Transmission system and a method for control thereof

A method for controlling a high voltage direct current transmission system, said system comprising at each end of a high voltage direct current transmission line a converter station for connecting said transmission line to an AC system, each converter station comprising a series connection of at least two converters having a DC side thereof connected on one hand to a pole of said transmission line on high potential and on the other to a neutral bus on zero potential by being grounded, a first of said converter stations being adapted to operate as rectifier and a second of said converter stations being adapted to operate as inverter, each converter comprising a by-pass DC breaker connected in parallel therewith in a current path forming a by-pass of said converter when said breaker is closed, said method comprising:

• controlling a DC current through said pole to be on a substantially constant, full current value under normal conditions of said transmission system,
• increasing a voltage between said neutral bus and said pole and thereby a power transmitted between said stations by starting operation of the converters being blocked, or
• reducing the voltage between said neutral bus and said pole and thereby the power transmitted between said converter stations by stopping operation of converters being active,
• wherein increasing the voltage between the neutral bus and the pole comprises unblocking a blocked converter by starting to control the blocked converter at a high delay angle and gradually decreasing the delay angle until substantially all the DC current flows through that converter and to then control said by-pass breaker in parallel with that converter to open at substantially zero current, and
• wherein reducing the voltage between the neutral bus and the pole the operation of said converter is stopped by controlling the converter at a gradually increasing delay angle until the voltage across the converter is substantially zero and then controlling the converter to be blocked by firing a by-pass pair thereof and then controlling said by-pass breaker in parallel with that converter to close for taking over all the DC current.

A computer program product, comprising

• a non-transitory computer readable medium;
• and
• computer program instructions recorded on the computer readable medium and executable by a processor for carrying out a method for controlling a high voltage direct current transmission system, said system comprising at each end of a high voltage direct current transmission line a converter station for connecting said transmission line to an AC system, each converter station comprising a series connection of at least two converters having a DC side thereof connected on one hand to a pole of said transmission line on high potential and on the other to a neutral bus on zero potential by being grounded, a first of said converter stations being adapted to operate as rectifier and a second of said converter stations being adapted to operate as inverter, each converter comprising a by-pass DC breaker connected in parallel therewith in a current path forming a by-pass of said converter when said breaker is closed, said method comprising:
• controlling a DC current through said pole to be on a substantially constant, full current value under normal conditions of said transmission system,
• increasing a voltage between said neutral bus and said pole and thereby a power transmitted between said stations by starting operation of the converters being blocked, or
• reducing the voltage between said neutral bus and said pole and thereby the power transmitted between said converter stations by stopping operation of converters being active,
• wherein increasing the voltage between the neutral bus and the pole comprises unblocking a blocked converter by starting to control the blocked converter at a high delay angle and gradually decreasing the delay angle until substantially all the DC current flows through that converter and to then control said by-pass breaker in parallel with that converter to open at substantially zero current, and
• wherein reducing the voltage between the neutral bus and the pole the operation of said converter is stopped by controlling the converter at a gradually increasing delay angle until the voltage across the converter is substantially zero and then controlling the converter to be blocked by firing a by-pass pair thereof and then controlling said by-pass breaker in parallel with that converter to close for taking over all the DC current.