HVDC transmission schemes

The present invention provides a high voltage direct current (HVDC) transmission system (300, 600) comprising: a first station (102) comprising series-connected first and second HVDC converters (110, 130); a second station (104) comprising series-connected third and fourth HVDC converters (150, 170), wherein a neutral node (164) coupling the third HVDC converter (150) to the fourth HVDC converter (170) is coupled to earth; a first transmission line (200) connecting a positive node (114) of the first HVDC converter (110) to a corresponding positive node (154) of the third HVDC converter (150), wherein a first pole (240) of the system (300, 600) comprises the first HVDC converter (110), the third HVDC converter (150) and the first transmission line (200); a second transmission line (210) connecting a negative node (138) of the second HVDC converter (130) to a corresponding negative node (178) of the fourth HVDC converter (170), wherein a second pole (250) of the system (300, 600) comprises the second HVDC converter (130), the fourth HVDC converter (170) and the second transmission line (210); characterised in that: a neutral node (124) connecting the first HVDC converter (110) to the second HVDC converter (130) is coupled to a parallel combination of a resistance (310) and a neutral bus ground switch (312) for coupling the neutral node (124) to earth.