltage divider circuit at given point of time after said instant, said means is arranged to calculate a value of the capacitance of the capacitor of the voltage divider on the basis of said values of two voltages and send information depending thereupon to an arrangement for supervising the valve arranged on low potential level, and members adapted to compare said capacitance value with a rated value of said capacitance and the supervising arrangement being arranged to determine whether a fault has occurred in the voltage divider on the basis of the comparison.