Abstract: An HVDC transmission system including a rectifier station and an inverter station each having a series connection of at least two converters. A by-pass DC breaker is connected in parallel with each converter. A control device is adapted to deblock a blocked converter by starting to control the converter at high delay angle and gradually decreasing the delay angle until substantially all DC current flows through the converter and to then control the by-pass breaker to open at substantially zero current, and to stop the operation of a converter by controlling the converter at gradually increasing delay angle until the voltage across the converter is substantially zero and to then control the converter to be blocked by firing a by-pass pair thereof and to then control the by-pass breaker to close for taking over all the DC current when the voltage between a neutral bus and a pole of a transmission line between the stations is to be increased and reduced, respectively.

Abstract: An HVDC transmission system including a rectifier station and an inverter station each having a series connection of at least two converters. A by-pass DC breaker is connected in parallel with each converter. A control device is adapted to deblock a blocked converter by starting to control the converter at high delay angle and gradually decreasing the delay angle until substantially all DC current flows through the converter and to then control the by-pass breaker to open at substantially zero current, and to stop the operation of a converter by controlling the converter at gradually increasing delay angle until the voltage across the converter is substantially zero and to then control the converter to be blocked by firing a by-pass pair thereof and to then control the by-pass breaker to close for taking over all the DC current when the voltage between a neutral bus and a pole of a transmission line between the stations is to be increased and reduced, respectively.