IMPROVEMENTS IN OR RELATING TO BIPOLE POWER TRANSMISSION SCHEMES

In the field of high voltage direct current (HVDC) power transmission networks there is a need for improvements in relations to bipole power transmission schemes.A bipole power transmission scheme (10) comprises a first converter station (12) that is positioned, in-use, remote from a second converter station (14). First and second transmission conduits (18, 20) and a return conduit (22) interconnect, in-use, the first converter station (12) with the second converter station (14) and thereby permit the transfer of power (PDC1, PDC2) between the first and second converter stations (12, 14). The first converter station (12) includes a first power converter (30) which has a first DC terminal (32) that is connected with the first transmission conduit (18), a second DC terminal (34) which is connected with the return conduit (22), and at least one first AC terminal (36) that is electrically connected with a point of common coupling (40) for connection to an AC network element which operates at a reference phase angle. The first power converter (30) further has a first converter controller (46) which is programmed to control the transfer of power between the first transmission conduit (18) and the point of common coupling (40) by having the first power converter (30) provide a first AC voltage (VAC1) at the or each first AC terminal (36) which is presented to the point of common coupling (40). The first converter station (12) also includes a second power converter (48) which has a third DC terminal (50) that is connected with the return conduit (22), a fourth DC terminal (52) which is connected with the second transmission conduit (20), and at least one second AC terminal (54) that is electrically connected with the point of common coupling (40). The second power converter (48) further has a second converter controller (56) that is programmed to control the transfer of power between the second transmission conduit (20) and the point of common coupling (40) by having the second power converter (48) provide a second AC voltage (VAC2) at the or each second AC terminal (54) which is presented to the point of common coupling (40). Also included in the first converter station (12) is a converter station controller (58) that is arranged in operative communication with each of the first and second converter controllers (46, 56) whereby the first and second power converters (30, 48) are controlled as grid forming converters such that the first and second AC voltages (VAC1, VAC2) presented to the point of common coupling (40) manage the AC frequency (f) and AC voltage (V) at the point of common coupling (40). The converter station controller (58) is programmed to operate the first converter station (12) in a current distribution mode (78) in which the distribution of DC current (IDC1, IDC2) between the first and second transmission conduits (18, 20) is controlled. The converter station controller (58), when operating the first converter station (12) in the current distribution mode (78), is programmed to instruct the first converter controller (46) to adjust the operating phase angle of the first AC voltage (VAC1) provided at the or each first AC terminal (36) in a first direction relative to the reference phase angle at the point of common coupling (40) and instruct the second converter controller (56) to adjust the operating phase angle of the second AC voltage (VAC2) provided at the or each second AC terminal (54) in a second direction opposite to the first direction relative to the reference phase angle at the point of common coupling (40).