Method for controlling hvdc transmission system to avoid line frequency overvoltage upon load dumping

Claim 1

In a high-voltage direct-current transmission system in which alternating-current power fed in from a polyphase voltage supply network is rectified by a converter of the thyristor valve type at a transmitting station and transmitted as direct current to a converter of the thyristor type located at a receiving station where it is then inverted into alternating-current power and supplied to a polyphase voltage network to which a consumer load is connected, the method for operating the system such as will diminish line-frequency over-voltage stresses on the thyristor valves particularly in the event of a disturbance of the type wherein partial or complete dumping of the consumer load occurs, which comprises the steps of progressively setting back the ignition angle of the converter at the transmitting station temporarily into the inverter range so as to progressively reduce the rectifier voltage, and thereafter re-setting the ignition angle of the converter at said transmission station into the rectifier range to prevent the rectifier current from dropping to zero during the disturbance, and the amplitude of the rectifier voltage following reset of said converter into the rectifier range being less than that which existed prior to the changeover of said converter into the inverter range, thereby to effect removal of reactive power from said polyphase supply network after the disturbance occurs.